|[AMRadio] Antenna Tuner Wonderings|
wa5bxo at pctechref.com
Mon Dec 13 19:47:44 EST 2004
What you have in mind is probably the best idea. I'm too am not
sure what is meant by "Crappy Antenna".
Tuning an 80 meter dipole fed with coax on 40 meters is a definite
"NoNo" because the antenna will represent many thousands of ohms Z to the
coax and depending on the length of line it could represent a very high
voltage node at the transmitter or tuner. Most coaxial tuners will arc when
trying to match this. At a low voltage node the Z is to low, it's all
current which is also very hard to match. The actual loss on the Xmision
line on 40 meters can be very high if the line is a few wavelengths long
other wise no to bad maybe only 1/2 power (if the connectors don't arc).
A balance network is best for a single ant to tune all bands or else
several antennas like what you may have is better for coax. A broad 40
meter bazooka will work 40, and 15. Use a separate dipole for 80 and one for
20. 10 meters can be worked with just about any thing. I have a dipole
about 250 ft from end to end and up about 100 ft. It is fed to a balanced
homebrew tuner mounted on the shack wall. The open wire feed line is made
of #12 wire spaced about 5.25 inches and droops to the side of the shack
wall where it is attached to strain relief insulators and then through the
Plexiglas window made just for the balanced tuner. BJ and I can tune any
freq on all ham bands 160-10 meters.
This ant system was a great amount of work and probably won't fit in
to most folk's idea of convenience and standard housing. This is why I say
maybe three or more antennas as you have is best for you.
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Merz Donald S
Sent: Monday, December 13, 2004 4:24 PM
To: glowbugs at piobaire.mines.uidaho.edu; 'Amradio (E-mail)
Subject: [AMRadio] Antenna Tuner Wonderings
As most of you know, I am a long time BA collector and inexperienced
operator. I am getting there. But it's a learning process <grin>.
I have the shack structured (in the loosest sense of the word...) into a
number of stations (20-25 with about 10 in actual operation). These mostly
consist of a transmitter/receiver pair with T/R relay and RX muting. The
stations are in the range of 40 to 250 watts max. I run these into your
choice of 5 or 6 crappy, mostly non-resonant antennas with connections being
made through a home-made SO-239 "patch panel".
Most of my interest is in AM operation on or near the supposed "AM windows"
on 80, 40 and 20. I have some rigs that I never change bands on. My
operation is often xtal-controlled but I have VFOs on some of the rigs. Most
of these old BA transmitters are pi-net outputs.
So here's the deal: I can tune any of these stations into a 52 ohm dummy
load and set up the rig easily enough. Now I switch to one of my crappy
antennas. And of course, it is nowhere near 52 ohms at the desired operating
frequency. If the mismatch isn't too bad, I can re-tune the rig's output
network. If the mismatch is ugly, or if the output network is limited in
range, then I can "insert antenna tuner here" to get a match.
After a while, laziness sets in. Why should I have an antenna tuner for each
rig?--especially if I never change bands on that rig? Wouldn't it make more
sense to dedicate an antenna tuner to each antenna? I can use my little
Palomar Tuner Tuner to match to pre-set each antenna to 52 ohms for each
frequency of interest and mark each antenna tuner accordingly. Then I can
simply load the rig up for 52 ohms into a dummy load, select an antenna, set
the antenna tuner to my previously-marked 52 ohm point for that frequency on
that antenna, patch the rig into the antenna tuner--and push-to-talk.
Ok, sure, this setup put the antenna tuner over at the patch panel and not
on top of the rig. Pretty inconvenient. But hey, there's 20-25 stations to
deal with here. I'm supposed to have 25 antenna tuners? OTOH, I have only 3
antennas that are sloppily non-resonant enough to require a tuner at the
frequencies I am interested in. So that's 25 tuners (okay, I'm
exaggerating--it would probably be only 10-15 tuners because the other
stations probably have adequate output networks) versus just 3.
This trade-off seems worth it to me. But experience is in short supply here.
So "pray tell me"** experienced ones, what am I missing? What is there about
this idea that is bad?
Any thoughts appreciated.
73, Don Merz, N3RHT
** When Winston Churchill was named First Lord of the Admiralty in 1939, he
was famous for firing off memos with questions to subordinates that began
"Pray tell me...". In the Royal Navy, these soon became known, obviously, as
"the Lord's prayers".
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