I have a situation that is similar! I have a 40 meter dipole in the attic
of a single story ranch type home. Diope elevation about 12 feet, with ends
bent at 90 degrees to make it fit in attic. It is fed with 75 ohm twin
lead.
I am using it as a dipole on 40/30/20 meters OK with a K1 and a T1 external
tuner. Results pretty good considering on those bands. I have also used it
on 80 and 160 meters by shorting the feeder and connecting to the "hot"
(center conductor on BNC jack) and a counterpoise to the ground connection
on the T1. Results were usable on 80 meters and 160 even though it is a
"stretch" on 160! Could fairly constantly work a station 200-250 miles
distant with weak signals on that band.
I haven't a lot of "radio time" available these days as my XYL had a stroke
last year and duties around the house keep me busy and away from getting on
the air.
I do have an "HF Packer" amplifier I built a few years ago, and it gives me
around 25-35 watts on all bands with around 2 watts drive from the K1.
Eventually I plan on getting an "Inverted L" antenna up on that side of the
house in the trees and working it against a counterpoise and ground rod.
73,
Sandy W5TVW
----- Original Message -----
From: "John Buck" <[EMAIL PROTECTED]>
To: "Frank MacDonell" <[EMAIL PROTECTED]>
Cc: "Elecraft" <[email protected]>
Sent: Sunday, October 26, 2008 4:19 PM
Subject: Re: [Elecraft] ATU vs. External Tuner
Frank,
Since the dipole is in the attic, you are probably not getting perfect
swr without a tuner. The antenna is a compromise unless you have a very
large attic with little metal near by. If you have trimmed it for
resonance at 40 meters I assume it is quite a bit shorter than a
standard dipole due the proximity of the roof and wood work.
If the dipole is actually resonant at 40 meters then it will work poorly
on 20 meters even if you can match it at the radio with some combination
of feedline and tuner. Please note that a perfect dipole up high in the
clear will not be 1 to 1 swr at resonance but will be 70 to 90 ohms. Do
not fall into the trap of trying to obtain a perfect match on every band.
I would try a non resonant length (g5rv style) fed with coax. It will
be more effective on multiple bands and the SWR will not be too high to
tune as it is on the second harmonic with a resonant dipole. The
antenna center impedance of the g5rv lengths (102' or 88' if in the
clear) avoids extreme high impedances on several of the HF bands. The
coax losses should be be acceptable on most bands. I would certainly
try that with the K3 tuner If you have carefully trimmed the 40 meter
for resonance in place in the attic, try adding about 25% length and if
necessary folding the ends to fit. There is nothing wrong with trying
fan dipoles if you have the space and can trim appropriately.
I am sure others will correct my ball park numbers. I also would not
get carried away about the perfect feedline length for feeding a g5rv.
The best feed line is the shortest one that goes from the antenna to the
tuner.
That said, if the coax line is very long, then a remote tuner is nice.
The K3 tuner is excellent. I would use it . If you fall into a good
automatic remote tuner use that.
But consider the fact that resonant antennas to not necessarily radiate
better than properly tuned non resonant lengths.
John KH7T
Frank MacDonell wrote:
I have a k3 with internal antenna tuner connected to a 40 meter dipole
in the attic. Will the internal ATU work for me visiting other bands
or would I be better off installing a tuner between the rig and the
antenna? Is an auto tuner more preferred than automatic? Thanks to all
for your time.
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