PA8W Amateur Radio

Wil, PA8W,  E-mail:           

A mobile UHF Array (380-470MHz)

A nice way to build an UHF array is illustrated in the above picture.
As in the VHF array, a very good capacitive coupling to the roof metal is essential.
Due to the shorter wavelength, another approach is feasable:

Simply take a single sided Printed Circuit Board as a ground surface.
Taped to the car roof it provides very good capacitive coupling, and it is very easy to solder the necessary electrical contacts to it.

Below some pictures to illustrate the process:

The base material: single sided epoxy PCB material.
Available in Europe in size 50x50cm, 1,6mm thick.
The ideal size would be 3/4 lambda x 3/4 lambda, which makes 52cm x52 cm at 70cm.
But size is not critical, since the car roof will take over the roll of a good conducting ground.

(this example is a little too small: 33x33cm and 3mm thick, just because it was in my junkbox)

We use it with the copper side up.

Mark the 4 antenna points at a spacing of 1/4  lambda.
Mark them evenly spaced from the sides, so the 4 antennas are nicely symmetrical.

Then, drill 3mm holes at the 4 markings.
The center marking was just to mark the spot for the summer and should not be drilled.
(And don't mind the holes in the corners: they are from a different experiment)

After drilling the 4 holes, a 1cm diameter grinder is used to remove the copper right at the 4 holes.
The copper is just 35 micron thick, so don't overdo it.

After that, polish the copper and seal it with a good layer of soldering laquer.

On the bottom side the holes need to be countersunk, so a countersunk screwhead will be flush with the bottom.
After mounting the antenna feet, I tape the bottom side to insulate the RF "hot" screw.

The 4 antenna feet are screwed in place,
Use 3mm metal spacers for that.
They are available in several lengths, and it is smart to pick a length that is similar to the inside height  of the PVC end caps that you use as a housing.

The actual screw-on antennas are made using identical metal spacers with a length of steel wire drilled and soldered in its top.
Total length including spacers should be 1/4 lambda.
I always cover the steel wire in heat shrink tubing.

Here the antennas have their switching diodes in series with the coax and a 1k resistor to mass.
The coaxes are identical length and meet at the summer PCB in the center.

Here, a piece of  control cable and coax is attached to do some initial testing.

Very important:
Take a good quality UHF coax and keep control wires short.
Tape coax and control cable together to the copper and also taped to the car roof.
All to prevent signal pickup by these cables, since it will reduce RDF accuracy.

The summer is soldered to the mass surface, the more soldering spots the better!
I left the control wires long but for experimental purposes but you really should keep things as short as possible.

This is the schematic of the antenna array.
For the RDF41 stick to this schematic.

Note that for the conventional V2,3 doppler the 1k resistors may be changed into 470nH inductors.

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. 

73, Wil.