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Author Topic: Interference to 915 MHz radio link...  (Read 8574 times)

Posts: 78

« on: February 03, 2011, 11:20:28 AM »

I purchased an Argo 650HD amphibious 6x6 for surveying that has a Briggs and Stratton Vanguard engine, Model 386447-0144-G1.  I use Topcon Hiper Lite Plus GPS equipment that consists of a base and rover that communicate via a 915 MHz spread spectrum radio link:

The GPS radio link signal is normally usable out to about 1.5 miles in the relatively flat farm country I am working in.  I am loosing the GPS radio link at a distance of about 1/2 mile when I rev the engine in the Argo.  I regain the GPS radio link when the Argo engine is at idle or, of course, when it is shut off.

The gasoline engine in the Argo has resistor type spark plugs.  I am now in the process of deciding what to purchase or build, as far as filters for the charging system, etc.

Any ideas or suggestions on where to start?  I have contacted Briggs and Stratton, but have yet to get in touch with anyone that is knowledgeable.  Topcon is of little help since this is a very specific case, works fine in my pickup.  I am on my own on this one so any help would be very much appreciated.  I thought about going to a two-way radio shop and using a spectrum analyzer to gain some insight into the problem.

Now to make this post related in some way to amateur radio...Before I setup my internet remote base, I ran my TS-480 in my pickup while I was surveying.  Listening to amateur radio helped pass the time while spending 12+ hours per day in my pickup.  During the winter while the farm fields are too soft for my pickup, I will be spending long days in the Argo.  I may throw the TS-480 in the Argo and mount a hamstick on the overhead bracket behind the GPS receiver and make a few contacts. Cool

Posts: 5639

« Reply #1 on: February 03, 2011, 12:44:39 PM »

I can be of absolutely no help in solving your problem, but I do want to say that is a very interesting vehicle - especially the "tracks" over the wheels.

A smoking section in a restaurant makes as much sense as a peeing section in a swimming pool.

Posts: 2566

« Reply #2 on: February 23, 2011, 08:42:16 AM »

Who makes the Spread Spectrum radio or is that integrated into the GPS unit?

I work with 902-928 MHz spread-spectrum radios constantly, your installation looks perfect. What kind of antenna do you have back at the base station? Is it also a small whip?

If it is a 1 watt transmitter, even with that antenna you should have a mile or two of range. If it is a screw-on antenna with some sort of standard connector you could go with a gain antenna.

The limitation is that you should not exceed 36 dB EIRP (about 4 watts effective power).

As all it is probably passing is NMEA GPS information it will run at a very low data rate of 4800 baud. Even the simplest Spread Spectrum radios are usually capable of that throughput.

If you wanted to stay spread spectrum you could always look for a pair of GE-MDS TransNet radios (up to 115 Kbps on a serial port) and with a decent omni antenna you could run 5-6 miles. If you had a permanent installation with Yagi antennas that range can be as high as 20 miles.

<smile> with 115 kbps you could even do ROIP (radio over IP) and have rig control of your base station from your Argo.

You may want to consider picking a better base station antenna or raising the height. Maybe a little bit of RFI is coming from the generator on the Argo.

Tisha Hayes
« Last Edit: February 23, 2011, 08:57:02 AM by AA4HA » Logged

Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
Free space loss (dB) = 32.4 + 20 × log10d + 20 × log10 f

Posts: 78

« Reply #3 on: February 24, 2011, 05:58:14 AM »

Who makes the Spread Spectrum radio or is that integrated into the GPS unit?...

The radios are integrated and made by Topcon with a power output of 1 watt.  The base and rover use the same rubber ducky type whip antennas.  The 1.5 statute mile range of my units is actually limited by software.  If I pay Topcon $1,200, they will provide me with the unlock codes (can you say, "rip off") to extend the range to about 3 statute miles.  The base is sending correction data to the rover at 9600 baud.

I don't notice the effect of the interference until I get out to about 1/2 statute mile, so the interference is relatively low level making it harder to detect.  At distances of around 1 statute mile, extending the legs on the tripod to increase the height of the base above ground doesn't help much.

I have thought about trading for new units anyway and will probably trade for units with 1 watt UHF radio links.  I couldn't complete a job the other day because the 915 MHz radios would not provide a link through a block of woods.  I am hoping that the UHF radio links will solve several of my problems.  I already have a UHF frequency licensed for this type use.

Thank you very much for sharing your knowledge and expertise!
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