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[TowerTalk] Re: Loss in Coax Cable

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Subject: [TowerTalk] Re: Loss in Coax Cable
From: (Jim Reid)
Date: Mon, 29 Jun 1998 14:15:27 -0900
At 12:57 6/29/98 PDT, you wrote:
>>Aloha Jim,

---some stuff snipped--

>> I will measure 100 watts from the 
>>exciter into into a wattmeter and dummy load. This will verify the 
>>wattmeter and the exciter. Then I will measure what comes out the other 
>>end with the the wattmeter and dummy load. Do you know how
>>to calculate the attentuation in db just using watts????? In this case 
>I know watts in and watts out and want to know how much 
>attentuation is present ?????? 

Hi Joe,  Well first to answer your dB question.  To get the dB loss
based upon the measurement you plan,  simply divide the
output power from the cable far end into the 100 watts
out from the exciter.  That will be a  number a fraction larger
than 1.0.  For example,  if you measure,  say 87 watts coming
out,  with 100 watts in,  the fraction of P1 divided by P2
will be equal to 100/87= 1.149 on your calculator.  With that
number showing on you calculator,  push the "log" button.
Up comes  0.0604.  Ten times that number is the loss in dB,
or 0.6 dB of loss in the cable.  You need a calculator that
can do logs and 10-to-the-x functions to do dB arithmetic,  hi.

Yes,  earlier today,  I posted results using a Bird meter doing
essentially as you suggested,  and unfortunately found the
same disappointing losses in the LMR-400.  I still wonder if
my kinking it at installation has anything to do with this
higher than expected loss -- though probably not,  no one
on the reflector has suspected it,  even though I explained
about the couple of kinks in an earlier post.

Know of a source of ready made 600 ohm ladder line;  no
way I could afford hardline to try to reduce my long
transmission line loss.  

In the end,  I may simply wind up living with it,  and work to
save nearly a dB of loss in the switches,  etc,  here in the
radio room.

73,  Jim

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