Common-Sense Rules for Long PA Life

by Adam M. Farson VA7OJ/AB4OJ

Click for MRF150 data sheet. Click for MRF448 data sheet.

  1. Do not tune your ATU (internal or external) at full output. Always reduce transmitter output to the minimum level required for accurate tuning*. Power can be increased to the desired level once the best match has been achieved.

  2. Do not operate with a poor match (VSWR > 1.5). Apart from the risk of damage, the complex load presented to the PA will degrade the transmitter's IMD performance. This is not a kindness to your neighbours on the band.

  3. Never cascade ATU's (internal autotuner and external manual or automatic tuner.) Cascading tuners can lead to component failure in the internal tuner, the low-pass filters and/or the PA stage.

  4. Do not "sweep" the VFO whilst transmitting. This is not only in breach of radio regulations; it can also cause component failure in the transmitter, especially if you sweep across an impedance "bump" in the antenna system or across a range boundary causing LPF selection relays to hot-switch.

  5. When using an external amplifier, always connect and configure the ALC line to level the amplifier's output at its design rating. This is especially true of a solid-state amplifier.

  6. Initial adjustments should always be performed with a 50-ohm resistive load at the transmitter output, to avoid radiating unwanted test signals.

  7. When driving a grid-driven (grounded-cathode) tube amplifier with an untuned, swamped input circuit, place a 30 MHz low-pass filter in the coax between the exciter and amplifier. This will protect the exciter from damage in the event of a VHF parasitic oscillation in the amplifier. Solid-state exciters are especially vulnerable in this respect.

  8. When driving a cathode-driven (grounded-grid) tube amplifier, ensure that the tuned input networks present a load with VSWR < 1.5:1 (if possible, but in any case < 2:1) to the exciter over each band in use. This will obviate the need for an ATU between the exciter output and amplifier input. Use of the exciter's autotuner to compensate for a grossly mis-tuned amplifier input network can generate dangerously high voltages and currents in the exciter's PA stage, LPF and autotuner, leading to catastrophic component failure.

  9. When a solid-state exciter drives an older tube amplifier fitted with relatively slow open-frame T/R relays, the exciter is unterminated for a brief interval during the RX/TX transition. For this reason, the exciter's internal ATU should always be disabled when driving this type of amplifier. Any tuned input networks should be aligned to present a 50Ω load to the exciter at midband on each band. If the exciter has variable RF onset delay, this should be adjusted to allow for relay settling time. The best solution is to retrofit the amplifier with vacuum relays.

* The Quadra autotuner requires approx. 75W for correct tuning. A typical transceiver's autotuner will tune correctly with 5 ~ 10W. When using an external manual ATU, 10W is a safe starting point.

when feeding an antenna

Copyright  2007-2009 A. Farson VA7OJ/AB4OJ. All rights reserved. Last updated 27/11/2015