[TowerTalk] Force 12 B-1/B-1S Baluns

[Ian White]

Below the feedline choke, the outside of the coax is (or should be)
almost completely dead to RF, so it can then be taped to the metalwork
without further problems. 

But taping the choke itself onto the boom is quite a different thing,
and wrong in several different ways. 

As Steve G3TXQ already said, it detunes the choke. An air-cored choke
has only a narrow parallel resonance where it will be effective as a
balun, and if you tape the choke onto the boom you have no idea where
that resonance has gone.

Added to that is the unbalancing effect of the one-sided capacitance.
Depending on the exact layout of the cable and the boom, the capacitive
unbalance could be anything up to a few tens of picofarads; we then have
to think about the feedpoint impedance, and of course the frequency...
but all of that is missing the point. It's a point of principle. Taping
the choke coil onto the boom is so obviously acting against what a balun
is trying to achieve, that the only worthwhile answer is: "Just don't do
that!"  

Coupling between the balun and the boom is going to be a problem for any
air-cored choke, because the electrical and magnetic fields extend some
way away from the coil itself. But if you have chosen one of the many
designs of  ferrite-cored chokes, this proximity problem mostly goes
away. As well as being far less sensitive to detuning, ferrite chokes
can be much more compact and have smaller stray fields, and can easily
be suspended inside a plastic box to space them further away from the
boom.

73 from Ian GM3SEK


>-----Original Message-----
>From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of
>Chris
>Sent: 14 May 2013 21:59
>Cc: towertalk at contesting.com
>Subject: Re: [TowerTalk] Force 12 B-1/B-1S Baluns
>
>Okay so how do you route the coax back to the mast from the driven
>element then?  Dangle the coax below the mast?  Add another choke at
the
>mast?  But wait!  The mast is metal too!   Just how much of a problem
is this
>stray C, anyway?
>
>Chris
>KF7P
>
>
>
>
>
>
>On May 14, 2013, at 2:50 PM, Tom H Childers wrote:
>
>>
>>
>> One reason those folks, which has included me have done that, is
>> because most loaded Yagi-Uda antennas sold by CushCraft and others
>> recommend doing just that in the assembly instructions!
>>
>>
>>
>>
>> On Tue, 14 May 2013 20:28:54 +0100, "Ian White"
>> <gm3sek at ifwtech.co.uk> wrote:
>>
>> [snip]
>>>> How many times have you seen a coiled-coax choke taped to a boom -
>>>> think about that in terms of a high-Q parallel tuned circuit!
>>>>
>>> Not only that, but taping the choke to the boom is adding extra
>capacitance between one side of the driven element and the boom. In
other
>words, taping the "choke balun" directly to the boom is making the
antenna
>more UNbalanced!  It's amazing to watch people doing this...
>>>
>>>
>>> 73 from Ian GM3SEK
>> [snip]
>> 73,
>> Tom
>> Amateur Radio Operator N5GE
>> ARRL Lifetime Member
>> QCWA Lifetime Member
>>
>> _______________________________________________
>>
>>
>>
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