Hi Bill,
Using Keven Schmidt's TL Calculator
http://fermi.la.asu.edu/w9cf/tran/index.html
I got 0.39 dB/100' for 450 ohm windowed line and 0.54 dB/100'
for 300 ohm tubular line. This is about what you would expect if
you assume that conductor loss is the dominant factor (probably
a good assumption at 28 MHz) and that the wire size (and type)
is the same in both cases (W9CF's applet doesn't tell you the
wire size of the lines). If, however, you are comparing 300
twinlead with stranded #20 AWG conductors to heavy duty 450
line with #16 AWG solid conductors, then its possible that the
450 ohm line might be closer to 0.15 dB/100' due to the lower
conductor losses of the bigger wire. Just comparing, 300 ohm
line with 20 AWG and 450 ohm 16 AWG solid conductors, (and
assuming 0.54 dB/100' for the 300 ohm line), you would expect
that the loss for the 450 ohm line would be around 0.22dB/100'.
I don't know how much additional difference there would be due
to the difference between the stranded and solid conductors.
73 de Mike, W4EF.........................................
----- 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.
>
>
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=15-11
75
>
> I have searched high and low for some believable
> loss numbers (20th edition Antenna Book, Wireman's
> Wirebook IV, Internet, etc) but have come up empty so
> far. Has anyone actually ever measured the Radio
> Shack Twinlead or does anyone know of published loss
> figures for this type of twin-lead?
>
> 73, Bill W4ZV
>
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