80/40M Mobile Antennas (RE: QRP mobile questions)

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From: schoon@amgt.com
Date: Mon Apr 02 2001 - 15:20:19 CDT


Good Afternoon!

        Being a very avid HF mobile op, I've followed this thread pretty close.
All in all, some good information out there! Most ops have no problem
running 20m or above while mobile. There is some additional challenges
in being successful on 40 & 80 though.... But where do you spend your
time in making improvements and how do you know if your improvements are
actually working?? The losses in physically shortened HF vertical
antennas go up as the freq goes down - but where?? The biggest losses
are in the ground and the coil. The ground on your vehicle is very
important and can lead to tremendous returns if done properly. They way
vehicles are built today leaves many 'opens' between body panels and
chasis parts. At times, these are hard to find, but once they are the
can easilt be 'jumpered' with heavy copper braid. I know, most folks
don't want a bunch of holes in their new vehicle but there are
techniques to do so and not suffer from corrosion or lower the value.
Spend time on scrapping paint and bonding all major pieces of metal.
Your noise level will drop and you'll get a better match on the low
bands. Don't go overboard as you can end up with ground loops and you
will run into diminishing returns for your effort. Suffice to say, a 1"
strap between hood/trunk and the body, plus all doors and the body
whould make a good setup. If you happen to be in a minivan, then there's
a very good chance that the roof isn't bonded all that well to the body.
You can find that out by removing some of the trim around th doors. If
you see what looks like stiff glue or sealant that has been painted
over, then those body parts are not connected electrically. I leave it
to the reader as an excersize to publish how they solved their
particular vehicles problems. Please post your results.

        Another place where grounds losses add up is mounting the antenna too
low on the vehicle. Most states have a limit as to maximum height on the
roadway, so check it out. Do your best to get the antenna as high as
possible - X/YL permission of course!! Bottom line is you want the
entire vehicle to couple with the ground below, not the puny little
feedpoint. Oh, also do you best to keep the loading coil atleast 18"
away from any part of the vehicle...... That helps in keeping bandwidth
narrow....

        What's that?? A narrow bandwidth?? Yes!!! Yes!!! Yes!!! My mobile setup
is a 96 Ford Ranger with a hacked up DK3 screwdriver. My 2:1 bandwidth
on 80m is 6 Khz!! No matter, I can resonate anywhere 3.5-29+ and on 6M
as well. Here's where people sometimes get confused. The second major
loss for 80/40M antennas is in the coil. Lets face it, a coil that has a
diameter about the size of your thumb, it's going to have quite a loss
associated with it. What's the best size?? Well, that all really
depends, but suffice to say a coil with about 2" diameter made from 16
ga solid will be good enough. If you happen upon some coil stock at a
swapmeet, get a chunk. You'll need about 18" or so to get to the bottom
of 80m. If you don't need to get that far, less will do. A quick and
dirty homebrew multiband HF mobile antenna can be made out of some of
this stock, a bicycle flag pole, wire and some heat shrink tubing. Yeah,
the flag pole will be pretty flexible, so you'll need to guy it off
below the coil. Put the bottom of the coil at the 36" mark and attach it
to the flag with some UV resistant wire ties. Using the heatshrink
tubing, run a wire from the base to the bottom of the coil. Prefer
atleast 16 ga. Do the same above the coil all the way to the tip - 65"
max if you need to make it to the top of 10m. Attach coax and prune for
resonance on 80m. You may need a 5 turn, 2" diameter coil at the base to
match, if your SWR is way off. If the SWR is below 1.75:1, don't sweat
it.... With a jumper piece of wire, attach one end to the base of the
coil and put an alligator clip on the other. Go up about half way on the
coil and you should be kinda close to 40m. Don't retrim the top section,
just keep moving the clip around until you find resonance on 40. Mark it
some way. Do the same for the rest of the bands. You'll end up with an
antenna that will smoke any Hustler or hamstick monoband all for about
the same price as two of them - if all parts are bought new! A well
stocked junk box will make it less... With this kind of setup, you won't
need a tuner because the antenna is resonant. Tuners don't work worth a
darn with very short antennas like this. Why?? Because, no matter what,
the antenna will only be resonant at one location and all the tuner is
doing it fooling the transmitter to put out full power.

        So, in QRP fashion, you two can have a lot of fun running HF mobile!
Comments welcome!!

>72 .mark


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