A SIMPLE ANTENNA ANALYSER
During attempts to tune and load the Decca antenna tower at Puckeridge using a SWR meter we were experiencing some difficulties. While Derek G3GRO and myself were scratchimg our heads, one of the operating team, Jim M0BMU dived into his bag and produced a small, “Black Box”. This, he assured us would quickly solve our problems, preventing us form going (prematurely?) bald. And so it proved.
Jim got the idea from a Swedish ham who’s name I regretfully don’t remember. The concept is childishly simple but very clever and useful. It tells you at a glance whether the antenna is properly tuned and loaded. If the antenna is not perfectly matched the box tells you whether it is tuned too high or too low in frequency or whether feed impedance is too high or too low and by how much.
Jim took the lid of his “black box” off and revealed the paucity of it’s content. There was a piece of 50ohms coax running through the box between two coax sockets marked “Antenna” and “TX”. One small toroid was threaded on the coax and another similar toroid was fixed flat to the bottom of the box. Two BNC sockets were fixed to the bottom of the box, labelled current and volts. About 3inches of hookup wire completed the count of parts. What could be simpler?
With Jim’s permission I made a copy of the box and it works a treat! Jim kindly gave his permission to describe this magic in the CARC Newsletter. A double beam scope is required for making measurements. Scope is availiable in the club if you do not have one.
The sketches below should make things clear. Try it. de G3KAU
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Lech G3KAU can be contacted by email