Topband: Air wound coil

[Paul Christensen]

Jim,

Just my 2-cents.  Here goes...

First, NEC 4.2 160m "T" modeling shows very little advantage of having 4 top-hat wires instead of just two.  The azimuth radiation pattern is almost a circle in the horizontal plane with two wires since very little field strength is radiated from symmetrical top hat wires.  OTOH, perhaps you need more for pole stability in which case you could get by with only three wires spaced 120 degs apart.  

Not sure how much horizontal room you have but top load an 80m vertical out as far as possible.  If you can, make the top wires long enough so that resonance occurs on a choice frequency as displayed on a vector impedance analyzer.  You'll be left with just the R value that can be brought up to 50+j0 with an unun or an L network.  

Let's say you can install those long wires into resonance.  4Nec2 shows that with a 68 ft. thin radiator, resonance can be achieved with wires roughly 41 ft. per side.  With a ground field of 60 radials, 90 ft long on sandy ground, 4Nec2 reports a base impedance of 15.4+j2.5.  That's probably close, real world will be different.  In that case, a high-pass L network can transform that impedance to 50+j0 with C = 3380 pF and L = 2.91 uH.  Low pass:  C = 2600 pF and L = 1.8 uH.

My preferred method with <1/4-wave verticals -- if there's a lot of space and tall supports -- is to get the top hat wires into an over-resonance condition into the top of the MW BC band.  Doing so increases base R and moves the maximum current point up and away from the ground.  It also often results in just one series capacitor to get 50+j0 at the base.  

Paul, W9AC




-----Original Message-----
From: Topband <topband-bounces at contesting.com> On Behalf Of Jim Garland
Sent: Sunday, September 2, 2018 6:04 PM
To: topband at contesting.com
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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