Radio Direction Finder uses some form of directional antennas.
In fact, the simplest example of this technique is a directional
antenna like a yagi, moxon or HB9CV, operated by hand to find the
direction in which the signal strength is maximum.
We can do the job automatically using an antenna configuration that can
change its directional pattern by switching on an off several elements.
Inspired by the experiments of K6TYO (silent key) I designed a
One receiving antenna in the center and four reflectors evenly
spaced around it at 1/4 wavelength distance.
On this website you will find a description how to build this very simple array.
A Front to Back ratio of more than 20 dB can be achieved over
a useful frequency span by
using two reflectors at the same time.
This way we can switch the antenna to 4 different directions, and use
the signal strength differences to calculate the Angle Of Arrival.
When we "rotate" the array at 500Hz as we do with an ordinary pseudo
doppler, the RF signal will show a 500Hz AM modulation.
So, an ordinary AM communications receiver (airband?) is used
to feed the RDF41 processor with audio and the RDF41 processes the signal as
usual, calculating the Angle Of Arrival.
Note that a conventional zero-crossing doppler RDF is less suitable
because its accuracy will depend largely on the shape of the
which -in the real world- can be distorted quite a bit.
Microcontroller based radio
direction finders however don't look for zero crossings but they
calculate the Angle Of Arrival directly.
They will only miscalculate in case the pattern is non-symmetric.
The suggested amplitude antenna array is much less complicated and critical to build compared to a doppler array.
An AM RDF is capable of finding EVERY type of signal.
AM, FM, SSB, RTTY it doesn't matter. Even broadband noise and electric
spark noise can be found.
The signal doesn't have to fit in the radio's bandwith. So you can even
hunt a 20MHz wide digital broadcast signal without a problem,
as long as the part you tune to is in the frequency span of the array.
You need a radio with AM detection.
The usable frequency range of the AM array is a little smaller compared
to a doppler array.
The size of an AM array is 1,4 x bigger compared
to a doppler array.
Elevation measurement is not feasable.
Using my RDF41 as
the RDF processor, tests show that the accuracy is somewhere
within +/- 5 degrees, which is excellent for mobile applications