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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|