HF

How do I get a hammerin' signal out on HF from my car? Simple! Make it BIG. For 80 through
10 meters I use a DK3 type antenna. Some call this a "screwdriver" antenna. This is an antenna
you can build yourself or there are several commercial models available. They're all based on
information in Don Johnson W6AAQ's book 40 +5 years of HF Mobileering (available from 
Radio Bookstore). My particular model is one built by H. Stewart Designs N7WMW, PO Box 643, 
Oregon City, Oregon 97045, 503-654-3350.

I looked long and hard at the competition and Henry's quality won out by a long shot. 
No question. I picked mine up from Henry last June at the NW Division Convention at Seaside,
Oregon. I chose this one because of it's superior mechanical construction. Henry uses a
combination of the absolute finest materials and excellent machine work to produce this
antenna. He charges a few more bucks than the rest, but I'm sure you'll agree that quality 
is worth a bit more. This is very important when operating in the harsh environment of mobiles.

These antennas are basically a mast of 3 to 5 feet, a huge center loading coil adjusted to
resonance with an electric screwdriver motor and topped with a "stinger" a short whip 4 to 8
feet long. It provides CONTINUOUS tuning between 3 and 30 MHz and at least twice the field
strength of any other type of HF mobile antenna. In fact, my tests indicate a solid 6dB
advantage over my HamStik 75m whip. Tests were done with a calibrated field stregth meter on
75, 40 and 20m both near and far field.

My antenna is mounted on a 1988 Chevy Blazer, up on the top mounting arm of the rear spare tire
carrier.  Side View  This puts the bottom of the loading coil well above the roof line. With 
the low band stinger (8 feet) this puts the top of the antenna well above the 13 to 14 foot
highway overpass and bridge height!  Rear View  Usually I run with a 4 foot stinger which provides me with
40 through 10 coverage.  Mount Closeup View

I cannot stress the importance of a good ground system. The ground system is related to antenna
radiation efficiency, matching network stability and receiver noise. Mobile antennas suffer
terribly from high ground losses to begin with and are usually physically short and have
additional losses in the matching networks. I use 1" silver tinned copper braid at the
tranceiver AND at the antenna base as well as several connecting points within the vehicle
frame and body system. Even with all this, huge choke-input filters, ferrite chokes, bypass
capacitors and 40,000 uF of filtering at the battery there's still a fair amount of ignition
and power line noise coupled into the receiver.

The coax used is very, very important also. Don't use RG-58, RG-8X or Radio Shack cables. Do
not use more coax than it takes to get from the rig to the antenna or matching device. DO
use high quality RG-8, RG-213 or RG-214. If you must use a smaller diameter cable, try RG-8M.
Never, ever use RG-58 or RG-8X! Never, ever use RG-58 or RG-8X! RG-58 may be used from the
antenna to the vehicle entrance, but keep it short. More than three feet and you're in for
trouble. Use the highest voltage rated cable available. This means RG-8, RG-213 or RG-214.
Don't use foam dielectric cables either. Use Teflon (PTFE) or solid polyethelyene. Try not
to use "UHF" connectors, PL-259's and SO-239's. They're not a constant impedance device and
are definately not wheatherproof. Use N type connectors where possible. They'll handle a kW
average and resist water quite well.

A bit more on RFI and receiver noise, etc.... My Blazer does have an ECM, Electronic Control
Module to monitor and actively adjust engine parameters. It's basically a microcontroller
with several sensors for telemetry and several remote controlled devices for engine adjustment.
Most newer vehicles have one of these or even a more complex computerized control system for
everything from climate control to emissions monitoring. I haven't experienced any trouble with
the ECM while operating HF mobile nor is their a great amount of computer hash generated from
the unit in my receiver. I have blown a few fuses on key down, probably due to stray RF summing
with some DC source to feedback and overload the circuit. An RF choke in the form of several 8
inch diameter turns of coax right at the antenna end of the coax fixed that. I've employed
ferrite beads and chokes on all DC connections as well as massive coke input filters for the

rig's DC supply. The rig is powered directly from the battery with a 40,000 uF capacitor across
the terminals and a .01 uF bypass cap to the vehicle chassis. All audio and auxillary lines
(such as the antenna remote control) are run through Radio Shack snap-on chokes and their cables
are shielded twisted pair with a drain. The cables are made in "shotgun" fashion. That is, one
end only of the cable is grounded via the shield/drain. The other end floats, giving no ground
return. This knocks RFI back considerably and reduces the likelyhood of ground loops.

The rig is an Icom IC-706. I can't say enough good about this little radio. All HF bands with
100 Watts plus 100 Watts on 6m plus 10 Watts on 2m. 'Nuff said! At some point I plan to build
a 600 Watt HF amplifier based on Motorola's MRF 154 transistor. 600 Watts from a single device!
That'll give me that extra punch on 75m. I've heard good things about Ameritron's ALS-500M amp
and a new solid state kilowatt amp from Henry Radio (800-877-7979). Sounds like good products
for those who'd rather purchase than roll their own.

VHF/UHF

As with HF operations, the antenna you select for VHF and/or UHF mobile work will determine
whether you get outstanding or merely mediocre results. My Dual band setup is fairly
straightforward. The rig is a Kenwood TM-731A, about as old as the Blazer. It's really not that
much different from any of the newer rigs in concept or application. The antenna is a Larsen
NMO-2/70, dual-band vertical, with an NMO or "Motorola style" mounting up on the cowling on the
drivers' side of the vehicle. Also employed are a 1/4 wave vertical for 6m and a mag mount 2m
antenna for the second 2m capability given by the IC-706. The mag mount can be mounted
horizontally for SSB contacts in a pinch!

Antenna placement is fairly critical for most gain type mobile antennas. A fair amount of
pattern distortion can be introduced by not locating the antenna properly. Generally, high
and in the center of the vehicle is the rule. On some larger vehicles, cowl mounting or on
the hood, rear deck, etc.. work very well well and the directional charistics are reduced.
Be wary of antenna products claiming more than about 5 to 6 dB gain. For most terrain, 
something between a unity gain (IE: 1/4 wave) and 5 or so dB in the vertical plane will
produce the best results. Higher gain antennas will work, but with crummy results. Common
problems are multipath, high take-off angle, vertical signal component compression. As with
HF antennas, a good ground system is critical to antenna performance. Don't assume that
because there's so much metal "ground plane" that you will have a great signal. Antenna mounts
which employ a through the body hole will generally give the best performance when installed
correctly.

Speaker placement, mixing audio signals, etc... 

Having a noisy vehicle, a need for some audio from the rigs other than the 2-3 Watts into
a 2-3 inch diameter top firing speaker was needed. I picked up one of those Motorola external
speakers, similar to the ones found in public safety vehicles. I tested it in my shop with
an audio analyzer and found that it could handle at least 10 watts without severe distortion.
The speaker is placed on the console in-between the two front seats and fires up and to the
rear of the vehicle. This makes plenty of audio in both the driver and passengers face.

Having two rigs, one of which allows dual-band monitoring, also presented a problem in
delivering the audio to my operating position. I wanted to have it loud, but didn't really
want a bunch of external speakers around the cabin. I experimented with several audio mixing
techniques and finally settled on one based on designs presented in the ARRL Handbook. My
mobile audio mixer/amplifier takes the external (muting the internal speakers) audio and
shunts an 8.2 Ohm resistor to ground so the rigs audio amps see a good load. Then the audio
goes to one leg each of a 5k Ohm pot for level control for each channel (the other leg is
grounded). The pots wiper legs are then passed through series 5.6k Ohm resistors (for
isolation)and summed into the input lead of an NTE-1375 5.8 Watt audio amplifier IC. This
device gives a good, clean 5 watts of output with a single ended 12 VDC supply and costs less
than $4.00. A few simple connections to the amp IC, DC and out to the Motorola outboard speaker
completes the audio mixer/amplifier. I'll put the schematic on this page as soon as I get
around to drawing it up!

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Copyright © 1997 & 1998 James S. Kaplan
Updated May 13, 1998