The Flattering SWR Meter
A station runs 50 W on 2 m through 50 m of RG-58C/U to a home-built vertical on the roof. At 145 MHz the cable's published matched loss is 4.3 dB per 50 m. An SWR meter connected at the transceiver through a short jumper reads a steady 1.5:1 across the band, and the operator concludes the antenna system is fine and plans to spend the weekend building a new antenna instead. A friend suggests first checking the feeder with a 50 ohm dummy load and a directional wattmeter. No antenna analyser is available, the antenna is 9 m up, and any adjustment at the antenna means a ladder job.
1. Estimate the SWR actually present at the antenna terminals, and explain why a meter at the transceiver understates it.
Read the worked answer
Convert the reading to a reflection coefficient
|Gamma_meas| = (SWR - 1)/(SWR + 1) = (1.5 - 1)/(1.5 + 1) = 0.20
Allow for the two-way trip through the cable
One-way voltage attenuation factor = 10^(-4.3/20) = 0.61. A wave travelling down and back is attenuated by 0.61^2 = 0.37. The reflection from the antenna is attenuated twice before it reaches the meter, so:
|Gamma_ant| = 0.20 / 0.37 = 0.54
SWR at the antenna = (1 + 0.54)/(1 - 0.54) = about 3.3:1
Why the meter lies
Every lossy feeder does this. The mismatch is real, but the reflected wave is absorbed on its way back, so the transmitter sees a mild mismatch. The more loss in the cable, the more flattering the reading. A 1.5:1 at the rig is not evidence of a matched antenna - it is evidence that the feeder is lossy enough to hide the truth.
2. Where should the operator's effort actually go? Estimate the power radiated.
Read the worked answer
Power arriving at the antenna
P = 50 W x 0.37 = 18.6 W
Mismatch loss at the antenna
10 log(1 - 0.54^2) = 10 log(0.71) = -1.5 dB
Radiated power
18.6 x 0.71 = about 13 W (a little more returns via re-reflection), i.e. roughly 5.8 dB below the 50 W leaving the transmitter.
Judgement
- The cable loss (4.3 dB) dominates the mismatch loss (1.5 dB). Replacing RG-58 with low-loss cable, or shortening the run, recovers several dB; perfecting the antenna match recovers only about 1.5 dB.
- The transmitter is in no danger: a 3.3:1 at the far end of 4.3 dB of cable looks like 1.5:1 at the PA, so there is no protection or arcing issue to chase.
- But do not build a new antenna and leave the feeder alone. A resonant, well-matched antenna plus a low-loss feeder is the only combination that gets most of the 50 W into the air. Fix the feeder and re-check the antenna match before spending the weekend on a new radiator.
3. Describe the dummy-load measurement and the figure you would expect to see.
Read the worked answer
Method
- Terminate the far end of the feeder in a 50 ohm dummy load rated well above 50 W.
- Measure forward power at the transmitter (SWR should be near 1:1).
- Measure the power delivered at the load end with the directional wattmeter.
Calculation
Loss (dB) = 10 log(P_in / P_out)
With 50 W in and about 18 W out: 10 log(50/18) = 4.4 dB, which matches the published 4.3 dB.
Interpretation
- If the measured loss matches the specification, the cable is healthy and the mismatch is at the antenna.
- If the measured loss is much higher, the cable or its connectors are water-damaged or corroded - that is the fault to chase first.
- Allow for wattmeter accuracy (typically +/-5%) and confirm the dummy load really is 50 ohm resistive; a poor load makes the loss calculation meaningless.