PA8W's Radio Direction Finding Technology

The UHF Amplitude Array (390-460MHz)

An UHF antenna array with switchable directional pattern for radio direction finding

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

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

This array is by far the simplest one to build, less complex and less critical than a dopper array.
So, if you have little RF experience and have a suitable AM receiver this array may be the best choice for you.

The array essentially exists of a center 1/4 wave whip antenna, surrounded by 4 whip reflectors.
These reflectors are electronically switched to the ground plate sequentially.
This yields a rotating directional pattern with a front to back ratio of 9dB @ 433MHz.
2000 times per second the next reflector is selected, so there are 500 full rotations per second.
This modulates the incoming radio signal in alplitude and that creates a solid 500Hz AM modulation in your receiver.
In the RDF, the phase of this audio signal is then compared to the phase of the reflector rotation to calculate the bearing.

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.

This array is virtually spinning around by activating the reflectors one by one in a fast sequence.
 (500 rounds per second)

As with the doppler array, a very good capacitive coupling to the roof metal is essential.
Therefore we take an aluminum sheet as a ground surface.
Taped to the car roof it provides very good capacitive coupling, and so the car roof will extend the ground surface.

A video of a test drive can be found here:

This is the schematic of the antenna array.
The center antenna is fixed directly to the receiver coax.
The four reflectors are numbered clockwise. (looking down on the array)
The control lines don't have to be equal length.
You can use 1N4148 diodes instead of PIN diodes without any drawback.

Right-Mouse-Click on the drawing, and choose "Save as" to save the drawing to your map or desktop.
The resolution will be doubled compared to the drawing on this page.

Take an aluminum sheet of 50x50cm.  Thickness 1 up to 2mm.
1mm thick is sturdy enough, it flexes enough to adapt to the curvature of most car roofs.
A 2mm thick sheet is a lot stiffer, but can still be pre-bent to fit the curvature nicely.

Mark a center point for the center antenna.
Mark the 4 reflector points towards the corners at 13cm from the center antenna.
Spacing between reflectors would therefore be around 18,4cm.
Mark them evenly spaced from the sides, so the 4 reflectors are nicely symmetrical.

Then, drill 3mm holes at the 5 markings. These are the antenna/reflector positions.

The reflector position in the bottom left corner is already drilled in this picture.

Note that the antenna center holes are 10mm, to avoid electrical contact between antenna screw and ground plate.
The fastening holes are 3mm. The fastening screws will have to provide electric contact from antenna PCB's to the ground plate.

On the back side the 3mm holes are countersunk to accept the countersunk screwheads.
The screwheads should not protrude because they will damage your car paint!


After all drilling the small antenna PCB's can be screwed in place.
Apply a good sealer between PCB and aluminum to prevent water to creep in.
I countersunk the bottom side of the center hole of the PCB to make sure that the screwheads have good clearance to the below car roof.

With a spacer bolt of the right length the total length matches the inside height of the cups I use as enclosure.
The actual antenna goes on top of all that.

After mounting these antenna PCB's and spacer bolts, I sealed the antenna base holes on the bottom side using silicone sealer.

The components at a reflector.
The control wire runs to the center pod, which houses the actual antenna element.
Length of this control wire is not critical at all.

The center antenna position:
The coax has to be soldered directly to the center antenna PCB.
No components necessary there.

I use a 70mm PVC cap as an enclosure for this center section.
This cap is higher, so I extended the metal stub to fit that height.

The actual screw-on antenna and reflectors are made using identical metal spacers with a length of steel wire drilled and soldered in its top.
Overall length including all spacers should be 17cm for the center antenna, the reflectors should be 18cm long from top to ground plate.
I covered the steel wire in heat shrink tubing, bare elements should be approx. 5% longer.

The picture shows 3 stadia of a reflector:
Bottom: Standard brass M3 spacer .
Middle: Threaded stub removed and 2mm hole drilled into the center, about 5mm deep.
Top: Steel antenna wire soldered in and covered in heat shrink tubing.

Fully wired and cups sealed with silicone sealer, ready for a paintjob.

The finished array.

Cheers, PA8W.