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 consists of a cluster of 6 or more directional antennas.
Actually, this method is used in a lot in SHF direction finding systems.

But for the 70cm and 2m amateur band these antenna clusters would be quite large,
so instead I designed a few simple antenna configurations that can change its directional pattern by switching on and off additional elements:
One receiving antenna in the center and four switchable reflectors evenly spaced around it at 1/4 wavelength distance.

On this website you will find the designs of these switchable directional pattern antennas.

Here's the simulation plot of a UHF version for mobile use:



As the simulation shows, 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?) can be used to feed the RDF processor with audio and the RDF processes the signal as usual, calculating the Angle Of Arrival.

Advantages:

The suggested amplitude antenna array is even 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.

Here's a short video showing that the Amplitude priciple can be used to track signals much wider than the receiver's bandwidth:

https://youtu.be/w3dX4ly06jU


Disadvantages:

You need a radio with AM detection (or a RSSI output).
"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 mistakenly seen as the true direction of the source.

Accuracy:

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.



Cheers, PA8W.