Radio Direction Finding Technology
A Homebase Preamped Amplitude Array (390-470MHz)
other frequencies: Check the table in this page.
A switchable directional pattern antenna array for UHF radio direction
finding is illustrated
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.
The above diagram shows the directional patterns on several frequencies
with 1 of 4 reflectors active.
The center conductor plus the 4 drooping radials are the actual antenna
elements, with a MMIC preamp in its feedpoint.
(I use a bias-T 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
This fast rotating directional pattern modulates the incoming radio
signal in amplitude,
so an AM recever connected to this antenna produces a massive 500Hz
The RDF41/42/43 compares the phase of this AM tone with the phase of
reflector commutation to calculate the Angle Of Arrival of the incoming
This array is meant for homebase applications,
in situations where we don't have a metal ground
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
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
So when no reflector is closed, we have a simple preamped GP antenna.
The measured figures are very close to the calculated ones.
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
So stick to the sizes I state below, and heat shrink cover the wire
like I did.
Sizes for several bands:
let's walk you throught the building process and start off with the
The center antenna doesn't necessarily have to be amplified.
But for UHF and up the extra gain is more than welcome generally.
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,
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
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.
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
And finally I sealed the end opening plus the hole where the
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.
The center hub, made of PVC tubing.
A mast fix is added, and the 4 ground plane elements are entering the
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.
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.
Now the preamp PCB is soldered onto the PCB material holding the ground
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
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.