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Old 13th Sep 2020, 6:33 am   #41
Radio Wrangler
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Default Re: Help identification of a military wavemeter poss from aircraft

Back then you would use a 'Heterodyne Wavemeter' like the British 'Class-D' or the gorgeously-made American BC221 to check that your transmitter or receiver was on the right frequency to a high degree of precision. The BC221 had a book built-in, hundreds of pages of data interpolating between the harmonic frequencies of its crystal oscillator. Each book custom printed for an individual BC221 by an automatic machine during manufacture.

The 'Absorption Wavemeter' was a lot cruder in frequency accuracy terms, its job was to allow you to check that your transmitter was not radiating too much signal on harmonic frequencies of the intended one.

Harmonic radiation is a well known problem with transmitters. Valved ones with tuneable Pi-tanks as their output matching section could even be mis-adjusted so that the power amplifier became a frequency multiplier. Useful if you intended this, problematical if you were unaware.

The frequency ranges of the amateur bands were once arranged as a harmonic series, so that the unwanted outputs from a bad transmitter landed in another amateur band and didn't upset any official service.

So if you were a radio amateur you needed to have two types of wavemeter. Something home-made as an absorption type as an harmonic indicator, and then a precision heterodyne one to prove your frequency accuracy if your transmitter wasn't crystal controlled. Even today, the licence obliges you to have equipment to prove that you are operating within the allowed band, and that you are not radiating excessively on harmonics. The authorities used to (and still reserve the right to) come visiting and want to check you have done these things from time to time and logged the results. They even used to sometimes ask to see the operator perform the checks.

If you've never seen a BC221, it's worth looking up.

David
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Old 13th Sep 2020, 8:08 am   #42
Luxman1050
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Default Re: Help identification of a military wavemeter poss from aircraft

Yeah I thought it was an absorbtion meter as I've seen a few modern ones for sale and look similar in design. One I saw has a meter on it and looks to have coils connected to the top the others have a dial.
Well she's had a gentle clean and looks much better. I had to change the outer screened cable as it basically fell apart. But it's silver braid military grade although covered in sleeve so I've got some tinned cooper braided sleeving coming so that will go over the top of that. 4 screws missing so got them on the way job done.
She's bit sqeally on turning the dial so was not sure if the thread was slightly oiled or greased. Or it's just a case of not being used for probably 60 years. It's not corrosion.
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Old 15th Sep 2020, 6:29 pm   #43
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Default Re: Help identification of a military wavemeter poss from aircraft

Well all cleaned up.
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Old 15th Sep 2020, 6:32 pm   #44
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Default Re: Help identification of a military wavemeter poss from aircraft

Some pics
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Old 15th Sep 2020, 6:33 pm   #45
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Some more
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