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Re: [TowerTalk] RE: [Antennas] loax 'sweet lenght'

To: "Al Williams" <>, <>
Subject: Re: [TowerTalk] RE: [Antennas] loax 'sweet lenght'
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Mon, 11 Oct 2004 14:25:48 -0400
List-post: <>
> What happens to the swr and feedpoint impedance at the
transmitter if a
> RESONANT dipole is fed with ladder line?

Ignoring some special cases for lossy lines, any time the
line has mismatch there will be a reactance change along the
length if the line has standing waves. That's because the
reflection and the time delay along the line causes current
and voltage to not be in phase at every point.

If the load is resistance 0j (no reactance) a line with
standing waves will have zero reactance only exact multiples
of 1/4 wl from the load. This is also why, when you have
mismatched lines, the phase delay in the line is not equal
to the length of the line except when the line is an exact
multiple of 1/4 wl long. The worse phase error occurs when
the line is odd multples of 1/8th wave long.

> However, does the resistance get tranformed--possibly
putting it out of
> range of solid state finals?

Sure, if the transmission line's input impedance falls
outside the rig's optimum load impedance.

> How much effect will rain on the line and imbalance of
each side of the
> line to nearby objects have on the resistance or introduce
> reactance?

I've been in disagreements about this, but the ladder lines
I've measured have significant loss changes when wet. The
loss increases, velocity slows,  and surge impedance change
in a wet line. This can produce significant input impedance
changes under certain conditions, and in other conditions
you won't notice it (although loss will change). It wouldn't
be likely to affect line balance, unless the line was not
installed properly in the first place.

73 Tom


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