[TowerTalk] common mode chokes, baluns and multiband doublets

[Kevin Stover, AC0H]

Bill,

Ditch the 450 Ohm window line. The antenna is not going to be balanced 
and the window line won't "balance" it for you. What happens to balanced 
line, doesn't matter the impedance, when it becomes unbalanced? It 
radiates.

Get  four or five 2.4 inch OD Mix 31 cores and run five or six turns of 
RG-213 through it K9YC style. Each pass through the core counts as a 
turn. If the antenna was for 20 meters and up I'd use Mix 43 instead.

You can run more than one choke in the line. Since they are in series 
the common mode resistance adds.

On 12/11/2017 3:33 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
> 
> 
> 
> _______________________________________________
> 
> 
> 
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
> 

-- 
R. Kevin Stover    AC0H
FISTS #11993
SKCC #215
NAQCC #3441
ARRL