[TowerTalk] common mode chokes, baluns and multiband doublets

[Grant Saviers]

It seems there is a good solution to a single common mode choke for 
balanced lines over all of HF.  See the article in November 2017 RadCom 
(RSGB) "An effective, easily made HF Balun".  G3LNP describes a 4:1 
balun for all band doublets fed with REAL open wire feeders, with CM 
rejection on the order of 40db.

Additionaly, consider G3TXQ's measurement of punched ladder line 
losses.  http://www.karinya.net/g3txq/wet_ll/

I evangelize using real open wire, easy to make or buy. Examples:

http://trueladderline.com/w7fg-design-600-ohm-open-wire-feedline/
http://www.73cnc.com/product_p/ls100.htm

Grant KZ1W



On 12/11/2017 16:15 PM, Bert wrote:
> We're talking about a common mode choke, not a balun/transformer.
>
> Bert, VE3NR
>
>
>
> On 12/11/2017 7:08 PM, Wes N7WS wrote:
>> Actually that's not true. A lot of baluns are wound with balanced 
>> transmission line.
>>
>> Wes. N7WS
>>
>> Sent from my iPhone
>>
>>> On Dec 11, 2017, at 3:49 PM, Bert <balmemo at sympatico.ca> wrote:
>>>
>>> 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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