I have built and measured several of the K9YC choke designs with similar
results to his. However, you can achieve resistances of several thousand
ohms in a less clumsy way. I use the small teflon dielectric coaxes like
RG142b/u and single or double type 31 2.4 OD cores. This coax is rated at
9KW at 10mhz per my old Times catalog. These mount easily in small plastic
boxes 6x6x5 from the big box stores. I have been using chokes of that type
for several years at legal limit power on bands from 160-10 meters with no
observed failures. I suspect that the big chokes with 5 or 6 cores and
large turns of RG8 size cable could handle more power dissipation if
needed.
I sometimes find the coax at flea markets, usually on tables with Motorola
equipment. There are other small teflon coaxes with different labels than
RG142b/u that would work.
I see that a company called Balun Designs now sells similar chokes. I
have had no dealings with them.
73, Dan, N5AR
On Tue, May 14, 2013 at 3:06 PM, Ian White <gm3sek@ifwtech.co.uk> wrote:
>
> 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@contesting.com] On Behalf Of
> >Chris
> >Sent: 14 May 2013 21:59
> >Cc: towertalk@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@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
> >>
> >> _______________________________________________
> >>
> >>
> >>
> >> _______________________________________________
> >> TowerTalk mailing list
> >> TowerTalk@contesting.com
> >> http://lists.contesting.com/mailman/listinfo/towertalk
> >>
> >
> >_______________________________________________
> >
> >
> >
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>
> _______________________________________________
>
>
>
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--
Dan Hearn
N5AR
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