Icom AH-4
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| Frequency | 3.5-54 MHz |
| TX power | 120 W (120 W PEP maximum input. Powered by the transceiver over the control cable (13.8 V DC +/-15%, under 1 A); no separate supply.) |
| Modes | all |
| Weight | 1.2 kg (2.6 lb) |
| Connector | SO-239 RF input (from radio); wire + ground binding posts; 4-conductor control cable |
| Year | 1997 |
| rf input | SO-239 (UHF female; 5 m raw coax with two PL-259 plugs supplied) |
| antenna output | binding-post / screw terminal (single long-wire) |
| ground | binding-post / screw terminal (RF ground / counterpoise - mandatory) |
| control cable | 4-conductor (START, KEY, +12 V, GND); 5 m (16 ft) supplied; optional OPC-420 10 m shielded |
| power req | 13.8 V DC +/-15% from the transceiver via the control cable; under 1 A |
It matches at the antenna feedpoint, so the coax back to the radio runs at low SWR instead of carrying a mismatch. The matching range is wide - 10 to 5,000 ohms - so it handles almost any random-wire impedance, and a wire of 23 feet or longer gets you 80 through 6 meters. Setup is simple: hang a wire, mount the box, run coax and a control cable, and let the radio drive the tune. It needs no separate power supply, stores 45 frequencies for sub-second retuning, and during the search it keeps the transmitter on a safe 50-ohm load while only a fraction of a watt reaches the wire. The sealed case is built to live outdoors.
It works only with Icom transceivers that have the tuner-control jack; other makers' radios need workaround interfaces. The 120-watt ceiling rules out running an amplifier through it. A control cable has to run alongside the coax out to the box. The wire must be at least 23 feet for the lower bands, and a good ground or counterpoise is mandatory - performance falls off sharply without one. Wire lengths near a half-wavelength on the operating band cannot be matched. And it is discontinued, so new stock is scarce.
For an Icom owner who wants multi-band HF off a single wire, the AH-4 is hard to beat for simplicity: mount it at the feedpoint, string out 23 feet or more, and work 80 through 6 meters. Its replacement, the AH-730, adds 160 meters, more power, and a splash rating, but the AH-4 is plentiful secondhand and does the same core job well. The limits are real - Icom-only, 120 watts, and a mandatory ground - but within them it is a clean, reliable antenna system.
The AH-4 solves an old problem in a tidy way: instead of matching your antenna back at the operating desk, it does the matching out at the antenna itself. You mount the weatherproof box at the feedpoint, run a single wire of 23 feet or more from its output terminal, and bring coax and a thin control cable back to the radio. Press the tuner button and a few seconds later you are on the air, anywhere from 80 through 6 meters.
Tuning at the feedpoint matters because it keeps the coax between the radio and the antenna running at a low SWR, where it is most efficient. The AH-4 pulls a quiet trick during the search: it routes the transmitter through an internal power divider so the radio always looks into a near-perfect 50-ohm load while the relays hunt, and only about a third of a watt actually reaches the wire. The 22 relays never switch under load, which is easy on them, and the matching network is a reversible L that can throw its capacitor bank to either side of the coil to cover impedances from 10 to 5,000 ohms.
A couple of design choices age well. The coils are air-wound rather than wound on ferrite, so they waste less power, particularly up on the higher bands. And the microprocessor shuts itself off between tunes, so it adds no digital hash to the receiver while you listen - the box stays silent until the next time you ask it to work. It carries no power supply of its own; the radio feeds it 13.8 volts and well under an amp down the control cable.
The catch is that it speaks only Icom. The AH-4 relies on Icom's two-wire START/KEY tuner interface, so it pairs with radios that have that jack - the IC-706MKIIG, IC-718, IC-7300, and IC-7610 among many others - and not with other makers' rigs. Power tops out at 120 watts, so an amplifier is out, and a solid ground or counterpoise at the box is not optional: skip it and the match suffers.
Icom has since replaced the AH-4 with the AH-730, which adds the 160-meter band, raises the limit to 150 watts, and carries an IPX4 splash rating the AH-4 never had. But the AH-4 ran in production from the late 1990s onward and turns up constantly secondhand, where it remains a simple, effective way to put an Icom station onto a wire across the whole HF spectrum.