----- Original Message -----
From: "Guy Olinger, K2AV" <email@example.com>
To: "TowerTalk" <firstname.lastname@example.org>
Sent: Friday, December 03, 2004 6:43 AM
Subject: Re: [TowerTalk] elevated short vertical dipole orquarterwave
> There are often unnoticed assumptions buried in VHF antenna design
> extrapolated to HF.
> 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.
Finally, the justification I need for the building permit review board!... I
have to have that 1000 ft tower, or my feedline won't be adequately
decoupled, and there might be RFI. I'm just trying to minimize the impact
on my neighbors.
> 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.
Of course, one could just let the coupling happen, and deal with its effect
. Say you're building a phased array, and perhaps (?) one could change the
phasing to compensate. Sure, you get funny lumps and bumps in the radiation
pattern, but it's not like you're expecting 50 dB sidelobe performance from
an HF phased array anyway. Certainly, you're not going to use a simple
phasing scheme like a quad hybrid either.
So, aside from the effects on the pattern (which can probably be down in
the -10dB range..), you're left with all the other inconveniences of RF on
neighboring conductors (in this case, the outside of the feedline): RFI, odd
impedance effects (because of the varying and uncertain coupling), things
that are RF hot, losses from all of the above (which might be no worse than
the near field losses from radiating into the earth).
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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