|PA8W Amateur Radio
Wil, PA8W, E-mail: PA8W@upcmail.nl
The UHF Amplitude Array
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
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.
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 a 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.
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.
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.
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
some pictures to illustrate the process:
This is the schematic of the
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.
drawing, and choose "Save as" to save the drawing to your map or
The resolution will be doubled compared to the drawing on this
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
Spacing between reflectors would therefore be around 18,4cm.
Mark them evenly spaced from the sides, so the 4 reflectors are nicely
Then, drill 3mm holes at the 5 markings. These are the
The reflector position in the bottom left corner is already drilled in
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
The screwheads should not protrude because they will damage your car
drilling the small antenna PCB's can be screwed in place.
Apply a good sealer between PCB and aluminum to prevent water to creep
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
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
length of steel wire drilled and soldered in its top.
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 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 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.