Also known as: feedline, antenna feedline, coax run, low-loss coax
Reader-supported. GopherTrunk is free, open-source software. Some
hardware links on this page go to Amazon with our Associates tag
(gophertrunk-20); if you buy through them we may earn a small
commission at no extra cost to you. It never changes which gear we recommend, and
the software stays free either way. How GopherTrunk is funded →
A coax feedline is the outdoor cable run that carries signal from a rooftop or mast-mounted antenna down to your receiver. Unlike a bench pigtail, a feedline covers real distance, so its one job is to deliver that distance with as little loss as possible — which means thick, low-loss coax (RG8X, LMR-240, or LMR-400) with proper connectors, not the thin RG58 or RG316 that bleeds signal away over a long run. This page is about buying and specifying a feedline; for the physics of why cable loss climbs with frequency, see the coaxial cable reference.
Choosing the cable
The trade is loss versus flexibility, and both are set by the cable’s thickness and construction:
- RG8X / LMR-240 — a practical middle ground: noticeably lower loss than RG58, still flexible enough to route easily. Good for runs up to roughly 50 ft at VHF/UHF.
- LMR-400 / RG213 — thick, stiffer, and lowest-loss; the choice for long masthead runs or the 700/800 MHz P25 bands where every dB counts.
- RG58 / RG316 — thin patch-lead cable. Fine for a short jumper, wrong for an outdoor feedline: it throws away too much signal over distance, especially at UHF.
Because attenuation grows with frequency, match the cable to both the distance and the band. A 25 ft RG8X run that is unremarkable at VHF becomes worth upgrading to LMR-400 at 800 MHz. And buy it made to length with connectors already fitted unless you enjoy soldering PL-259s — a factory-terminated jumper is weatherproof and correctly matched out of the bag.
Connectors and weatherproofing
For the UHF land-mobile trunking bands, order the feedline with N connectors rather than PL-259: N is weatherproof and holds a clean 50 Ω match, while the non-constant-impedance UHF connector adds reflection and loss above a few hundred MHz. For HF/VHF, PL-259/SO-239 is fine and common. Either way, the last short hop into the dongle steps down to SMA with a pigtail or adapter.
Whatever connector you use, weatherproof every outdoor joint with self-amalgamating tape or a proper boot. Water in coax raises loss permanently and corrodes the connectors — the slow death of many rooftop antennas.
Relevance to GopherTrunk
GopherTrunk decodes whatever the SDR captures and has no view of the cable, so the feedline is precisely where a good rooftop antenna’s signal is most often lost before digitisation. A long or low-grade run quietly undoes the antenna: 6 dB of feedline loss discards three-quarters of the signal power. Two levers fix it — keep the run short and thick, or mount a low-noise amplifier at the antenna (fed via a bias tee) so its gain lands before the cable loss. Better still, where the antenna can be near a PC, skip the long RF run entirely: put the SDR at the window and send samples back over a USB extension, since USB length costs nothing while coax length costs signal. None of this changes what is decodable — encrypted traffic stays encrypted — but it sets the signal-to-noise every clear decode depends on.
Where to buy
For a real rooftop or mast run, buy a made-to-length low-loss feedline with connectors fitted — a factory-terminated LMR-240/RG8X jumper (around $30 for 25 ft) covers a typical run, stepping up to LMR-400 for long runs or the 700/800 MHz bands. Order the length you actually need and, for UHF, choose the N-connector variant over PL-259.
For the loss physics behind the cable choice, see coaxial cable; for the whole RF chain, the SDR cables and connectors guide.
As an Amazon Associate, GopherTrunk earns from qualifying purchases — at no extra cost to you. It never changes what we recommend.
Sources
Frequently asked questions
Which coax feedline should I buy for a rooftop SDR antenna?
Buy a made-to-length low-loss run — RG8X or LMR-240 for moderate distances, LMR-400 for long runs or the 700/800 MHz bands — with connectors already fitted. A factory-terminated 25 ft LMR-240/RG8X jumper (around $30) covers a typical run and spares you soldering PL-259s. For the UHF trunking bands, order it with N connectors rather than PL-259, which gets lossy above ~300 MHz. Do not use thin RG58/RG316 for an outdoor feedline.
How much cable loss is too much?
Six dB of feedline loss throws away three-quarters of your signal power before it reaches the SDR, and loss climbs steeply with frequency — a cable that is fine at VHF can be a poor choice at 800 MHz. Keep the run as short as you can, use thick low-loss cable, and if the run must be long, put a mast-mounted LNA at the antenna so its gain is applied before the loss.
RG8X, LMR-240, or LMR-400 for a feedline?
RG8X and LMR-240 are a good middle ground — noticeably lower loss than RG58, still flexible enough to route easily, fine for runs up to roughly 50 ft at VHF/UHF. LMR-400 is thicker, stiffer, and lowest-loss, the choice for long masthead runs or the 700/800 MHz P25 bands where every dB counts. Match the cable to the distance and frequency rather than overbuying stiff cable for a short bench run.
Should my feedline use PL-259 or N connectors?
For HF/VHF, PL-259/SO-239 is fine and common. For the UHF land-mobile trunking bands, prefer N: it is weatherproof and holds a clean 50 Ω match where the non-constant-impedance UHF connector adds reflection and loss above a few hundred MHz. Whichever you choose, weatherproof the outdoor joint — water in coax raises loss permanently.
Feedline or USB extension — which is better?
If you can, avoid a long RF run altogether: put the SDR at the window on a short pigtail and send the digital samples back over an active USB extension, since USB length costs almost nothing while coax length costs signal. Use a real low-loss feedline when the antenna genuinely must be far from any PC — a rooftop or mast install — and then keep it short, thick, and sealed.