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Re: Topband: 160 meter band Antenna.

To: "Rune Øye" <>,
Subject: Re: Topband: 160 meter band Antenna.
From: "Dan Zimmerman N3OX" <>
Date: Sun, 30 Sep 2007 18:28:41 -0400
List-post: <>
>When I shorten my coil the antenna is resonant around 1.830 Mhz
(strange with 15+14.5 >meter) the swr value is something around 3+
from my MFJ analyser I measure around

>12-15 ohm on meter.

Rune, you need a bigger capacitor.

I think you're measuring the natural feed impedance of the vertical,
and your shunt capacitor is too small to do anything.  12 ohms
resistive in parallel with -252 ohms (350pF) is like 12-j0.5 ohms, and
that's what you're measuring.

.I think you actually need something like 3000 pF instead!!!  The
shunt reactance needs to be somewhere around 25-30 ohms.

A 3000pF variable capacitor is probably not worth it... but you could
parallel a couple of doorknob capacitors with the shunt cap for fine
tuning, or you could use a shunt coil instead.

You only need 2.6 microhenries inductance for 30 ohms... this sounds a
lot like my shunt coil, and it's a 9 turns of #12 (2mm) wire about
50mm in diameter spaced over maybe 60-70mm .

You'll need to tap for slightly less inductance on the series coil.

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