VHF Net Degradation at a Forward Post: Reading Power, VSWR and Feedline Loss
The VHF net at a forward border post works on 150 MHz. Each 5 W handheld feeds a mast-mounted 3 dBi collinear through 18 m of RG-58 coaxial cable. Over three weeks operators report that transmissions reach battalion headquarters only up to about 6 km, against 15 km earlier, and that the set grows warm during long transmissions. A technician runs two measurements. On a dummy load at the radio's antenna socket the set delivers 5.0 W forward with VSWR 1.1, so the transmitter itself is healthy. At the mast base, with the antenna connected, the same set shows 3.2 W forward and 0.7 W reflected. RG-58 is rated at about 8.5 dB loss per 100 m at 150 MHz, and the connectors plus one lightning arrestor add roughly 0.5 dB. The post faces a 12-hour listening watch beginning in two days, during which it must be able to break silence. No spare antenna is held locally. A spare 5 W handheld with a rubber-duck antenna of minus 2 dBi is available, and a hilltop relay site 4 km away can be manned by a two-man detachment.
1. From the two sets of power readings, work out the VSWR, the percentage of power reflected, the mismatch loss and the feedline loss. On that evidence, state where the fault lies and whether the set should be run at full power.
Read the worked answer
Reflected fraction = 0.7 / 3.2 = 0.219, i.e. about 22 per cent of forward power comes back.
VSWR = (1 + sqrt 0.219) / (1 - sqrt 0.219) = 1.468 / 0.532 = 2.76, say 2.8.
Mismatch loss = -10 log10 (1 - 0.219) = 1.07 dB.
Feedline loss = 10 log10 (5.0 / 3.2) = 1.94 dB. Expected cable loss = 18 x 0.085 = 1.53 dB, plus about 0.4 dB for connectors and the arrestor = 1.93 dB. The measured figure matches the datasheet almost exactly, so the coax, joints and arrestor are behaving normally.
Diagnosis: the transmitter is proved good by the dummy-load test, and the feedline is proved good by the loss calculation. The remaining element is the antenna itself - typically water ingress at the masthead connector, a corroded joint, or a partially open radiating element. The total loss from radio to radiated signal is about 1.94 + 1.07 = 3.0 dB, which is exactly the order of loss that halves a working range from 15 km to 6 km.
Judgement: do not keep transmitting at full power into VSWR 2.8. Protection circuits fold the output back, the PA heats up, and the coverage obtained is wasted. Drop to low power, keep transmissions short, and treat the antenna as unserviceable until the masthead is opened and dried.
2. Quantify the effective radiated power now, and the effective radiated power if the RG-58 is replaced by LMR-400 (about 3.5 dB loss per 100 m at 150 MHz) and the antenna is repaired to VSWR 1.5. Express the improvement in dB and say whether the expenditure is justified.
Read the worked answer
Convert to a common unit: 5 W = 10 log10 5 + 30 = 36.99 dBm.
Present state: ERP = 36.99 - 1.94 (feedline) - 1.07 (mismatch) + 3 (antenna gain) = 36.98 dBm = about 5.0 W ERP.
After the change: cable loss = 18 x 0.035 = 0.63 dB, plus 0.5 dB connectors = 1.13 dB. At VSWR 1.5 the reflection coefficient is 0.2, so 4 per cent is reflected and mismatch loss = -10 log10 0.96 = 0.18 dB. ERP = 36.99 - 1.13 - 0.18 + 3 = 38.68 dBm = about 7.4 W ERP.
Improvement = 38.68 - 36.98 = 1.7 dB, which in field terms is roughly a 20 per cent increase in range on a clear path, and considerably more in marginal, foliage-covered terrain where every decibel counts.
The gain is real but modest. The dominant fault is the antenna mismatch (1.07 dB, avoidable entirely by repair), not the cable (1.94 dB, of which only 1.3 dB is saved by the upgrade). So the correct order of work is: repair and reseal the antenna first, replace the cable only if the post's range requirement still fails the link test afterwards. Spending on LMR-400 while leaving a waterlogged antenna in place would recover under half a decibel.
3. With the listening watch two days away and no spare antenna locally, what do you do? Give your priority of actions and the reasoning behind each.
Read the worked answer
1. Protect the equipment immediately. Take the unserviceable antenna out of circuit and run the set on low power into a known-good dummy load for any test transmission. Continuing into VSWR 2.8 risks the PA, and a burnt-out set would leave the post with nothing.
2. Restore a working net for the watch by the cheapest reliable means. The relay site 4 km away is the best option: it is close enough for a 5 W handheld with a minus 2 dBi rubber duck (ERP about 3.2 W, with no feedline and no masthead connector to fail) and it can be manned by the two-man detachment. Put the post on the relay channel for the 12 hours and keep the mast antenna isolated.
3. Fix the root cause in parallel. Send the masthead connector and antenna down for inspection and drying, reseal with self-amalgamating tape, and re-measure forward and reflected power before reinstalling. Raise an indent for a spare antenna and a jumper so the post is never again single-point.
4. Manage the risk formally. Any mast climb needs a second man, a safety belt and the officer's clearance; record the readings, the temporary relay arrangement and the indent in the station log, and inform battalion so that the reduced coverage is known before the watch begins rather than discovered during it.