MF/LF OPERATING: Life Below the AM Broadcast Band
This month’s discussion begins with a description of real-time monitoring of antenna impedance (both resistance and reactance) while transmitting. While an antenna analyzer is an essential tool for implementation and practical setup of a reactive antenna system that would commonly be used at medium-wave (MF) and longwave frequencies (LF), it is advantageous to have feedback on the state of the antenna and feed line while on the air.
Amateurs commonly observe standing wave ratio (SWR) via a simple metering circuit, often believing that a one-toone value means that all is well at the antenna and on the feed line itself. All may, in fact, be in order but when a one-to-one SWR is not achieved, what next? Impedance is a complex value made up of resistance, R, and reactance, X. Often, adjustment of reactive antenna systems then becomes a matter of trial and error, as the amateur hunts for the minimum SWR value. In simple terms, SWR is really only a way of representing the magnitude of reflection coefficients and does not readily gives clues about whether the problem contributing to a high SWR measurement is due to the resistance, reactance, or a combination of both. However, the astute amateur may be able to
