Topband: 160m Ant on 80m?

[David Gilbert]

Tom,

I definitely agree.  I played around with different loading coils in the
simple EZNEC model I created and the base impedance swings all over the
place, both real and imaginary components.  I distrust top loading coils
anyway since they model weirdly when they're located near current minimums,
where capacitive effects can have a big, maybe even greater, influence.

Dave

----- Original Message ----- 
From: "Tom Rauch" <w8ji at contesting.com>
To: "David Gilbert" <rimradio at direcway.com>; "Thomas Hoeppe"
<thomas.hoeppe at asamnet.de>; "Topband Reflektor" <Topband at contesting.com>
Sent: Friday, April 16, 2004 2:42 PM
Subject: Re: Topband: 160m Ant on 80m?


> > I believe your antenna should simply look like a vertical halfwave
dipole,
> > except electrically off-balanced by the coil at the top end.  In theory,
> any
> > simple vertical one half wavelength or less and close to the ground
should
> > have a single lobe at a takeoff angle determined by radials and ground
> > characteristics.
>
> It is really hard to predict what will go on with 80 meters without
modeling
> the antenna or trying it.
> It would all depend on what it looks like on 80, and that would depend on
> how much L the antenna has and what is above the L.
>
> If it was a 100 ft vertical with all hat it would indeed behave like a
> near-1/2 wl on 80.
>
> A modest loading coil of 170uH 90 feet up would make the base impedance 95
> ohms +220j on the second harmonic. That isn't acting like a half-wave by
any
> means!
>
> There are combinations of coil size and capacitance above the coil that
> could seriously hurt 80 meters, and some that won't bother it at all.
Short
> verticals are the worse ones for having noticeable problems. It should be
> pretty safe with a tall vertical, but the base impedance might really be
> surprising.
>
> 73 Tom
>
>
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