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Re: [TowerTalk] new Steppir antenna

Subject: Re: [TowerTalk] new Steppir antenna
From: Jim Lux <>
Date: Tue, 05 Oct 2004 13:18:17 -0700
List-post: <>
At 12:44 PM 10/5/2004 -0700, Rick Karlquist wrote:
Al Williams said:
> A relay switchable linear  loaded wire to add use on 80/75 meters.  I
> haven't done the calculations but expect that the  range of the
> extendable elements would cover the entire band in conjunction with the
> linear loaded wire?
> What do you all think?
> k7puc

You would have to have a bunch of relays in the linear
loaded wire to make it disappear all the way to 54 MHz.
Having thought about schemes like this for a long time,
I would say it is not practical.

If you try to put loading coils at the feedpoint, the
dozens of pF of capacitance between elements (due to the copper
tape feed reel) is a showstopper, not to mention the
high voltage that would be present.

Rick N6RK

So, electrically, the SteppIR looks like a dipole with a 10-20 pF capacitor across the feedpoint? In the usual scheme of things, I assume that you just make the elements a bit different length to tune out the C.

Then, if you're attempting to go down to 3.5 MHz, the loading L interacts with the C, which superficially, you could tune out (make the L a bit bigger), but then, because the Q is high, you're going to have big currents and voltages in the LC combination?

Of course, doesn't the antenna look pretty capacitive already at 1/4 wavelength long?

I am working on a spreadsheet (which probably has bugs still) for just this sort of thing. For a 20 meter long dipole I get a X of about 900 ohms (say, 40 uH at 3.5 MHz). this would resonate with about 50 pF. Winding that coil with 2mm diameter wire on a form 10cm long and 10cm in diameter (about 25 turns), you'd get a AC resistance of around 1 ohm. a Q of 900!

Just a bit of resonant rise there...


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