Master Mobile Antennas

[Steve Harrison]

At 08:25 AM 12/13/98 -0500, Rick Markey wrote:
>Anyone remember the Master Mobile antennas or perhaps have any literature?
 I friend picked up what appears to be 2 coils from the HY "Q" AB series
but has no literature.  He's trying to figure out what the length of the
bottom and top whips should be.  I have some catalog info from a 55
handbook, but it doesn't have the individual dimensions, just the overall
length.

I've got a bunch of the coils. Use a standard Hustler bottom mast (about 5
feet long); actually, any bottom mast is OK, the length has very little
effect itself, primarily changing the capacitance of the coil and top whip
to the car body which changes the resonant frequency slightly. For the top
whip (I've also got a couple of the original Master Mobile tops), he
probably should start with a telescoping auto whip so he can adjust the
length. I can't give exact lengths; the problem is all of my coils have
been modified with either fewer or more turns added and that changes the
length of the top whip. In addition, the top whip length will vary with its
capacitance to the car body; different locations on the car will require
readjusting the length somewhat.

A grid-dip meter is ideal because the only other way is to transmit into
the antenna and try to find the lowest SWR across the band by sweeping your
VFO, and that's not cool! But if he doesn't have a GDO or antler analyzer
(MFJ or AEA or whomever), he'll have to sweep and adjust the whip length
accordingly. This can be done at just enough output power to be able to see
the reflected power on the SWR meter. As he comes closer to the resonant
frequency and the reflected power dips, he can increase the output power
slowly to find the exact resonant frequency of the antenna. But it's still
a crummy (and illegal, too!) way to tune an antenna.

Eventually, he may find the whip length for minimum SWR and be deeply
chagrined to find the SWR is still very high, like 3:1 or worse. The
analyzer will show that the base impedance is very low, much lower than 50
ohms, particularly on the lower bands. Master Mobile had an adjustable coil
with a tap that could be used for both precise resonance and impedance
matching. I have one but never got it to work well. Instead, I use a shunt
variable capacitor at the base of the mast inside the trunk, a multisection
receiver type. Made a panel with a couple of tip jacks connected to each
section of the variable cap so I could set up the cap for each band. Later,
when I mounted several antlers on the car and selected them with a surplus
multiposition coaxial relay, I tuned each antenna at their base with an
individual cap. Initially, to tune the same mast (with different coils) For
160, 80 and 40 meters, I had a few high-voltage silver mica caps inserted
into tip plugs that could be plugged into the panel.

If you read some of the earlier ARRL antenna manuals and especially the
mobile manuals from the '60s and '70s, you will find some formulas from
which you can calculate the required capacitance and other such parameters;
I found that to be useful and helpful.

The collapsible auto whip works very nicely but will have to be replaced
after a year or so of driving as it hits trees. To reduce such damage, I
used Hustler whip springs on top of the Master Mobile coils and guyed the
antenna with fishing line tied around the base of the spring on top of the
coil. The whip also wears out, gradually losing its ability to hold the top
sections in place as the whip suffers high-frequency vibration on the top
of the coil plus the tree whacks.

Later, so I could operate all HF bands from home and not disturb my
existing high-performance V/UHF antennas, I mounted antennas on each fender
of the car for each of four bands. These were all matched with shunt C at
their bases and the feedlines led to the coaxial relay in the trunk. The
control cable and main feedline to the relay was then unrolled and run into
the shack in the house while the car was parked at the curb.

Then I borrowed a Henry 2K-Classic. Running 1500 watts to a mobile antler
was an interesting experience (both for me and the kids who skated up and
down the street, running their hands along the car as they passed!). To
stop corona discharge, I had to make corona balls for each whip,
particularly the low bands (80-20). The only band I actually kept on the
car when driving was 40m although I could also run 80 if I wanted.
Radiation efficiency of the antlers was greatly increased as the top whip
length was increased and the coil tapped to resonate the antenna; this
caused the base feedpoint impedance to rise which decreases base resistance
loss. The Henry didn't cover 160m so corona wasn't a problem there. The 80m
setup had a large tophat consisting of a round aluminum plate around 6"
diameter on top of which were bolted a number of 19" VHF whips I found
somewhere, I think 8 total. I can't remember for sure but believe I used
the same top hat on 80m, too. The top hat was mounted on top of the whip
spring on top of the coil and would sway in the wind as I drove, causing
the SWR to wobble around as the resonant frequency changed. Both the 80 and
40 meter top whips were cut-down CB whips inserted into original Master
Mobile top whip adapters which screw into the top of the coils; four set
screws tighten against the whip bases to hold them. I had to make clamps to
fit over the top of the whips onto which I could mount corona discharge
"devices". These consisted of six 3/16"-wide straps of copper bent into a U
shape, the ends of which were bolted onto the adapter which clamped onto
the top of the whip. The straps were arranged into a ball shape. The
purpose of the straps was to provide sufficient area to reduce corona
discharge to a mere glow on the edge of the straps when running 1500 watts
and simultaneously reduce wind resistance while driving so the whip didn't
bend back too far on the highway. Early experiments with other various
"skinnier" versions of the discharge ball using wire were short-lived as
the wire burned up from the heat of the high-voltage discharge! And I
actually MELTED the top of one whip when I first tried tuning up at high
power before biting the bullet and making the discharge devices.

The Master Mobile coils allow one to build mobile antennas whose
performance with a 100 watt radio appeared to be about equivalent to most
home stations using anything except beams. When I ran 1500 watts on 80
meters, it was possible to top many pileups during DX contests. The
very-low radiation angle of a large, older car and optimumly-tuned Master
Mobile coil with long top whip constantly provided exceptional performance
beyond that of most home stations, particularly when working stations on
the "gray line". There is no comparison with any of the slimmer mobile
antennas.

Hope this has been of some help. I'd like to find more Master Mobile coils
and accessories, myself. I put them to use, not just collect them (I doubt
that any of my Master Mobile stuff is unmodified).

73, Steve Ko0U/1

P.S. I've never seen ANY literature for Master Mobile other than
advertisements in old handbooks or QST.

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