Feedline Physics | Signal Lab | Greyline Performance
The Wire Between You
and the World
Your antenna is half the system. The other half is the wire that carries its signal to the shack. Most operators give feedline about five minutes of thought — and then wonder why their system underperforms. This is the piece Bob Zavrel's textbook spends chapters on. We'll do it justice here, without the algebra. The goal: you understand exactly what to buy, why, and how to run it.
01 — The ProblemEvery Foot Costs You Something
Feedline loss is real, continuous, and additive. A coaxial cable doesn't just pass signal — it absorbs a fraction of it on the way through, converting RF power to heat in the dielectric (the insulating layer between center conductor and shield). That heat isn't going to your antenna. It isn't going to Japan. It's gone.
The number that matters is dB loss per 100 feet at your operating frequency. Small cables have high loss per foot. Large cables have low loss per foot. The frequency matters too: loss rises as frequency increases. A run that's negligible on 40M becomes noticeable on 10M.
Three dB is the number operators quote glibly. In real terms: running 100W through a bad feedline is the same as running 50W through a good one. You could double your amplifier budget and recover the same ground that better coax would give you for $80.
02 — Your Three OptionsCoax, Twin-Lead, or Ladder Line
There are exactly three feedline families an HF operator should consider. Each has a physics reason for existing. Here they are, honestly scored.
450Ω Ladder Line
Open-wire or window-line. Signal bounces between two parallel conductors in open air. Air is the best possible dielectric — its loss is essentially zero. Lowest loss of any practical feedline, by a wide margin.
Two-Wire HV Silicone
Two stranded conductors in flexible silicone jacketing, run parallel and spaced with non-conductive spacers every 6 inches. Essentially a DIY ladder line with a weatherproof jacket. Lowest-cost, very low loss. Requires ATU and careful routing.
50Ω Coaxial Cable
The universal standard. Shielded, impedance-controlled, weather-tolerant, buriable. Loss is higher than open-wire but manageable when you choose the right diameter. The single variable that matters most: go bigger.
03 — The Coax DecisionGo Big. No Exceptions.
If you choose coax — which most Greyline operators do, because you can bury it and forget it — the single most important choice is diameter. Larger diameter = lower loss. The cost difference between RG-8X and LMR-400 on a 100-foot run is roughly $60. The dB difference is 2 dB or more on 10 meters. Two dB is approximately 60% more signal delivered to your antenna.
This is not a close call.
| Cable | OD | Loss @ 10M (100 ft) | Loss @ 20M (100 ft) | Loss @ 40M (100 ft) | Buriable | Verdict |
|---|---|---|---|---|---|---|
| RG-58 | 0.19" | 4.9 dB | 3.6 dB | 2.3 dB | No | Avoid |
| RG-8X | 0.24" | 3.5 dB | 2.5 dB | 1.6 dB | Marginal | Short runs only |
| RG-8U | 0.405" | 2.5 dB | 1.8 dB | 1.1 dB | Limited | Dated |
| LMR-400 | 0.405" | 1.5 dB | 1.1 dB | 0.7 dB | Yes | Minimum standard |
| LMR-600 | 0.59" | 0.9 dB | 0.7 dB | 0.4 dB | Yes | Recommended |
| 7/8" Hardline | 0.875" | 0.4 dB | 0.3 dB | 0.2 dB | Yes | Contest/agency grade |
LMR-400 is the floor. LMR-600 is worth the upgrade for any run over 75 feet. If you're building a permanent installation — buried conduit, professional connectors — hardline or Heliax makes the feedline effectively invisible in the system budget. That's the goal.
04 — The ATU Question50% of Operators Are Right
Roughly half of Greyline buyers run a shack-side desktop ATU. This is a completely valid configuration — and in some ways superior to remote ATU setups, particularly for operators who work multiple antennas, run high power, or prefer to keep electronics out of the weather.
The shack-side ATU does introduce one variable: if the feedline sees significant SWR, loss increases. This is the argument for remote ATUs — they present a matched load at the antenna base, minimizing SWR on the cable run. But the math on this is subtler than it seems:
The operator who uses a shack-side desktop ATU and LMR-600 is in excellent shape — better shape than the operator using a remote ATU with RG-8X, in most band/power combinations. The wire wins.
05 — Noise Is the Other SideWhat Comes Into the Shack Matters Too
Feedline is bidirectional. The same physics that sets your loss on transmit sets your noise figure on receive. But coax doesn't just pass signal — it acts as a receive antenna for every noise source it runs past. Power supplies, switching regulators, LED drivers, solar inverters — all of it couples onto the outer jacket of your coax and rides it back to your receiver.
This is why RF chokes matter, and why feedline routing matters. The VDA's no-radial architecture already removes the primary noise-coupling path that afflicts conventional verticals (a 200-foot radial field pointed directly at every switching supply in your house). The feedline routing is the remaining variable.
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Noise audit first. Before you trench, map noise. Walk the property with a battery-powered AM radio, tuned off-station. You're listening for the buzz of switching supplies. The quiet corner of the yard is where the antenna goes.
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Route away from AC wiring. Run feedline perpendicular to power wiring where paths must cross. Never parallel to house wiring for any meaningful run length.
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Choke at the antenna base. The Greyline 5-bead choke at the feedpoint isolates the antenna from common-mode current on the coax outer shield. Non-negotiable. This is in every Greyline kit for a reason.
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Second choke at shack entry. For operators in RF-noisy environments, a Maxi Line Isolator at the shack entry provides additional rejection — -38 dB CMR across 1–61 MHz. Belt-and-suspenders for serious stations.
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Bury if you can. Earth provides additional shielding. A few inches of burial turns the outer shield's noise exposure from continuous to negligible. LMR-400 and LMR-600 are rated for direct burial.
06 — What Should I Buy?Decision Tree
07 — The Short VersionWhat Jon Runs
The practical summary: LMR-400 is the minimum for any permanent installation. LMR-600 is the right answer for most operators. If your run is long or your power is high, hardline is the professional choice. Don't bury RG-8X and wonder why the bands feel dead.
Questions on feedline for your specific QTH? Call the shop at 435-200-4902. We'll walk through it with you.