|PA8W Amateur Radio|
A mobile UHF Array (380-470MHz)
|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 the center of the sheet, and draw a cross to the corners.
At 13 cm distance from the center, draw markings for the antenna positions.
The antenna 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 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!
||Now, the small antenna PCB's
can be screwed in place.
Apply a good sealer between PCB and aluminum to prevent water from creeping 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.
I use a 15mm spacer bolt so 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.
|Then, the center PCB
(combiner) is glued in place using silicone sealer.
The yellow line points at the spot where the outgoing coax core should be soldered.
Note that there's room for a MMIC there in case the combiner is used in a passive array.
The red lines indicate the spots where the antenna coaxes should be soldered.
Of course the coax shields are soldered to the nearest ground surface.
The blue lines point at the pads where the antenna control lines should be soldered.
|All antenna PCB's are screwed and glued in place using silicon sealer.|
|All wired up.|
|All caps glued in place.|
Ready for a paint job.
|The actual screw-on
antenna are made using identical brass spacers
length of steel wire drilled and soldered in its top.|
Overall length including all spacers should be 17cm for all 4 antennas, top to ground sheet.
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 5mm deep.
Top: Steel antenna wire soldered in and covered in heat shrink tubing.
|As pointed out earlier, the antenna array is one of the most important parts of the RDF system.|
And it's really not that easy to construct an UHF array that only picks up signal via its antennas, with the coax and controll cable entirely dead.
Good coax, keeping both cables together, using ferrites to reduce mantle currents, taping all cables flat to the car's metal can make the system perform well, but it's not an easy job.
And still it turns out that generally the bearing is more or less influenced by movement of coax and control line.
As long as that is the case, we know that there's some degree of direct RF pickup of the cable system, which adds to the signal received by the antennas, reducing system accuracy.
Ok, a good coax helps, but it is not as good as you would expect in a simple antenna concept.
First of all, a 1/4 wave whip has an impedance of around 37 ohm, not 50.
So there is some mismatch from the start.
Secondly, if we use the system on a frequency that's way off the designing frequency the antenna has serious capacitive or inductive properties, which introduced even more mismatch.
Coax shielding isolation in a mismatched system will NOT be anywhere near the value stated by the manufacturer, because this is specified under matched conditions.
For that reason I developed the following solution:
The preamped UHF array:
Two big advantages:
1, It has about 18dB gain, that's more than welcome on UHF bands!
2, Since the preamps are located directly at the antenna feedpoints, the coax and control lines are less sensitive to the RF field since they are 18dB less sensitive compared to the antennas.
Due to this, the received signal is less polluted by radiation picked up by the cables.
This serves accuracy quite a bit and makes routing of the cables much less critical.
On the left picture you see the finished preamped Array during a field test @433,2MHz.
Averaged error turns out to be 2,2 degrees, with a measured Peak error of 4,5 degrees.
And that without ferrites, and simple RG58 cable.
I won't publish the design, since there is some commercial interest from two parties right now.
But I am willing to supply radio amateurs with a set of 4 antenna PCB's with all parts soldered in place, plus the combiner PCB, also with all parts soldered in place.
So you only have to build these PCB's in your array and connect control cable and coax to the combiner, plus 4 short coax cables to the preamp PCB's.
For those of you who have really payed attention reading my pages:
Yes, it is still very important that a non- selected antenna is decoupled from the system so it won't act as a parasitic element.
And yes, the preamps do just that, they are fed by the control signal through their coax, and they let go of the antenna as soon as they are switched off, virtually making the antenna "disappear".