Also known as: coax, coaxial cable, feedline
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Coaxial cable (“coax”) carries RF between the antenna and receiver. A centre conductor runs inside a tubular shield, separated by a dielectric, which keeps the signal contained and the impedance constant (commonly 50 Ω).1 Every metre and every connector adds loss — and, crucially, that loss grows with frequency, so the same cable that is fine at 30 MHz can be a poor choice at 1 GHz.
How it works
The defining property of coax is that the signal travels as a field between the inner conductor and the surrounding shield, fully enclosed. Two geometric facts follow. First, the ratio of the shield’s inner diameter to the core’s diameter (and the dielectric constant between them) fixes the characteristic impedance — the impedance a matched line presents regardless of length.2 Radio gear standardises on 50 Ω for transmit-capable systems and 75 Ω for video/broadcast reception; mixing them causes a mismatch and reflections. Second, the shield keeps external interference out and the signal in, which is why coax outperforms open wire in noisy environments.
Loss comes from two mechanisms: conductor loss (resistance in the copper, which rises with the square root of frequency as current crowds into the skin) and dielectric loss (the insulator absorbing energy, rising roughly linearly with frequency). The combined effect is that cable attenuation, quoted in dB per 100 ft or per 100 m, climbs steeply with frequency. A thin RG-174 patch cable might lose a fraction of a dB at HF but several dB at 1 GHz over the same length. The cable’s velocity factor (typically 0.66–0.85) also means the signal travels slower than in free space, which matters when cutting phasing lines or stubs.
Variants
- RG-58 / RG-174 — thin, flexible, lossy; fine for short patch leads only.
- RG-6 — 75 Ω, low-loss, cheap (TV cable); excellent for receive-only VHF/UHF if you accept the impedance mismatch, which is minor for reception.
- LMR-400 / RG-213 — thick, low-loss 50 Ω runs for masthead installs; the go-to for any cable over a few metres at UHF.
- Hardline / semi-rigid — solid copper shield, lowest loss, used for long or high-frequency feeds.
- Cable terminates in connectors — SMA on most SDR dongles, the larger N-type on antennas and low-loss runs — and each junction adds its own small loss and potential mismatch.
In practice
A long or low-grade cable can quietly undo a good antenna: 6 dB of feedline loss throws away three-quarters of the signal power before it ever reaches the SDR. Operators counter this two ways — keep the feedline short and thick, or mount a low-noise amplifier at the antenna (fed via a bias tee) so the LNA’s gain is applied before the cable loss and suppresses it, per the Friis budget. A poor match at either end also raises SWR and, for receive, wastes signal; weatherproofing outdoor connectors matters because water in coax raises loss dramatically.
Relevance to SDR
Feedline choice is the most common overlooked variable in an SDR install: a hobbyist adds a better antenna but leaves 15 m of thin RG-58 in place and sees no improvement. GopherTrunk decodes whatever the SDR captures and has no view of the cable, but the coax is where a weak trunking signal is most often lost before digitisation. On a UHF or 800 MHz P25 site, using proper low-loss cable — or moving the SDR to the antenna and streaming IQ back over the network — is frequently the single biggest improvement to GopherTrunk’s decode reliability.
Where to buy
For the short SMA-terminated patch leads an SDR bench setup needs, a NooElec SMA coaxial cable connectivity kit (around $18) bundles common lengths and adapters in one box. For an actual feedline run of more than a few metres — especially at UHF or 800 MHz P25 — buy proper low-loss RG8X or LMR-400 by the length instead of relying on thin RG58.
For connectors, adapters, and full cable choices, see the SDR cables and connectors guide.
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Sources
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Coaxial cable — Wikipedia, on coax construction, characteristic impedance, and frequency-dependent loss. ↩
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Characteristic impedance — Wikipedia, on the length-independent impedance set by a line’s geometry and why matched lines avoid reflections. ↩
Frequently asked questions
What coax should I buy for an SDR?
For short bench and patch leads with the SMA connectors SDRs use, a NooElec SMA cable connectivity kit (around $18) covers the common lengths and adapters. For any run over a few metres, especially at UHF or 800 MHz, step up to low-loss RG8X or LMR-400 rather than thin RG58, which throws away too much signal over distance.
How much does cable loss matter for scanning?
A lot on the higher bands. Loss grows with frequency, so a long thin cable that is fine at HF can cost several dB at 1 GHz — and 6 dB of feedline loss throws away three-quarters of the signal before it reaches the SDR. Keep runs short and thick, or mount an LNA at the antenna so its gain is applied before the cable loss.
RG58 or RG8X for an SDR feedline?
RG58 (or thin RG174) is fine only for short patch leads. For a real feedline run use RG8X for moderate lengths or LMR-400/RG213 for masthead installs — the extra thickness cuts loss sharply at VHF/UHF. RG6 (75 Ω TV cable) is also excellent and cheap for receive-only use if you accept the minor impedance mismatch.
Do connectors and adapters add loss too?
Yes — each junction adds a small insertion loss and a potential mismatch, so use the fewest transitions you can. Weatherproof any outdoor connectors, since water in coax raises loss dramatically.