[BC] Slant wire feeds vs series fed

[Phil Alexander]

On 25 Jan 2006 at 8:34, Cowboy wrote:

> On Wednesday 25 January 2006 08:08 am, Richard Fry wrote:

> >Does anyone know of a shunt-fed, monopole installation where the feed wire 
> >is inserted at the tower base, makes a 90-deg bend, and extends up in the 
> >vertical plane on the tower vertical C/L to attach at the feedpoint?
> 
>  While I don't know of any, it's something I've wanted to try for some time.
>  I really don't see much difference between that, and a standard skirt.
> 
>  In this case, that the "outer conductor" is grounded rather than the "inner
>  conductor" should really make little difference in the current distribution
>  in the actual radiating element, the tower.
> 
>  While I don't know of any coaxial sleeve antennae ever being used in
>  the standard broadcast band, the principal is well understood in other
>  services, and works quite well. ( though usually a full 1/2 wave vertical
>  dipole, rather than something shorter )

This idea is quite a bit different from a 1/2 wave vertical dipole in
that it has no insulator between upper and lower sections of the
radiator if I understand what is being proposed.

While I can see several advantages, to the "inside out" skirt, correctly
tuning it might prove more difficult than tuning a conventional skirt
and, a skirt permits considerably greater flexibility regarding inherent
Q etc. as the support arm length is easily manipulated. 

This antenna should be an attractive one for Cowboy because more climbing
probably would be involved for setting the feed attachment point. It seems
to me you would want to tweak the attach point to achieve zero leg current
at ground level.

It looks like an interesting science project where we might learn a new
trick or two, but if compared with a high power (six wire) skirt, I don't
see a substantial advantage to offset the downside of higher cost, more 
difficult installation and reduced flexibility. It would be interesting
to model this and compare, but my gut reaction is you will need a larger
tower face to get the same bandwidth results as a conventional external 
skirt.


Phil Alexander, CSRE, AMD
Broadcast Engineering Services and Technology 
(a Div. of Advanced Parts Corporation) 
Ph. (317) 335-2065   FAX (317) 335-9037





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