Mick and Smithy Talk Antennas
Chapter 1
By a club member who (currently) wishes to remain anonymous
The first of (hopefully) a series of encounters between Mick and Smithy. Both have amateur radio licences, Mick is a relative newcomer whilst Smithy has had his for a good many years. Any passing resemblance to Dick and Smithy who featured in the old “RadioConstructor“ magazine is purely coincidental. They both belong to their local radio club and although the club meets weekly, Smithy doesn’t get down there quite so often these days.
In this episode, Mick has gone to Smithy's place to borrow the club’s MFJ antenna analyser, which Smithy looks after. It should be a quick visit - but you don’t usually get away from Smithy in under two hours!
ick pressed the doorbell and from within the house came an electronic noise defying description, and then seconds later a bell rang rather feebly from down the garden. Minutes later Smithy appeared at the door and led Mick down the garden to the radio shack. Smithy is one of the last of those generation of amateurs who occupied garden sheds. “Come on in Mick” said Smithy and never having been to Smithy’s before, Mick was in for a surprise. The amount of equipment piled in there was fantastic, but it would have been difficult to say what if anything was actually operational equipment. There were numerous cardboard boxes stacked on shelves along one wall, but what Mick noticed was a pervading smell of paraffin.
“You know Mick, before we had these analysers, it was really difficult to check antennas out” said Smithy reaching up to pull down a cardboard box. Mick noticed it had a ‘Cadburys Penny Bars’ label on it. Smithy blew dust off the box whilst Mick tried to avoid breathing for a minute or so. But Smithy didn’t notice for he was extracting a battered old biscuit tin from the box, “this is my old dip oscillator, this is what we had to use, we had to couple the aerial to the dip meter coil with a couple of turns of wire, then search for a dip on the meter current as we tuned the dip oscillator with the dial marked in Megacycles, course you had to know roughly what frequency to start at.” (Smithy sometimes slipped back into aerials and kilocycles without realising it.) “You see Mick, when you got the frequency right, the antenna extracted energy from the dip meter tuned circuit and this showed up on the meter.” “That sounds tricky” suggested Mick. “Absolutely right - I well remember trying to tune a top-band whip up on my old Triumph Herald, as you altered the coupling so it altered the whip resonant frequency - a real nightmare” said Smithy, reaching for another cardboard box.
“What’s that”asked Mick as Smithy pulled out a tobacco tin from the box. “This was a great improvement, it’s called an antennascope, I first came across it in ‘Short Wave Mag’, you coupled the dip oscillator into a simple bridge circuit, which was nothing more than a carbon composition 50-ohm and a carbon low-value pot, the other bridge connections were a diode and meter for the RF detector and the antenna input.”
“How did that work then” asked Mick, slightly puzzled by its simplicity, despite its impressive title. “Well, look I’ll just try a resistive load” said Smithy, selecting a resistor out of an open ‘Oxo’ tin. “ Look, you offer up the dip meter coil to the antennascope coupling coil, clip the resistor across the antenna terminals, switch on dip meter. Now, because our resistor load isn’t an antenna, we don’t need to bother about the frequency, as long as its not too high. Now Mick see if you can balance the bridge.” Mick did as instructed and cautiously rotated the knob on the tobacco tin, and as he did so, Smithy pointed out that the miniature centre-zero meter also on the tobacco tin went through a minimum and then increased again. Dick tried to zero it but it would only settle at a minimum. “Why’s that” asked Mick. “Well, I purposely chose a wire-wound resistor to check out. What you see is that because there is some slight reactance there, the meter will null but not zero, but you can still get a good idea of the resistive value if you read the pot setting.” “It’s about 200 ohms” said Mick. “Yes - this sort of bridge would usually read from about 10 ohm to about 500 ohms, depending on the pot, which should be a linear carbon type” replied Smithy. “This is a really simple RF bridge, using the dip oscillator as the signal source. You could use a sig gen to power it, provided it would deliver a little bit of power. That is what the professional radio engineers used, RF bridges, but they wouldn’t touch a dip oscillator with a barge pole - that was definitely confined to ham radio circles.”
“How do these analysers work then Smithy” asked Mick. “That’s another story, perhaps I’ll tell you down the club some time. Would you like a cuppa?” asked Smithy. The reason for the paraffin smell now became obvious, as Smithy pulled a camping stove out from under the bench and lit it. (It seems as if Smithy had been given several large jerricans of the stuff, which is now hard to get, and this was why he was using the stove for a brew up - Mick found this out later down at the club). But Mick had surreptitiously looked at his watch and, politely declining the offer of tea, carefully took the club analyser and promised to let Smithy know how he got on with it.