[222mhz] Cost/performance comparisons of transmission lines.

Mon, 01 Dec 2003 06:32:23 -0700

I have been asking about  RG-213, 9913 & LMR-400 transmission line 
comparisons on several repeater owner/builders forums. I was seeking 
feedback for what to use for a repeater.  I got many replies. All are in 
agreement on hardline being the most cost effective when repeat site visits 
for troubleshooting and maintenance were factored into consideration.  Here 
is an interesting response from a RF Engineer that has been installing and 
maintaining government/commercial/amateur sites for 42 years.  His was the 
most comprehensive explanation.  
__________________________________________________________________________

The problem with aluminum foil coax is hard to discover on paper or in a 
lab, if you have a chance to meet this person take along some chewing gum 
and offer him/her a piece. Then take the foil wrapper and roll it around a 
pencil, remove it from the pencil and try to fold a corner.  It tears open 
on the outside and smooshes together on the inside.  What you are left with 
in feedline is now just plain old 70-80 percent shielded coax leaking rf all 
over everything.
But wait, now you want this to go out of a building, and up the tower to the 
antenna.  You need to seal it to prevent insect intrusion.  You carefully 
unroll everything in very large radius's and pull.  Finally you are near the 
top.  How much foil has been torn open going up the side of a tower?  I 
don't know either, but every time I have done this there is bad reciever 
desense or mysterious swr conditions and crackling noises when it rains.
9913 must have support fingers every 15 - 20 feet of pulling up the tower, 
LMR a minimum of 40 feet.  If you are actually pulling it up you will need 
to prepare it on flat ground and make connections so that you can pull it 
using wire-rope.  If the wire-rope whips in the wind and touches the 
metallic tower structure, you now have an intermittent diode grounding 
something and making noises for everyone at the site.  Use vinyl covered 
locksmith originated cable.  If you add up all this special support hardware 
it is often cheaper to get a vinyl jacketed hardline that exhibits lower 
loss and lower cost in the end.
9913 is a spec for lightwieght interior computer transmission cable  Someone 
saw the 50 ohm rating and instantly applied it to radiation field usage.  It 
does work well inside a building where nobody will be walking, leaning on it 
or trying to form tight radius corners. Once it has been abused it quickly 
turns into grapevine support when it goes outside.

Don't argue with an idiot. The eavesdroppers may not be able to tell the 
difference.

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----Original Message Follows----
From: "Dr. Gerald N. Johnson, electrical engineer" <[email protected]>
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Subject: Re: [222mhz] Cost/performance comparisons of transmission lines.
Date: Sun, 30 Nov 2003 20:53:17 -0600
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RG-58 and RG-8X at VHF are the most expensive of coaxes. They are big
attenuators. Likely LMR-200 is also expensive because of loss not much
better than RG-8X. RG-8 and RG-213 work half way decently, 9913 is a bit
better but prone to gathering water if not sealed perfectly at the
connectors. LMR-400 seems to be good.

73, Jerry, K0CQ
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Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.

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