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@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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