Hi Bill,
Choking the window line is not a good idea - you'll choke the signals
too. Common mode chokes are mainly used for coax cables and
multi-conductor cables. I'm sure you'll get many more detailed replies
from the experts.
Good Luck!
Bert, VE3NR
On 12/11/2017 4:33 PM, k7wxw wrote:
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 of 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. Will 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
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