[TowerTalk] Coax Losses on 160 and 75?

[Jim Brown]

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