Topband: T Vertical feed

[Charles Moizeau]

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 at arrl.net
> To: topband at 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 
> 
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