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measuring cable loss
"Jimmie D" wrote in
: I think I have given enough info. But I will try yo expess it in another way. Power delivered to the antenna but be maintained with in +- 1 db in this case that power is 100 watts. Power is normally checked at the TX and recorded after allowing for line loss as "power at the antenna". Power checks are done on a weekly basis. Once a year the line loss is measured and this value is used to subtract from the power at the transmitter for the rest of the year. With this in mind it would be most prudent to measure the cable loss accurately. to establish the annual benchmark. Ok. You haven't mentioned how you intend to deal with the likely case where VSWR1. Considering the test equipment I have available to use in a temperature stablized building an Agilent network analyzer or use an old HP power meter at the top of the tower I am thinking that measuring rho of the cable while terminated in a short may be the more accurate way to go. Yes, especially if the NA is calibrated against o/c, s/c and Zo locally. If for example, you discover that the one way matched line loss is 6.75/2dB (3.375dB), and you measure the fwd and reflected power at the tx end to be say 200W and 15W, you could use the calculator I mentioned to determine that the VSWR at the antenna was 1.36. Using that, and setting forward power at the antenna to 102W for a net power to the antenna of 99.6W, forward and reflected at the tx end of the line should be 222W and 1.1W. Of course, if the line was perfectly matched, you could just set the tx end forward power to 100*10^(3.375/10) or 217W and reflected would be zero... but that is unlikely. You could take the easier, simpler and conservative way out and just set it for 217W forward irrespective of mismatch. It is splitting hairs, but sometimes precision in the method distracts attention from accuracy! Owen using the calculator I mentioned, and setting the line loss to the tx to 3.375dB, loss to antenna to 0, tx power to |
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