----- Original Message -----
From: "Tom W8JI" <w8ji@w8ji.com>
To: "'TopBand'" <topband@contesting.com>
Sent: Saturday, February 15, 2014 12:08 PM
Subject: Re: Topband: Palomar R-X Noise Bridge
The lowest loss cable I have here is 75 Ohm 1" General Cable Fused Disc;
its under a differnt name these days. Mostly air with poly discs and used
for the 200' runs for 10M, 2M, and 222 MHz.
For the 160/80 inverted vee it is 450' of regular foamed 3/4" 75 Ohm CATV
hardline with a RG-11 jumper and plenty of ferrite to the feed point. Ive
been using ferrite sleeve baluns since the mid 70's; I was introduced to
them by the company I worked for who was building equipment for the joint
CIA/DOD Tempest program.
The lowest loss cables have large, smooth conductors that are the maximum
possible size for the cable impedance. Dielectric is largely meaningless,
except as it might affect conductor size.
** Which directly affects loss. While meaningless at 160 even in long runs
the difference between the 1" Fused Disc and 1" solid foam Commscope is .5
vs .65dB/100' at 222 MHz which can add up to no contact in long lengths.
We can argue this point endlessly, but it will always come back to the
conductors.
** As close to an air dielectric as possible will have the largest diameter
center conductor and lowest loss. Adding any other dielectric requires a
smaller conductor to maintain the same impedance and with its own extra loss
caused by the dielectric choice so it will always be a contributing factor
since its dielectric constant and capacitance per foot varies and is not a
lossless medium. It is also frequency sensitive.
A nitrogen pressurized coax is about as good as it gets.
The exception would be some horrible dielectric or operation way
up above normal VHF/UHF with marginal dielectrics.
It is the way it is. The confusion probably occurs because dielectrics
with more air allow a larger conductor to be used for a given cable
diameter and impedance, it is not because the dielectric has less loss.
** Im not confused but for the sake of the forum it is not exactly related
to 160 so lets leave it for elsewhere.
Carl
KM1H
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