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[TowerTalk] Coax

To: <>
Subject: [TowerTalk] Coax
From: "Jim Thomson" <>
Date: Mon, 9 Apr 2012 06:04:21 -0700
List-post: <">>
Date: Sun, 08 Apr 2012 21:11:58 -0700
From: Jim Brown <>
Subject: Re: [TowerTalk] Coax
On 4/8/2012 9:01 PM, Jim Thomson wrote:
> LMR-400db .666 db
> LMR-400UF .799 db
> RG-213U 1.024 db

RG213 is not a spec, it's a very broad generic description. There are 
RG213s built with thin copper braid and others with heavy copper braid. 
The loss in cable is a direct function of how much copper they use to 
build it. Based on resistance data, Davis's 213 is directly equivalent 
to LMR400 for use on the HF bands.

## say what. Run 200 feet of 213-U into a dummy load at the far end, with a 
wattmeter 1 foot before the dummy load.

You'll need far more precise instrumentation than a Bird to measure a 
difference of 0.1 dB Rather, you'll need a scope, RF voltmeter, or 
spectrum analyzer that can resolve 0.01dB, you'll need to measure by 
substitution, and you'd better make a bunch of measurements and compare 

73, Jim K9YC

##  Andrew  .5 inch heliax  uses a copper clad solid aluminum  center 
conductor.  Andrew is the not the only game in town.
Other makers  will offer  .5 inch heliax  in both copper clad Al  and also  
solid CU.   Are you trying to tell us that the solid Cu
variety will result in lower loss.  .875 inch heliax uses a hollow copper tube. 
  LMR-600  uses 5.5 gauge  copper (7 x strands)
for the  center conductor.   Which one do you think will have the lowest dc 
resistance for the center conductor. 

##  If Davis RF’s  version of  213 has way lower dc resistance than belden 213, 
then  the strands would have to be  a LOT  bigger.   If that was true,
the OD of the cable would have to be a lot bigger, and all those 213  cables  
appear to be .405 inch OD.   You can’t just use heavier gauge strands
for the braid and be able to use the same pl-259.   Well  maybe you can, if you 
made the sheath from thinner material to maintain the .405 inch
OD. t 

##  LMR cables use a tinned outer braid...on top of a  360 deg  AL wrap of 
foil.    Most eng notes will tell you that  90%  of losses  in coax cable are
dielectric losses..and not  dc resistance loss’s.   

##  Bird products have limitations to them.  Any array solutions power master  
wattmeters  will read in .1 watt  increments, but only below 100w.  
Above 100w, they read in 1 watt increments.   Pretty easy  to see the 
difference between  1500w  and  1498 w.  (.00579 db)   or  the diff
between  1500  and 1499 w  (.002896 db)   Pretty easy to measure the diff 
between  1500w  and  1467w    (.00966 db)     

## It would not be rocket science  to measure the  difference   between davis 
213, Belden 213..and  LMR-400.  Use  200-300 feet of each
of the 3 x cables, as long as they are all identical length,   and use the same 
freq to test em.... like  29.0  mhz   

##  The problem with using any scope is... if the scope is off by say 5%, your 
results will be off by twice that.   Voltage squared divided by  50 ohms
= power.   The bottom line is..even a bird will show the big diff between  300 
ft of belden 213   and  300 ft of .875 inch heliax on 29 mhz.  Its like
apples and oranges. 

## If you are implying that  dc resistance is the predominant factor in coax 
cable loss, then  all these various  formulae  + online loss 
calculators  must be out to lunch, and I find that hard to believe.      

##  Bigger coax is cheaper than a bigger amplifier.   I want a bare min of 2.5 
kw at the feed point of the ant  on any band.  With a crank up tower
I am limited to flexible coax up the side of the tower.    The best I can come 
up with is  LMR-1200DB  to the base of the tower..and  RG-393
up the side of the tower. 

Jim  VE7RF      

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