Radio Direction Finding Technology
Hunting Down Radio Transmitters...
Ever since the use of radio transmissions, there was a need to locate
the radio sources, or at least a need to know from which direction the
signal was coming.
For radio navigation purposes for example, or for military intelligence.
Additionally Radio Direction Finding is used to locate local noise
sources and illegal transmitters.
The Radio Amateur community is also quite active in this field: ARDF
competitions are organized all over the world,
and almost every local Radio Club has at least one Radio-Fox-Hunt each
For the sheer fun of it, amateurs also hunt balloon-radio-sondes
(mostly meteorological radiosondes).
In fact they hunt all signals they find interesting.
So the reasons for Radio-Hunting are quite diverse, and so are the
We will take a look at some of these tools within (financial and
practical) range of radio amateurs.
The simplest locating method is using a portable radio with a proper
signal strength indicator and simply walk around the suspect room,
building or city block so see where the signal is the strongest.
With a nearby radio or noise source this can be a feasible method, but
it is time-consuming can be quite frustrating...
If we add a directional antenna to our radio we can do a 360 degrees
sweep to see from what direction the signal seems to originate.
Again we need a proper signal strength indicator on our radio.
Such a directional antenna saves a lot of time and is also practical
for distant signal sources.
Taking a few sweeps from different locations can give us a good
impression where the signal source might be.
Operator skills may be even more important here than equipment quality.
Radio Amateurs use all kinds of contraptions for this DF-method:
A popular antenna is the homebrew "tape measure yagi", using flexible
tape measure material as elements, often fixed to a PVC tube frame.
In Europe the HB6CV phased array antenna is very popular as well, yet
more difficult to homebrew:
Personally I prefer an modified moxon beam with Gamma-match; it's
smaller in size and has no sharp ends that may pierce anybody's eye...
when hunting in corn fields or in dense undergrowth where
the longer ends of any antenna get stuck with every step you take this
is a big advantage.
And a well designed moxon has a HUGE front to back ratio.
With that deep notch at the back of the antenne it is even quite
practical to hunt strong sources looking for the minimal signal
The field strength RDF method has one major drawback: Getting close to
a powerful radio source, you may find that your radio's S-meter will
show nothing but full scale.
De-tuning the radio may help you a bit but there's a more elegant
Tune to the second or third harmonic of the hunted signal, that will
bring the signal down to a level that your S-meter is usable again.
The very best solution however is an "offset attenuator", that shifts
your received frequency with say 1 or 4MHz, and enables you to bring
the signal level down to practical zero.
A Google search will bring you plenty examples of that technique.
But now let's talk Business:
What if we automate the process...
What if we have a system connected to our radio that can read a bearing
from the incoming signal in less than half a second?
Without the need for a highly trained operator?
Would that give us an advantage?
Does a bear shit in the woods?...
Yes, a bit of affordable electronics and a specially designed antenna
array can make a HUGE difference.
Spoiler: This is a preferred appoach for mobile (car) hunting, or from
a fixed (home) location.
Real portable use is not that easy due to the necessary bigger antenna
We can use 4 directional antennas (or an antenna with switchable
directional pattern) and do some electronic switching to simulate a
rotating directional antenna.
Or we can
put 4 antennas in a square and switch them to simulate a circling
antenna. (Pseudo-Doppler method):
The RDF processor that controls the antennas does the rest and
calculates a quite precise and generally very accurate bearing
analyzing the audio from the attached receiver.
Electronic switching can easily be done very fast so generally we sweep
the antenna around about 500 times per second.
That's 2000 steps to the next antenna each second.
For a 145MHz pseudo-doppler array that's a rotational speed of 3600km/h
or 1.6x the top speed of a F16 fighter jet...
Now you know why we rotate electronically, not mechanically.
The virtual rotating of the antenna produces a "doppler" tone in your
And we feed the receiver's audio into the RDF processor for analysis:
This is how a RDF processor looks like.
The RDF41 is designed to be very user-friendly, having only 2 controls
at the backside:
The Calibration potentiometer and the 3-position mode-switch.
And even this switch is not stricktly necessary.
Over the years the RDF4x family has grown with the RDF42, RDF43mono and
All described here on this website.
The latter models are menu based and have a lot of settings that can be
changed by the user. Because I am still not done experimenting.
The circle in the screen (called pelorus) has a bearing line that
points into the direction where the hunted signal originates from.
we can go even more sophisticated:
Especially for the RDF41/42/43 family, there's software to plot your
bearings on a map!
And it can upload your bearings to a server, and read other
stations bearings from that server.
This way, a team of mobile and fixed hunters can share their bearings
and build a very powerful Signal Hunter Group.
Just look for MapApp and RDF-Mapper on this site!
Generally an hunting sequence may look like this:
First you become aware of an interesting signal.
For example the pirate that only coughs and sneezes over the local
repeater, just to feel alive.
Let's call him the patient.
So you want to bring the poor basterd his medicine.
First of all you'd be very happy with a first bearing from home.
you don't have a fixed RDF setup you could drive your mobile setup
preferrably to a high, open point with an unblocked view over the area
There you wait for his next sneeze and maybe plot the measured bearing
on the map.
This will give you a good idea where to drive to.
So you close in on him, not too fast, and process his next sneeze.
Not too fast I said, because an extra wait at a good open spot will
bring you more information than speeding.
Step by step you get closer, and see the bearing turn, slowly but
A look on the map may help you decide at what point you should leave
this road and take a turn towards the changing bearing.
Now things start to speed up; you are closing in and the bearing turns
more rapidly, or maybe even quite rapidly.
That means you just passed the patient.
you are that close, you slowly circle the block once more, to gather as
much directional information as possible,
in the mean time looking for
antenna structures etcetera.
If you drive by very close, you may
even hear the whining of the "doppler" tone faint a bit because you're
partly underneath his antenna.
This way you will get a good "feel" where in the block your patient is.
The last step is to park your car around the corner, walk back, maybe
through a back alley
with your old fashioned handheld foxhunt set and confirm the exact
location of your patient, so you can hand over his medicine.
So what medicine am I talking about?
Anything non-violent to persuade the patient to stop fooling around on
a post-it note in or on his mailbox with the remark that he is hunted
down and caught red handed does the job quite well.
Use your imagination, and stay peaceful.
The above story (=a quite accurate report of a real hunt I did) is just
one of many examples I could give you.
Radio Hunting is entertaining, sometimes frustrating, sometimes
rewarding, but always exiting!
You never know what awaits you.
time I close in on a "patient" or drive below a descending weather
balloon waiting for its landing I notice my heart rate goes up.
Yes, even after 130 succesful recoveries...