Pete, you were fully correct - this one has the potential to
generate lots of opinions.
I only partially agree with you, Pete. Your suggested feed
method is perfect for feeding the resonant loop (say, on 40m) and
it will give a good match on 15m, but I think Paul will encounter
too high of swr on a coax feedline on some bands. If his
objective is to work all (or as many as possible) bands, the
openwire is absolutely the very best way to go.
The biggest problem is indeed heating the matchbox instead of
radiating power.
I would suggest that if this antenna is to be a long-term
antenna, Paul would be better off looking around for an old DECCA
"KW E-ZEE Match" matchbox. It is a Z-match and is symmetrical
from the get-go. It requires openwire feed, of course.
By the way, this is not "theory". I've built loops of either 40m
or 80m total lengths in at least a dozen different QTH's. It
never failed to work great - but I always fed it with open wire.
73
Rick
-----Original Message-----
From: tentec-bounces@contesting.com
[mailto:tentec-bounces@contesting.com]On Behalf Of
ac5e@comcast.net
Sent: Monday, January 05, 2004 12:48 AM
To: tentec@contesting.com
Subject: Re: [TenTec] Not TT - Wire antenna question.
HI Paul: This is one that will probably catch a lot of people
with their opinions up. My take on your loop is first, by
formula, your loop will be a full wave just below 7 mHz, and
should load fairly well on 60M and very well on 40 and up.
On the 3.5 mHz band your loop is close to a half wave OA, and
tuning is going to be touchy. Extremely touchy. So touchy you
often run into the same problems you get on 160.
I suppose it's possible to load a quarter wave loop on 160 but I
have not had any luck doing so. You can "load up the tuner"
giving yourself an apparent match since the MFJ tuners are
notorious for that - but all you are really doing is heating up
the tuner. And tuners don't make good artificial antennas. The
cost of replacement parts is pretty high.
I would reccommend both coax and a "balun" at the loop feedpoint.
The first because it will make a better match to your loop on
40/60/80 than most open lines and I have not had wonderful luck
with the typical built in toroidal impedance matching coil in
most tuners. Especially feeding comparitively low impedance
loads.
There's some controversy about the "current balun" at the
feedpoint. Some say there's no need of any sort of choke if your
antenna is balanced, and a loop is about as balanced as you can
get. However, I well remember my old professor telling the class
that we need not memorize all the formulas in Terman, we only had
to know where they were. And the second bit of advise was to
measure the critical stuff, not infer results from secondary
measurements.
>From my measurement of RF current on the coax shield, a coax fed
loop has quite a bit of RF on the shield. Enough to cause
problems if they aren't choked off at the loop feedpoint. You can
use 10 to 15 turns of coax solenoid wound on a 4 inch, 100 mm,
tube, which should work well for 60 M through 30. And again, some
swear by scramble wound coils, or bunched coils, but my
measurements lead me to avoid such.
Being a safety type, a "belt and braces man," I generally use a
choke balun at the antenna feedpoint and then back that up with a
ferrite balun somewhere fairly close. No RF in the shack, and the
tuner likes the result much better.
I hope this helps;
73 Pete Allen AC5E
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