Measuring Transmission Line Loss
Posted: Sat Aug 16, 2025 9:37 am
My recently purchased VHF directional wattmeter has become a very useful testing device. I used it to determine the transmission line loss of a coaxial cable I had on hand.
The test set-up was as follows: (pardon the ASCII artwork)
Initial set up:
TX = VHF Marine Band 25-Watt radio
WM = Directional Wattmeter
DL = 50-Ohm dummy load
CBL-1 = 2-foot-long RG400 Teflon 50-Ohm coaxial cable, UHF plug to N plug
The transmitter was tuned to 156.8-MHz and a short transmission was made into the dummy load. The wattmeter measured the incident power (P1) as 26.86-Watts, and reflected power as 0-Watts. This indicates the dummy load is essentially a perfect match to the transmission line impedance.
The cable under test (CBL-2), a 50-foot length of RG-8/U with PL-259 plug and N receptacle, was then added to the test set up; one adaptor (A), was also needed, a N-plug to UHF receptacle. These are shown in red below. The new test configuration was:
Note that the position of the wattmeter relative to the dummy load remains unchanged, however the wattmeter is now located about 50-feet and one adaptor farther from the transmitter than in the earlier test. Any change in power delivered at the dummy load is then inferred to be caused by loss in CBL-2.
The wattmeter measured (P2) incident power as 19.65-Watts and 0-Watts reflected power.
The loss in the cable in decibels was then calculated:
The exact type of RG-8 cable and the manufacturer were obscured from age and also from the somewhat thorough washing I gave the cable to remove dirt from the jacket (in order to bring the cable into the antenna laboratory whose floor is white carpet). I suspect the cable was Belden 8214 RG-8/U. Proceeding on that assumption the specified loss when new at 150-MHz for 100-feet was -2.15 dB. For 50-feet the specified loss would then be half or -1.075 dB.
The measured cable loss was -1.35 dB or 0.275 dB greater loss than the manufacturer specified. That seems quite reasonable for 20-year-old cable that spent at least 15-years outdoors in sunlight and weather.
On the basis of this test, the measurement technique appears to be reasonably accurate. Note that the use of the 50-Ohm dummy load insures that the transmission line is terminated in the proper impedance and there is no reflected power. Any influence of the wattmeter itself or the RG400 short cable is present in both measurements, as that portion of the test assembly does not change.
More information about the wattmeter is given in an companion article.
The test set-up was as follows: (pardon the ASCII artwork)
Initial set up:
TX--< <---CBL-1---> >--WM--< <--DLTX = VHF Marine Band 25-Watt radio
WM = Directional Wattmeter
DL = 50-Ohm dummy load
CBL-1 = 2-foot-long RG400 Teflon 50-Ohm coaxial cable, UHF plug to N plug
The transmitter was tuned to 156.8-MHz and a short transmission was made into the dummy load. The wattmeter measured the incident power (P1) as 26.86-Watts, and reflected power as 0-Watts. This indicates the dummy load is essentially a perfect match to the transmission line impedance.
The cable under test (CBL-2), a 50-foot length of RG-8/U with PL-259 plug and N receptacle, was then added to the test set up; one adaptor (A), was also needed, a N-plug to UHF receptacle. These are shown in red below. The new test configuration was:
TX--< <------CBL-2------< <--A--< <--CBL-1--> >--WM--< <--DL Note that the position of the wattmeter relative to the dummy load remains unchanged, however the wattmeter is now located about 50-feet and one adaptor farther from the transmitter than in the earlier test. Any change in power delivered at the dummy load is then inferred to be caused by loss in CBL-2.
The wattmeter measured (P2) incident power as 19.65-Watts and 0-Watts reflected power.
The loss in the cable in decibels was then calculated:
dB = 10 * LOG10(P2/P1)
dB = 10 * LOG10(19.65/26.86)
dB = 10 * LOG10(0.731)
dB = 10 * -0.135
dB = -1.35The exact type of RG-8 cable and the manufacturer were obscured from age and also from the somewhat thorough washing I gave the cable to remove dirt from the jacket (in order to bring the cable into the antenna laboratory whose floor is white carpet). I suspect the cable was Belden 8214 RG-8/U. Proceeding on that assumption the specified loss when new at 150-MHz for 100-feet was -2.15 dB. For 50-feet the specified loss would then be half or -1.075 dB.
The measured cable loss was -1.35 dB or 0.275 dB greater loss than the manufacturer specified. That seems quite reasonable for 20-year-old cable that spent at least 15-years outdoors in sunlight and weather.
On the basis of this test, the measurement technique appears to be reasonably accurate. Note that the use of the 50-Ohm dummy load insures that the transmission line is terminated in the proper impedance and there is no reflected power. Any influence of the wattmeter itself or the RG400 short cable is present in both measurements, as that portion of the test assembly does not change.
More information about the wattmeter is given in an companion article.