PA8W Amateur Radio

Wil, PA8W,  E-mail: PA8W@upcmail.nl           
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The UHF Amplitude Array (400-440MHz)



 
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. 


As with the doppler array, a very good capacitive coupling to the roof metal is essential.
Again we 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.

Field tests show a FB ratio of 12dB up to 28dB @ 433MHz, depending on the angle, very close to the theory.

A video of a test drive can be found here: https://youtu.be/0ECTJc8r3l0 

Below some pictures to illustrate the process:
  


The base material: single sided epoxy PCB material.
Available in Europe in size 50x50cm, 1,6mm thick, which I would recommend as the minimum size.
I made my prototype from a 60x60cm sheet.
But size is not critical, since the care roof will take over the role of a good conducting ground.

(this example is not to scale, just used to show how it's done)

We use it with the copper side up.

Mark a center point for the center antenna.
Mark the 4 reflector points towards the corners at 15cm from the center antenna.
Spacing between reflectors would therefore be around 21,2cm.
Mark them evenly spaced from the sides, so the 4 reflectors are nicely symmetrical.

Then, drill 3mm holes at the 5 markings.

(And don't mind the holes in the corners: they are from a different experiment)


After drilling the 5 holes, a 1cm diameter grinder is used to remove the copper right at the 5 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 sitting flush with the bottom.
After mounting the antenna feet, I tape the bottom side to insulate the RF "hot" screw.


Screw all 5 antenna feet 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.


At the base, the reflectors have their switching diodes run to ground and a 1k resistor in series with the control wire.

The control side of the control wire has a capacitor running to ground to bleed off RF.

The coax runs directly to the center antenna.


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 actual screw-on antenna and reflectors are made using identical metal spacers with a length of steel wire drilled and soldered in its top.
Total length including spacers should be 17cm for the center antenna, the reflectors should be 20cm long.
I cover the steel wire in heat shrink tubing, in that case you may shorten the alements by 5%.


The RDF41 control lines only activate one antenna -or in this case reflector- at the time.
Therefore, the center pod has a simple diode matrix inside to activate two reflectors at the same time.

 

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 at the array)
A simple piece of experimenting PCB can be used to hold the 8 diodes in the control lines.
The control lines don't have to be equal length.

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.


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