Re: [TowerTalk] elevated short vertical dipole orquarterwave monopole?

[Gary Schafer <garyschafer@comcast.net>]

Tom,

Are you saying that it is not possible to decouple with a coaxial sleeve 
such as is commonly used on VHF or UHF collinear antennas ? It was used 
for many years quite successfully as a quarter wave VHF sleeve decoupled 
mobile antenna by Bell telephone. Or are you referring only to a 
vertical dipole sleeve antenna that is mounted close to the ground?


73
Gary  K4FMX

Tom Rauch wrote:
> > I assume you meant "without" making the feedline part of
>
> the antenna, and,
>
> > yes, that is a problem. But, for the sake of discussion,
>
> assume you run the
>
> > feedline up the inside of the lower half of the dipole, so
>
> the "exposed"
>
> > part of the feedline is basically the same for both..
>
> something coming out
>
> > at the bottom of the antenna near the ground.
>
>
> The feedline shield exits the antenna at a point with the
> full potential of the lower tip of the antenna, and that
> isn't good. You might as well tape the feedline to the
> outside of the antenna as the inside, since the bottom of
> the antenna is NOT an electrical 1/4 wl with respect to the
> shield length along that area and thus there is almost no
> decoupling at all.
>
> Decoupling the shield is a real major PITA. Ferrites will
> overheat and fracture, air-wound coiled coax is pretty bulky
> and restricted BW.
>
> You need many many kilo's of impedance on the shield, or the
> shield to the rig is a major part of the radiating system
> ala EH antenna and Isotron antenna.
>
>
> > Interesting.  Did you ever contemplate where the
>
> difference was coming from?
>
> > Ground losses? A pattern difference because the phase
>
> center is at a
>
> > different height?
>
>
> No idea. That was beyond the scope of what I could measure,
> and no one cared. Loss is loss.
>
> The two major vertical "dipole" problems I found were
> decoupling the feedline (big PITA to do correctly) do to the
> fact the electric field was so high (which means big
> dielectric losses in anything lossy around the antenna base)
> and loss of signal strength compared to a moderate ground
> system with the same overall peak height.
>
> I never tested a hat loaded 1/4wl monopole that would
> increase voltage and decrease current at the antenna base
> while increasing radiation resistance. The feedpoint was a
> major headache because somehow you have to get the feedline
> past that very intense electric field at the antenna base
> when the antenna bottom is floated from ground.
>
> The common solution is to simply ignore the problem and
> pretend like the feedline isn't radiating.
>
> 73 Tom
>
> _______________________________________________
>
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See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.

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