[TowerTalk] Coax Losses on 160 and 75?

[TexasRF at aol.com]

This sounds like more fly excrement in the pepper. 52R +j2 equates to a  
vswr of 1.06 or a reflection of -30.7 dB. The resulting mismatch loss is only  
.08 dB and even that gets mostly canceled when a tube type amplifier is 
peaked  for max power out as we typically do.
 
If I owned the equipment to resolve differences this small I  would 
certainly use it but with the knowledge that less expensive methods  would result 
in answers so close it would not matter. 
 
What would be useful here is some actual measured data at the lower  
frequencies using some of the more accurate measurement tools. At this point it  
is unknown if the manufacturer's published data is from actual tests or  
extrapolated from higher frequency data.
 
The next step in measurement precision is taking the physical temperature  
of the cable into account. After all, copper R changes at the rate of about 
20  parts per million per degree C. I said this somewhat in jest but in 
another life  time copper conductors were used for open wire lines in long 
distance telephone  and telegraph transmission. I well remember having to factor 
in the outside  temperature when trouble shooting line problems with a 
Wheatstone Bridge and  130vdc at the toll test board of the local telephone 
company. That was a looong  time ago!
 
73,
Gerald K5GW
 
 
 
 
 
 
 
In a message dated 8/6/2016 12:36:57 P.M. Central Daylight Time,  
jim at audiosystemsgroup.com writes:

On  Sat,8/6/2016 4:35 AM, Jim Thomson wrote:
> My dummy load does not have a  mismatch, coax is 50 ohms, swr is  1:1
> at each end of the coax,  and there is no reflected power..at either end. 
 T

Jim,

The  actual Zo of coax is NOT constant with frequency, and it is not a 
REAL  number -- it is complex, of the form R +/- jX. The value of Zo 
CONVERGES  to its nominal value at VHF, but at lower frequencies, it's a 
different  value. You can see computed numbers for many common cables in 
N6BV's TLW  software, which comes free on the CD that is packed with the 
ARRL Antenna  Book. The accuracy of Dean's data has been questioned, but 
the concept is  quite correct.

SO -- coax that we CALL 50 ohm coax can have a Zo of 52  + j 2, so a 50 
ohm resistor is NOT a perfect match. This variation from  the "nominal" 
value is greatest at lower frequencies.

73,  Jim

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