Wednesday, June 21, 2017

APRS on motorcycle vs. RF density levels

I recently sold my off-road motorcycle and replaced it a few days ago with a Kawasaki Versys 650
LT. The LT stands for "light touring" and I plan to do a bit of that once our Texas heat subsides in late September. That gives me three months to familiarize myself with the bike on morning rides. And to install an APRS transmitter.

Fortunately, I already have what I believe may be the most suitable APRS set-up for the limited space available - a Byonics Micro-Trak RTG.

But then I started looking at recommended RF density safety levels and am re-thinking the whole idea.

The FCC has published Maximum Permissible Exposure (MPE) limits and expresses those values in mW/cm². To convert a known power output in watts to a density level requires antenna gain and distance to "area of interest" to also be known. This online calculator simplifies the calculations. A downloadable Excel spreadsheet can be used as well.

The results are such that it doesn't seem possible to operate a 10-watt APRS transmitter on a motorcycle without exceeding the recommended limits. I wonder how this compares to the RF density level of a cell phone held directly to the head...


  1. A better comparison would be a 5 W HT next to your head. That is running FM, 100% duty cycle, and you might talk for a minute.

    APRS packets are very short transmissions, right? Maybe one or two seconds? Doesn't seem like a big risk.

    1. You're right, a couple seconds every 5 minutes is pretty insignificant - certainly less than what we get by talking on a 2m HT. Now to find a way to mount an antenna...

  2. Thats the fun part! I've seen them on the windscreen but mine are on a mount behind a radio box just behind the pillion's seat...