UHF Amplitude Array (390-470MHz)

Amplitude Radio Direction Finder Array Ready.jpg

A simple Amplitude UHF array is illustrated here.

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.

Originally, looking for best front to back ratio, I designed an array in which two reflectors were activated as a pair, resulting in a front to back ratio of over 25dB.
Until my friend Goos, an expert in this field showed me in a spread sheet that this type of array probably has a peak error of almost 8 degrees.
So I changed the design into a “one reflector at the time”  approach .
This yields a FB ratio of 9dB @ 433MHz which is still enough to create a solid 500Hz AM modulation in your receiver.
Goos his spreadsheet showed that the peak error of this design was limited to a little under 4 degrees.
As a bonus this array maintains its directional properties over a considerably wider frequency span than the first approach.

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: https://youtu.be/0ECTJc8r3l0

Below some pictures to illustrate the process:

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.

Amplitude Radio Direction Finder UHF Array Schematics.gif

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.

Note that this undersized ground plate should be taped flat to the car roof, because only this way the roof will extend the ground surface.

Mark a center point for the center antenna.
Mark the 4 reflector points towards the corners at 15,5cm from the center antenna.
Spacing between reflectors would therefore be around 22cm.
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!
UHF Radio Direction Finder Array Baseplate.jpg

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.
UHF Radio Direction Finder Antenna Cap.jpg    UHF Radio Direction Finder Antenna Base.jpg

The components at a reflector.
The control wire runs to the center pod, which houses the actual antenna element.
Length of this wire is not critical at all.Amplitude Radio Direction Finder Array Reflector.jpg

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.
Amplitude Radio Direction Finder Array Center.jpg

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 20cm long from top to ground plate.
I covered the steel wire in heat shrink tubing, in that case you may shorten the elements by 5%.

The picture on the left shows 3 stadia of a reflector:
Bottom: Standard brass M3 spacer .
Middle: Threaded stub removed and 2mm hole drilled into the center, about 4mm deep.
Top: Steel antenna wire soldered in and covered in heat shrink tubing. UHF Radio Direction Finder Antenna Detail.jpg

Fully wired and cups sealed with silicone sealer, ready for a paintjob.Amplitude Radio Direction Finder Array Before Paint Job.jpg

The finished array….

Amplitude Radio Direction Finder Array Ready