Choose a Feed-Line for a 2 m Station

On HF, almost any reasonable coax works for a short run. On VHF and UHF, cable loss rises steeply, and the feed-line can throw away more than half the transmitter power and the same share of every received signal. This guide compares four common cables for a typical home 2 m station: a vertical on the roof, 75 ft of cable to the shack, a 50 W transceiver, and an SWR of 1.5:1 at the antenna.

The comparison

The coax loss calculator gives the matched loss for 75 ft at 146 MHz and the total including the 1.5:1 SWR. RG-58 loses 4.20 dB matched and 4.35 dB in total, so of 50 W only 18.4 W reaches the antenna. RG-8X loses 3.38 dB matched and 3.52 dB in total, delivering 22.3 W. RG-213 loses 1.80 dB matched and 1.90 dB in total, delivering 32.3 W. LMR-400 loses 1.13 dB matched and 1.20 dB in total, delivering 38.0 W. The difference between the cheapest and the best cable is more than a factor of two, about 3.2 dB: half an S unit at the other station.

The SWR is the smaller problem

Notice how little the 1.5:1 SWR adds: 0.07 to 0.15 dB depending on the cable. Spending an afternoon improving the antenna match from 1.5:1 to 1.1:1 would gain less than a tenth of a decibel, while changing from RG-58 to LMR-400 gains over 3 dB. On VHF, the cable is the first place to look, then connectors, then the match. A common misconception is that a low SWR reading at the radio means the system is efficient; with 4 dB of cable loss, a 1.5:1 SWR at the antenna reads about 1.2:1 at the radio, which looks excellent and is not.

Receive matters as much as transmit

Cable loss works in both directions. A signal arriving at the antenna at −110 dBm reaches the receiver at −114.2 dBm through the RG-58 and at −111.1 dBm through the LMR-400. For weak-signal work near the noise floor, those 3 dB can decide whether a contact is possible at all. A preamplifier at the antenna end recovers some of the receive loss on a long run, but it does nothing for transmit, and good cable is simpler and more reliable.

Practical trade-offs

LMR-400 is thicker, stiffer and more expensive than RG-8X, and needs proper connectors; RG-213 is heavier still. For a 10 ft jumper to a mobile antenna, RG-58 loses only about 0.6 dB on 2 m and its flexibility is worth more than the loss. For a 75 ft run to a roof, the low-loss cable pays for itself on the first weak contact. At 440 MHz on the 70 cm band, the differences grow: 75 ft of RG-58 loses 7.5 dB, more than four-fifths of the power, while LMR-400 loses 2.0 dB. Seal outdoor connectors against water; wet cable can double its loss.

Common mistakes

• Using HF habits on VHF. Cable that is fine on 20 m can lose half the power on 2 m.
• Trusting the shack SWR. Cable loss masks the mismatch at the antenna.
• Ignoring connectors. A poorly fitted PL-259 or an adapter chain adds loss and intermittent faults.
• Forgetting receive. Every decibel lost on transmit is also lost on every received signal.
• Letting water in. Seal outdoor joints and replace cable that has been wet.

Feed-line checklist

1. Measure the cable run from the radio to the antenna, including any slack.
2. Compare cables at your highest operating frequency, not the lowest.
3. Check the total loss with your expected SWR and the power reaching the antenna.
4. Choose the thinnest cable whose loss you can accept for that run.
5. Fit connectors properly and weatherproof every outdoor joint.