Sausages and radio frequencies

What connections do radio frequencies have with sausages?  Quite a lot if you are an engineer and if you believe my story. Long, long ago, when I worked for the BBC Research Department, I was involved in developing a system to add data to analog radio transmissions. Having sorted out RDS on VHF (FM) the next challenge was medium and long wave (AM). And now all those years later the system is still in use though, as far as I know, only on the BBC long wave (Radio 4) transmission at 198kHz.  Most of the UK has Radio 4 long wave coverage from a single 500kW transmitter at Droitwich near Birmingham, with two subsidiary transmitters in Scotland to fill in otherwise weak reception areas there.  All three stations transmit on the same frequency and are now synchronised in phase.

If you are interested you can find out about the compatibility tests I was involved in by reading my report "Radio‐data: Mush‐area reception tests" here. To go into more detail would be a digression but I can hardly omit that my office companion at the time was also involved and also wrote a report.

Back to the subject: to carry out these tests we had to replace the 198kHz frequency source at Droitwich (and later at the other two stations) with a rubidium standard followed by our experimental phase modulation equipment. And this of course involved a visit to Droitwich and getting the inevitable tour where I heard the story.

The partial null I found at ~ 4300MHz

I must digress to explain a principle. Just days ago (which was what brought all this to mind) I was testing a radio receiver operating at microwave frequencies (around 4GHz in fact) and, on performing a frequency sweep, observed that there was an unwanted partial null at around 4300MHz. At radio frequencies (RF) electricity has behaviour which appears strange to those of us brought up on battery and light-bulb circuits. Signals have to travel along transmission lines and any discontinuity in impedance causes reflections just like what happens after jiggling a taught rope anchored to a building so that a wave propagates along it and is reflected at the wall. Add another wall the other end of the rope and the reflection is reflected back to combine to give stationary waves, the things that violins and guitars are made of. And transmission lines when mismatched.

The likely culprit

I found out the propagation velocity for the particular transmission line (called a co-planar waveguide) that I was using and thus calculated the distance that a reflection might result in a null and this turned out to be 10.6 mm and thus I was able to identify the likely culprit as the transmission line between the marks '10' and '11' on the ruler in my picture. Whilst tuned to the offending frequency of the null I lightly pressed my finger at this location and, hey presto, the null disappeared. I then tested to make sure in other locations the application of my finger was relatively benign. This, then, is the classic RF finger test. The human finger acts like a capacitor and resistor and thus affects the RF circuit it is close to or touching. A frequency null is very sensitive to the exact electrical properties (capacitance and inductance) of the circuit and thus even a small change can have a significant effect. And a similar effect is used by one and all when navigating on our smart phones.

Stock photo inside the Droitwich station

Back to the BBC station at Droitwich - if you'd like some history about the place see here. Inside the transmitter building there were and for all I know may still be rows of windowed grey cabinets. These contained parts of the transmitter circuits which included water cooled valves aka vacuum tubes for example as in the stock photo below.

Stock photo of a water cooled transmitting valve

The story, that I was told on that tour, goes that a former engineer was making adjustments in one of these cabinets to diagnose or fix some maloperation which might have been RF instability that manifested as distortion or some such. The engineer found that when he placed his finger in a particular location the problem went away, but it reappeared as soon as he removed his finger. After making various circuit changes to emulate what his finger did to no advantage, he eventually gave up the bitter struggle and went to the canteen to procure a sausage. And of course after wedging said sausage just where his finger had been the problem was solved.

Later the engineer's senior, whilst doing his rounds, noticed the sausage and the rest is history. I have often pondered what happened to that sausage. It can hardly still be there.

Memories of the BBC Research Department 2

Now where did I get to?  Ah, yes, I had just become a fully fledged Research Engineer in Special Projects section under Stan Edwardson.  I found this video of Stan, strange how now he looks so young yet then he seemed relatively ancient.

If Stan ever reads this I trust he will forgive me, but we used to joke endlessly about about his vetting of our Tech Memo's and Reports. As section head he had, of course, to vet everything that left the section, and after a vetting session one's style and sometimes one's content was hardly recognisable so the game used to be, how little could one get away with writing seeing as Stan would re-write it all anyway?

Here I designed a prototype receiver for the CARFAX radio traffic warning system which Stan so loved to demonstrate, see R&D Report 1979-10.  The photo in my previous blog shows one of the RF transistors used in this "straight" or TRF design. The CARFAX system was cellular in operation - a bit like today's cell-phones only medium wave, so susceptibility to interference from adjacent cells was an important consideration. Some rather tedious work followed R&D Report 1982-05.  Sadly the system was never adopted. The original Carfax is, of course, the cross-roads at the centre of Oxford.

In a later project the CARFAX receiver was integrated into a "car radio" design intended to demonstrate some digital techniques like PLL tuning, a digital bar-graph tuning indicator, a hand-made angle encoder for volume control and so forth.  Nowadays all that is standard fare but back then... The whole thing was shoehorned into standard car radio width and height, but it was two or three times as deep as a normal car radio. So Stan referred to it as the "long bonnet (or hood) car radio".

I shared an office with Andrew Lyner whose humour and no-frills view of life I greatly appreciated.

Another engineer, Dr Susans or Susie to us, was somewhat longer in the tooth than Andrew or I, indeed he was almost part of the Special Projects section furniture. His office seemed to be the height of disorder and yet he could find a relevant document within seconds if you asked him a technical question.  He had a voice a bit like a rasp and, for a hobby, made lace bobbins from exotic timbers.

The other main work of the section at this time was on radio data on VHF f.m. (now known as RDS) and on a.m. I was particularly involved in the latter which ended up being applied to Radio 4 long wave (then 200kHz, now 198kHz) for teleswitching electricity meters. The system involved phase modulating the carrier at subliminal frequencies, whilst not interfering with the long term frequency precision as this transmission is used as a frequency standard. This system was awarded the Queen's Award for Innovation in 1987 and I was kindly invited back for the ceremony as I had been part of the team which invented it.

We carried out compatibility trials which meant designing special test gear and a fun trip to Scotland driving around in a van operating it. Our work in Research Department often involved adding more features without affecting existing viewers / listeners, hence stereo had to be compatible with mono, colour TV with monochrome, and now radio data with radio.  Phase modulation is orthoganal to a.m. (amplitude modulation) so will not interfere with it in an ideal world. But the world is not ideal, especially in this "mush area" inbetween co-channel transmitters in Scotland as explained in R&D Report 1982-22.

And then I handed my notice in...