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 9 dB can be achieved over
a useful frequency span by activating one of the 4 reflectors.
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.
The suggested amplitude antenna array is much less complicated and less 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.
"Listening through" is difficult when hunting FM modulated signals, since the receiver has to be in AM mode.
Elevation measurement is not feasable.
Compared to the pseudo doppler, this AM mode RDF is a bit more sensitive to multipath disturbances.
In a highly reflective area it simply points at the direction with the highest signal level, so that may be tree tops, road signs, buildings etc.
doppler array (at least using my RDF41) in the same conditions will
show you that the signal Quality is very poor but it won't easily "see"
the reflection as a true bearing.
A consistant and strong reflection would have to come from a large metal object to be seen as the truth.
Using my RDF41 as
the RDF processor, tests show that the accuracy is somewhere
within +/- 5 degrees, which is excellent for mobile applications