Radio Direction Finding Technology
What receiver is suitable for my Radio Direction Finder?
I've been using several conventional receivers with my RDF's and almost
all worked fine.
In fact, all "normal" conventional receivers I know work fine in
conjunction with a peudo doppler RDF.
For example I've got great feedback from radio hunters using an Icom
R20 and R30,
but even a cheap radio like the Uniden Bearcat UBS 125XLT should do an
The only radio that I found was useless was the Baofeng UV3 as
Generally, we use the audio (earphone) output of a receiver to feed
into the RDF41 or RDF42.
But quite often a discriminator output works fine as well for RDF work.
For mobile use I have a Stabo XR2000 handheld scanner, which is compact
and very versatile.
It has NBFM as well as AM so it can be used with pseudo dopplers arrays
as well as amplitude arrays.
For fixed applications on UHF and SHF I use the AOR2000, the
predecessor of the AOR3000 in the picture.
It also has NBFM and AM mode.
I added a discriminator output to this radio and that output is equally
useful for RDF work.
This second output may even come in very handy when you're hunting AM
signakls such as Airplane communications.
If I switch the radio to AM to hear the pilots speak the discrimination
output will still give out FM output so my pseudo doppler will have a
fine signal to work with.
For fixed applications on VHF I use the Icom IC-746pro, which has a DSP
last IF stage.
An excellent transceiver but beware not to hit the transmit button if
you don't want to blow the electronics in the array,
as it may output about 100 watts in the 2 meter band!
I've tried to make RTL-SDR receivers work with the RDF42 direction
instead of using a standard radio scanner or communications receiver.
Basic setup: RTL dongle RTL2832U from RTL-SDR.com, connected to a
laptop running SDRsharp.
The RTL dongle is connected to the coax output of a standard pseudo
The RDF42 controls the array and gets audio from the laptop.
Result: it works, sort of....
Issue: using an older Acer laptop I noticed a slow but serious drift in
the bearing showed on the RDF42.
That means the latency (time-lag) in the laptop is not constant enough.
A brand new Lenovo laptop showed similar issues but with additional
Maybe at points where the laptop decides to turn its processor speed up
So, a standard RTL stick receiver setup is simply NOT an alternative
for a classic receiver.
However, I didn't yet test a Raspberry Pie with RTL dongle, so maybe
that could work.
Beware of radio's with a "smart" squelch: A squelch that doesn't simply
open or close but uses a few stages of audio filtering depending on
The idea is that the weaker the station, the more receiver noise needs
to be suppressed, and so the more agressive the audio high cut filter
Since audio filtering introduces phase shift, it will immediately
result in an serious error in the calculated bearing of the RDF.
Phase shift is fine if it is constant, so you can calibrate it out, but
when it adapts itself during signal fluctuations you're cooked...
The Baofeng UV3 is one of these ugly examples...