Date   

Re: multipath

Mike WB2FKO
 

John's explanation has cleared up the physical picture I have of what is happening.  Two or more coherent signals traveling over different static paths and interfering at the receiver can't produce new frequencies.  All they can do is enhance or reduce signal strength, but there won't be any new harmonic content.  The path length has to be changing/moving to induce a Doppler shift.

If I assume a steady 10mph wind, pushing an air mass that can somehow reflect/scatter RF, I estimate a Doppler shift of about 7 Hz at 432 MHz. Stronger winds and higher carrier frequencies induce bigger frequency shifts. I am unsure if atmospherics can do this, but such a shift is consistent with the fuzz I remember seeing on the FT8 waterfall.  If the wind is steady, the signals should still decode. If the air mass is not moving steady, then the Doppler shift will fluctuate and and decodes may not happen. FT8 is not tolerant of frequency instability beyond ± 1 Hz. 

This problem should have been reciprocal, however, and Jay seemed to be decoding us just fine. This makes me suspect there was a problem in our setup.  I'm quite sure the audio level coming into WSJT was set below the point of clipping, although it might have been getting close. The indicator on the GUI turns red to warn the op of this condition, but I don't recall seeing that.

Didn't save any traces, so just going by memory.  I think the JT65 ghost signals (yes, they were weaker) were spaced at periodic intervals in frequency, suggesting a nonlinearity like clipping somewhere in the demodulation or decoding.

Mike

On 9/17/19 11:02 PM, John Klem wrote:
Actually, I'm saying the multipath you describe (time-invariant coherent interference) should not produce what you saw.  If I understand the physics correctly... 

If you consider a signal transmitted at a single frequency, you can scatter it however many times from however many static (not changing in any way with time) scatterers, but when you recombine all those linearly at any point (specifically the receiver), the sum will have the same frequency as the signal originally transmitted.  Only the amplitude and phase will be different in the final sum signal.  I think you can effectively consider any of the low-symbol-rate modes single-frequency for this purpose.  So I don't think time-invariant multipath explains what is happening here.

If your scatterers start changing slowly, you will see the sum signal at the receiver slowly changing in phase and amplitude, which would be typical fading.  The frequency remains essentially constant.  Again, not what you saw.

If the scatterers change rapidly, you could modulate the sum signal fast enough to create obvious sidebands.  It seems unlikely there would be anything like a natural modulator generating carrier plus one sideband, which is similar to what we've seen, but I don't know the math behind that.

I suspect there's some sort of moving atmospheric refractor creating a Doppler-shifted version of the original signal, but what is it and what is its path?  Maybe some sort of moving density boundary?  Maybe a big blob of something that is expanding or contracting?  Waves of some sort in the troposphere?Beats me, but I imagine someone with a good understanding of atmospheric science would have some ideas.

Do you remember, in the case where you saw four signals, were they equally spaced in frequency?

John AA5PR


Re: Our rove for the 2019 September VHF contest was in turn, rewarding…

Mike WB2FKO
 

Her are some photos of the rover, taken at the wrecking yard in Chinle, Arizona on Monday morning. WB2FKO

On 9/18/19 11:49 AM, James Duffey wrote:
Our rove for the 2019 September VHF contest was in turn, rewarding, frustrating, educational, and catastrophic. I roved with WB2FKO In this contest in part to learn WSJT and in part to have two operators, which is a big help in roving. I will deal with the catastrophic first; near the end of the contest, on our way back to Albuquerque we hit a horse with the rover just west of Whippoorwill, AZ. It was dark and the horse was black. We were not in open range, that is, the range in that area is fenced with gates and livestock is not supposed to be on the road, so it was unexpected. The horse is dead and the rover is likely totaled, although that is still up to the insurance company. It was (is) undrivable. Mike and I were examined by EMTs at the scene and other than a few scrapes and light bruises appear to be OK. Olivia’s brother Alex came up on Monday to pick us up and empty gear from the rover. We ended up back home about 1630 Monday.

This was to be a trip to learn how to use WSJT in the rover. Lots learned in that area. We spent the Thursday before the contest in my driveway doing a dry run. After a few issues we finally got everything working and made an FT8 QSO and an MSK144 QSO, so things appeared to be OK.

We started in DM64 and things went well, working a number of stations on SSB and CW. Signals seemed down from what I expected. We then switched to FT8 and picked up an Es opening and worked several stations. Signals were strong so we went to SSB and CW, but there was nothing. Calling CQ did not raise anyone. There were lots of looky-loos there, so we got in some good PR for ham radio. WW6V was among them and he stayed for a while to chat and watch us work. We got on the road and Mike worked several stations from DM65 while in motion.

The next stop was DM55 and we started having problems with the interface sending continuous dahs, no matter which mode we were in. It was intermittent, but the next day I discovered the paddle cord was frayed, so we solved that problem, too late, by carefully positioning the cord.

We then moved to DM44 in Holbrook, AZ, but couldn’t raise anyone on the way. We decided to have dinner before getting on the air. Denny’s seemed like a good choice, but it turned out to have a dysfunctional kitchen. We got on the air from the motel semi parking lot, which was pretty nice with clear horizons and low noise. The IsoPwr battery charger/isolator failed, so we had to hook up directly to the car battery and run the car. The battery booster also blew a fuse. I think there is something in the setup that draws more current than it should. When I disconnected the 222 gear it seemed to go away, so there may be a problem there. We worked some down into Phoenix, but signals were never strong.

After charging the battery overnight, We got up the next morning, with the purpose of working meteor scatter in the morning. For reasons that are still not clear to us, we didn’t work anyone in over an hour. PSK reporter indicates we were heard over a wide area, and we had lots of solid decodes. Nobody came back to us. I am sure it wasn’t a multipath problem. After a morning of this, we got back on the road to DM45 and set up in a nice spot with clear horizons. We got on the air and worked a few SSB/CW QSOs. With not much in the way of expectations, we switched to MSK144. Here, unlike in DM44, we had success immediately and put a half dozen stations in the log in half an hour, including some good grids. We then conducted the infamous “multipath” experiments with W9RM using a variety of WSJT modes and beam headings. 45 minutes later we decided to bag the WSJT modes and worked on CW. Too many scientists/engineers involved and not enough sense of contest urgency.

We then got on the road to DM45. I decided to take a BIA road through and above Keams canyon to avoid an extra hour on paved roads and that turned out to be a mistake. Never take an unpaved BIA Road! It had started to rain, which made the road muddy and slippery, but that wasn’t a problem until we had to descend down the other side of the canyon. It got steep and very slippery, similar to driving on ice. There were no guardrails and a very steep and high drop off, so we decided to stop until the road dried off. Despite not being able to point the antennas, which were pointed straight into the mountain, we got on the air, but I don’t think we worked anyone. We were still in DM44. After three hours it stopped raining and 45 minutes after that the road dried enough to travel and, although the sun had set, it was light enough for us to continue down the mountain. It was late by then so we decided to head back to Albuquerque.

About a half hour after that disaster struck and we hit the horse. This, from talking to people there, is a fairly common occurrence in this area. One of the EMTs said it was her third day on the job and this was the third time she had to deal with people in cars hitting livestock. No one seemed surprised that we had hit livestock and almost everyone had a story to tell of them or someone in their family hitting livestock.

With many (most?)people working WSJT modes, it seems like the VHF contests seem to have shifted from rate-oriented to activity-oriented events. Although many times rates could have been higher on SSB/CW, there was no activity to support them.

Not sure why, but it seemed after DM64 we never developed a rhythm. Some of this may have been due to having to shift from WSJT to the analog modes and some of it may have been that Mike and I weren’t familiar with each other’s operation styles. Anyway, I learned much about operating the digital modes that I can use going forward.



James Duffey KK6MC
Cedar Crest NM


Re: Our rove for the 2019 September VHF contest was in turn, rewarding…

Steve London
 

Glad to hear that you and Mike had only minor injuries.

Thanks for the FT8 QSO from DM55. One of the few times I was on during the 18 continuous hours of rain on Saturday/Sunday. You were weak (around -18, if I recall), but there was Es to southern CA, so you may have been strong, but 18 dB weaker than the strong Es signals.

73,
Steve, N2IC

On 09/18/2019 11:49 AM, James Duffey wrote:
Our rove for the 2019 September VHF contest was in turn, rewarding, frustrating, educational, and catastrophic. I roved with WB2FKO In this contest in part to learn WSJT and in part to have two operators, which is a big help in roving. I will deal with the catastrophic first; near the end of the contest, on our way back to Albuquerque we hit a horse with the rover just west of Whippoorwill, AZ. It was dark and the horse was black. We were not in open range, that is, the range in that area is fenced with gates and livestock is not supposed to be on the road, so it was unexpected. The horse is dead and the rover is likely totaled, although that is still up to the insurance company. It was (is) undrivable. Mike and I were examined by EMTs at the scene and other than a few scrapes and light bruises appear to be OK. Olivia’s brother Alex came up on Monday to pick us up and empty gear from the rover. We ended up back home about 1630 Monday.
This was to be a trip to learn how to use WSJT in the rover. Lots learned in that area. We spent the Thursday before the contest in my driveway doing a dry run. After a few issues we finally got everything working and made an FT8 QSO and an MSK144 QSO, so things appeared to be OK.
We started in DM64 and things went well, working a number of stations on SSB and CW. Signals seemed down from what I expected. We then switched to FT8 and picked up an Es opening and worked several stations. Signals were strong so we went to SSB and CW, but there was nothing. Calling CQ did not raise anyone. There were lots of looky-loos there, so we got in some good PR for ham radio. WW6V was among them and he stayed for a while to chat and watch us work. We got on the road and Mike worked several stations from DM65 while in motion.
The next stop was DM55 and we started having problems with the interface sending continuous dahs, no matter which mode we were in. It was intermittent, but the next day I discovered the paddle cord was frayed, so we solved that problem, too late, by carefully positioning the cord.
We then moved to DM44 in Holbrook, AZ, but couldn’t raise anyone on the way. We decided to have dinner before getting on the air. Denny’s seemed like a good choice, but it turned out to have a dysfunctional kitchen. We got on the air from the motel semi parking lot, which was pretty nice with clear horizons and low noise. The IsoPwr battery charger/isolator failed, so we had to hook up directly to the car battery and run the car. The battery booster also blew a fuse. I think there is something in the setup that draws more current than it should. When I disconnected the 222 gear it seemed to go away, so there may be a problem there. We worked some down into Phoenix, but signals were never strong.
After charging the battery overnight, We got up the next morning, with the purpose of working meteor scatter in the morning. For reasons that are still not clear to us, we didn’t work anyone in over an hour. PSK reporter indicates we were heard over a wide area, and we had lots of solid decodes. Nobody came back to us. I am sure it wasn’t a multipath problem. After a morning of this, we got back on the road to DM45 and set up in a nice spot with clear horizons. We got on the air and worked a few SSB/CW QSOs. With not much in the way of expectations, we switched to MSK144. Here, unlike in DM44, we had success immediately and put a half dozen stations in the log in half an hour, including some good grids. We then conducted the infamous “multipath” experiments with W9RM using a variety of WSJT modes and beam headings. 45 minutes later we decided to bag the WSJT modes and worked on CW. Too many scientists/engineers involved and not enough sense of contest urgency.
We then got on the road to DM45. I decided to take a BIA road through and above Keams canyon to avoid an extra hour on paved roads and that turned out to be a mistake. Never take an unpaved BIA Road! It had started to rain, which made the road muddy and slippery, but that wasn’t a problem until we had to descend down the other side of the canyon. It got steep and very slippery, similar to driving on ice. There were no guardrails and a very steep and high drop off, so we decided to stop until the road dried off. Despite not being able to point the antennas, which were pointed straight into the mountain, we got on the air, but I don’t think we worked anyone. We were still in DM44. After three hours it stopped raining and 45 minutes after that the road dried enough to travel and, although the sun had set, it was light enough for us to continue down the mountain. It was late by then so we decided to head back to Albuquerque.
About a half hour after that disaster struck and we hit the horse. This, from talking to people there, is a fairly common occurrence in this area. One of the EMTs said it was her third day on the job and this was the third time she had to deal with people in cars hitting livestock. No one seemed surprised that we had hit livestock and almost everyone had a story to tell of them or someone in their family hitting livestock.
With many (most?)people working WSJT modes, it seems like the VHF contests seem to have shifted from rate-oriented to activity-oriented events. Although many times rates could have been higher on SSB/CW, there was no activity to support them.
Not sure why, but it seemed after DM64 we never developed a rhythm. Some of this may have been due to having to shift from WSJT to the analog modes and some of it may have been that Mike and I weren’t familiar with each other’s operation styles. Anyway, I learned much about operating the digital modes that I can use going forward.
James Duffey KK6MC
Cedar Crest NM


Our rove for the 2019 September VHF contest was in turn, rewarding…

James Duffey
 

Our rove for the 2019 September VHF contest was in turn, rewarding, frustrating, educational, and catastrophic. I roved with WB2FKO In this contest in part to learn WSJT and in part to have two operators, which is a big help in roving. I will deal with the catastrophic first; near the end of the contest, on our way back to Albuquerque we hit a horse with the rover just west of Whippoorwill, AZ. It was dark and the horse was black. We were not in open range, that is, the range in that area is fenced with gates and livestock is not supposed to be on the road, so it was unexpected. The horse is dead and the rover is likely totaled, although that is still up to the insurance company. It was (is) undrivable. Mike and I were examined by EMTs at the scene and other than a few scrapes and light bruises appear to be OK. Olivia’s brother Alex came up on Monday to pick us up and empty gear from the rover. We ended up back home about 1630 Monday.

This was to be a trip to learn how to use WSJT in the rover. Lots learned in that area. We spent the Thursday before the contest in my driveway doing a dry run. After a few issues we finally got everything working and made an FT8 QSO and an MSK144 QSO, so things appeared to be OK.

We started in DM64 and things went well, working a number of stations on SSB and CW. Signals seemed down from what I expected. We then switched to FT8 and picked up an Es opening and worked several stations. Signals were strong so we went to SSB and CW, but there was nothing. Calling CQ did not raise anyone. There were lots of looky-loos there, so we got in some good PR for ham radio. WW6V was among them and he stayed for a while to chat and watch us work. We got on the road and Mike worked several stations from DM65 while in motion.

The next stop was DM55 and we started having problems with the interface sending continuous dahs, no matter which mode we were in. It was intermittent, but the next day I discovered the paddle cord was frayed, so we solved that problem, too late, by carefully positioning the cord.

We then moved to DM44 in Holbrook, AZ, but couldn’t raise anyone on the way. We decided to have dinner before getting on the air. Denny’s seemed like a good choice, but it turned out to have a dysfunctional kitchen. We got on the air from the motel semi parking lot, which was pretty nice with clear horizons and low noise. The IsoPwr battery charger/isolator failed, so we had to hook up directly to the car battery and run the car. The battery booster also blew a fuse. I think there is something in the setup that draws more current than it should. When I disconnected the 222 gear it seemed to go away, so there may be a problem there. We worked some down into Phoenix, but signals were never strong.

After charging the battery overnight, We got up the next morning, with the purpose of working meteor scatter in the morning. For reasons that are still not clear to us, we didn’t work anyone in over an hour. PSK reporter indicates we were heard over a wide area, and we had lots of solid decodes. Nobody came back to us. I am sure it wasn’t a multipath problem. After a morning of this, we got back on the road to DM45 and set up in a nice spot with clear horizons. We got on the air and worked a few SSB/CW QSOs. With not much in the way of expectations, we switched to MSK144. Here, unlike in DM44, we had success immediately and put a half dozen stations in the log in half an hour, including some good grids. We then conducted the infamous “multipath” experiments with W9RM using a variety of WSJT modes and beam headings. 45 minutes later we decided to bag the WSJT modes and worked on CW. Too many scientists/engineers involved and not enough sense of contest urgency.

We then got on the road to DM45. I decided to take a BIA road through and above Keams canyon to avoid an extra hour on paved roads and that turned out to be a mistake. Never take an unpaved BIA Road! It had started to rain, which made the road muddy and slippery, but that wasn’t a problem until we had to descend down the other side of the canyon. It got steep and very slippery, similar to driving on ice. There were no guardrails and a very steep and high drop off, so we decided to stop until the road dried off. Despite not being able to point the antennas, which were pointed straight into the mountain, we got on the air, but I don’t think we worked anyone. We were still in DM44. After three hours it stopped raining and 45 minutes after that the road dried enough to travel and, although the sun had set, it was light enough for us to continue down the mountain. It was late by then so we decided to head back to Albuquerque.

About a half hour after that disaster struck and we hit the horse. This, from talking to people there, is a fairly common occurrence in this area. One of the EMTs said it was her third day on the job and this was the third time she had to deal with people in cars hitting livestock. No one seemed surprised that we had hit livestock and almost everyone had a story to tell of them or someone in their family hitting livestock.

With many (most?)people working WSJT modes, it seems like the VHF contests seem to have shifted from rate-oriented to activity-oriented events. Although many times rates could have been higher on SSB/CW, there was no activity to support them.

Not sure why, but it seemed after DM64 we never developed a rhythm. Some of this may have been due to having to shift from WSJT to the analog modes and some of it may have been that Mike and I weren’t familiar with each other’s operation styles. Anyway, I learned much about operating the digital modes that I can use going forward.



James Duffey KK6MC
Cedar Crest NM


Re: multipath

Scott K5TA
 

Just got around to reading these messages —

A couple of questions come to mind:

Is there any noticeable difference in the appearance of the waterfall traces between the fundamental signal and the ghosts?  Signal strength?  Freq stability?

Was the only available spectrum display at audio?  It would be neat to look at an RF panadapter display alongside of the WSJT audio waterfall to compare.  A local RX issue could be in play, which would also provide a possible explanation for the non-reciprocal observations.

-TA


On Sep 18, 2019, at 8:34 AM, Keith Morehouse <w9rm@...> wrote:

There are, most certainly, two different phenomenon with regard to "multi-path" (I'll put that in quotation marks).

Bruce describes one, which is heard frequently.  Rapid fading due to out of phase paths mixing at the receive end.  There is another, which manifests itself as multiple coherent signals.  I see both, although the latter is much more obvious on CW or FT8.

-W9RM

Keith Morehouse
via MotoG

On Wed, Sep 18, 2019, 6:36 AM Bruce Draper <bruceaa5b@...> wrote:
What I saw -- and heard -- a few times was multipath fading of the single-frequency type described by John. It was plenty strong enough to be decoded, but failed. There was a single trace on the waterfall, but visibly a little different than normal. In the headphones, there was the sound of phase-shifted signals interfering with each other (hear this occasionally on DX signals on 20 CW, too).

Could've completed a CW or SSB QSO. Just sayin'.

    -Bruce AA5B

On Tue, Sep 17, 2019 at 11:02 PM John Klem <klemjf@...> wrote:
Actually, I'm saying the multipath you describe (time-invariant coherent interference) should not produce what you saw.  If I understand the physics correctly... 

If you consider a signal transmitted at a single frequency, you can scatter it however many times from however many static (not changing in any way with time) scatterers, but when you recombine all those linearly at any point (specifically the receiver), the sum will have the same frequency as the signal originally transmitted.  Only the amplitude and phase will be different in the final sum signal.  I think you can effectively consider any of the low-symbol-rate modes single-frequency for this purpose.  So I don't think time-invariant multipath explains what is happening here.

If your scatterers start changing slowly, you will see the sum signal at the receiver slowly changing in phase and amplitude, which would be typical fading.  The frequency remains essentially constant.  Again, not what you saw.

If the scatterers change rapidly, you could modulate the sum signal fast enough to create obvious sidebands.  It seems unlikely there would be anything like a natural modulator generating carrier plus one sideband, which is similar to what we've seen, but I don't know the math behind that.

I suspect there's some sort of moving atmospheric refractor creating a Doppler-shifted version of the original signal, but what is it and what is its path?  Maybe some sort of moving density boundary?  Maybe a big blob of something that is expanding or contracting?  Waves of some sort in the troposphere?Beats me, but I imagine someone with a good understanding of atmospheric science would have some ideas.

Do you remember, in the case where you saw four signals, were they equally spaced in frequency?

John AA5PR


Re: multipath

Keith Morehouse
 

There are, most certainly, two different phenomenon with regard to "multi-path" (I'll put that in quotation marks).

Bruce describes one, which is heard frequently.  Rapid fading due to out of phase paths mixing at the receive end.  There is another, which manifests itself as multiple coherent signals.  I see both, although the latter is much more obvious on CW or FT8.

-W9RM

Keith Morehouse
via MotoG


On Wed, Sep 18, 2019, 6:36 AM Bruce Draper <bruceaa5b@...> wrote:
What I saw -- and heard -- a few times was multipath fading of the single-frequency type described by John. It was plenty strong enough to be decoded, but failed. There was a single trace on the waterfall, but visibly a little different than normal. In the headphones, there was the sound of phase-shifted signals interfering with each other (hear this occasionally on DX signals on 20 CW, too).

Could've completed a CW or SSB QSO. Just sayin'.

    -Bruce AA5B

On Tue, Sep 17, 2019 at 11:02 PM John Klem <klemjf@...> wrote:
Actually, I'm saying the multipath you describe (time-invariant coherent interference) should not produce what you saw.  If I understand the physics correctly... 

If you consider a signal transmitted at a single frequency, you can scatter it however many times from however many static (not changing in any way with time) scatterers, but when you recombine all those linearly at any point (specifically the receiver), the sum will have the same frequency as the signal originally transmitted.  Only the amplitude and phase will be different in the final sum signal.  I think you can effectively consider any of the low-symbol-rate modes single-frequency for this purpose.  So I don't think time-invariant multipath explains what is happening here.

If your scatterers start changing slowly, you will see the sum signal at the receiver slowly changing in phase and amplitude, which would be typical fading.  The frequency remains essentially constant.  Again, not what you saw.

If the scatterers change rapidly, you could modulate the sum signal fast enough to create obvious sidebands.  It seems unlikely there would be anything like a natural modulator generating carrier plus one sideband, which is similar to what we've seen, but I don't know the math behind that.

I suspect there's some sort of moving atmospheric refractor creating a Doppler-shifted version of the original signal, but what is it and what is its path?  Maybe some sort of moving density boundary?  Maybe a big blob of something that is expanding or contracting?  Waves of some sort in the troposphere?Beats me, but I imagine someone with a good understanding of atmospheric science would have some ideas.

Do you remember, in the case where you saw four signals, were they equally spaced in frequency?

John AA5PR


Re: multipath

Bruce Draper
 

What I saw -- and heard -- a few times was multipath fading of the single-frequency type described by John. It was plenty strong enough to be decoded, but failed. There was a single trace on the waterfall, but visibly a little different than normal. In the headphones, there was the sound of phase-shifted signals interfering with each other (hear this occasionally on DX signals on 20 CW, too).

Could've completed a CW or SSB QSO. Just sayin'.

    -Bruce AA5B

On Tue, Sep 17, 2019 at 11:02 PM John Klem <klemjf@...> wrote:
Actually, I'm saying the multipath you describe (time-invariant coherent interference) should not produce what you saw.  If I understand the physics correctly... 

If you consider a signal transmitted at a single frequency, you can scatter it however many times from however many static (not changing in any way with time) scatterers, but when you recombine all those linearly at any point (specifically the receiver), the sum will have the same frequency as the signal originally transmitted.  Only the amplitude and phase will be different in the final sum signal.  I think you can effectively consider any of the low-symbol-rate modes single-frequency for this purpose.  So I don't think time-invariant multipath explains what is happening here.

If your scatterers start changing slowly, you will see the sum signal at the receiver slowly changing in phase and amplitude, which would be typical fading.  The frequency remains essentially constant.  Again, not what you saw.

If the scatterers change rapidly, you could modulate the sum signal fast enough to create obvious sidebands.  It seems unlikely there would be anything like a natural modulator generating carrier plus one sideband, which is similar to what we've seen, but I don't know the math behind that.

I suspect there's some sort of moving atmospheric refractor creating a Doppler-shifted version of the original signal, but what is it and what is its path?  Maybe some sort of moving density boundary?  Maybe a big blob of something that is expanding or contracting?  Waves of some sort in the troposphere?Beats me, but I imagine someone with a good understanding of atmospheric science would have some ideas.

Do you remember, in the case where you saw four signals, were they equally spaced in frequency?

John AA5PR


Re: multipath

John Klem
 

Actually, I'm saying the multipath you describe (time-invariant coherent interference) should not produce what you saw.  If I understand the physics correctly... 

If you consider a signal transmitted at a single frequency, you can scatter it however many times from however many static (not changing in any way with time) scatterers, but when you recombine all those linearly at any point (specifically the receiver), the sum will have the same frequency as the signal originally transmitted.  Only the amplitude and phase will be different in the final sum signal.  I think you can effectively consider any of the low-symbol-rate modes single-frequency for this purpose.  So I don't think time-invariant multipath explains what is happening here.

If your scatterers start changing slowly, you will see the sum signal at the receiver slowly changing in phase and amplitude, which would be typical fading.  The frequency remains essentially constant.  Again, not what you saw.

If the scatterers change rapidly, you could modulate the sum signal fast enough to create obvious sidebands.  It seems unlikely there would be anything like a natural modulator generating carrier plus one sideband, which is similar to what we've seen, but I don't know the math behind that.

I suspect there's some sort of moving atmospheric refractor creating a Doppler-shifted version of the original signal, but what is it and what is its path?  Maybe some sort of moving density boundary?  Maybe a big blob of something that is expanding or contracting?  Waves of some sort in the troposphere?Beats me, but I imagine someone with a good understanding of atmospheric science would have some ideas.

Do you remember, in the case where you saw four signals, were they equally spaced in frequency?

John AA5PR


Re: [AOCC] ARRL Sep VHF K7HP Single Op LP

Mike WB2FKO
 

Yes, offsets in frequency in the waterfall.  To confuse the FT8 decoder would require two signals offset in time by more than 40ms.  If my math is right, this corresponds to a path length difference of 12,000 km.  That's why I think it's multipath coherent interference, which only requires a path length difference on the order of a wavelength.  I suspect greater phase difference (multiple wavelengths) would make it worse and also explains why it would be more problematic as the radio frequency goes up.

If I understand you correctly, multiple static paths would produce multiple ghost traces that should still all decode.  It's the dynamic path difference that causes sufficient signal distortion to prevent decoding.

I should mention that in the ~30 minutes of trying various things with W9RM on 432, I did actually get one FT8 decode.  But we had already agreed to switch modes at that point and start over.

Mike WB2FKO

On 9/17/19 6:14 PM, John Klem wrote:
If I correctly understand, you saw multiple traces offset in frequency, not time?  I think many people have now seen this on 6 m (I'm among them), and it's not hard to believe the effect could be worse on 432.

In order to produce this frequency offset, I believe you must have some sort of dynamics in your path to induce Doppler or something similar.  Multiple static paths shouldn't do it.  I trust your receiver is linear enough that you shouldn't have observable mixing products (unless you have multiple strong signals already on different frequencies and an FT-817 like mine).  As Keith suggested, aircraft would do it, but the shift would vary with time (which I presume you didn't see).  I suppose some sort of atmospheric dynamics could produce this effect as well, and someone invoked that explanation in something I read recently.

I did a quick search on nonreciprocal propagation, and it appears this is generally recognized for ionospheric paths, but not obviously so for tropospheric paths.

Really interesting stuff.  It would be fun to do comparisons on different bands when you observe this, to see if the frequency offset scales with carrier frequency.

John AA5PR



Re: [AOCC] ARRL Sep VHF K7HP Single Op LP

John Klem
 

If I correctly understand, you saw multiple traces offset in frequency, not time?  I think many people have now seen this on 6 m (I'm among them), and it's not hard to believe the effect could be worse on 432.

In order to produce this frequency offset, I believe you must have some sort of dynamics in your path to induce Doppler or something similar.  Multiple static paths shouldn't do it.  I trust your receiver is linear enough that you shouldn't have observable mixing products (unless you have multiple strong signals already on different frequencies and an FT-817 like mine).  As Keith suggested, aircraft would do it, but the shift would vary with time (which I presume you didn't see).  I suppose some sort of atmospheric dynamics could produce this effect as well, and someone invoked that explanation in something I read recently.

I did a quick search on nonreciprocal propagation, and it appears this is generally recognized for ionospheric paths, but not obviously so for tropospheric paths.

Really interesting stuff.  It would be fun to do comparisons on different bands when you observe this, to see if the frequency offset scales with carrier frequency.

John AA5PR


Re: [AOCC] ARRL Sep VHF K7HP Single Op LP

Keith Morehouse
 

Mike gave a good explanation of what our problem was.  The multi-path really became obvious when we tried JT65 and KK6MC was seeing two synch tones.  I moved my antenna 10 degrees west and the result on the paths south end was FOUR synch tones.  Your question about reciprocity is valid.  I don't know why I wasn't being affected - the signal strength explanation put forward by Mike is certainly possible.  When we went to CW, I could detect a slight buzz or beat-note on his signal that was probably multi-path induced.

I have terrible multi-path problems on 2M when working rovers from a couple of commonly used locations to the south of me.  There is one place in particular were signals are very loud but the distortion is so bad on the direct path I need to turn the antenna 20-30 degrees east or west to copy anything on SSB.  On CW, it sounds like a DX pileup on 20M.  The 14,000' peaks of the San Juan range lay just 40 miles south of here.

On 6M, it is very common at this QTH to see multiple FT8 traces on 6M from 'local' stations (50-60 miles away).  The are usually offset by at least 1/2 of the FT8 signal bandwidth (so, maybe 15-20 Hz).  Sometimes, depending on who I'm hearing and where the respective antennas are pointed, they are far enough apart to see two distinct signals.  The result is usually TWO decodes of the same station, reported on two discreet frequencies, not a total failure.  It appears signals offset a smaller amount at much higher frequencies can cause problems.

A good example of multi-path can be viewed if you're near a major airport and can hear a CW beacon on 432 that's out a way - maybe 100 miles.  If the air traffic flow is correct, and the incoming or outbound traffic passes across your path to the beacon, you can watch a spectrum display and see the multiple signals caused by the aircraft reflections.  When I lived in Illinois, I used to monitor a 432 beacon in Michigan, northeast across the lake about 200 miles.  The direct path was right across the traffic flow in and out of O'Hare airport in Chicago.  The patterns of multi-path and Doppler shifted signals caused by the aircraft was amazing.  This is also a good demonstration of a technique called 'passive bi-static radar' that can be used to detect stealth aircraft without having to emit a signal of your own.

-W9RM

Keith Morehouse
via MotoG


On Mon, Sep 16, 2019, 9:40 PM John Klem <klemjf@...> wrote:
I am very curious - what characteristics of these signals led you to conclude multipath was the problem?  What was painfully obvious about it?
Should multipath be non-reciprocal?

Getting an audio recording of this kind of signal for analysis would be really interesting.

John AA5PR


Re: [AOCC] ARRL Sep VHF K7HP Single Op LP

Bill
 

Nicely done guys.

Bill


On Sep 16, 2019, at 10:54 PM, Mike WB2FKO <mph@...> wrote:

Duffey and I discussed this on the long ride home today.  Thinking about it some more, the idea of multipath is that two or more (coherent) signals from the same source travel by slightly different paths to the receiver.  This path difference only has to be a fraction of a wavelength. This causes interference and mixing in the detection circuit, producing ghosts of the digital signal in the audio waterfall.  This is exactly what I saw when we were running FT8 and especially JT65 (the synch signal is very obvious in the traces).  Seems this would be become a greater issue as the frequency goes up into UHF. All that said, FT8 should have been able to successfully decode the baseline signal and any harmonics.  Or maybe the frequency separation of the ghost signals was not sufficient and confused the decoder.

I am convinced the equipment was not at fault, as we had been working very successfully with various digital modes throughout the weekend.

The question of reciprocity is a good one.  W9RM's signal was very up and down, but his digital signals refused to decode independent of the widely varying receive level.  My hunch is that the reflection(s) occurred closer -- much closer -- to our QTH than his.  Beam divergence and diffraction would substantially weaken the reflected/scattered signal by the point they reached him, allowing decodes there.  Also the lower gain, wider aperture of the rover antenna may make us more susceptible.  But we were at an ideal location with almost nothing on the near horizon that suggested a reflection source. We experimented over the course of about 30 minutes with different digital modes and especially beam headings to no avail.  I would have wagered that moving the beam around would have fixed it.  And lost.

I have never had this problem (that I can remember anyway) on 6 or 2.  My experience with FT8 on UHF has been entirely this past weekend, or about 3-4 QSOs total.

Mike WB2FKO


On 9/16/19 9:40 PM, John Klem wrote:
I am very curious - what characteristics of these signals led you to conclude multipath was the problem?  What was painfully obvious about it?
Should multipath be non-reciprocal?

Getting an audio recording of this kind of signal for analysis would be really interesting.

John AA5PR


Re: [AOCC] ARRL Sep VHF K7HP Single Op LP

Mike WB2FKO
 

Duffey and I discussed this on the long ride home today.  Thinking about it some more, the idea of multipath is that two or more (coherent) signals from the same source travel by slightly different paths to the receiver.  This path difference only has to be a fraction of a wavelength. This causes interference and mixing in the detection circuit, producing ghosts of the digital signal in the audio waterfall.  This is exactly what I saw when we were running FT8 and especially JT65 (the synch signal is very obvious in the traces).  Seems this would be become a greater issue as the frequency goes up into UHF. All that said, FT8 should have been able to successfully decode the baseline signal and any harmonics.  Or maybe the frequency separation of the ghost signals was not sufficient and confused the decoder.

I am convinced the equipment was not at fault, as we had been working very successfully with various digital modes throughout the weekend.

The question of reciprocity is a good one.  W9RM's signal was very up and down, but his digital signals refused to decode independent of the widely varying receive level.  My hunch is that the reflection(s) occurred closer -- much closer -- to our QTH than his.  Beam divergence and diffraction would substantially weaken the reflected/scattered signal by the point they reached him, allowing decodes there.  Also the lower gain, wider aperture of the rover antenna may make us more susceptible.  But we were at an ideal location with almost nothing on the near horizon that suggested a reflection source. We experimented over the course of about 30 minutes with different digital modes and especially beam headings to no avail.  I would have wagered that moving the beam around would have fixed it.  And lost.

I have never had this problem (that I can remember anyway) on 6 or 2.  My experience with FT8 on UHF has been entirely this past weekend, or about 3-4 QSOs total.

Mike WB2FKO


On 9/16/19 9:40 PM, John Klem wrote:
I am very curious - what characteristics of these signals led you to conclude multipath was the problem?  What was painfully obvious about it?
Should multipath be non-reciprocal?

Getting an audio recording of this kind of signal for analysis would be really interesting.

John AA5PR


Re: [AOCC] ARRL Sep VHF K7HP Single Op LP

John Klem
 

I am very curious - what characteristics of these signals led you to conclude multipath was the problem?  What was painfully obvious about it?
Should multipath be non-reciprocal?

Getting an audio recording of this kind of signal for analysis would be really interesting.

John AA5PR


From Utah JS8Call on 144.176

George Gallis
 

Greetings,

Just found this group when looking for an email address for W9RM.

I have dabbled in 2m weak signal for many years and now trying to get serious about it.

I have at least temporary access to an unused 2m packet antenna at a site at 10,200 ft.  It is a 20ft StationMaster vertical. Location DM37NO62 on Blowhard Mountain just east of Cedar City, UT.

About a month ago we put an FT857 on it with the program JS8Call running on a Raspberry Pi.  JS8Call is built on the FT8 mode but has several other functions that allow what is in effect a beacon to run continuously. Also it can do some of the things that a packet digi does.  A distant station can query it to respond to a few questions or to relay to another station. It seems FT8 needs close operator attention.

Right now once every 10 minutes it sends the 15 second ID and Grid message. It is heard from Las Vegas to Central Utah so far. If you would check Pskreporter for the mode JS8 on 2m you would see recent activity.  The station IDs as AL7BX/11.

N6KOG in No Calif was heard once by this last week from 460 miles away. I had asked him to transmit for several hours one afternoon.

The receive may at times be poor since there is a 2m repeater right next to it that could see a lot of use.  However it is serving as a beacon for now and will respond to anything it does hear.

The dial frequency is 144.176 mhz is not the program's default since there was a birdie at 144.174.  Housed in that building are VHF and UHF repeaters and 7 UHF link transmitters since it is a major hub for two major systems.

There is no internet there so it can't report what it hears, I can query it for a stations heard list. But distant JS8 stations would report to Pskreporter if it is heard.

I'd appreciate any reports or comments.

KG7PBX and I are going to the ABQ Duke City Hamfest this week, so may meet a few of you.


Tnx
George
AL7BX
Cedar City, UT







W9RM - September VHF

Keith Morehouse
 

Summary:
BAND     QSOs     MULTs
6                67            54
2                38            26
222              4              4
432             11            11

Total Score:  120x95 => 12,285

Pretty good Rover turn-out for this contest and a little unusual conditions. Had a short Es opening on 6 to south TX and almost tropo-like conditions set in over the desert valleys due to some monsoonal moisture over the four corners. Because of that, it appeared I had better coverage into the eastern plains and was able to work rovers & a few fixed stations heretofore unreachable. Thanks to those same Rovers for a multitude of grid multipliers. Although, as usual, I didn't have much luck with local grids. I worked ONE DM58 station on 6M FT8 (of all modes - jeeze) and NOBODY in the surrounding DM59, DM69 & DM68. Even a basic rover with a tri-band radio and 222 add-on up at the grid corner (which I can see out the window) would have given me FIFTEEN more mults. Add on 902 and 1296, which I have full capability on and it would have been an extra TWENTY THREE mults, almost 10,000 more points AND a multiplier count close to the top scores out of New England. Lame.... Had to shut down a few hours early to get the towers and antennas squared away before the heaviest of the monsoons hit tonight and tomorrow. But, the same storms have rained out the four corners rover groups so not too much loss. EQUIPMENT 6M - 7 el @ 65', 7 el @ 40' & 5 el @ 30' fixed NW + KW 2M - 5WL yagi at 50' + 600W SSPA 222 - 5WL yagi @ 70' + 400W 432 - 9WL yagi @ 75' + KW SSPA
902 & 1296 on the shelf with nobody to work. -W9RM DM58xn  



Keith J Morehouse
Managing Partner
Calmesa Partners G.P.
Olathe, CO


Re: [AOCC] ARRL Sep VHF K7HP Single Op LP

Bill Mader
 

Sadly, the same is true on HF now!  There will be lots of FT8 signals on the bands and few, sometimes none, SSB and CW signals.  We operated the WAE SSB Contest from VY2TT as VY2AT this past weekend.  There were many times our CQ's went unanswered because there were no new ops to work.  

It's far worse on a weekday for example when I'm running a county, chasing parks or SOTA, etc.  These smaller groups often have more activity than the bands in general.  Although, it seems a lot of County Hunters have "drifted" over to FT8.

One of our team members here this weekend who is just short of the DXCC Honor Roll operates FT8 almost exclusively.  He chases DX entities with that mode for missing band-slots.  Often, FT8 is the only mode that propagation supports.

Another old friend, and now former contester, skips contests in favor of operating FT8 exclusively!  Just a year older than I, I hope I never get to the point where all I can handle, or all that interests me, is a couple of clicks to make a contact.

I will likely wait for the dust to begin to settle on WSJT-X contesting.  I think it's great that Joe and his team plus others are working hard on making those modes work.  However, it will require an incredible amount of training and education to make them work among the masses.  I will briefly cover some of these concepts at HF University at the DCHF this coming weekend.

Good luck in the contest!

73, Bill Mader, K8TE
W6H NM Coordinator, Route 66 On-the-Air 7-15 Sep 2019
ARRL New Mexico Section Manager
ARRL - The national association for Amateur Radio
Duke City Hamfest BoD www.dukecityhamfest.org 20-22 Sep 2019
President, Albuquerque DX Association 


Re: [AOCC] ARRL Sep VHF K7HP Single Op LP

KD
 

I have only been on VHF (6 and 2) since late May.  My first experience on 6M is hundreds of signals at most times of the day, only on FT8.  I have listened and called CQ on CW and SSB, but to this day, no takers.  So the vast majority of my 6M contacts are FT8.  During August I drifted to MSK since it seemed that FT8 was dead, and I did make contacts.  Fast forward to this weekend, and I was puzzled why so many people wanted me to try FT8 with them on 6M.  Anytime that I listened to the FT8 frequency on 6M, I got a burst every now and then.  No full period signal with one exception.  I worked W7DHH Saturday night.  He had a huge signal on the 600+ mile path and we tied up the QSO in normal time.  I heard him for at least 30 minutes longer calling CQ.  I don't know what the propagation mode was but that was the only good signal I heard on 6M FT8 the whole weekend.  My point is that I'm flabbergasted that anyone was using FT8  except to work very close stations.  2M on the other hand, was great for FT8.  All but 2 of my contacts on 2M were FT8 and they were in the 200 to 400 mile range.

This was my very first contest in my 49 years of ham radio.  It was definitely a huge learning experience.

73 de N5KS - KD


Re: [AOCC] ARRL Sep VHF K7HP Single Op LP

Keith Morehouse
 

I agree with K7HP's assessment of FT8.  For contesting, not only is FT8 intolerant of static crashes, but also of meteor pings, of which 6M has in abundance.

 Yesterday, KK6MC Rover op Mike, WB2FKO and I also found another problem with FT8. It is totally intolerant of multi-path.   We attempted a ~300 mile 432 MHz path over the mountains from DM58 to DM44.  We made a snap decision to use FT8 because of experience earlier that morning on a similar path.  Even though my signal was very loud in DM44, Mike could not get a decode.  We cycled through JT65 (where the multi-path problem was painfully obvious) to MSK144 and finally just went to CW (they were having equipment issues which took them off CW without a re-cable of their 432 system, so that mode wasn't immediately available).  Why didn't we just choose CW as our starting point, you ask ?  It was a combo of the aforementioned equipment problem AND the fact I was running 13 dB more power on my end (1000 vs 50 watts - the allowed maximum in NM/AZ).  A good signal on the south end of the path did not necessarily equate to easy copy on the north end.

The take away lesson here is obvious.  Intermountain paths are full of multi-path situations.  This multi-path was not immediately obvious on the FT8 waterfall display and no amount of digital fiddling about could produce a decode on a loud signal.  Meanwhile, on my end, with a considerably weaker signal and no multi-path, I was decoding 100% of the time.

A smart operator must consider all of the WSJTX modes as tools in the box.  No one tool is good for all circumstances.  This is MY major issue with FT8.  Most casual operators on 6M have given up on SSB operation, even during band openings and are exclusive users of FT8.  During a contest, when the goal is working as many stations as possible, they are limiting the run rate for everyone, serious and non-serious ops alike.  Unfortunately, many serious ops are dragged along in the rush, saying 'Well, we need to abandon SSB also and follow the contacts'.  This is impacting scores, impacting the 'fun factor' of 6M contesting for many and, most worry some to me, moving a lot of very experienced operators away from VHF contesting and off 6&2 in general.

-W9RM

Keith Morehouse
via MotoG


On Mon, Sep 16, 2019, 8:53 AM Steve London <n2ic@...> wrote:
On 09/15/2019 11:21 PM, K7HP wrote:
>
> ARRL September VHF Contest - 2019
>
> Comments:
> I just cannot get much interested in contesting using FT8 .
> Have no particular issue with using FT8 day to day but
> as a contest mode ESPECIALLY when I can hear by ear
> at least 75 percent of the signals I work on FT8 and could work
> them on CW in 5 -10 seconds .
This seemed especially true on 6 meter FT8. A static crash during the RX cycle
would eliminate any chance of a decode. My brain would have easily filled in the
missing dot or dash. Also, there was rapid QSB on most FT8 signals (weak Es ?
meteor trails ?). WSJT-X was also intolerant of that, whereas my brain and CW
would have had no problems.

MSK144 would have been far more appropriate than FT8.

73,
Steve, N2IC




Re: [AOCC] ARRL Sep VHF K7HP Single Op LP

Steve London
 

On 09/15/2019 11:21 PM, K7HP wrote:
ARRL September VHF Contest - 2019
Comments:
I just cannot get much interested in contesting using FT8 .
Have no particular issue with using FT8 day to day but
as a contest mode ESPECIALLY when I can hear by ear
at least 75 percent of the signals I work on FT8 and could work
them on CW in 5 -10 seconds .
This seemed especially true on 6 meter FT8. A static crash during the RX cycle would eliminate any chance of a decode. My brain would have easily filled in the missing dot or dash. Also, there was rapid QSB on most FT8 signals (weak Es ? meteor trails ?). WSJT-X was also intolerant of that, whereas my brain and CW would have had no problems.

MSK144 would have been far more appropriate than FT8.

73,
Steve, N2IC