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 <[email protected]> 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" <[email protected]> > To: [email protected] > 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 > > > > _________________ > Topband Reflector Archives - http://www.contesting.com/_topband > > _________________ > Topband Reflector Archives - http://www.contesting.com/_topband > _________________ Topband Reflector Archives - http://www.contesting.com/_topband
