A Clean SWR Reading That Hides a Bad Antenna
A 25 W VHF marine transceiver on 156.800 MHz feeds a masthead antenna rated 3 dBd through 35 m of RG-58C/U coaxial cable; the cable's published attenuation at 150 MHz is 0.20 dB per metre. At the radio end the technician measures 25 W forward and 0.2 W reflected, and computes a healthy 1.2:1. Borrowing a second meter, he climbs the mast and measures 3.5:1 at the antenna's own connector. The owner complains that working range has dropped from about 20 nm to about 8 nm. The top connector shows green corrosion; all connectors at the radio end are clean and tight, and the antenna is a half-wave vertical with radials.
1. Compute the one-way line loss, the power delivered to the antenna, and the effective radiated power. Is the cable the dominant problem?
Read the worked answer
Line loss: 35 m × 0.20 dB/m = 7.0 dB one way (about a factor of 5 in power).
Power at the antenna: 25 W × 10^(−7.0/10) = 25 × 0.20 = 5.0 W.
Mismatch loss at the antenna for 3.5:1: |Γ| = (3.5−1)/(3.5+1) = 0.556, so mismatch efficiency = 1 − Γ² = 1 − 0.309 = 0.691. Radiated ≈ 5.0 × 0.691 = 3.5 W, and with 3 dBd gain the ERP is only about 7 W - versus roughly 25 W at the antenna (≈50 W ERP) with a proper feed.
Verdict: yes, the cable dominates. A receive-side penalty of 7 dB also explains the apparent loss of range on both transmit and receive, and 7 dB on both paths is roughly a 14 dB round-trip shortfall. Replacing RG-58 with a low-loss cable of about 0.07 dB/m at 150 MHz cuts the loss to ~2.5 dB; the corrosion still has to be fixed, but it is the second-order problem.
2. Explain quantitatively why the meter at the radio reads 1.2:1 while the meter at the antenna reads 3.5:1.
Read the worked answer
The reflected wave is attenuated on the way down as well as up, so the transmitter only ever sees a fraction of the true reflection.
- At the antenna, 3.5:1 → |Γ| = 0.556.
- Round-trip loss = 2 × 7.0 dB = 14 dB, a voltage factor of 10^(−14/20) = 0.20.
- Apparent |Γ| at the radio = 0.556 × 0.20 = 0.111.
- Apparent SWR = (1 + 0.111)/(1 − 0.111) = 1.25:1, which matches the measured 1.2:1 within meter accuracy.
Judgement: a good-looking SWR at the transmitter is not evidence of a good antenna system on a lossy feed line. Lossy line masks mismatch - 6 dB of loss makes any antenna look better than 2:1. The honest measurements are the one at the antenna, a time-domain reflectometer sweep, or a return-loss measurement made with the line loss properly accounted for. The corollary is the trap: never "tune" the antenna to make the shack meter read flat.
3. The owner proposes adding an antenna tuner at the radio to bring the 1.2:1 to 1.0:1. Evaluate this and list what you would actually do.
Read the worked answer
Reject it. An ATU at the radio can only transform the impedance it sees at that point, which is already 1.2:1 because the line hides the mismatch. It cannot recover the 7 dB of dissipative loss, cannot improve the radiation efficiency of the corroded joint, and it wastes nothing because there is nothing to match. It would also add insertion loss and give a false sense of improvement.
Do instead:
- Replace the RG-58 run with LMR-400/RG-213 class cable (≈2.5 dB at 35 m, 150 MHz) and use proper waterproofing (self-amalgamating tape) at the masthead.
- Cut back and re-terminate the corroded top connector, then re-measure at the antenna.
- Verify the antenna itself: radials/ground plane, coax choke at the feedpoint, physical damage, and whether the "3 dBd" figure is honest.
- Only after the line is good does the antenna's own 3.5:1 need correcting - check the feedpoint connector, the radiating element length, and nearby metal.
- Re-measure forward and reflected power at both ends and confirm the improvement on air with a real distant station.