Wavemeter Type 2

This is an early RAF wavemeter, or to be more precise a Royal Flying Corps wavemeter

 It's a very old device of which I know very little except that recent research has led me to the fact that there were three similar equipments around during WWI. Below is a picture of the other two.

The set of models were designed by a Professor J.S.Townsend.

The last owner said his father-in-law got it from a disused RAF Station in the 30's

I imagine this must have been before re-armament started so it must have been early 30's

From the label inside the lid (see below) it must have been RAF Kidbrook in Kent.

The dial is marked from 1000 to 2000 metres and a multiplier switch engages a second band, 2000 to 4000 metres.

The wavemeter uses either a lamp or a buzzer, selected by the switch at the top right, to indicate when it is tuned to a local transmitter. I wonder whether it was used with spark transmitters?

An impressed marking on the mahogany case says it was patented in May 1917 and from its construction, it must date from the period 1917 to say the mid-20s. The manufacturer's mark can be seen as "JP Ltd". Who were they?

Where was RAF Kidbrook and when was it open for business?

 On the left is a miniature buzzer and on the top are what looks like a couple of ancient condensers.

The rubber wiring insulation has completely perished so probably predates all my older stuff from the late 20's

A front panel switch selects either the buzzer or the lamp which is fitted on the front panel. This is a type I haven't seen before and has a small reflector. The filament is an S-shaped length of shiny wire rather than the usual coiled-coil of tungsten.

 Tuning is not by variable condenser but by a variometer. This comprises two coils one of which is connected to the tuning knob. This is an underside view showing the pairs of twin windings for the two longwave bands.
A larger view

Two wavebands:- 1000 to 2000 metres = 300 to 150KHz and 2000 to 4000 metres = 150KHz to 75KHz


Here's Wavemeter Type C

 There are two wavebands marked "Coupling Range 1" and "Coupling Range2" marked in Metres and Mc/s

Coupling range 1: 80 Metres to 20 Metres and 3.8 Mc/s to 15 Mc/s

Coupling range 2: 300 to 80 Metres and 1 to 4 Mc/s

The two coupling ranges are selected by the switch and then orientating the wooden box to align with the markings

Coupling range coil 1 uses 4 turns of wire and 2 uses 20 turns and they are tuned by a

The bulb would illuminate when the wavemeter was tuned to the transmitter

The rear cover is missing and there's a threaded aperture in the side. What's this for?

 

 Here are two other wavemeters dating to the same period as the No.2 shown above.

On the left is a No.3 covering 300-4,000metres in two bands and on the right a No.50, made by ATM and covering 3,000-12,000meters also in two bands.

I spotted one in a "radio" junk shop but despite going back I didn't manage to make my visits coincide with those of the proprietor so I departed empty handed.


Artificial Aerial Type 1

 As you can see above, the thing is tunable over two wavebands viz.

136KHz to 10MHz

10MHz to 15MHz

Different earth points are provided for the ranges 136KHz to 1MHz and 1MHz to 15MHz

It's made slightly puzzling because the dial is calibrated in M/M/Fds, which is a technical author or panel designer's way of stating micro micro Farads or in modern parlance, picofarads.

Strictly speaking "M" stands for Meg and "m" for milli whilst the greek mu symbol is used for micro. The dial shows two different ways of stating KHz before the powers that be decided to change things.

 Definitely something missing behind the front panel. I can see a ginormous differential tuning capacitor good for a couple of thousand volts and a fixed capacitor, but nothing else.

See a document relating to this ex-Air Ministry kit.

 I understand the Artificial Aerial Type 1 dates from the mid 1930s and was used in an installation comprising the R1083/T1082 in aircraft such as the Vickers Wellington being the forerunner of the R1155/T1154.

 Alas, as you can see above R1 & R2 from my example have been removed, I thought these may be "inductive" resistors but are probably just wirewound resistors having very little inductance at the T1082 frequency ranges. I understand R1 is 8 ohms and R2 is 7 ohms. Interestingly, as you can see, my example has marked end-capacitance values which are slightly different to those on the drawing above. Are the dials calibrated and etched after assembly? Not individually, but in batches because the picture of a second example I've seen is marked exactly like mine.

The artificial aerial, given the resistance values above has an HF load of 8 ohms + series capacitance of C1 and C2, and an LF load of 15 ohms + C1. Both loads are tunable by setting the condenser dial to the recommended capacitance at the operating frequency.I measured the fixed capacitor and it was 50pF. Looking in my junk box I found a 100 watt wirewound ceramic resistor exactly the right size. It had a centre tap and measured about 10 ohms and 9 ohms so should be just about perfect. I fitted three drilled copper strips and screwed these to the rear of the terminals. Looks OK except it's not supposed to be wired to those terminals so I think I'll do the job properly and make a proper pair of resistors instead.

If you look at the picture of the original dummy load resistors (click the picture below and scroll down) you may see something strange. Well you WILL see something strange in that uninsulated wires criss-cross over each other. Also, if you try and figure out exactly how the resistors are wired, you'll something else that's odd. Each of the two resistors, R1 and R2 is made up of two coils of resistance wire wound in opposite directions, one clockwise and the other anti-clockwise. This technique was invented by a pair of electrical engineers Messrs Ayrton and Perry who probably wore top hats, being born before Queen Victoria's reign and giving lectures by 1880. They discovered that winding coils in anti-phase reduced their self-inductance by as much as 80%. In those days of course dummy load resistors weren't used for connecting to radio transmitters but more likely low inductance networks were used in telegraph circuits.

The fact that the pair of wires comprising R1 or R2 come into contact isn't important because the voltage at the same distance from the connection to the two windings for R1 or R2 is more or less identical, in fact if made precisely symmetrical the voltages will be identical so it doesn't matter if the wires come into contact.

In fact, now I understand the design of R1 and R2, I realise that the nice brown resistor I found in my junk box should be returned there forthwith as its useless as a dummy load.

To make a new R1 will require two identical lengths of nichrome wire each having a resistance of 16 ohms and for R2 14 ohms. A search of the Internet yielded an interesting fact. Since the smoking ban electronis cigarettes have become popular and these use resistance wire in their manufacture. The good news is that this has depressed the price of resistance wire quite dramatically and as a side effect has resulted in a newer wire composition with a more stable resistance at high temperatures. I've calculated that I need about 11 metres of 0.6mm Kantal resistance wire. This will give me two 16 ohm and two 14 ohm resistors making the 8 ohm and 7 ohm values for R1 and R2.

 

 Now, if you'd like to see what this thing used to look like, click the picture to see

 

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