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.