Random wire antenna question

[Craig Rairdin]

The only antennas I have experience with are dipoles and commercial
multi-band verticals. When I built my K1 I took the manual at its word and
strung 30' of hookup wire around the room and shoved one end in the antenna
socket and got 1.2:1 or better on all bands with the ATU. So I got ambitious
yesterday and ran a 50' copper wire between a couple peaks on the roof,
about 3' above the roof. One end is connected to the center conductor of a
25' piece of RG-58 I had laying around and the other end of the RG-58 is
plugged into the K1.
 
The new antenna tunes up fine on 30, 20, and 15 meters, but on 40 meters the
power bounces randomly from 0.1W to 5.0W and I never see an SWR reading. The
manual says I'm getting too much RF back into the radio. It suggests I
reduce power and try again. That doesn't help.
 
I find it weird that I can transmit on 30' of hookup wire tossed around the
room but can't get an acceptable SWR from 50' of wire 25' in the air (albeit
close to the house). Any suggestions? Longer? Shorter? I have the option of
putting a full 40M dipole on the roof but it will violate my wife's "I don't
want to see it from the driveway" rule, and that's important to me. :-) So
I'm somewhat limited to the current length.
 
When I get my 2-band board built, it will have to work on 80M, too.
 
I'm eventually going to talk her into some kind of multiband vertical but
until she gets used to this piece of wire, I'd like to be able to transmit
on 40M. :-)
 
Craig
WB0GUU
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[Ron D'Eau Claire-2]

Craig WB0GUU wrote:
I got ambitious yesterday and ran a 50' copper wire between a couple peaks
on the roof, about 3' above the roof. One end is connected to the center
conductor of a 25' piece of RG-58 I had laying around and the other end of
the RG-58 is plugged into the K1.
 
The new antenna tunes up fine on 30, 20, and 15 meters, but on 40 meters the
power bounces randomly from 0.1W to 5.0W and I never see an SWR reading. The
manual says I'm getting too much RF back into the radio. It suggests I
reduce power and try again. That doesn't help.

--------------------

What's happening is that you don't have a decent RF ground at the rig.
Connect a 33 foot length of wire to the case of your K1. That's 1/4 wave
'counterpoise' on 40 meters. You can just lay it on the floor or anywhere
convenient. The only requirement is to keep the far end insulated because
it's hot with RF. That should provide a ground reference for the rig on 40.

The way a 'counterpoise' wire like that works is this. If the end of a wire
is insulated from any other conductor, it must be a high impedance (high
voltage) point for RF. After all, no current can flow at that point if it is
insulated. Moving from the end of the wire toward the rig you will have a
low impedance point 1/4 wave later (about 33 feet on 40 meters) which makes
a decent RF ground. If the wire is longer, 1/2 wave from the insulated end
you'll repeat another high impedance point (that's why the ends of a dipole,
which is 1/2 wave end-to-end, are high impedance points and the center, 1/4
wave from each end, is a low impedance point). Moving on, you'll have
another low impedance point at 3/4 wavelengths, and so on.

The braid on the coax would have worked as well but obviously it's not close
to 33 or 99 feet long (1/4 or 3/4 wavelengths on 40). That coax will
introduce some significant losses, especially on the higher frequencies, due
to the high SWR on it. Remember, your ATU will match the antenna connected
to the rig to the rig itself, but it has no effect on the SWR on the coaxial
line leading away from the rig.

Ron AC7AC



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[Leigh L. Klotz Jr WA5ZNU]

Craig,
You might like reading the articles at http://www.cebik.com to get a
feel for what things to try in your next experiments.
If you don't like the results of Ron's suggestions (which by the way is
what I used in FYBO this weekend) you might find you can fit a
similarly-sized doublet center fed with twinlead outside the house to an
Elecraft balun and then a short run of coax through the wall to the
rig.  The WireMan has good twinlead for not much money, and black teflon
wire that is extremely hard to see from the driveway.  Emtech makes a
small, cheap clamp that will hold twinlead to the wire mechanically.
Here's an old discussion on this topic:
http://www.ac6rm.net/mailarchive/html/elecraft-list/2001-10/msg00931.html

73,
Leigh WA5ZNU
 >The only antennas I have experience with are dipoles and commercial
multi-band verticals.
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