PA8W's
Radio Direction Finding Technology
Building a Mobile VHF Array (120-180MHz)
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 coax shield should be hard wired to the magnet mount cup.
Below you can find the right schematic.
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 design 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 antenna housings are made of 32mm PVC end caps with a short
piece of pipe to connect the lower end upper cup.
The lower cup has a M10 bolt screwed in the bottom which
fits the M10 bus of the actual magnet.
This M10 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!
Above 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
or polymere sealant.
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 higher speeds.
Thouroughly check if the use magnet mounts are suitable for your speed,
given the used antennas.
As stated at the beginning of this page, a good alternative are
the
Sirio PM-100 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 almost 1100pF due to their
size and very thin rubber bottom.
Good enough for VHF above 100MHz and excellent for UHF.
They even have a larger model, the PM125 Power Mag, for max
performance below 100MHz.
The
PL259 connectors that come with the magmounts are transformed into whip
antennas:
This is the center PCB (combiner) like the one in the black box.
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 all coax shields are soldered to the nearest ground surface.
The blue lines point at the pads where the antenna control lines should
be soldered.
Don't forget to connect the ground of this PCB to the ground of the RDF
(the center pin of the 5 pole DIN)
So, using a 8 wire network cable, 4 wires are used for controlling the
4 antennas and the 4 remaining wires are used as ground connection.
The combiner pcb measures 42x42mm = 1.64 inch
square.
The schematics of the array:
For the RDF41/42/43 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.
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 design frequency,
but also over a considerable frequency span up to 20% above and 50%
below the design frequency.
Cheers, PA8W.