On Mon, 23 Oct 2006 23:52:49 GMT, John Phillips
wrote:

John,

.... clipped dissertation on how feedline loss reduces VSWR at the tx
end of the line, and how it can feed urban myths.

to reach the shack. Of all my G5RV antennas that I have built, and I
have built many, My conclusion is that it is a good tri-band antenna.
80,40 and 20 mtrs. Not an "all band" antenna. 300 ohm ladder line is a
better choice than 450 ohm. The reason is that on 40 mtrs the 1/4 wave
length better transforms the high center impedance nearer to 50 ohms
than 450 ohm line. On 75/80 mtrs it is a 1/8 wave length feeding a very
low impedance which results in adding inductive reactance to the system.
And again 300 ohm line is better than 450 ohm which adds a bit too much
inductive reactance. It would make no difference on 20 mtrs since it is
a 1/2 wave length there and WYSIWYG on both ends of the line. Your

I ran a series of models of a typical G5RV in inverted V
configuration, modelling feedpoint Z using NEC2, and transmission line
and tuner losses. The models are reported at
http://www.vk1od.net/G5RV/index.htm .

I refer to Figure 3 which shows nominal 450 ohm ladder line and 15m of
RG58C/U feedline.

I have rerun the model with a feedline that is the same copper
dimensions as TV ribbon available here in Australia, but air
dielectric (in the absence of reliable measured loss characteristics
of the feedline). Real TV ribbon will be very slightly lossier.

Examining the loss on 80m, 40m, and 20m, the nominal 300 ohm line is
not a clear improvement over the nominal 450 ohm line in the model
scenario.

Here are the total loss figures (open wire line, coax, tuner) for the
model scenario:

450 300
3.6 2.4 3.3
7.1 1.9 1.4
14.2 1.0 1.7

Though the 300 line is a little better at 40m, it is a little worse on
80m and 20m.

The interaction of components of a G5RV antenna system are
interesting... the case of 300 line at 3.6MHz is increased loss in the
open wire section (no surprises there), and decreased loss in the coax
section compared to 450 line.

A lower loss 300 ohm line (ie one with thicker conductors) would
perform differently. One just doesn't know unless one runs the
numbers.

Perhaps a "feeling" that something works better leads to urban myths?

The optimum length of coax in a G5RV feed is zero (metres or feet).

Owen
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