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[TowerTalk] Lossy coax

To: <towertalk@contesting.com>
Subject: [TowerTalk] Lossy coax
From: W8JI@aol.com (W8JI@aol.com)
Date: Thu, 2 Jul 1998 12:26:14 EDT
In a message dated 98-07-01 17:00:48 EDT, you write:

<< 
 Following Autek's instructions, I tuned for minimum Z. At 28 Mhz, 
 according to their formula, my loss was 1.3 db and at 7 Mhz it was 0.7 
 db! I used the formula 0.17 x Z = loss in db (for 50 ohm coax). This 
 seems awfully high, or does it? >>
 
I'm trying to understand the formula you posted Barry. I like to take formulas
to the extremes to check accuracy.

With an infinite loss the measured Z would be 50 ohms.  0.17 times 50 is not
infinity, it is 8.5 dB.

With zero loss the impedance at minimum Z frequency would be zero ohms. It
works at zero loss.

Ten dB loss. The impedance could be as low as 40.9 ohms. 40.9 times .17 is
6.95 dB.

Looks like the approximation has errors in the ideal case even without
counting all the other potential errors, although it does appear to work
pretty good under some conditions at lower values of loss (like around 3 dB
where minimum Z is about 16.6 ohms in the ideal case). 16.6 times .17 is 2.822
dB. 

SWR should be more accurate. Convert unterminated SWR at ANY frequency or Z
into return loss, and take half the return loss to determine transmission
loss. If you tell me the true SWR with an unterminated cable, I'll tell you
the loss. ;-)

73 Tom

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