Mick and Smithy Talk Antennas
Chapter 9
By a club member who (currently) wishes to remain anonymous
If you are new to our saga, click here to start at episode 1
An Interlude.....
Smithy’s intention of describing antennas in a ‘back to basics’ way had to be postponed, for it was now the summer holidays when the club met up for outdoor activities until the AGM at the start of the school term. The settled weather (amazing as it was) prompted Smithy to ask Mick around - for the run of cold winters had finally decided a chilblain-afflicted Smithy to forsake his back-garden shed and move his station indoors. Mick arrived promptly for he was intrigued to find out the contents of all those cardboard boxes in the shed. The idea was to go through the stuff and re-construct Smithy’s station indoors. Smithy had made a list of the stuff he would need for his station indoors, but the trouble was he couldn’t find it. While he was scratching around for this, Mick said “Did you see that Maradona woman is directing a film at Eastnor Castle, apparently she flies in every day in her private helicopter?” As he said this, fully aware of Smithy’s confusion on the matter, he couldn’t help but smirk. Smithy however, just grunted, seeming to let the remark pass.
A tidy HRO - missing its nameplate - was the first item to go indoors. “What’s this?” enquired Mick, pointing at the HRO. Smithy looked at his friend with amazement “Do you mean to say you don’t know what this is? This is a classic communications receiver of World War Two. There must have been thousands and thousands of these used in the war.” It came as a shock to Smithy to realise that there were people in amateur radio who had no idea what an HRO was. However, in his youth Smithy had not regarded HROs as desirable receivers, with their plug-in coil assemblies they had seemed archaic. But CR100 and US Navy TCS 12 receivers of about the same era, with troublesome multi-way switches and monstrous cable looms had shown him the error of his ways and he was now an HRO convert. He had also discovered the great thing about HROs - turn them upside down, remove the base plate and there was almost everything spread out before you for fault finding and repair.
Various home-made transmitters followed the HRO indoors. Next were a couple of class A linear amplifiers that he had made. (Smithy had discovered that a tuned class A valve [807 and 5B254M] amplifier cleaned up the less-than-perfect outputs from his attempts at solid-state TXs.) Bustling about, Mick had managed to drop a small power supply onto Smithy’s foot (but unknown to Mick, Smithy was wearing his old factory-issue safety shoes) and appeared to take no notice, again just grunting as Mick apologised.
Mick had found something that puzzled him! It was a small plastic box, with a knob, dial and one of those meters from an old Japanese multimeter. Out of one end poked a tube ending in a circular piece with a hole through the middle. “What an earth is this?” asked Mick. “Ah” said Smithy, snatching it away, “it’s an intelligence detector, Mick.” He then waved it around Mick’s head saying as he did so” Look - no reading!” - Yes, he had noticed that smirk of Mick’s, but couldn’t quite put his finger on what lay behind it. “Very funny” snorted Mick “ No come on, what is it?”
“It’s a RF current monitor, you just thread the wire through that hole and you can monitor RF current. It’s not calibrated, I always used it for adjusting antennas and earths for maximum RF current. End-fed antennas on 160 and 80 metres are sometimes short of the L/4 length, so you need to load them with series inductance. That meter will tell you when you’ve reached the L/4 electrical length. It’s quite simple really, that end bit is a ferrite-cored RF transformer and there is a diode RF detector and a pot to limit meter drive.”
“What do you mean, RF current to earth, why would you want to know that?” queried Mick. “Well” said Smithy “end-feds also need a good earth so you can check earth currents with this meter as well. Incidentally, you sometimes find end-feds can be lengthened to say 3/8L, thus having higher input impedance; so people say that you then don’t need much of an earth connection. But I’ve always maintained that this is a superficial way of treating the antenna. Always arrange the best earth you can manage, spread around the vicinity of the antenna so that return currents are maximised and the necessary ‘image’ of the antenna is formed in the ground itself. Antennas are NOT simply characterised by input impedance (or any other single parameter): RF currents have to be converted into radiated RF energy and the earth image affects the radiation pattern. In any case, a good RF earth is a sensible precaution for station wiring. It will guard against outer cable RF on coax and equipment cases, and make RF feedback less likely.”
“Image, what is that? “asked Mick. “OK” said Smithy “ With an end-fed antenna, because it is erected over a conducting surface, earth; the fields around the antenna behave as though there was an identical antenna beneath the surface, which can then be considered as a non-conducting surface. This is useful in analysing the performance of the antenna, but it is entirely imaginary and allows the reflected field from the ground surface to be visualised as emanating from a suitable image antenna located below the surface of the ground. But it is quite complicated, Mick, because the image of a vertical antenna provides in phase radiation, whilst that of a horizontal antenna is in anti-phase. This explains why the height of horizontal antennas above ground is critical to their performance. Anyway I found that meter useful in checking the earth currents for a loaded L/4 end-fed on top band. For local ground-wave working, the idea is to get minimum ground resistance for return currents underneath the antenna rather than worrying about radiation patterns. Incidentally, Mick, even balanced antennas such as dipoles have to be regarded as producing images in the ground beneath them, and this of course effects their radiation patterns and input impedance. Then of course many people can't put up straight runs of wire anyway, but the same general rule applies: images of vertical sections reinforce the radiation and those of horizontal parts oppose radiation from the antenna itself.”
“Phew, I’ve got an image of tea right now Smithy, all this humping stuff indoors is thirsty work.” gasped Mick. And so we take our leave of the pair, supping tea and crunching Smithy’s cut-price biscuits, with Smithy grimacing as Mick spins the dial of his beloved HRO violently backwards and forwards.
If you have missed our other episodes:
Episode 1.
Episode 2.
Episode 3.
Episode 4.
Episode 5.
Episode 6.
Episode 7.
Episode 8.
next episode (Chapter 10).