|Subject:||Re: [TowerTalk] new Steppir antenna|
|From:||Jim Lux <firstname.lastname@example.org>|
|Date:||Tue, 05 Oct 2004 13:18:17 -0700|
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
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...
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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