Signal Lab · Feed System Physics

Where your tuner lives is half a decision. The line is the other half.

Ask ten operators whether the ATU belongs at the antenna or in the shack and you will start a fight that has run for decades. Here is the secret: both camps are right — each for their own station — because tuner location and feedline quality are one decision, not two. There are three architectures the published physics fully supports, one combination it does not, and a free calculator that settles your specific case in thirty seconds. Let us do this with respect for everybody’s shack.

What a tuner actually does (and does not do)

First, the part nobody disputes. An antenna tuner — an ATU, automatic or manual — does not tune your antenna. The antenna’s feedpoint impedance on a given band is set by its geometry, not by the box. What the tuner does is transform impedance so your transmitter sees something close to 50 ohms and delivers full power happily. That is a real and necessary job on any multiband, non-resonant antenna — including ours. The interesting question is what happens on the line between the matching point and the antenna.

The physics that ends the argument

SWR — standing wave ratio — lives on whatever section of line runs between the tuner and the antenna, and that section pays additional loss above the line’s matched loss. Here is the part most arguments miss: the additional loss scales with the line’s matched loss. The ARRL Antenna Book’s curves show it plainly. High SWR on lossy budget coax is expensive. The same SWR on very low-loss line — LMR-400, LMR-600, 7/8-inch hardline, or ladder line — costs remarkably little, because there is almost nothing to multiply.

Walter Maxwell, W2DU, wrote the canonical treatment in Reflections, and his central point is the one the myth gets backwards: reflected power is not power lost. The reflected wave is re-reflected at the matching point and delivered; what the system actually pays is the line’s attenuation, round trip. Keep that attenuation low — by matching at the feedpoint, or by running line good enough not to care — and the tuner can live wherever your station wants it.

Three architectures the physics respects

One: remote ATU at the feedpoint. Match at the antenna and the mismatched section shrinks to inches; your entire coax run operates near 1:1, where its loss is just the number on the spec sheet. Ordinary coax becomes good enough, the system is set-and-forget, and the match travels with the antenna. This is the configuration our bundles ship, with the LDG RT-100 powered over the coax and a common-mode choke keeping the line clean.

Two: shack ATU over low-loss coax. Roughly half the operators we ship to run a desktop tuner — and with the right feeder, that is not a compromise, it is a choice the math supports. Feed the antenna with LMR-400, LMR-600, or 7/8-inch hardline, and the SWR your tuner sees from the shack rides on a line so quiet that the additional loss stays small across the bands. You keep full match control at the desk, your tuner’s memories and integration stay in the station where you want them, and the line quality buys back what the geometry would otherwise spend. This is exactly why our feedline teaching and kits run the full spectrum up through hardline.

Three: shack ATU over ladder line. The classic. An RF Choke at the antenna, open wire to the shacks desktop tuner inside. Open-wire line is the lowest-loss feedline in HF service and shrugs at SWR that would cook budget coax. ARRL Handbooks going back nearly a century have shown this iconic setup. NO4ON runs his 39-foot Greyline exactly this way into a 4O3A Tuner Genius XL — 1.1:1 across the bands and a 20-contact international streak in one sitting.

The one wrong answer is the combination the bands are full of: long, lossy budget coax, high SWR, and nothing at the antenna. The radio reads 1:1 because the shack tuner made the radio happy; the line quietly converts a slice of every transmission into heat. The meter is telling the truth about the radio and nothing at all about the line.

Do not argue. Calculate.

Your run length, your line, your band: the free Feedline Loss Calculator covers RG-58 through LMR-600, hardline, and 450-ohm ladder line, and answers in dB and watts. Run your station both ways — tuner at the base versus your line at its real SWR — and the decision usually makes itself in thirty seconds. The deeper teaching behind the numbers lives in Feedline Physics.

Match at the feedpoint, run line good enough not to care, or run the line that never cared in the first place. Three right answers, one free calculator, and a DX window that does not ask which camp you came from.