|PA8W Amateur Radio|
Building a Mobile VHF Array (120-180MHz)
The housing is made of 32mm PVC end caps with a short
||I glued a sheet of 1,5mm thick felt to the bottom of the magnet.|
||Then I covered it with alu-foil.
When the switcher is bolted in, the alu foil provides massive capacitive coupling to the car roof.
One magnet mount with 1700pF coupling capacity!
||This picture shows a somewhat
different approach just to make it look better:
I cut the alu-foil half way the side of the magnet cup, and fixed it using black gaffer-tape.
The rest of the magnet and switcher is simply painted black.
This way it has a lower profile, especially on my black car.
The alu foil at the bottom will wear pretty quickly if you don't take proper precautions handling it,
but a new sheet of aluminum is taped on in less than a minute.
The whips itself? Just cut equal lengts of about 1/4 wave of steel wire (or copper clad welding wire!) for every band you want to use the RDF for, and solder them to the core of the PL239 plug.
The length is not too critical, just make them identical.
I cover my whips with heat shrink tubing and seal them using silicone glue or polymax. Also the space between whip and the PL259 housing is filled with this mass, to make it really stable and waterproof.
Warning! Never rely on the magnetic force of the magnet mounts if you might drive at speeds higher than 50kmh.
Securing them with some strong dralon or nylon or whatever rope will keep things safe for you and the other traffic members.
|As stated at the beginning of this page, a good alternative are the Sirio Mag H12-PL magmounts.
They can be opened to accept the diode and resistor, or even a preamped switcher.
They have a proper capacitive coupling of around 550pF due to their size and very thin rubber bottom.
Good enough for VHF and excellent for UHF.
|And this is how they look.
It is good practice to tape all cables flat to the car roof b.t.w.
This will reduce the effect of cables picking up RF.
|Here's the mobile antenna
combiner, with 4 antenna switchers at the ends of the 4 equal length
Use a good sealer to seal the coax entries.
|The schematics of
For the RDF41 stick to this schematic.
Note that for the conventional V2,3 doppler the 1k resistors may be changed into 1uH inductors, like stated in the V2,3 schematic.
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.