Charles, Joe listed some symptoms of excessive feedine common-mode current
such as RF bites and RF feedback. If you do not suffer from any of these
effects a balun may be of no use.
If one does experience CM problems that might be cured by a balun (CM
choke) by all means give it a try.
In my NEC model a CM choke having an impedance magnitude of 300 ohms
reduced the coax CM current by one half.
My anecdotal evidence from a dozen vericals is that no balun has been
needed. Even in the case of a 12' top loaded 1.8 MHz vertical having a base
current of 10 amps.
Dave WX7X
On Jan 27, 2012 9:40 AM, "Charles Moizeau" <w2sh@msn.com> wrote:
>
> My radial field consists of 55 radials, 75' to 150' in length, buried 0.5"
> to 1" deep. My coax feedline, encased by a 1.25" gray pvc conduit, is 12''
> deep and 80' long. It passes beneath several radials between the shack and
> the antenna base.
>
> I don't use a common-mode choke at the base feedpoint of my inverted L,
> where the only matching element is a series-connected capacitor to cancel
> out the inductive reactance of the antenna's total length of 170'.
>
> I am willing to insert a common-mode choke, but don't know what to measure
> beforehand to learn if one is needed. Nor do I know what changed
> indications to look for after such a choke has been installed.
>
> I'd be grateful for any advice.
>
> 73,
>
> Charles, W2SH
>
> > From: w9ac@arrl.net
> > To: topband@contesting.com
> > Date: Fri, 27 Jan 2012 10:20:16 -0500
> > Subject: Re: Topband: T Vertical feed
> >
> > > This is a terrible error in logic. Current on the radials will divide
> > > based on the impedance of each radial. If the feedline happens to be
> > > a "pathological" length its (outer) shield can carry *all* of the
> > > antenna return current.
> >
> > To Joe's point, I don't think we want the feedline to become a radial.
> It
> > also seems that placement of the line should occur under the radial field
> > and not on top of it, but I have not seen any studies that compare
> > measurements. Anyone have this data? My initial thought for base-fed
> > verticals is to use a CM choke at the base and also at the perimeter of
> the
> > radial field, unless by placing the line under the field significantly
> helps
> > to reduce coupling to the line.
> >
> > Paul, W9AC
> >
> > _______________________________________________
> > UR RST IS ... ... ..9 QSB QSB - hw? BK
>
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
>
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
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