Re: [TowerTalk] new Steppir antenna

[Jim Lux <jimlux@earthlink.net>]

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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