LPB Communications, Inc.
ATU-20 Antenna Tuning Unit
Introduction
The ATU-20 Antenna Tuning unit is a matching device with impedance compensation and reactance cancellation
capability, along with a high pass filter.
A cascaded impedance correction system uses two high permeability ferrite coils (T2 and T3) to provide three and
five impedance taps for a total of 1 5 ranges which are selected through two switches (S5 and S6). Each reactance
cancellation is accomplished by selecting combinations of high voltage mica capacitors (C3 through C10) which
are switched in series with the RF output by two decade-type switches (S3 and S4). Finally, capacitors (Cl1, Cl2
and Cl3) block the 60 cycle energy from entering the coupler (when used for carrier current applications) and fuses
(Fl, F2 and F3) protect the line in case of capacitor failure.
The “Match-Operate” switch (Si) activates the bridge circuitry and, through a transformer (T1) which lowers the bridge
driving impedance, dummy loads 90% of the RF input power for protection during setup. The “Forward-Reverse” switch (S2)
enables the DC milli-ammeter to look at reflected or forward power through the bridge, so that SWR can be optimized by
selecting the proper cancellation components. The calibration pot (R8) enables calculation of actual SWR by setting a forward
power reading reference point.
Installation and Operating Instructions
ATU performance and radio reception are highly dependent on proper system design and proper pre-installation
tests.
1.
Connect RF and AC cables as described on the following pages.
2.
Place S1 in Match position
3.
Place S2 in Forward (FWD) position. Meter should indicate full scale. If not, adjust R8 to achieve full-scale reading.
4.
Place S2 in reflect (REF) position.
5.
Set S3, S4, S5, and S0 for minimum meter reading. A patient, systematic approach must be taken to test all possible
combinations of impedance compensation and capacitance cancellation to achieve the lowest meter reading.
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