PA8W Amateur Radio

Wil, PA8W,  E-mail: PA8W@upcmail.nl           
Back

The 23cm Preamped Amplitude Array (1150-1300MHz)

200205



  
A simple Amplitude 23cm array is illustrated here.

This array is suitable (using the amplitude method)  for hunting ATV-signals in the 23 cm band.

Of course also digital and analog voice transmissions can be hunted.

The center pin is the actual antenna, with a MMIC preamp in its base.
I added a bias-T to my AOR receiver to feed the preamp over its coax.

The antenna is surrounded by 4 reflectors that can be switched on and off.
A selected reflector changes the omnidirectional pattern of the central antenna into a directional pattern with a front to back ratio of around 10,7dB.
This directional pattern is
"rotated" simply by switching the reflectors on and off so that all the time at least one reflector is active.
This is done in a fast pace: 2000 steps per second = 500 cycles per second.
The rotating directional pattern modulates the incoming radio signal in amplitude, so an AM recever hooked up to this antenna produces a massive 500Hz tone.
(In a pseudo doppler RDF we would call this a doppler tone but in this case this really is the wrong terminology)

The RDF compares the phase of this AM tone with the phase of the antenna commutation to calculate the Angle Of Arrival  of the incoming signal.


As with the doppler array, a very good capacitive coupling to the car 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.



Below some pictures to illustrate the process:
  

 

This is the schematic of the UHF amplitude array.
The 23cm array only differs from this schematic by the MMIC preamp directly at the base of the antenna, and the use of 220pF capacitors instead of 1nF.

(In the UHF version 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.

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 16x16cm.  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 4,4cm from the center antenna.
Spacing between reflectors would therefore be around 6,2cm.

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

Add two extra 3mm fastening holes at 1 cm distance of the antenna/reflector holes.

Then, the antenna/reflector center holes are enlarged to 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!



Drop in the 3mm screws and then cover the bottom side with tape.
Then fill the holes with silicone sealer.

Now the small antenna PCB's can be screwed in place before the sealer is cured.


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.

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.

The components for the reflector switchers and the tiny preamp are already in place.
Now it's time to add the coax and control cable.

The components at a reflector.
Note that this is the UHF situation, for 23cm the capacitor and PIN-diode are smaller than on this picture.

This is the center antenna pcb:
The coax has to be soldered directly to the small pad in the top right hand corner.
Solder the coax shield to the mass surface.
Keep connections short!


I use 32mm PVC end-caps as enclosure for all 5 antenna/reflector positions.

They are glued to the ground plate using a black polymer sealer.

The thin control wires running to the reflectors are not visible since they are completely covered by sealer.


The actual screw-on antenna and reflectors are made using brass spacers with a length of
57mm for the center antenna and 67mm for the reflectors.
That' s total length, from ground surface to tip.

That sums it up, I hope to do some accuracy measurements when weather allows...


73, Wil.


Back