PA8W's Radio Direction Finding Technology

Some thoughts on Amplitude Direction Finder Antenna Arrays

In an Amplitude Radio Direction Finder system the Antenna Array must be well designed for the frequency band you are going to monitor, and for the specific type of use.
On this page we are going to zoom in at some specific applications.

First of all we have to distinguish two main groups of amplitude arrays:

1, Arrays existing of multiple directional antennas, to be sequentionally switched to the receiver.
These types of arrays are often used in UHF and SHF applications because the size would still be practical.
Advantage: They often cover a wide frequency span.
Disadvantage: Rather big and cumbersome, difficult for amateur applications.

2, Arrays existing of a single antenna with switchable parasytic elements
to give it switchable directional properties.
The Amplitude arrays on this website are of this second category.
Advantage: Simple, easy to build, just as compact as a doppler array.
Disadvantage: Reduced frequency span, generally 0.15 x design frequency, but wide enough to easily cover an amateur band.

A few design parameters of this second category are:
1, The amplitude-array should have a clean, predictable directional radiation pattern with a F/B ratio >5dB preferrably.
All switched-off elements should be much shorter than 1/2 lambda to prevent them mutulating the desired directional pattern.
2, The array should  be sensitive enough to receive weak signals, especially in mobile applications.
3, The array should be reasonably tolerant to strong signals, especially in fixed applications.

To achieve this, element length, low switcher stray capacity and -in case of full size reflectors- switched off top+bottom elements are very important.

Now let's look into the specific applications:

Mobile UHF:
A mobile UHF Amplitude array is relatively small in size, so it can be designed as a single ground plate with whip-like antenna and reflectors attached on it.
The size (50x50cm or a little smaller) allows to simply tape it to a car roof in which case the car roof extends the ground surface enhancing the performance of the array.
If needed, the center antenna can be preamped.
On the other hand, excessive gain and the necessary open, unfiltered antenna design will make it a bit sensitive to very strong signals.
18dB gain has proven to be a fair compromise on UHF.

Here's the design:

Mobile SHF:
The smallest mobile array I ever built: For the 23cm band.
I built the center antenna preamped and used the array for tracking the video signal of amateur balloons.

Here's the design:

Mobile VHF:
In theory the above UHF array can be scaled up for VHF as well, using Magnet mount antenna and reflectors.
Due to lack of time I haven't built one yet.

Fixed UHF/VHF:
For the fixed UHF application I designed a sleek Groundplane antenna surrounded by 4 switchable reflectors.
The groundplane antenna has a 18dB preamp because I use it for High Altitude Balloon tracking as well; These signals can be tiny...
On the other hand these preamps directly at the antenna feedpoints may not survive thunderstrikes in the direct vicinity. (Had that once...)
The reflectors are dipole types with diode switchers in both top and bottom parts.

The UHF design including a table with sizes for other frequency bands can be found here:

Cheers, PA8W.