Mick and Smithy Talk Antennas
Chapter 3
By a club member who (currently) wishes to remain anonymous
If you have missed episodes 1 or 2, Episode 1 or Episode 2
Mick had endured another week of evenings spent fruitlessly shouting into his microphone on 20-metres (he was no cw operator, to him cw was just QRM.) He was anxious to find out why Smithy had expressed doubts about his vertical antenna, despite the low SWR and impedance around 50ohm that he had measured on the club’s MFJ analyser. At last the pair met down at their local club to continue their discussion.
“No luck?” asked Smithy. “No” replied Mick, “I can receive ok but it’s just as if the TX is going out into a dummy load.” “Well” said Smithy, “you mightn’t be too far wrong there Mick.” “Look, that vertical against a good earth system should have measured about 35ohm. Your measurement of 50ohm or so suggests to me that you have a very poor earth which is absorbing a lot of your RF power. End-fed antennas need a really good earth spread around the vicinity of the antenna. Can you string out some L/4 radials around the base of the vertical.”
“Yep, I can manage that ok - actually you’re right - all I did was to stick an earth rod down at the base of the vertical.” Smithy winced at this. “What you will find next”, Smithy continued ”is that with those radials in place, the impedance at the base of the vertical will drop to about 25ohm and you will have acquired a 2:1 SWR. An easy way out is to use 75ohm TV type coax and then put in an 11ft length of 50ohm coax to the actual antenna feed. That will give you a nice match to the vertical.” “Amazing” said Mick “how did you work all that out?” “Well” admitted Smithy ”when I first went on HF I used a home-made vertical on 20 - tilting up/down over the garage roof, with radials coming back into the garden and foil inside the garage roof. But the feed impedance was too low. I remember measuring it with a home-made mains-powered GDO balanced on a step-ladder. That was the first time I used that antennascope I showed you the other week. Anyway I worked out what was wrong and then used that L/4 matching section of 50ohm coax and it worked a treat.”
Mick looked impressed. “Well anyway” said Smithy “over the years I’ve seen one or two very similar examples of end-fed antennas failing because of poor earths. I remember our club station operating a 160metre contest once with an end-fed. We could hear stuff coming in but just like you described Mick, we had a terrific struggle to actually work anyone - you see all we had was a mains earth!”
“Do you know, Smithy” said Mick in a rare flash of perception “these same old problems must be happening to people all the time.” “That is certainly true, and a good many people have wasted a good many hours trying to get no-hope antennas to perform” said Smithy, passing Mick a sheet of paper. It said at the top:-
SMITHY’S THIRTEEN RULES FOR TRANSMITTING ANTENNAS
- There is no single measurement, characteristic or physical property (including SWR) that defines how good, in absolute terms, an antenna is.
- Like boxers, a good big ‘un will always beat a good little ‘un.
- A commercial antenna is not necessarily better than a home-made one. Price is no guide to performance.
- Buying a book on antennas is often better value than buying an antenna.
- An antenna is best erected out in the open and as high as possible.
- Each radiating element should be at least L/4 long.
- Any antenna whose radiating element(s) is/are less than L/10 long is likely to be inefficient
- Balanced antennas usually outperform unbalanced (end-fed) antennas
- Antenna tuning units at the transmitter do not alter the SWR on a coax line feeding an antenna, neither do they “tune” the antenna
- Extra power radiated because of improvements to the antenna is usually significantly greater than any achieved by adding an RF power amplifier. Increasing transmitter power is unlikely to compensate for a poor antenna
- All antennas are affected by their surroundings, especially indoor ones and those requiring earths or groundplanes.
- Field strength measurements close to an antenna are meaningless.
- A poor/inefficient/lash-up/con-trick or otherwise imperfect antenna will always work to some extent.
“Tea I think” said Smithy, and once again the workings of the MFJ analyser went unexplained.
If you have missed our other episodes:
Episode 1.
Episode 2.
next episode (Chapter 4).