|PA8W Amateur Radio||
|What's an Amplitude Radio Direction Finder|
An Amplitude 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 and off several elements.
Inspired by the experiments of K6TYO (silent key) I designed a 5-antenna directional array:
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 RDF processor with audio and the RDF 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 almost any type of signal.
AM, FM, SSB, RTTY it doesn't matter. Even broadband 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,
as long as the part you tune to has some degree of continuity.
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 feasible.
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.
A doppler array (at least using my RDF41/42/43) in the same conditions will show you that the signal Quality is very poor and 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 true direction of the source.
Below graphics show the absolute error of this array, laying flat on the grass, measured at 433MHz.
Peak error stays within 3,3 degrees, mean error is within 1,5 degrees.
This is excellent for a 4 (or 5) antenna RDF.