|Subject:||Re: [TowerTalk] elevated short vertical dipole orquarterwave monopole?|
|From:||"Guy Olinger, K2AV" <firstname.lastname@example.org>|
|Date:||Fri, 3 Dec 2004 09:43:23 -0500|
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.
----- Original Message ----- From: "Gary Schafer" <email@example.com>
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?
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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