3:1 SWR on 2182 kHz After an Antenna Insulator Repair
After a rigger replaced a cracked ceramic insulator at the base of a ship's 10 m MF/HF whip and re-taped the connector where the coaxial feedline joins the antenna, the ship's 150 W MF/HF transmitter no longer matches its antenna. The installation uses a base-mounted automatic antenna coupler. With an inline meter between the transmitter and the coupler, a technician reads 150 W forward power and an SWR of 3.0:1 on 2182 kHz; the same meter read 1.3:1 at that point before the insulator work. The coupler hunts briefly and then stops tuning without reaching a match. The feedline is 25 m of RG-8X with a matched loss of about 1 dB at 2 MHz. The vessel is scheduled to sail at 0600 and the master wants to know whether he can keep the 2182 kHz watch and make his scheduled radiotelephone calls, or whether he must stay alongside.
1. Calculate the reflection coefficient, the reflected power and the mismatch loss at 3.0:1, and explain whether 37.5 W is simply thrown away.
Read the worked answer
For SWR = 3.0:
ρ = (SWR − 1)/(SWR + 1) = 2/4 = 0.5
Reflected power = ρ² × forward = 0.25 × 150 W = 37.5 W
Power delivered to the load = 150 − 37.5 = 112.5 W
Mismatch loss = 10 log₁₀(1/(1 − ρ²)) = 10 log₁₀(1/0.75) = 1.25 dB, and 10 log₁₀(150/112.5) = 1.25 dB, so the two routes agree.
The 37.5 W is not simply radiated as heat or lost. It is reflected at the coupler, travels back toward the transmitter, is re-reflected by the transmitter's output network or the coupler, and most of it reaches the load on the next pass. The genuine penalties are:
- the 1.25 dB mismatch loss - about 19% of the forward power never reaches the antenna;
- extra dissipation in the feedline, small here because the matched line loss is only about 1 dB;
- the risk that the transmitter's protection folds back the output. If it folds back to 90 W forward, the delivered power is about 68 W, a 3.4 dB loss against 150 W - which on 2182 kHz materially shortens the communications range.
So the correct answer is 'about 1.25 dB, plus fold-back risk', not '37.5 W lost'.
2. What is the diagnostic plan, and what should you tell the master about sailing?
Read the worked answer
The fault appeared immediately after work on the antenna, so the new work is the prime suspect, but prove it in order from cheapest to hardest:
- Inspect the repaired point. A partly pulled braid, a whisker of shield touching the centre conductor, tape trapping moisture, or an insulator not fully seated so water reaches the base - any of these raises the feedpoint impedance.
- Isolate transmitter from antenna. Put a 50 Ω dummy load on the transmitter output. If the SWR there is flat, the transmitter is healthy and the fault is in the connector, feedline, coupler or whip.
- Sweep the band with a portable analyser rather than measuring one frequency. A fault that appears at 2182 kHz but not elsewhere points to a resonance or ground problem rather than a dead short.
- Check the ground strap and bonding to the hull or ground plate. A corroded strap raises the feedpoint impedance exactly as a bad connector does, and it is the classic cause of 'it tuned before the rigger came'.
- Interrogate the coupler. If it hunts and stops with 3:1, either the antenna impedance has moved outside its tuning range or its control circuit has failed.
Advice to the master: an installation that cannot achieve a match on 2182 kHz has not been shown to meet its technical performance, and sailing with it means the MF distress and calling capability is degraded at exactly the moment it might be needed. At minimum, complete the dummy-load test and a working-frequency check with another station before departure. Note that the 2182 kHz watch is provided by a separate receiver and is not affected by the transmitter mismatch - receive capability and transmit capability are separate questions.
3. If you make test transmissions to prove the repair, what rules apply, and what must be done if the fault cannot be cleared before sailing?
Read the worked answer
Test transmissions: every one must be brief, must identify the station by call sign, and must not be made on 2182 kHz or on channel 16. Use a working frequency or a designated test frequency, keep the transmission to the minimum time needed to take the measurement, and log the test - frequency, power, time and result. Radiating a carrier on the distress frequency to 'see whether it works' is a rule violation and will be heard by every ship within range, quite apart from the risk of masking a real distress call.
If the fault cannot be cleared: the equipment is taken out of service rather than operated at reduced performance. Then:
- Record the failure, the measurements and the remedial action in the radio log.
- Inform the master in the form your procedures require, so the decision to sail or stay is his with the facts in front of him.
- Inform the nearest coast station or the rescue coordination centre if the failure degrades the ship's GMDSS capability, because they may be asked to provide an alternative means of distress alerting for the passage.
- Keep the work order, the analyser sweeps and the service-monitor readings with the log; they are what a surveyor or inspector will ask for.