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5/8 wave ground plane

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May 14th 04, 06:30 PM

Ken Fowler

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On 13-May-2004, (JGBOYLES) wrote:

You may be thinking of a vertical close to 3/8 wave which
will be 50 -jx. The -jx (capacitance) can be resonated with a +jx in series
for a 50 ohm feedpoint.

Ken, we may be talking about two different things, but a 3/8 wave vertical
has inductive reactance, and the R component is no where near 50 ohms. A 2/8,
or 1/4 wave vertical has no reactance, shorter has -jx, and longer, as in 3/8
wave has +jx.
A .26 lambda vertical has about 50+j43 impedance, and the +j43 can be
cancelled with series 154pf at 24 MHZ. This is a common method of matching a
1/4 wave vertical to 50 ohms. Make it a little longer to look inductive,
50+jx, and cancel with a series cap.
According to all the theory I know on this stuff a 3/8 wave vertical will not
be as you describe.

Sorry. I mispoke on that. The 3/8 wave will indeed be inductive. However its resistance will be
closer to 50 ohms than it would be for a 1/4 wave. A 1/4 wave would have a resistance of 37 ohms.
I'm not sure about your 50 + j43 for a 0.26 wave. Is that from EZNEC? With what kind of ground?

I didn't take the time to model the 3/8 wave. Sorry.

Ken

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