[TowerTalk] common mode chokes, baluns and multiband doublets

[Bert]

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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