Jim Lux wrote in news:f9gg4i$7q9$1
@nntp1.jpl.nasa.gov:

....

Jim good points and all noted.

Jimmie hasn't give a lot of detail about the specification he is
apparently trying to meet. Reading between the lines, it might be an
EIRP, and assuming a given antenna gain, he is trying to calculate the
permitted transmitter power output.

Not only is the uncertainty of practical service equipment an issue in
tenth dB accuracy, but no mention has been made of transmission line loss
under mismatch conditions, and mismatch loss.

Jimmie, if you have a plausible story to tell the regulator, then that
might suffice.

If you have assessed the Return Loss of a rho=1 termination, then you
could use that and the measured Forward and Reverse power using say a
Bird 43 at the transmitter end of that known line loss (being half the
return loss) to calculate the power absorbed by the load. The calculator
at http://www.vk1od.net/tl/vswrc.php does just that. The calculator at
http://www.vk1od.net/tl/tllc.php could be used to calculate the expected
RL of the o/c or s/c line section, just specify a load impedance of 1e6
or 1e-6 for each case. For example, at 1GHz, the RL of 200' LDF4-50A with
a 1e-6 load is 8.9dB, and if you got much higher than that, you might
suspect the cable to be faulty.

Tenths of a dB, remember that most service type power meters are probably
good for 6% to 10% of FSD, so I will go with Jim's 1dB accuracy.

BTW, directional wattmeters for the ham market are often not capable of
reasonable accuracy on loads other than the nominal 50 ohm load. There
are a range of tests that such an instrument should satisfy, but for
hams, it is usually considered sufficient if the "reflected" reading is
approximately zero on a 50 ohm load.

Owen