Michael:
Some have actually designed switches to put in
different lengths of ladder or window line, or
twin lead to adjust the impedance. I have even
used a large toroid at the TX with a few loops of
balanced line to choke out most common mode
currents in the feed line unless you want the
vertical feed line to radiate.
Gary, K5AMH
get a good match. On 12/20/2015 22:15, Joshua Rose via BVARC wrote:
Michael,
Try adding another 5 to 10 feet of ladder line. I had a similar
issue with my 80m doublet. After adding just a bit more, it was
able to find a match across all bands.
Just one of the drawbacks of not having dedicated resonant antennas
for each band!
Josh
KB3VQQ
On Dec 20, 2015 10:10 PM, "Michael Rapp via BVARC" <[email protected]
<mailto:[email protected]>> wrote:
Hi all,
My last antenna experiment was a center-fed zepp cut for 80
meters. (Dipole fed in the center with 450 ohm feed line,
requiring a tuner.)
I found this antenna to be a decent multi-band antenna, with one
exception: I never could get my MFJ manual tuner to find a match
on 40 meters. For the other bands, it seemed to find a match
fairly easily. As I had another antenna -- an inverted L vertical
-- for 40 meters, I shrugged this off as something to look into
later.
Well, my inverted L has succumbed to the elements and I need to
rebuild it, so I turned my attention to trying to figure out my
center-fed zepp 40 meter problem.
After much research (remember, I'm a beginner at this!), the
problem has become somewhat obvious. One drawback to a center-fed
zepp, I have learned, is that the impedance at the second harmonic
of the lowest frequency will be very, very high. The second
harmonic of an 80 meter antenna is 40 meters. Oops. Apparently
the impedance is so high the MFJ tuner can't match it.
I thought about purchasing a fancy tuner, but I remembered that
tuners aren't miracle devices. They don't change the impedance of
the antenna and I am concerned about power loss at the tuner. It
seems the only thing to do is to find a way to temporarily
electrically shorten the dipole when I want to operate on 40 meters.
The 'zepp is an inverted V, so the ends of each leg of the dipole
are fairly low to the ground and easily accessible. (They
actually run along the top of my backyard fence for about 15 feet
on each side!) What if I coiled enough of the wire such that the
second harmonic of the new length was off the 40 meter band? Is
coiling the excess wire enough to electrically shorten the
antenna, or do I really need to run the excess wire back along the
elements (parallel to them) for this to work?
I like the coiling idea as I could just uncoil the wire on each
end, stretch it back to its original length when I want to operate
on 80 (or 60) meters.
--
/*/-=[Michael / KT5MR]-=/*/
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