[AOCC] ARRL Sep VHF K7HP Single Op LP
|
Steve N2IC
On 09/15/2019 11:21 PM, K7HP wrote:
ARRL September VHF Contest - 2019This 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 |
|
|
|
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: |
|
|
|
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 |
|
|
|
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! 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 |
|
|
|
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 |
|
|
|
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? |
|
|
|
Bill
Nicely done guys.
toggle quoted message
Show quoted text
Bill On Sep 16, 2019, at 10:54 PM, Mike WB2FKO <mph@...> wrote:
|
|
|
|
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? |
|
|
|
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 |
|
|
|
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.
toggle quoted message
Show quoted text
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. |
|
|