I have had an 80 meter loop running around my house at a height of 35
feet now for 5 years. It was fed with a 75 foot section of 450 ohm
window line. This served me well during this time. I know very little of
antenna theory. I do know some electronics and repair my own
transceivers and amplifiers.

I wanted to do a little improvement to this loop to stop the thing from
changing tune every time it rained or snowed.

A fellow ham urged me to use open wire 600 ohm line to stop this
detuning from happening and to improve my losses with this antenna.

I already felt I had a pretty good antenna because it really lowered my
receiver noise over a dipole that I had originally. Anyway, two days ago
I finally did the work to put up a 600 ohm line using #14 insulated
house wire and spacers from www.73cnc.com.

I was shocked to see my antenna change tune radically. I thought I had
made a mistake and was going to have to replace the open wire line with
the 450 again. I was worn out after the day of changing this line. So I
spent the next day using the antenna anyway. My VSWR on 75 meters had
gone from 3 to 8. Yikes. However, all the higher bands looked a lot
better. 15 and 10 meters showed an almost perfect match. The antenna
became much quieter. My friends on 75 said they heard no difference in
my signal strength even with the 8 to 1 VSWR.

I have an Imax 2000 that I use for 15 and 10. This antenna always showed
an increase in signal strength on those bands over the loop. Now the
reverse is true. I hear signals on the loop that are buried in the noise
on the Imax. In 1 short hour I worked Cuba, Belfast, Nurenburg, and
California getting great signal reports from them all and seeing
beautiful S meter readings from them.

I cannot explain this effect with any kind of theory. I simply don't
know antenna theory. But it happened. My friend cannot explain it either
but he tried using open wire feeds 30 years ago and will not use
anything else.

On 9/1/2012 8:08 AM, Boomer wrote:
I have had an 80 meter loop running around my house at a height of 35
feet now for 5 years. It was fed with a 75 foot section of 450 ohm
window line. This served me well during this time. I know very little of
antenna theory. I do know some electronics and repair my own
transceivers and amplifiers.

I wanted to do a little improvement to this loop to stop the thing from
changing tune every time it rained or snowed.

A fellow ham urged me to use open wire 600 ohm line to stop this
detuning from happening and to improve my losses with this antenna.

I already felt I had a pretty good antenna because it really lowered my
receiver noise over a dipole that I had originally. Anyway, two days ago
I finally did the work to put up a 600 ohm line using #14 insulated
house wire and spacers from www.73cnc.com.

I was shocked to see my antenna change tune radically. I thought I had
made a mistake and was going to have to replace the open wire line with
the 450 again. I was worn out after the day of changing this line. So I
spent the next day using the antenna anyway. My VSWR on 75 meters had
gone from 3 to 8. Yikes. However, all the higher bands looked a lot
better. 15 and 10 meters showed an almost perfect match. The antenna
became much quieter. My friends on 75 said they heard no difference in
my signal strength even with the 8 to 1 VSWR.

I have an Imax 2000 that I use for 15 and 10. This antenna always showed
an increase in signal strength on those bands over the loop. Now the
reverse is true. I hear signals on the loop that are buried in the noise
on the Imax. In 1 short hour I worked Cuba, Belfast, Nurenburg, and
California getting great signal reports from them all and seeing
beautiful S meter readings from them.

I cannot explain this effect with any kind of theory. I simply don't
know antenna theory. But it happened. My friend cannot explain it either
but he tried using open wire feeds 30 years ago and will not use
anything else.
I have a similar antenna at 35 feet, except is it a 160 meter loop fed
with 225 feet of 600 ohm feed line. I use a Dentron MT-3000A antenna
tuner with a built-in balun.

I tune the tuner using a MFJ 259B and get almost 1-1 SWR on all bands
through 6 meters. On some bands and at some frequencies, the tuning is
very narrow band. Particularly on 75 meters. If you are not using
something like an antenna analyzer, you may be missing the correct spot
for a 1-1 SWR.

Also, I always have to adjust the tuning for moisture, even here in the
desert. Rain, ice and snow have a big effect.

The other side of the picture is the antenna really loves thunder storms
and precipitation static. I have a big DPDT switch to open the feed line
to the tuner and ground it directly. sometimes 1/4" arcs! You probably
have already discovered this!

I love my loop! Even work 6 meter E's and contests with it.

73, Paul, KD7HB

On Saturday, September 1, 2012 10:08:23 AM UTC-5, Boomer wrote:
I was shocked to see my antenna change tune radically.

An antenna analysis program like EZNEC would have predicted those results. Since the SWR, Z0, and losses are different going from 450 ohm ladder-line to 600 ohm open-wire line, the 50 ohm SWR and impedance will be different at the shack. There's absolutely nothing to be shocked about - it's just relatively simple physics in action. Here's a transmission line calculator that might help:

http://www.vk1od.net/calc/tl/tllc.php

A free demo (learning) version of EZNEC is available at: www.eznec.com
--
73, Cecil, w5dxp.com