A field guide to coax
Guide · updated July 2026
The feedline between your radio and antenna isn’t just wire — it’s a tuned transmission line, and every foot of it costs you signal. Here’s what actually matters: sizes, loss, impedance, connectors, and sealing it against the weather.
What coax actually is
Coaxial cable is four layers sharing one axis: a center conductor, a dielectric (the insulating foam or solid plastic around it), a shield (braid and/or foil), and an outer jacket. The spacing and materials set two numbers that define the cable: its impedance and its loss.
The thing to internalize: coax is a transmission line, not a plain wire. It carries RF as a wave, it has a characteristic impedance the rest of your system has to match, and it turns some of your signal into heat on every foot. Radio settled on 50 Ω as the standard — a compromise between the lowest-loss geometry (~77 Ω) and the highest power-handling one (~30 Ω). TV and satellite use 75 Ω, tuned purely for lowest loss where power is tiny. Mixing the two is the classic beginner mistake.
Sizes & types
Fatter cable = lower loss, but stiffer, heavier and pricier. Pick the smallest that keeps loss acceptable for your run.
| Cable | Diameter | Z | Vel. factor | Where it fits |
|---|---|---|---|---|
| RG-174 | 0.10″ / 2.6 mm | 50 Ω | 0.66 | Pigtails & patch leads — thin and lossy; keep it short. |
| RG-58 | 0.20″ / 5 mm | 50 Ω | 0.66–0.79 | Cheap and everywhere; fine for short VHF/mobile jumpers. |
| RG-8X (“mini-8”) | 0.24″ / 6 mm | 50 Ω | ~0.82 | The flexible all-rounder — a big step up from RG-58. |
| RG-213 / RG-8 | 0.40″ / 10 mm | 50 Ω | 0.66 | Thick, rugged, low loss — the HF/VHF base-station staple. |
| LMR-400 | 0.40″ / 10 mm | 50 Ω | ~0.85 | Low-loss workhorse for long runs and UHF; foam core, spec-controlled. |
| ½″ hardline (LMR-600 / Heliax) | 0.5–0.6″ / 13–15 mm | 50 Ω | ~0.85+ | Lowest loss going — tower and long base runs; stiff and dear. |
| RG-6 / RG-11 | 0.27″ / 0.40″ | 75 Ω | ~0.83 | TV / satellite / cable. Not for a 50 Ω radio. |
Buy by spec, not by name. “RG” (Radio Guide) numbers are a loose old military convention — two spools both stamped “RG-58” can lose very different amounts. Look at the published loss, impedance and velocity factor. Foam dielectric beats solid on loss and has a higher velocity factor, but crushes and wicks water more easily — and for anything buried or outdoors, use a cable rated for it (UV-resistant jacket, flooded/direct-burial).
Loss: the number that matters
Loss is rated in dB per 100 ft and it climbs with frequency — gentle on HF, brutal at UHF and above. It scales with length (200 ft loses twice as much as 100 ft), and it costs you both ways: watts on transmit, and weak signals on receive.
| Cable | ~30 MHz HF/10m | 146 MHz 2m | 446 MHz 70cm | 915 MHz mesh |
|---|---|---|---|---|
| RG-174 | 5.5 | 10 | 17 | 28 |
| RG-58 | 2.5 | 5.8 | 11 | 20 |
| RG-8X | 1.8 | 4.3 | 7.9 | 12.6 |
| RG-213 / RG-8 | 1.1 | 2.6 | 4.8 | 7.7 |
| LMR-400 | 0.7 | 1.5 | 2.7 | 3.9 |
| ½″ hardline | 0.4 | 1.0 | 1.8 | 2.6 |
Nominal dB per 100 ft (≈ 30 m). Real cable varies with brand, dielectric and age — treat these as ballpark.
The 3 dB rule. Every 3 dB of loss halves your power; 6 dB loses three-quarters; 10 dB loses 90%. A 100-ft run of RG-58 at 70 cm (~11 dB) delivers under 10 watts of a 100-watt signal to the antenna — the rest is heating your coax. Swap to LMR-400 (~2.7 dB) and roughly 54 watts gets there. Same radio, same antenna, 5× the radiated power.
Two takeaways: keep runs short, and go bigger as frequency climbs. On HF, thin RG-58 barely stings; at 915 MHz mesh or 70 cm, cable choice can matter more than the radio. When loss is unavoidable, mount the low-loss cable — or a mast-head preamp/amplifier — as close to the antenna as you can.
Match & mismatch
Your radio, your coax and (ideally) your antenna all live in a 50 Ω world. When impedances match, power flows out to the antenna. When they don’t, some of it reflects back down the line — that reflection is what an SWR meter reads as SWR (Standing Wave Ratio), or equivalently as return loss.
- Grab the wrong coax and you build in a mismatch. Feed a 50 Ω radio through 75 Ω TV coax and you’ve created a 1.5 : 1 SWR before the antenna is even in the picture.
- SWR is about the match, not the cable length. A perfectly matched 50 Ω system reads 1 : 1 whether the coax is 3 ft or 300 ft. Length changes loss, not the match.
The sneaky part: loss flatters your SWR. A lossy feedline attenuates the reflected wave on its way back, so the SWR you read at the radio looks better than the SWR at the antenna. A rig showing a suspiciously perfect 1.1 : 1 through a long, lossy run may be hiding a poorly matched antenna — the coax is quietly eating both the forward and reflected power. Measure at the antenna with a NanoVNA when it matters.
Velocity factor
RF travels slower in coax than in free space — the ratio is the velocity factor (VF), typically 0.66 for solid-PE cable and ~0.80–0.85 for foam. It doesn’t affect loss, but it matters any time electrical length counts: cutting phasing lines, matching stubs, or 1/4-wave transformers. A ½-wavelength of coax is VF × shorter than a half-wave in air, so always multiply by the cable’s VF when you cut to length.
Connectors, HF to UHF
A connector is just another bit of transmission line — a bad or wrong one adds loss and mismatch. The usual suspects:
| Connector | Good to | Where you’ll see it |
|---|---|---|
| UHF (PL-259 / SO-239) | ~300 MHz | The big silver screw-on on nearly every HF/VHF radio and mobile antenna. Rugged, cheap, everywhere — but not a constant-impedance design (ironic name). |
| N | ~11 GHz | The outdoor/UHF pro pick — true 50 Ω, threaded, with a weather gasket. Best choice for anything on a mast. |
| BNC | ~4 GHz | Bayonet quick-connect on test gear, scanners and older handhelds. Fast on and off; fine through VHF/UHF. |
| SMA | ~18 GHz | The small screw on HTs, LoRa/mesh boards and GPS. Tiny; easy to cross-thread — finger-tight plus a nudge. |
| RP-SMA | ~18 GHz | Reverse-polarity SMA on WiFi gear and some LoRa boards. Looks identical to SMA and will not mate with it. |
| TNC | ~11 GHz | A threaded BNC — the screw-on version, better under vibration and weather. GPS and vehicular installs. |
| F | ~1 GHz | 75 Ω TV/satellite/cable. Not for 50 Ω radio work. |
| 7/16 DIN | ~7.5 GHz | Big, low-PIM, high-power — cellular and commercial towers. Overkill for the hobby, but you’ll spot them up high. |
Watch the RP-SMA trap. Standard SMA and reverse-polarity RP-SMA look the same at a glance but swap the male/female pin — a very common headache on LoRa and WiFi antennas. Check which your board and antenna use before you order, and remember there’s a separate SMA-vs-RP-SMA and male-vs-female question.
Solder or crimp? A properly crimped connector (with the right die) is fast, repeatable and excellent for the outer braid; a good solder joint is bomb-proof if you can heat the shield without melting the dielectric. Whichever you use, the enemy is a cold joint or a nicked braid — take your time, and tug-test every finished end.
Weatherproofing an outdoor joint
Water is the number-one killer of a coax run. It wicks into the cable, corrodes the shield and raises loss — and the symptom is a slow, mysterious decline (or an SWR that creeps up after rain). Seal every outdoor connector.
-
Leave a drip loop
Route the coax so it makes a downward U before reaching the connector. Water runs to the bottom of the loop and drips off instead of tracking into the joint.
-
Seat the connector first
Make sure it’s properly tightened (and soldered, if applicable) before you seal it — a bad joint wrapped in tape is worse than a visible one. Point connectors downward where you can.
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Layer 1 — a base wrap of vinyl tape
One layer of ordinary electrical tape over the connector body. It protects the metal and lets you cut the seal off cleanly later.
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Layer 2 — self-fusing rubber tape
Stretch self-amalgamating (self-fusing / self-vulcanizing) tape to about double its length and wrap from the bottom up with 50% overlap. It fuses into one solid, waterproof rubber mass. A butyl mastic like Coax-Seal molded over the joint does the same job.
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Layer 3 — UV-resistant outer wrap
Finish with a layer of UV-resistant vinyl tape (or the same rubber tape’s recommended cover). Bare self-fusing tape degrades in sunlight; the outer layer is its sunscreen.
Choose weather-friendly hardware. N connectors ship with a gasket and seal far better than a bare PL-259. Add a smear of dielectric grease on the threads, use heat-shrink boots on straight sections, and inspect yearly — if you ever see green corrosion on the braid, cut it back and re-terminate.
The 60-second version
50 Ω, always
Match the radio: 50 Ω coax and connectors. Leave the 75 Ω TV cable for the TV.
Size to the frequency
RG-58 is fine for short HF; step up to RG-8X, RG-213 or LMR-400 for long runs and anything at UHF or 915 MHz.
Keep it short
Every foot is loss. Mount the radio, or a preamp, near the antenna when the run has to be long.
Right connector for the band
PL-259 through VHF, N for outdoor/UHF, SMA on handhelds and boards — and mind RP-SMA.
Seal every outdoor joint
Drip loop, then base tape → self-fusing rubber → UV tape. Water is the silent killer.
Coax question, or a run that’s misbehaving? Bring it to #antennas in the RF Lab Discord — someone here has cut, crimped and cursed at the exact cable you’re holding.
Go deeper
Less heat in the coax, more in the air.
Cable picks, connector crimps and weatherproofing wars — #antennas is the place.