The companion to feedline, SWR and tuners: the physical side — what to buy, how to terminate it, and what to do about the current flowing on the outside of the braid.
Choosing cable
| Cable | Loss at 14 MHz | Loss at 440 MHz | Use |
|---|---|---|---|
| RG-58 | ~1.0 dB/100 ft | ~6 dB/100 ft | Short jumpers only |
| RG-8X | ~0.8 dB/100 ft | ~4.7 dB/100 ft | Portable, short HF runs |
| RG-213 | ~0.6 dB/100 ft | ~3 dB/100 ft | Permanent HF runs |
| LMR-400 | ~0.4 dB/100 ft | ~2.2 dB/100 ft | Long runs, VHF/UHF |
| LMR-600 / hardline | ~0.25 dB/100 ft | ~1.3 dB/100 ft | Towers, repeater sites |
Figures are approximate and worsen with SWR, age and water ingress. Buy from a known supplier — copper-clad-aluminium cable sold as RG-213, and cable with a partial braid, are both common.
Connectors
- PL-259 / SO-239 (UHF) — the HF standard. Not constant-impedance, and fine to about 150 MHz.
- N — weatherproof and constant-impedance; the right choice above VHF and for outdoor joints.
- BNC — quick-connect, small, common on test gear and portable rigs.
- SMA — handhelds and SDR dongles. Note reverse-polarity SMA on some radios, and that the
- Keep a small box of adapters, but minimise them in a permanent run; every joint is a potential
connector on a handheld is easily destroyed by a heavy antenna.
water path.
Installing a PL-259
Solder-type on RG-213 with a reducer for RG-8X: strip carefully so no braid strands short to the centre, tin the centre pin, and solder the braid through the shell's holes with enough heat to flow properly but not so much that the dielectric melts and shifts. Crimp connectors are perfectly good and faster if you own the right die. Test every connector with a meter before it goes up the tower.
Weatherproofing
Water in coax is invisible, permanent and the most common reason a station "used to work". Over every outdoor joint: electrical tape, then self-amalgamating tape stretched over the whole connector, then more electrical tape as a UV cover. Butyl mastic is better still. Drip loops below every connector, and never leave a coax end pointing upward.
Common-mode current and chokes
Coax is meant to carry current on the inside of the braid only. Any imbalance at the feedpoint puts current on the outside, which turns the feedline into part of the antenna. Symptoms: a hot microphone, erratic SWR that changes when you touch the radio, computer resets on transmit, and a noisy receive.
Cures:
- A 1:1 current balun at the feedpoint of any dipole, OCF or beam.
- A choke of type 31 ferrites on the coax at the antenna, and often a second at the shack
- Good grounding and bonding — related, but not a
entrance. Ten to fifteen mix-31 clamp-on beads, or a coil of coax through a large type-31 toroid.
substitute. Chokes fix common-mode current; grounds do not.
Entering the building
Bring feedlines to a single grounded entry panel with arrestors, bonded to the house electrical ground. This is a lightning and safety requirement as much as a performance one — see grounding and lightning.