Hi Bill,
No, don't try to even think about running an balun "at the end of a window
line" if that line is not matched. I don't understand why you are trying to use
a center feed dipole as a multiband antenna as you will deal with plenty of Z
variations. A common mode choke will have "plenty of work" as it has to work
across a wide frequency range and a wide impedance range. You can, within
practical means, chose one of the two but I have a hard time to believe you can
combine both. (I tried at one point but failed).
I, now-a-days, run an off-center-fed wire. It's fed at the 1/3 point and I am
happy with it. It's tuned for 160 meter in my case. I use a home-made ladder
line and feed it directly into my shack where my tuner is located. The tuner is
located just where the feed line enter. The tuner is "isolated" from ground by
a air-choke "de grande". I staggered the winding, 50 feet of coax, around a 4"
pipe. That gave me a reasonable high Z. The other end of the coax is grounded
very solid to my shack to avoid RFI. Seems to work well.
Good luck with your installation and 73 de,
Hans - N2JFS
-----Original Message-----
From: k7wxw <k7wxw@arrl.net>
To: towertalk <towertalk@contesting.com>
Sent: Mon, 11 Dec 2017 16:34
Subject: [TowerTalk] common mode chokes, baluns and multiband doublets
Yesterday the wind brought down my doublet, which I mostly see as an upgrade
opportunity, at least once I finish pulling tangles of wire out of trees.
While a window line fed doublet is a well-known beast, and the one I had up
worked okay, I hope the new version will perform better by taking into account
my particular installation limitations. As I worked on the design, I came up
with a couple of questions...I apologize if this is too much detail, or
whatever. I am putting up a centerfed doublet to cover 80 through 10 meters,
focusing on 80, 40, 30, and 20 meters. I plan on feeding it with a random
length of 450 ohm window line. I have a Johnson Matchbox and Dentron Supertuner
and can use either to match the antenna to my rigs. My antennas come to a
common connection panel, which is mounted on an ground rod. I rarely run more
than 25W and never more than 150W. For a number of reasons - including the fact
that it won't be quite level, it is only up about 30 feet and part o
f it runs over the house - the antenna will not be balanced. If I install a
1:1 current choke built using two stacked FT-140-43 toroids at the antenna feed
point, is it likely to survive given the wide variation in feedpoint impedance
over the operating frequency range?I originally planned on using a Morgan
balanced line lightning supressor. Unfortunately, it is only rated for input
and output impedances of 300 to 600 ohms, a condition that is impossible to
meet in a multiband antenna. My alternative is using a 4:1 balun, followed by a
polyphase lightning suppressor. Same question about this balun (stacked
FT-140-43)... how will it do given the variation in load impedance over the
operating range? The 4:1 balun at the base of the feedline has the added
advantage that I can avoid running window line into the house (I am doing that
now) which is a PITA. Bonus question: I know that expecting baluns and common
mode chokes to work well over 3 to 30 Mhz is, well, optimistic. Wi
ll I get better common mode suppression at the feedpoint by using two chokes,
material chosen to optimize different portions of the operating range?I have
read K9YC's guides, along with a lot of material from W8JI and others but I
haven't quite figured out how to figure out the answers to my questions.Thanks
for any help and 73 de bill
K7WXW______________________________________________________________________________________________TowerTalk
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