Power measurement in a transmission line:

In the transmission line, power can be measured by means of the voltmeter between the conductors, and an ammeter in the series with one of the conductors. Then the power, P is equal to the product of voltage and the current I. This technique can be used in any transmission line, be it for 60-Hz utility service, or in the radio transmitting station.

Figure--   Power measurement in a transmission line.

But is this indication of power the same as the power dissipated actually by the load at the end of line? Not essentially.

Any transmission line has characteristic impedance. This value, Z_o, depends on size of the line conductors, the spacing between conductors, and the type of dielectric material which separates the conductors. For the coaxial cable, Zo can be anywhere from around 50 to 150 Ω. For the parallel-wire line, it can range from around 75 Ω to 600 Ω.

If load is a pure resistance R having no reactance, and if R = Z_o , then the power indicated by voltmeter or ammeter scheme will be same as the true power dissipated by load. The voltmeter and ammeter should be placed at the load end of the transmission line.

If load is a pure resistance R, and R < Z_o  or R > Z_o, then voltmeter and ammeter will not give an indication of true power. And, if there is any reactance in load, voltmeter or ammeter method will not be accurate.

You should always remember that it is desirable to have the load impedance be a pure resistance, a complex value of R = j₀, where R = Z_o. Small discrepancies, in = form of a larger or smaller resistance, or the small reactance, can at times be tolerated. But in very high frequency (VHF), ultra high frequency (UHF) and microwave radio transmitting systems, even small impedance mismatch between load and line can cause excessive power losses in line.

An impedance mismatch can be corrected by means of matching transformers and reactances which cancel out any load reactance.