PA8W Amateur Radio

Wil, PA8W,  E-mail: PA8W@upcmail.nl           
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Building a Mobile VHF Array (120-180MHz)

Revised 190310

This page describes how you can construct a high performance VHF mobile array.
Instead of the DIY magnet mounts in the description, you may as well use the 92mm Sirio Mag H12-PL magmounts.
They can be opened to add the necessary switching diode and resistor, and the capacitive coupling to the car roof is good enough to ensure good performance.
Always pick an oversized magmount to ensure solid capacitive coupling to the car roof.

The car array exists of 4 quarter wave whips on magnet mounts.
That would be 50cm whips on 145MHz.
Element spacing should be approx. 0,25 wavelength, 50cm on 145MHz.
Be sure to have the antennas in a symmetric square, preferrably centered on the car roof.
The third switching diode in the classic V2,3 schematics -the one that switches the bottom leg in the homebase version antenna- should be deleted.
The coax shield should be hard wired to the magnet mount cup.
Below you can find the right schematic.

The spacing should not come anywhere near 0,5 wavelength.
Better keep it quite a bit smaller.
With the 145MHz array on the photo I had good results even tracking down 35MHz RC transmitters for model airplanes.
In spite of the constant digital pulse (FM) on these signals, and the fact that the frequency is 1/4 of the array designing frequency, 
it was easy to hunt down these transmitters.
However, being so far off the design frequency, you may have to recalibrate the RDF to be spot on.

 

Considerations:

In the real world, multipath signals are a constant menice, resulting in erratic readings on the RDF display.
There is quite a bit of difference in performance between different arrays, some are more resistant to multipath than others.
A good symmetric radiation pattern for all 4 whips is very important.
On average, any directivity will enlarge the amplitude of multipath errors.
Therefore you should put the array at the center of the car roof, with plenty roof metal around it.
Additionally, a poor ground coupling to the car roof will cause mantle currents in the coaxes, which in turn will distort the measured bearings.

 N0QBF did some interesting tests with standard and modified magnet mount solutions.

First he tested a standard mag mount system with aluminum crossarms, and recorded the bearing deviation in 200 points over a measuring route.
Losts of severe deviations are clearly visible on the below chart:



Only after: 
1, replacing the aluminum crossarms by non-metallic versions and 
2, using alufoil under each magnet for improved capacitive coupling to the metal roof of the car,

the following improvement was recorded on the same test route:



The deviations are much smaller and much less common compared to the first test.

So, very good capacitive coupling is essential for good performance of the mobile array.
This is what we have to keep in mind.

A standard mag mount generally may have pretty poor capacitive coupling, and the coupling is easily degraded further by the curvature of the roof and by 
dirt particles between the roof and the magnet. 
Only the edge of the metal cup in which the actual ceramic magnet is held, is really close to the roof metal.
This is easy to improve: Just glue a thin layer of felt or thick cloth to the bottom of the magnet.
Put a clean flat sheet of alufoil beneath it, (no folds!) and wrap the foil upwards around the metal cup of the magnet.
Make sure there's good, reliable electrical contact between foil and cup, and we have created a magnet mount that has a very high and constant capacitive coupling to the metal roof.
Thanks to the layer of felt or cloth in between, the foil will adapt to the curvature of the underlaying roof and at the same time provide a softer surface which is less likely to damage the car paint.
I measured the coupling capacity of a 80mm diam. cup magnet put directly on a painted steel sheet with an absolutely clean and flat surface, it was 473pF at best, not that bad after all. 
Only realize this was a best case situation.
With the usual rubber sheet in position the capacity dropped to 155pF, way too low for a good RF ground. 
But with the above construction using felt and aluminum foil it was almost 1700pF, which is excellent for 145MHz and higher.

Ok, now let's start constructing!

In the following pictures it is illustrated how I made my magnet mounts.
The magnet itself is a 80mm ferrite bus magnet with a theoretical pulling force of 70kg on a massive, polished block of iron.
You will need a strong magnet if you want to follow through with the above description of a good magnet mount.


The housing is made of 32mm PVC end caps with a short
piece of pipe to connect the lower end upper cup.
The lower cup has a M8 bolt screwed in the bottom which
fits the M8 bus of the actual magnet.
This M8 bolt is also the grounding point.
The coax mantle is soldered to this grounding point, and the coax core runs via a switching diode to the center contact of the PL259, which accepts the actual antenna whip.
Between the center and ground of the PL259 there's a 1k resistor.


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.

Et voilá! 
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 coax arms.
Use a good sealer to seal the coax entries.

The schematics of the array:
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. 

Summary
Of course, arrays may be scaled up or down for other frequency bands.

With the above technology and dimensions, good results will be achieved not only on the designing frequency, but also over a considerable frequency span up to 20% above and 20% below the designing frequency. 


73, Wil.


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