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[TowerTalk] Cable loss revisted.

To: <>
Subject: [TowerTalk] Cable loss revisted.
From: "Jim Thomson" <>
Reply-to: "Tower and HF antenna construction topics." <>
Date: Fri, 5 Feb 2010 05:34:31 -0800
List-post: <">>
  I visited some contest stations, and after having a look at things,  cringe 
at the thought of the loss on the coax cables used. 
Some had really long runs of coax from station to base of tower.. then more up 
the tower, etc.  300-400'  was typ. 
Biggest cables used were 1/2" heliax.     A lot of it was 213-U    Some was 
LMR-400 or eq. 

I don't see why one would want to try and squeak out another .5 db  of gain on 
a longer boom yagi,
[ then repeat on every other yagi].. then turn around and lose a whole bunch  
from the long run's
of coax being used !!   Sure... 1db  doesn't  sound like much.. but  that's 21% 
of ur power gone.... and it
drives me nuts to think I burned up 21% of the power coming out of the  linear. 
  That equates to 472 watts
burned up.  [ 2250w pep out  x .21 = 472 w]  

Here in Canada, we are allowed  2250 w pep out on SSB.,,,....but  only 750W  on 
CW.    [ and 750 W  CXR on AM]
Power is measured ... 'across a  Z matched load' .    Well the load IS the feed 
point of the ant.  We all interpret Z matched load
as  'forward pwr - rvs power'  .   W0XB's wattmeter  will even do the math's 
for you....  ['net power'] 

The end result ?     Now I have to crank out  2250/.789 = 2851 watts pep ..... 
just to get my 2250w pep
at the ant  feed point.       And that's  with JUST  1 db of loss ! 

IMO... investing in lower loss coax is the real answer.  TX/RX signals lost in 
the coax, are lost forever. 

Either that, and/or... put the 10m yagi's / upper band stuff  close to the 
shack... and the low band stuff  out in the back 40.

Sure you can use on on line calculator's and various schemes to measure coax 
loss.  Humour me.. and haul a bird /CD
meter up the tower.. and get someone to stuff exactly  1kw of cxr into the coax 
in the shack.   Then measure at the
top of the feedpoint,  or close to it... like the  top of tower / remote switch 
box / stack matcher etc.  

For the folks  with 1.5 kw....... 1 db loss  means you only end up with  1171 
watts at the feed point.  So you are no where 
near  1.5 kw.    Stick 1.5 kw into the coax... and  see what you actually get.  
The results will sometimes make you gag.
I had moisture get into a connector... and didn't realize it.... till I 
measured  50 k ohms across the coax... in the shack
[ gamma  matched yagi yrs ago]    Removed the coax seal glop  off the pl-259  
at feedpoint of 20m  yagi.. and water came out! 
I had  foolishly not used any vapor wrap + /88/33 tape.   If I hadn't of  
measured the dc resistance of the coax on a hunch, [should
have read infinite ohms]   I would  have kept on going.. with lousy 

I took a hint  from W2PV... and all cables are now well documented.   I measure 
everything, including Capacitance  between  center conductor
and braid, dc resistance, calibrated watt meter reading at far ends [ on at 
least 80-20-10m], initial install date, when coax was initially purchased,
from who, and how much $$,  which brand connector's used,  exact water proof 
method employed, etc.    You can laugh, but 6  yrs down the road,
when you suspect something is amiss, you now have a bench mark /reference.. to 
compare to.   Ditto for all control cables too. 

later...... Jim  VE7RF

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