Re: Pointing Antennas - Car or Rotator?


Hi Alden,

There are two questions here:

1) What's better for the rover, fixed antenna pointing or independent antenna rotation, usually via remote controller rotor?


2) Is there a better material for mobile antenna structures than PVC tube/fitting systems?

We could spend a lot of text on the first subject but I'll try to keep this short:  Roving involves managing constraints and challenges.  As an example, a successful rover/antenna mount scheme used in the southwest desert would be completely unusable in the Pacific Northwaest due to typical tree (and limb) cover. Obviously if you carried a stack of antennas at a legal limit of 14 feet, that would be a bad idea in the east where rail crossings over highways are only 10' 6".  So the point is: It depends on the rove location  to make the best argument for antenna configuration.

There are some advantages to fixed mounting an antenna system on the rover vehicle  -- 

There are better materials than PVC pipe for mechanical structures; fiberglass, carbon fiber, aluminum and steel to name a few.  The trades often center on cost as a driver for amateurs.  Correctly engineered, PVC tube and fitting structure can be perfectly adequate so long as the vehicle to antenna support structure interfaces are managed correctly.

Mechanically and electrically simple, and therefore easier to fabricate and assemble.  Fixed mounting involves a structure, Any other system including a rotor, or armstrong rotation requires a structure and a mechanism. Mechanisms, by their nature, can become a reliability problem.  I'll chat more about that in a few more lines.

A second advantage for fixed mount systems is that one may mount longer yagis along the major axis of the vehicle without violating any Dept of Transportation (State or Federal) requirements.  Managing a 12 foot yagi while in motion becomes much simpler, as an example.

Generally being lower than mast rotor systems, the fixed structure, often surrounds the antenna(s) and tends to protect them from foliage.

A popular rove adage says that you're always going either to or from a city so the rover only needs two antenna bearings; forward and backwards.  A fixed structure antenna system will provide you at least 50% of the needed antenna bearings.  Some rovers have added individual front and rear pointing antennas to cover most of the needs.  I saw a yagi set up for front/rear aiming on a common boom.  It had only one reflector......

In the quest for the ideal operating location I have been up some unimproved roads which are not wide enough to maneuver the vehicle to a desired aim point.  Murphy sez you will not reach a turnaround until the road leads you to the other side of the mountain.  Grrrrr. Blocking a Forest Service road, let alone an Interstate Highway while you run a couple of schedules will make you unpopular and could cost you a fine if you're un-lucky.

For rotating antenna options we've pretty much covered the pros and cons.  Just multiply the above reasoning times a negative and that becomes the pros and cons for the rotating antenna system.

If you're driven to win, roving in the southwest demands that the rover cover a maximum number of grids because it takes only a short time to work the bands out because the density of workable stations is low.  Working in motion is an advantage because it allows you to do two or more things at once, and often one that makes a score difference. That reality drives an approach which demands an ability to point the antenna(s) while in motion while on the way to a new grid.  That means that one must use shorter yagis to allow legal passage even when the yagi boom is crossways to the direction of travel.  Some southwest rovers do well by avoiding the complication of moving the antennas while driving by being patient and waiting for a good location, stopping and manually moving the antennas to the correct bearing.

A third option consists of building the antenna stack each time the rover is stopped.  It's an easy option for beginning rovers and can be effective if the antenna system is designed to minimize the time, tools, connectors and  fasteners required to complete setup and teardown/stow cycle.  That's a recommended method for first time rovers.  Some have combined that approach with using loop antennas while driving then stopping and erecting better antennas.  In some areas that works well. As Keith W9RM points out, a tall 6 meter antenna is a distinct advantage.  That requires the antenna be well in excess of 14 feet, the DoT vehicle maximum height.  For those able to erect a 20+ ft high 6 meter antenna while stopped can overshadow the advantage of operating in motion.  There are tradeoffs to be made and that's what makes roving interesting.

I hope these opinions help you decide a path to follow.  If you have questions do not hesitate to ask.

73 Bill W7QQ

On Wed, May 22, 2019 at 7:23 AM Alden Oyer <atoyer@...> wrote:
This antenna support structure, made of pvc tubing, was on KF8QL’s vehicle at the Hamvention last week. Antennas range from 144 MHz to 3.4 GHz. My question to you is “What are the pros and cons of pointing the car to aim the antennas instead using a rotor to aim them?”
Is there a better material for the structure than pvc?

Good luck in the VHF/UHF contest in a few weeks.

Alden Oyer, AG5S

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