[TowerTalk] Radio Shack 300 Ohm Line Characteristics

[Tom Rauch]

> 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'.

Bill,

The primary loss mechanism in any transmission line cable
with even a reasonable dielectric at HF is conductor loss.
Foam or air dielectric allows the conductor size to be
increased for a given cable size and impedance, and the
increased conductor size for a given impedance reduces loss.

The main problem with 300 ohm transmission lines is the
close spacing makes conductors smaller. They are also
stranded, and that hurts a little bit also. A 300 ohm TV
line using 20-24 ga conductors should have quite a bit more
loss than a 450 ohm line using 14-18 gauge conductors.

Belden lists 22 AWG 300 ohm line as 1.4dB/100 ft at 100Mhz
while stepping up to 20 gauge reduces loss to 1.1dB/100ft @
100MHz. That's  the only data I have here in my catalogs.

The rule of thumb for cables like this is at about 1/3 the
frequency you should have about 1/2 the loss. So that would
put Belden at .55dB loss per 100 ft at 33 MHz.

It sounds like TLA is right on, based on what I see. Quite
honestly when I measure lines with my network analyzer and
compare to TLA's calculations they are almost always within
a few percent of each other.  I've pretty much quit double
checking TLA after years of not finding any flaws in
anything it does.

73 Tom