Perhaps the dielectric loss isn't the dominant effect. Looking at the tables
in XLZIZL, it looks like the big change between generic 300 and generic 450
(which calculate out to the numbers you give in your post) is in the K1
term, which goes as sqrt(f). That's primarily the resistive component, and
strongly affected by conductor diameter (skin effect...).
What's the conductor diameter on the two cables you're interested in? To a
first order, the loss will be proportional to the AC resistance, which will
be proportional to the diameter (not the area... the current flows in a thin
layer on the surface) of the conductor.
Let's look at the ratio in losses: .146 and .541... (in dB)...
If the AWG 20 wires in the RS 300 ohm wire are compared to the AWG 16 wires
in the wireman 450 ohm stuff... there are two factors, both helping reduce
loss:
1) The AWG 16 has 2 dB more diameter.. A 3dB change would cut the loss in
half.. (from .541 to .27)
2) The 450 ohm has about 20% lower current than the 300 ohm (because the
impedance is higher.. so I^2 R losses are lower).. That would get the loss
from 0.27 to 0.18, which is close enough to the reported figure to be
reasonable.
There are other more subtle factors.. the current's not evenly distributed
around the circumference of the conductor because of the proximity of the
two conductors, so the "tubular current sheet" approximation is less valid
for the smaller cable. They're both stranded wire, which has it's own skin
effect peculiarities, depending on the number and size of the strands.
Overall, I'd believe the reported numbers. By the way, if you haven't
downloaded XLZIZL from the website, you should. It's very slick for this
sort of analysis.
Jim, W6RMK
----- Original Message -----
From: "Bill Tippett" <btippett@alum.mit.edu>
To: <towertalk@contesting.com>
Sent: Sunday, June 13, 2004 6:38 AM
Subject: [TowerTalk] Radio Shack 300 Ohm Line Characteristics
> I'm planning to try an 80m dipole fed with Radio
> Shack 300 ohm twin-lead for Field Day. I only plan to
> run QRP so power handling is not a consideration. In
> using the method on p.34 of April 2004 QST, I am first
> using Eznec to determine feedpoint impedance for each
> band (80-10m) and then plugging that into TLW (N6BV's
> transmission line analyzer) to look at line loss issues.
>
> The data N6BV has loaded into TLW for "300
> ohm tubular" line does not seem right for the RS feedline.
> It show loss of 0.541 dB/100' at 28 MHz, but 450 ohm
> ladder line shows loss of 0.146 dB/100'. Since the
> 300 ohm line uses foam insulation between conductors,
> I'm finding it hard to believe there is so much difference
> in losses. The RS feedline appears to be of fairly high
> quality, with a heavy-duty brown polyethylene covering
> two stranded #20 wires with foam insulation surrounding
> and between the wires. This is not the junky white stuff.
_______________________________________________
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Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
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