AWA Longwave transmitter inductance coil with 12 taps and central rotatable coil segment from circa 1930-40's
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Location: Melbourne, VIC
Member since 11 July 2012
Member #: 1179
Postcount: 56
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I have a Type 7744 inductor assembly with markings Ref No. Y10D/70224. It is the same assembly as shown in the Powerhouse museum collection - link to photo https://api.maas.museum/object/263493 -, and identified as an AWA Longwave transmitter inductance coil from circa 1938.
The size is about 28cm tall (11 inches), so not small. The insulated wire is likely 18-19 AWG, so maybe used up to 10Arms. I have yet to measure its tapping inductances.
Anyone able to comment on its original application?
It was repurposed to act as the reference inductance arm in a very capable DIY inductance bridge.
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Location: Melbourne, VIC
Member since 11 July 2012
Member #: 1179
Postcount: 56
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I was able to measure the tapped/variable inductor and it has a span from 142uH to 12.5mH. The variable portion is from zero to 656uH, and the taps are mainly 1 to 1.3mH steps (so not accurately 1.0mH steps).
The DIY inductance bridge has dial scales for the first two tap ranges with markings from 150 to 800uH, and 0.7 to 1.8mH, and those markings are not too far off. But with 12 taps total it would require a look up table to make useful. Anyway it was nice to see a slidewire pot for coil Q/phasing balance fine matching, along with switchable coil Rs for a wider range of Q/phasing matching. The oscillator coupling transformer has a sizeable turns ratio and its impedance indicates it should be ok up to 10kHz. The x0.1 and x10 multiplier settings suggest it should be able to measure down to 15uH and up to 125mH.
The bridge has excitation and detection nodes as often seen in references to Maxwell's inductance bridge, but they are arse-about for commercial inductance bridges I've seen schematics for like the GR 667, so I'll have to check that out further. Anyone in deep on this particular type of bridge?
Schematic of bridge:
https://www.dalmura.com.au/static/Inductance%20Bridge%20Schematic.PNG
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Location: Melbourne, VIC
Member since 11 July 2012
Member #: 1179
Postcount: 56
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Just identified that this coil was originally part of the RAAF's AT5 M/F aircraft communication installation's Aerial Coupling Unit that was made by AWA in large numbers for WW2 (photos and manual on-line). The manual only identifies the coil is made from litz wire, with no inductance values.
The bridge was DIY made back in 1978 using the original AT5 chassis, with a paper and Perspex cover over the original front, and retains the location and mechanicals of a few internal parts.
The bridge does a pretty good job of measuring my 99uH inductance standard accurately, so that's pleasing.
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Location: Linton, VIC
Member since 30 December 2016
Member #: 2028
Postcount: 472
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That is one heck of an impressive coil Trobbins.
And I'm not surprised. AWA made splendid equipment during that era and beyond. Test gear was their speciality, no doubt about that.
It would make an excellent antenna for locating NDB's (Non Directional Beacons) for navigational purposes. Mounted on a turntable, the radio operator would swing the coil around in search for the strongest signal to the pre-tuned beacon.
Could it have been enclosed in an aerodynamic cover and mounted outside the aircraft? Just a thought---.
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Location: Melbourne, VIC
Member since 11 July 2012
Member #: 1179
Postcount: 56
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