Mick and Smithy Talk Antennas
Chapter 2
By a club member who (currently) wishes to remain anonymous
If you have missed episode 1, click here to read it.
Smithy pushed open the door into the meeting room of his local club. The room was buzzing with activity. He opened the large sports bag he had brought down and took out a cardboard box with a ‘Farleys Rusks’ label on it. His friend Mick had been looking out for him and came straight over.
“Mick, when you came to my shack to pick up the club’s MFJ analyser, you left before I had time to tell you about a useful RF bridge that is relatively easy to build and use.” Smithy pulled out what had obviously once been a toffee tin, but was now thinly disguised with grey paint and sprouting knobs and sockets. “This is a noise bridge and this one was built for HF use.” “In a noise bridge” continued Smithy “ the signal source is a wideband RF noise generator, often a zener diode, with a couple of stages of wideband RF amplification. This is usually input to the bridge through a toroidal transformer wound on a ferrite ring. The bridge detector is simply an HF receiver. As in the other bridges I told you about, there is a balance pot and a couple of fixed resistors in the bridge arms.”
The toffee tin had an SO239 socket bolted onto its lid and there were also a couple of ‘banana plug’ sockets. Smithy had also brought one of his many homemade receivers (a valve one) and he connected it up to the banana sockets and switched on. Into the SO239 socket labelled ‘ANT’ he plugged the club’s dummy load. As the receiver warmed up, background noise became evident and Smithy flicked a toggle switch on the toffee tin and a sudden woosh of noise erupted from the receiver. He indicated to Mick to try balancing the bridge for minimum noise in the receiver. Mick did this and remarked how easy it was. “Yes” said Smithy “ of all the homemade bridges I’ve played about with, I’ve always found the noise bridge easy to use. Of course, if you were using an antenna, you’d have to tune the receiver to the appropriate frequency.”
“Could you use a transceiver?” queried Mick. A smile spread across Smithy’s face. “Oh yes, of course, but once you’ve done your measurement or antenna work, you must remember to take the bridge out of circuit before you transmit. I do remember when I used a short antenna on 80 metres and brought it to resonance by series loading and matched it to my Heathkit 200 watt transceiver by checking it on receive with this noise bridge. Then one Sunday morning I slipped up and left it in circuit. I actually had a QSO with a station in South Wales through the noise bridge! It wasn’t badly damaged though. If I remember it didn’t take me long to fix it. Those BC108s/BC109s are tough little beasts and they get up to VHF too.” (It must be noted here that Smithy had long regarded the BC108/BC109 transistor as the finest type ever produced and had not progressed much beyond them in terms of contemporary devices. Likewise he had only recently found out that Madonna and Maradona were actually different people.)
Smithy could see that Mick was impressed by the noise bridge, so he added “ You know Mick, if you are measuring a reactive load across the antenna terminals, you can insert a calibrated capacitor or inductor as appropriate in series and cancel out the reactance- so finding out how reactive the load is. But it will only work with a fairly limited value of reactance, say about X= +/- 200 ohm. Look at the time and we haven’t had any tea yet” said Smithy. “Anyway Mick, these homemade bridges are not so good now we have the club’s MFJ analyser. How did you get on with it?” “Magic” said Dick, “my 20-metre vertical came out at R=56ohm and X= 10ohm. I can’t remember the exact SWR but it was fairly low.”
Mick was surprised by Smithy’s reaction. “Hmm” said Smithy “Look Mick that sounds suspicious.” “Funny you say that” Mick replied “Even though that MFJ gave a good result, I don’t seem to do very well with that vertical on the band, you know how competitive 20-metres is.”
“Right” said Smithy “ Next time we’re both down here at club I’ll explain the myths that have grown up about SWR and the probable reason why your vertical is not working well. Come on let’s get some tea and finish the biscuits up then I can have the empty tin.”