[TowerTalk] RG-Numbers don't tell us much

[Jim Thomson]

Date: Thu, 04 Feb 2010 11:36:50 -0800
From: "Jim Brown" <jim at audiosystemsgroup.com>
Subject: Re: [TowerTalk] Coax Loss -- RG-Numbers Don't Tell us Much
To: "Tower and HF antenna construction topics."
<towertalk at contesting.com>
Message-ID: <20100204193655.2952457C7C at gw1.nlenet.net>


>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 only
1/2  of what is was originally. [same deal when using tubing for tank coils on an amp]. 

###  DC resistance of the center conductor has  nothing to do with it.  My LMR-1200
uses a hollow  copper tube for the center conductor ! 

##  I read someplace [ dipped if I can remember, think it was buried in the Andrews
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 lot 
bigger.

###  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, and move 
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