[amsat-bb] DFing Terrestrial QRM
bruninga at usna.edu
Fri Sep 21 06:16:11 PDT 2007
>>...if you listen to 145.85 and hear something...
>>... from that point, some knowledgeable and
>> experienced sleuths may be required to help...
And that is where we need to educate all VHF'ers, and that is
that we don't need DF equip, we don't need beams, and we don't
need experts. Anyone with a mobile radio and/or an HT with
stock omni antenna, and a little spare time for some fun
driving, can find a transmtter ... the closer you get to a
transmitter the stronger it is.
Scan quickly down the page about 2/3rds to the most recent
Balloon recovery and you can see how easy it is..
The web page says "APRS Dfing" but ignore the APRS part. You
don't need APRS and you don't need a PC. Just drive while
watching the AVERAGE VALUE of the S meter... And visualizing
what's going on. There are two easy techniques for "DFing"
using signal strength alone.
Average Signal Strength is a very significant indicator that can
be used to home in on a signal. Combined with local terrain
knowledge, a little driving and chris-crossing can quickly give
knowledge of the signal profile and allow the driver to just
keep narrowing down on the signal and with enough time, arrive
at the door.
Just remember the Smeter average value where you first start
hearing the signal and then drive straight watching this
indication improve and then fade back to this level. These two
points are more or less equidistant from the source. Now drive
back to the middle where now you know it was strongest, and make
a 90 degree turn. See the new Smeter average. Again, drive
straight through the peak to the same reading on the other side.
Again, go back to the center of that leg. See the new Smeter
average. Turn another 90 degrees. Repeat. Eventually you
arrive at the door of the signal.
Spot signal strength reported by a large number of ham mobiles
or fixed stations can instantaneously narrow down the search to
a neighborhood. The BIGGEST mistake made here is the fact that
most people collecting reports tend to -ignore- the null reports
of nothing-heard. When in fact, that is often where the most
useful initial data is! There are usually far more reports of
nothing-heard than heard. And it is that large amount of data
that shows where the signal -ISN'T- that is the biggest
contributor to knowing where not to search! Focus then on more
reports from other areas not reporting -nothing-heard. This
should only take a call up on the local voice repeater asking
for people to listen for the signal and make spot reports.
Once an area is found with reports of -HEARD- then go drive, and
use the individual technique of signal strength and drive right
to the door. Its fun, anyone can do it... If you have enough
These techniques are built into APRS, but that is only for easy
visuilization of location on the map as the solution develops.
(And everyone watching can see it in real time...) But this
hardware and APRS and maps are absolutely not needed. Just
drive and watch the Smeter.
When you get to MAX signal and the mobile Smeter is max'ed, then
you need your HT sitting on the seat with its ruber duck becomes
the benchmark. Once it is full quieting, you can usually remove
its antenna and stick in shorter and shorter paper clip as you
get closer and closer. Eventually the HT will hear the signal
with no antenna. Now it is time to get out of the car and walk.
Holding the HT on your chest. You have a NULL out your back.
Use it to get walking directions and by now you are within yards
of the source.
More information about the AMSAT-BB