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[KD8PGB]

 I am interested in reliable point to point APRS over a 25mile range without using digipeaters.

Due to obstacles and interference standard 2m won't work reliably.

So I am wondering if we switched to 2m SSB would this work better than FM ? In theory it sounds promising, but in practice what are the pitfalls? Due to being mobiles, we would have to use vertically polarized antennas,  obviously a no-no for SSB operations, would that preclude reliable 25 mile APRS ? The lack of error correction seems like it would play a role...would Robust Packet be a better option on 2m SSB?

Does anyone here have actual experience with 2m SSB and APRS?

Dan W5DMH (new vanity callsign)

[KT4NR]

Might work try it. This really could be an interesting experiment.

Obstacles:

* Finding a frequency where you won't bother anyone. You might want to stay high in the SSB sub band but below the Satellite guys. (make sure all emissions avoid their sub band.)

* Vertical is NOT an obstacle so long as both stations are vertical. Were you to want others to join in, then it could become a path loss issue, not a death blow but definitely not helpful.

Robust Packet would work well (with such low noise and stable conditions it would tear through pretty fast) but again, need to find a frequency where you won't bother folks.

Consider 432 MHz SSB as well. A path that does not work on 2m might work on 432 or even 220.

I've never heard of anyone running APRS on SSB above 30 MHz, however, so long as you are polite and don't bother anyone why not give it a shot. SSB is sadly under utilized above 6m. 

FWIW, I once asked WB4APR why 2m and not 220 or 440. He replied that it was the one most people would use and what he had gear for at the time. No reason not to try running it elsewhere. I know some folks in this area try 6m APRS on FM once. Not sure whether it is still going on or not.

[KD8PGB]

 I guess I am going to find out the hard way, I may spend some time testing on Sunday and see what I get, obviously I will report back here when I have some results.

Dan W5DMH

[K0JEG]

You might want to try using FLDIGI's KISS modem or maybe APRS Messenger:

http://www.w1hkj.com/FldigiHelp-3.22/config_io_page.html
http://www.crosscountrywireless.net/aprs_messenger.htm

AFSK on sideband isn't as good as other modulation modes. Since you're going down the SSB route why not try to optimize everything?

GMSK modulation can use FM PAs and will probably work much better than AFSK (it's used for 9600 baud packet, D-Star and DMR, as well as the old GSM cellular standard). If you have 9600 baud compatible radios you're all set.

Since you're not going to be using the normal APRS infrastructure anyway, you get a lot of flexibility. Make sure you post your results.

[KD8PGB]

 So for inquiring minds,  my APRS testing on 2m USB seems to have  provided about a 20% improvement in range over 2m FM,  unfortunately while a 20% improvement is substantial, its not enough as 20% equates to an additional 2 miles. Another downfall is that I am getting substantially more garbled packets due to noise on the band especially out in the fringe coverage area.

Today we will start trying 6m SSB and see what that yields.

[KD8PGB]

 So some initial tests of 6m SSB are yielding slightly better results than 2m SSB...starting to make me think going back to 2m FM and using a 200watt amp.....

Obviously my frustration is showing, it would seem that consistent mobile to mobile communication over a 20 mile path would not be this tough...however it seems that no matter what band and mode, I seem to max out reliability around 12-14 miles.

[KT4NR]

Thanks for following up. This is really interesting.

Could you share some more details on the topography and the station operating conditions?

Have you tried going up?

Also, might want to try more than one day for each band due to propagation variations. For example, lots of fog in the Mid-Atlantic this morning so that may enhance propagation versus a flat day.

I know this can be frustrating but hopefully you can find the fun in experimentation and discovery.

Thank you for taking the lead and sharing.

[KD8PGB]

 Topography is gently rolling to flat...worst hills in the area are about 400ft, most of the area is flat...as for going up, yes testing from overpasses does work better than ground level, which would be expected, but the reliability cannot be reliant on available overpasses

I am really starting to think since we cannot truly optimize antennas on mobiles, maybe the right answer is to go back to 2m FM and use a 160-200watt amp

I am going to keep testing, tomorrow we will be doing the rolling hill area, should prove interesting to see if 6m propagates better than 2m in those areas....

Please don't get me wrong either, testing in each mode / antennas has been going several days with each experiment and in different topographical areas. By no means this is one and done...For instance today's tests were over pretty much flat ground...almost no hills at all,  but very suburban, lots of power lines and obstructions (buildings & RF). Tomorrows testing will be over in the hilly area where we have the most trouble communicating.


I would be interested in hearing from anyone who is running 2m mobile with an amplifier and how the amplifier impacts their simplex propagation.

[KT4NR]

I should have clarified, going up refered to up in frequency.

Also, antennas play a role. I have a Larsen NMO270. That antenna center mounted on my old Ford Escape roof played much nicer than the Japanese high gain antennas anywhere on the vehicle. The takeoff angle of the Larsen is higher, thus keeping it closer in. The higher gain antennas had lobes with a very low takeoff angle. Those antennas let me work repeaters far away easily but not hear some of the closer in machines. Thus it appeared to suggest some shadows and odd reflections played differently with different antennas.

Maybe play with the antenna you use at both ends?

Topography was very hilly urban jungle.

In the end it may not work without higher power or a beam/horizontal loop and that is equally fine.

[K7AAT]

Interesting thread.  I am not active on APRS but I like the concept the original poster raised.  I wonder if he has the equipment to run 300 Baud Packet instead of 1200 Baud?  If so,  could try his experiments on 10M FM too ....

[KD8PGB]

Interesting thread.  I am not active on APRS but I like the concept the original poster raised.  I wonder if he has the equipment to run 300 Baud Packet instead of 1200 Baud?  If so,  could try his experiments on 10M FM too ....

Interesting, I had not thought about using 300baud packets....that might make the packets more successful and less subject to the static and noise. Although the bandwidth of FM is quite a bit wider than SSB, therefore a larger noise floor is included with the FM signal...FM receivers on the other hand require a fairly low SNR before the signal is accepted, this of course causes further limitations in range, basically about 12db more signal than SSB is required to pass through an FM receiver.

So all of that said, 300 baud packets over 6m or 10m SSB might be the answer, although I am not sure if I can generate 300baud APRS packets and decode them..I will have to look at the software and see what I can do.


6m SSB was pretty reliable at 12 miles yesterday. Today we are trying 2m FM again with better antennas,  initial results where not impressive...8 miles in the rolling terrain area....

Over the weekend I am going to install a 200watt 2m linear in one of the vehicles. In theory this should easily reach the other vehicle on 2m FM over a 15 mile path, the receiver pre-amp in the linear "may" allow the weaker mobile to be received at the high power mobile...we will test this using FM on Monday and then try SSB the following day.

I have always been a believer that if you can raise an antenna to at least be clear of the trees, power is not really an issue....but with two mobiles, we are limited with what we can do with antennas and modes....the next logical step is to "pour the coal" to it and a 200 watt amp is certainly going to pour the coal to it!

[K0JEG]

The downside of using 300 baud packet is that it is a very long transmission, compared to a 1200 baud packet. Any brief interference will destroy the entire packet since there's no forward error correction (only a checksum) and since APRS is a UI packet no way to ask for a retransmission.

[AA4PB]

Given a mobile antenna height of 6 feet, the line of sight to the horizon is 3 miles. The line of sight distance between the two mobiles is thus 6 miles over flat ground on UHF or VHF. Given that the radio horizon is usually about 60% greater, the maximum mobile to mobile distance is about 9.5 miles. That can be extended if either vehicle is located on higher ground or there are reflections from tall buildings or other structures.

In my experience (which included some rather extensive testing of packet protocols in a very similar condition), more power, antenna differences, and bandwidth differences made very little improvement in range. The only thing that made a huge difference was antenna height. Your distances seem about right for a mobile over flat ground. Drive one of the vehicles on a 15 foot hill and you'll see a major increase in range.

In my testing I was not required to use the official APRS protocol so using compression I was able to shrink my total packet size to only 16 bits. That helped considerably in reducing errors with a weak signal BUT when you are beyond line of sight nothing helps except to increase the antenna height. Put one end in an aircraft at 10,000 feet and suddenly you're looking at 50-60 miles reliable with only 5 watts output.