Radio Direction Finding Technology
of a pseudo doppler Radio Direction Finder
The accuracy of a pseudo doppler directio finder is generaly stated as
5 degrees for a
4 antenna array and around 2,5 degrees of an 8 antenna array.
Of course this depends on a lot of factors:
For a stationary Radio Direction
How clean is your site? An elevated array with a free line of sight up
to the horizon is a dream that will hardly ever be forfilled.
A flat desert or a calm ocean would do the trick.
In that case you may achieve up to 2 degrees error for a 4 antenna
array and around 1 degree for an 8 antenna version.
This is confirmed in real life:
NCI-Folkestone uses one of my dopplers on the very best site I know:
Looking down on the sea from a 60m high cliff: They achieve around 2
degrees of error with the 4 dipole VHF array.
But this is clearly an exception.
Even if the antenna is well above surrounding buildings and trees there
will still be a lot of scatter from these obstacles.
These reflections add up to the direct signal and this will increase
the overall error of the direction finding system.
For an antenna that just peeks over the roofs of your neighborhood you
expect something like the below table,
made with my 8-antenna V3 conventional doppler direction
finder in the 2m band:
For a mobile Radio
First of all the antenna is generally much lower than surrounding
buildings and trees.
This obviously makes things worse.
On the other hand a mobile RDF can move, and if it moves enough in
resonable circumstances we can gather enough information to be able to
calculate a fair bearing.
In the conventional dopplers we achieve a pretty good result using a
very high Q of the digital fiilter.
This makes the RDF slower but it will generally point more or less into
the right direction, because a lot of faulty bearings add up and cancel
each other partially.
In the microcontroller based RDF's we can check the reliability of
incoming bearings and ignore the ones that are obviously corrupted by
This improves the bearing estimate accuracy considerably.
But there will always be a chance that a reflection from a large
building is more powerful than the original signal; in that case every
Direction Finder will point at that building...
With my RDF40 I drove a route back and forth, tracking a wellknown
424MHz source, in the below picture marked as TARGET.
On three straight roads on that route I took 2 good bearings driving in
Back at home I used Google Earth to project my bearings on the map, and
to see how much they were off.
As you can see the overall average error is not bad at all, if you
manage to ignore the total chaos during most of the route.
The algorithms in the RDF41/42/43 help you to do just that, though I
that a bit of training surely helps. ;)
For precision measurements it is much better to use only the
few measurements in the best conditions,
rather than polluting the
outcome with an abundance of crooked measurements.
Having said that, a mobile RDF will generally be used just to
approximately point at the right direction.
Errors of up to 45 degrees don't really bother then, since at the end
they will still lead you to the radio source.
But also in this type of use the very steady Long Time Average of the
RDF41 will help you navigate to the radio source with a minimum of
You can focus on the traffic and just now and then take a short peek at
the RDF to know where to go.
For those interested in the absolute accuracy of the RDF41 with a UHF
mobile doppler array, here's the outcome of a field test performed on
december 4 2017:
Environment: lawn, no objects closer than 80m.
Source: 433,2MHz low power beacon, no modulation, @15m distance, 40 cm
Array: Preamped doppler with antenna spacing =17cm, ground plate =
50x50cm. Placed flat on grass.
Direction Finding Processor: RDF41.
I calibrated at 0 degrees and checked readings @ 90, 180 and 270
Then I checked one quadrant in increments of 11,25 degrees.
The average error of these 10 measurements is 2,2 degrees, with a peak
error of 4,5 degrees.
These figures are pretty close to figures stated by professional
Real life causes of severe bearing deviations:
Note that reflections may lead to severe bearing
deviations. (multipath signals)
For example if the transmitter is behind a big obstruction or below the
Especially when the transmitter has horizontal polarisation these
effects can be severe.
In these cases there 's a bad or non-existing direct path between
transmitter and RDF.
A horizontally polarized signal for example will produce a poor signal
on the RDF antenna,
however any reflection against a bridge or crane may produce enough
vertically polarised signal to be seen as the main signal.
Unfortunately, there 's very little we can do to solve this, take a
look at the following picture:
Just for simplicity, I have drawn an incoming radio "beam" as a fat red
(In real life, it is a wavefront of course, but this simplification
will help to explain.)
The signal comes in from the right, where obviously the transmitter is
For the RDF car, the blue one in the picture, this signal is totally
blocked by the buildings on the right.
The higher buildings on the opposite side of the road however reflect
and scatter the incoming signal, illustrated by the thinner red lines.
Repeat this idea a million times and you will have a good idea of the
truth; total chaos!
For the hunting car, the only signals coming through are false,
scattered, multiple reflections.
Sometimes even crisp and clear, when the building turns out to be a
good reflector, but always coming from a false direction.
There's no algorithm that can solve this, only operator skill will help
you make the right decisions.
A rooky in this situation is tempted to throw the doppler out of the
Another phenomenon that often occurs in a city- or industrial
A signal that is striking a city with avenues running more or less in
the same direction will appear to stick to that avenue.
Because that avenue offers a path with reatively low attenuation,
signals seem to originate somewhere from the end point of that road.
Once the road leads you out of the city, the true bearing will reveal
The bearing error due to this effect may well run up to 45 degrees.
Note that this effect will be observed only with long and good
averaging settings of your RDF.
A fast responding RDF in a city environment will show total chaos.
I can't judge you eyeballs, but mine are not at all capable of "eyeball
averaging" such a mess.
So, as a rooky, pick a nice quiet open road and a loud stable signal to
improve your skills, and keep your eyeballs on the road.
With the proper settings and a lot of averaging, a quick look at the
RDF will tell you all you need to know to take the right turn.