PA8W Amateur Radio

Wil, PA8W,  E-mail: PA8W@upcmail.nl           
Back

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 an off several elements.
Inspired by the experiments of K6TYO (silent key) I designed a 5-antenna 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 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 directional pattern,
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.

Advantages:

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.

Disadvantages:

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.


Accuracy:

Using my RDF41
as the RDF processor, tests show that the accuracy is somewhere within +/- 5 degrees, which is excellent for mobile applications


73, Wil.


Back