[TowerTalk] common mode chokes, baluns and multiband doublets

[Bert]

The balun in Radcom article is not a common-mode choke - it's a voltage 
balun transformer.

Bert, VE3NR


On 12/11/2017 8:04 PM, Grant Saviers wrote:
> 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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