PA8W's Radio Direction Finding Technology

A Homebase Preamped Amplitude Array (390-470MHz)

For other frequencies: Check the table in this page.


A switchable directional pattern antenna array for UHF radio direction finding is illustrated here.

This array is suitable (using the amplitude comparison method) for hunting narrowband and wideband signals in the UHF band.

Of course also digital and analog voice transmissions can be hunted.

Antenna array with switchable radiation patter for radio direction finding

The above diagram shows the directional patterns on several frequencies with 1 of 4 reflectors active.

Antenna array with switchable radiation patter for radio direction finding

The center conductor plus the 4 drooping radials are the actual antenna elements, with a MMIC preamp in its feedpoint.
(I use a bias-tee to feed the preamp over its coax.)
The antenna is surrounded by 4 dipole type reflectors that can be switched on and off.
An activated reflector changes the omnidirectional pattern of the central antenna into a directional pattern with a front to back ratio of around 11 or 12 dB.
This directional pattern is "rotated" simply by switching the reflectors on and off sequentially by the RDF processor.
This is done in a fast pace: 2000 steps per second = 500 cycles per second.
This fast rotating directional pattern modulates the incoming radio signal in amplitude,
so an AM recever connected to this antenna produces a massive 500Hz audio tone.

The RDF41/42/43 compares the phase of this AM tone with the phase of the reflector commutation to calculate the Angle Of Arrival of the incoming signal.

This array is meant for homebase applications, in situations where we don't have a metal ground plane.
Therefore the actual antenna is built as a sleek Ground Plane Antenna,
with the 4 drooping radials acting as a ground plane.
The 4 reflectors are built like dipoles which can be electronically closed and opened in the center.
In a opened state, the reflector breaks up in two 1/4 wave conductors which are too small to have an impact on the radiation pattern of the actual antenna.
When closed, a reflector is a conductor of a little more than 1/2 wavelength and thus it acts as a reflector.

The achieved (measured) front to back ratio is approx. 12dB.
That is about 5dB gain at the frontside, and 7dB suppression at the rear.
So when no reflector is closed, we have a simple preamped GP antenna.
The measured figures are very close to the calculated ones.
I only noticed a downward shift in tuning due to the heat shrink covering of all steel wire elements, which was to be expected.
So I reduced element length to get the desired 400-470MHz with uniform pattern properties.
So stick to the sizes I state below, and heat shrink cover the wire elements like I did.

Sizes for several bands:

Antenna array with switchable radiation patter for radio direction finding

Now let's walk you throught the building process and start off with the electric diagram:

Antenna array with switchable radiation patter for radio direction finding

The center antenna doesn't necessarily have to be amplified.
But for UHF and up the extra gain is more than welcome generally.


Antenna array with switchable radiation patter for radio direction finding

The 4 switcher PCB's:
On the left is the control wire that feeds the 1N4148 diode (or PIN) via two 470ohm resistors.
Red is the "hot" wire and grey is ground.
So when say 10V of control voltage is applied to the hot wire,
the diode current will be close to 10mA and so the diode will be conducting and closing the gap between the two reflector halfs.
These reflector halfs are soldered to the pads on the right side of the PCB.
The black ring on the diode should be the bottom side of the reflector.

The PCB's fit into a tube with 13mm inside diameter.

Antenna array with switchable radiation patter for radio direction finding

The switcher PCB is put in an oversized piece of PVC 3/4" tubing.
Then both reflector halfs are soldered in place and covered in heat shrink tubing.
And finally I sealed the end opening plus the hole where the top-reflector-half enters.
I never seal the bottom element to have a tiny opening where condensed water can escape.
Additionally I drill an extra 1,5mm hole in the botton of the tubing and hub for the same reason.

Antenna array with switchable radiation patter for radio direction finding


The center hub, made of PVC tubing.

Antenna array with switchable radiation patter for radio direction finding

A mast fix is added, and the 4 ground plane elements are entering the hub.

Antenna array with switchable radiation patter for radio direction finding


The ground plane wires are soldered into a piece of Printed Circuit Board that doubles as a washer to clamp down on the tube arms.

Antenna array with switchable radiation patter for radio direction finding  

Then the radiator is prepared and soldered in a hole in the side of a brass spacer bolt.
The M4 bolt is fastened to the center pad of the preamp PCB.
The tiny black thing on the PCB is the actual preamp.
Any MMIC will do fine here.

Antenna array with switchable radiation patter for radio direction finding

Now the preamp PCB is soldered onto the PCB material holding the ground plane wires.

Antenna array with switchable radiation patter for radio direction finding

This picture shows the complete UHF array just before the wiring is done and the top cap over the center hub is glued in place.

I used 2mm steel wire to make all elements:
The reflectors are made of 2 pcs of 19cm.  (8pcs in total)
The ground plane is made of 4 pcs of 18cm.
The radiator is a piece of 16cm long.
All elements are covered in heat shrink tube after they were soldered in place.

The distance radiator-to-reflectors is 10cm.
The actual ground plane is rotated 45 degrees so these elements are at maximum clearance distance from the reflector elements.

When installing the array, it's best to use a glassfiber, wooden or plastic mast top, and put a ferrite clamp over both the coax and the control cable, at approx. 10 - 16 cm below the hub.

I use this antenna formany years now for radio direction finding as well as for decoding amateur ballon foxes,
welcoming the extra 5 dB gain into the direction of interest.


Cheers, PA8W.