Date: Thu, 04 Feb 2010 11:36:50 -0800
From: "Jim Brown" <firstname.lastname@example.org>
Subject: Re: [TowerTalk] Coax Loss -- RG-Numbers Don't Tell us Much
To: "Tower and HF antenna construction topics."
>Bottom line is that DC resistance is only loosely related
>to RF loss.
Yes, but at MF and HF, it's a VERY good starting point when COMPARING cables.
## say what ?
## If you DOUBLE the diam of the center conductor, it's DC resistance
will drop to just 1/4 of what is was originally.
## IF you DOUBLE the diam of the center conductor, it's RF resistance is now
1/2 of what is was originally. [same deal when using tubing for tank coils on
### DC resistance of the center conductor has nothing to do with it. My
uses a hollow copper tube for the center conductor !
## I read someplace [ dipped if I can remember, think it was buried in the
catalog]... that 90% of coax loss.. is dielectric loss !
## be careful when comparing 50 and 70 ohm cables. RG-11 and RG-8
both are .405" OD RG-11 has a SMALLER ga center conductor.
Reason is... so the PL-259 will fit either cable.
## if both the 50 and 70 ohm cables had the same ga center conductor,
the OD's of the outer jacket would be different, and the 70 ohm would be a
### don't confuse loss with power handling capability. 35 ohm coax will
handle more power than 70 ohm. 50 ohm coax is just a compromise between
35 and 70 ohm coax.
## If you really want to measure loss.... stick a bird lne section at each end
of the cable. Terminate the far end with a dummy load. Use the SAME slug,
it from one end to the other. You will get a real eye opener.
## I once had a buddy who had aprx 350' of RG-213 on 10m... going
up a 100' tower. With exactly 1000 w applied on 28.5 mhz... he was lucky to
get 350 watts at the top of the tower! After that, cable swapped out to
7/8" heliax. Then he got well > 900 w at top of tower.
Later... Jim VE7RF
TowerTalk mailing list