Antennas

摘要

This month, G3LDO unravels some of the finer points of SWR - what it means and how it is measured. When I first came to amateur radio back in 1955, most radio equipment was homemade and all radio equipment used valves. The most common method of matching the transmitter PA to the antenna via coaxial transmission line was the variable π-tank circuit as shown in Fig 1. This arrangement allowed the tuning and matching of quite a range of antenna impedances. Matching was achieved by first tuning the PA to resonance with C1, indicated by a sharp dip in the anode current. The value of C2 was then reduced causing the anode current dip to become less sharp at resonance as the loading was increased, and was set so the PA was operating at the correct rated anode current. The higher the feed impedance of the antenna and feeder the lower the value of C2. The dial of C2 could be calibrated using a number of dummy loads of different resistive values. At this stage I hadn't heard of the SWR meter.