[TowerTalk] elevated short vertical dipole orquarterwave monopole?

[Guy Olinger, K2AV]

There are often unnoticed assumptions buried in VHF antenna design 
extrapolated to HF.

In the sleeve antenna, the relationship of the sleeve to the coax is 
mechanically controlled. The generous separation of the sleeve from 
the mounting pipe FURTHER isolates the coax from the sleeve, which is 
INSIDE the mounting pipe.

A sleeve will work, but what difference does it make if at or before 
the bottom of the sleeve the coax runs away at right angles, heavily 
coupling it and the coax beyond to the primary vertical radiator.

If ALL of the conductors in the HF surrounds are modeled, including 
the entire coax run, grounds at the house, tower, gutters on the 
house, yada, yada, an HF antenna will often show behaviors never 
encountered with a VHF scale down.

The VHF version usually has the luxury of the coax dropping many 
wavelengths straight down before bending out and/or disappearing into 
a virtual shield. Scaling something like that to 80m would be an 130' 
sleeve vertical on top of a 500 or 1000' tower. The coax sleeve will 
work just fine up there, IF you can find a way to make it stand 
straight up and you have a good brute force decoupling method.

Scaling down the usual 80 meter circumstances to VHF produces a sleeve 
dipole with the sleeve bent horizontal a few inches below the upper 
portion. With the top of the thing barely over a foot off the ground.

Then since the thing is probably on a tower with the sleeve running 
down, one has now heavily coupled the tower and any control leads not 
carefully RF-grounded at the base, and one is also at the mercy of 
whatever true grounding exists at the base of tower.

The basic sleeve principle is proven at VHF, but the practicalities at 
HF overwhelm it, and one is left with all of Tom's objections.

Unless one has the advantages of very short wavelengths, decoupling IS 
a royal PITA.

73, Guy

----- Original Message ----- 
From: "Gary Schafer" <garyschafer at comcast.net>
 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?