The Autotuner as Preselector

by Adam M. Farson VA7OJ/AB4OJ

Many modern HF transceivers and solid-state amplifiers are fitted with an internal T-network autotuner. If the autotuner is left in the RF signal path on receive as well as on transmit, it will provide some additional preselection. (The same applies to an external manual or auto ATU, as long as it is in the receive signal path.)

A swept-frequency test was conducted on the internal autotuner of a YaesuQuadra 1kW HF/6m amplifier. The purpose of the test was to measure the tuner's amplitude/frequency response on each amateur band from 160m to 6m, so as to evaluate its usefulness as a preselector.

An HP/Agilent 8935/E6380A communications test set was connected to the Quadra as follows: DUPLEX OUT via 6 dB pad to Quadra INPUT 1, ANT IN via 6 dB pad to Quadra ANT1. On the test set, the RFTOOLS suite was started. The instrument's GPIB port was connected to a laptop computer via aProLogix 6 GPIB/USB controller for screen capture.

The Quadra autotuner was first tuned into a 50Ω resistive termination on a midband frequency in each amateur band (160 - 6m) in turn, with the PA stage in standby. Then, on the test set, the Swept Insertion Loss tool was run on each band in succession, and the screen plot captured using thePrintCapture program.

It will be seen that the passband is asymmetrical, with a sharp cutoff below the tuned frequency and a gentler roll-off above it. This is to be expected, as the T-network tuner is in effect a high-pass filter. Typically, on 20m the -3dB point is at -2 MHz on the low side, and at +6 MHz on the high side. This is admittedly a rather modest sort of preselector, but I have found it to be helpful under strong-signal conditions.

The Swept Insertion Loss test has 15 dB maximum expected insertion loss. Thus, the trace levels off as this loss value is approached at the low end of the frequency sweep.

Copyright  2006-2017 A. Farson VA7OJ/AB4OJ (incl. images). Last revised: 09/25/2019.