1. Overview
The AH-4 is a wide-range antenna tuner capable of matching a 50 ohm feed line to an antenna feed
impedance in the range of 10 to 5,000 ohms. The tuner is designed to operate with up to 120 watts of
power. It incorporates 22 relays that switch combinations of inductance and capacitance to achieve
tuning – typically within 1 to 3 seconds.
The AH-4 is enclosed in a weatherproof housing and is capable of being permanently installed
outdoors. It interfaces to the radio with the coaxial feed line and a 4-conductor control cable. The
cable carries the 12 volt power and ground, and the KEY and START control lines. The peak current
demand is less than one amp, but typically requires less than 300 ma for operation.
The tuner consists of a microprocessor control system, a power divider, impedance and power
measurement circuitry, and the matching network. The RF can be switched between straight thru
(power o
ff
state or disengaged state); through the power divider network, measurement circuitry, and
tuning network (tuning mode); or just thru the tuning network (normal operation).
I feel that the AH-4 o
ff
ers advantages over other tuners on the market for the following reasons:
•
During tune the radio always sees a low SWR, typically 1.1:1 or less. This is because the RF is
switched to a 10:1 power divider during the tune operation. Only approximately 350 milliwatts of
power is used during tuning. 50 ohm resistors swamp the remaining power so that the radio never
sees a high SWR.
•
The tuning relays are never switched under power. This results in long relay life since you don’t have
to worry about arcing while tuning.
•
The microprocessor is shut o
ff
except when tuning. This means that you won’t hear digital noise from
the AH-4 while listening.
•
Weatherproof enclosure.
•
Low cost – typically in the $330 price range.
•
Well designed – filters on all control lines, protection circuitry, mechanical construction, solid
electrical design, etc.
•
Works better than Icom says – with the right antenna it will operate on 160-6 meters.
The AH-4 has some disadvantages of course. They are:
•
Only works with certain Icom radios out of the box.
•
The instruction manual is badly done. It o
ff
ers very little information about the tuner and gives bad
suggestions regarding the best antennas to use with the tuner.
•
Is limited to 120 watts.
In this paper, I hope to minimize the cons by providing the reader with additional information about the
tuner as well as some suggestions of antennas that can be used with the AH-4.
AH-4 Operation
K9EQ
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