Jim,
@ say, 1.83 MHz, the electrical length of your vertical is about 47.4 deg,
necessitating a top hat of about 42.6 deg. I followed the prescription of
John, ON4UN and calculated out a surge impedance of 377 ohms based on an
average radiator diameter of 2.25 inches; the hat capacitance would be
about 212 pf. If you did a disc cap hat, it would need to be maybe 6m
diameter, pretty big. If you just used T-wire loading you need maybe 35m
of top hat.
Not sure how you would use this antenna on 80m.
I use base loading on 160m, thanks to a motorized mobile screwdriver coil
at the base of my 50-foot radiator. And NO problem running 350 watts AM
through the system.
73
Eugene/AF9O
On Sun, Sep 2, 2018 at 7:30 PM <donovanf@starpower.net> wrote:
> Hi Jim,
>
>
> If you base load a 68 vertical on 160 meters it will create very
> high voltage at the base of the vertical which may damage the
> base insulator. Much better to use top loading to avoid high
> voltage at the base.
>
>
> 73
> Frank
> W3LPL
>
> ----- Original Message -----
>
> From: "Jim Garland" <4cx250b@miamioh.edu>
> To: topband@contesting.com
> Sent: Sunday, September 2, 2018 10:03:32 PM
> Subject: Re: Topband: Air wound coil
>
> On this general topic, I'm erecting a 68 ft aluminum self-supporting
> pole (DX Engineering) that is four inches OD at the bottom, tapering to
> about 1/2 inch at the top. It is designed as a full size 1/4 wave 80m
> vertical. I'd like to use it on 160m. If I add four capacity hat wires
> at the top, would that sufficiently lower the resonant point? The
> antenna tilts at the base with a hand winch, so it's not hard to add or
> remove wires if I want to restore the antenna to 80m. The other option
> is to add inductance at the base (easy to do), but I don't know if that
> is the preferred solution or not. BTW, there are sixty 30m radials
> already installed at the base, from a previous antenna. I only care
> about resonating the antenna near the low end of the band. Except for
> cacti and coyotes, the antenna is 200m from any structure, fed with
> buried hardline. Any suggestions? Thanks!
>
> 73,
>
> Jim W8ZR
>
>
>
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