Radio Direction Finding Technology
thoughts on Amplitude Direction Finder Antenna Arrays
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
On this page we are going to zoom in at some specific
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
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:
The amplitude-array should have a clean, predictable directional
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 Amplitude array is relatively small in size, so it can be
a single ground plate with whip-like antenna and reflectors
attached on it.
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.
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:
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:
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.
For the fixed UHF application I designed a sleek Groundplane antenna
surrounded by 4 switchable reflectors.
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
thunderstrikes in the direct vicinity. (Had that once...)
The reflectors are dipole types with diode switchers in both top and
The UHF design including a table with sizes for other frequency bands
can be found here: