The BC221 Wavemeter

This is probably the most well known piece of test equipment of WWII.

Before the days of digital displays it was pretty difficult to work out one's exact frequency, especially if the transmitter you were using was homebrew. Receiver calibration accuracy was determined by a combination of its initial cost, the age of the set or whether someone had fiddled with the trimmers inside its case. Because of this the Post Office, who issued radio amateur licenses, and whose responsibility it was to check that people were transmitting inside the designated amateur bands, insisted on every licensed amateur to be in a position to check that they were operating inside band edges.

Some receivers had built-in crystal calibrators and these offered some confirmatory evidence of one's frequency, however something better than this was demanded by the Post Office Technical Branch. In the appropriate space provided on one's license application therefore one would write the word "BC221" and all was well. If you were inspected, which used to be a regular thing, and is now exceedingly rare, if at all, one could always say "I lent my BC221 to a friend a few days ago". If you had to prove you had one, all that was necessary was for you to know someone from whom you could borrow one if the necessity arose.

Hence loads of people knew the name "BC221" but few had actually seen one.

This example was tropicallised and fungus-proofed in 1944.

Here is the BC221 with its front door open.

Below is the calibration chart.

Batteries are held inside the bottom of the case.

The equipment consists of a stable variable frequency oscillator having a vernier dial, a crystal oscillator and some circuitry, which after twiddling the appropriate knobs, enable a squeak to be heard in headphones when coupled to a transmitter. By comparing the dial reading of the squeak with a set of tables carried in the lid a pretty exact frequency reading of the transmitter output may be obtained.

Alternatively one could set the dial to a precise setting and tune a transmitter or oscillator, to which the BC221 is connected, to a pre-determined frequency.

The set of cards in the lid were very important as without them the wavemeter would be useless.

Shown here is a typical page from the booklet held in the drop-down flap.

The information is very interesting because when one studies its content it is apparent that a lot of effort must have gone into its preparation. In fact the information is essentially unique because each equipment has its own set of cards prepared specifically for that equipment and no other. The numbers of BC221's produced was enormous and it must have been a trifle disconcerting, to the people selling them to the military, when it was found exactly how much labour was needed to prepare the charts. In fact so much effort was needed that the whole job had to be automated and this was probably the first time a computer had been used in a manufacturing process.

How was this achieved?

A mechanical arm was coupled to the BC221 tuning knob and a custom made electro-mechanical computer twiddled, measured and typed the charts. Sometime I'll research the job and note some of the statistics. Without this embryonic computer wartime communications would have been much more difficult for many radio operators.

With the passage of time I wonder how the calibration of the equipment has stood up?

Sometime I'll set it up and carry out some measurements to see how the passing of nearly 60 years has treated the capacitors, resistors and valves inside the box.

The valves are a VT116 (6SJ7) and a VT167 (6K8) which is a little odd as the batteries need to be fairly potent to handle the heater current. Presumably the time the set was turned on was deemed to be normally short otherwise lower consumption battery valves would have been a better bet.

The British equivalent wasn't much better in this respect... the Class D wavemeter also uses a mains valve but requires a 6-volt accumulator to power its built in vibrator pack.

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