[Elecraft] dipole antenna efficiency

Sun Jan 6 11:58:24 EST 2008

I'd go "mobile" Bill. Like an automobile (at least when they had significant
metal), the frame of your trailer will provide capacitive coupling to ground
and you can use a short, loaded  vertical "mobile antenna" such as sold for
use on automobiles. 

It will be a very inefficient antenna, compared to a half-wave long antenna
up 1/2 wavelength or more in the sky, or a vertical that's a full 1/4
wavelength long working against an ideal ground, but you will make contacts
- lots of them when band cdx are decent. Hams have made contacts all over
the world at QRP power levels for decades with just such setups, even on
phone.

I know many operators have excellent results with the "screwdriver" types of
mobile whips. Those have motors to adjust the loading coil according to the
frequency of operation so you can "tune" from inside the trailer. The better
your installation, the sharper the tuning. As others noted, a reasonably
efficient shortened antenna has high "Q" - that is the bandwidth between,
say, SWR of 2:1 is much, much smaller than a full sized antenna so you want
to be able to adjust it easily. I know one or two operators who routinely
use that type of antenna at their homes, where they have little space or
time to fiddle with other options.  

I would also plan to mount the antenna on the roof, even if that means
taking it down (unscrewing it from a mount affixed permanently and
water-tight on the roof) when traveling. After all, that should only require
a moment's work to erect or stow it and it's well worth the effort. The
majority of the radiation from such an antenna will be at its base. Keeping
the base away from "ground" (the metal parts of the  trailer) and as high as
possible will help your signal a lot. Putting it in the center will produce
the most uniform pattern. Putting it one end will produce a stronger signal
in the direction of the trailer. 

Once you do that and you are happily operating QRP from your trailer, you'll
find endless things you can "do" with your antenna installation, depending
upon your circumstances and interests. Some of them might be:

1) Improve the ground system. Instead of relying on the capacitive coupling
between the metal and ground, arrange to string some "radials" on the ground
or insulated slightly above the ground that are easy to deploy and not a
trip hazard. 

2) Improve the antenna. You might find you are in situations where you'll be
spending a few days with a handy tree nearby, etc., and you get the wire,
etc., needed to throw a longer antenna over a tree limb and connect it to
the mount on top of the trailer in place of the normal antenna. You'll want
an ATU at the rig to do that, and  that means your coaxial feeder will be
operating at a very high SWR (which produces loss) but those things should
be minimal because the coax shouldn't be so terribly long in a trailer! Some
mobile ops do this with a wire and a clip lead that they just clip onto
their regular whip for when they can add it. Every foot of wire will improve
your antenna's efficiency. 

3) Another way you might considering improving your antenna would be to
investigate surplus military "whip" antennas that show up on the market from
time to time. Some of these are 15 or 20 feet long! Over the years more than
one trailer traveler has arranged such a whip, mounting at the top of his
trailer but at one end. Then he could either tilt the mount or just bend the
tip over and tie it at the opposite end when in motion, making the whole rig
only a couple of feet higher than without the antenna. A number of hams have
bought a "full sized" loaded vertical intended for home use and arranged a
tilt-over mount for it on their trailer. After all, most of those antennas
are never more than 12 or 18 feet tall and the upper section can often be
disassembled if needed for transport. 

4) Consider *where* you go in your trailer! A trip to the seashore, mountain
or open desert can produce unexpected and amazing DX from even the simplest
locations by reducing the sort of losses to surrounding objects and the
earth itself for several wavelengths around us that most of us in fixed
locations cannot do anything about. 

Those things all depend upon how your interests and opportunities develop
over time. One thing for sure is that you'll have a lot of fun operating and
make lots of contacts even if you just use the basic mobile antenna
installation. 

Ron AC7AC

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of bill KE5KWE
Sent: Sunday, January 06, 2008 7:53 AM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] dipole antenna efficiency

Let me bring this down to the practical world.  My QTH is a 42.5 ft. Travel
Trailer which is insulated from the ground by 6 Rubber Tires and Air.  It
has Metal, I presume Aluminum, siding and a Rubber over Wood Roof which is
approx. 10 ft. off the ground.  My K2 is almost built and I will be
operating CW exclusively QRP.  Are there any suggestions for either a
commercially built antenna or a homebrew. (Recognizing that I am not a EE
and have no formal electrical background!)

Thanks

Bill Fogel, KE5KWE
"On the Road in the USA"

This thread started with a person trying to find an
antenna that would work reasonably well that does not
take up much space. 
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View this message in context:
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