bunnell: Jove Receiving Transformer 8834: Details from Serioja Tatu

Details of Serioja Tatu's receiver setup:

1. This is an loose coupler manufactured by the well known J.H. Bunnell & Co. pre 1919. The tuner comes from the Harold Greenwood wireless radio collection. Its paper Greenwood tag is undamaged and affixed to the underside of the base.
2. The crystal receiver that use this coupler (see schematic) tunes from 600 KHz to 1410 KHz with the indoor 10 ft. antenna and a ground connection. With the moving coil completely outside and higher impedance static coil (left side) and moving coil switch at 0 position (right) the receiver tunes at 600 KHz. When moving sliding coil inside, the tuned frequency increase without touching any switch or variable capacitor. The tuned frequency increase also when the switch of the static coil is moved toward right, decreasing his inductance.
3. A vintage “de Forest Crosley” 2 tube amplifier (2 x 201A tubes) is used on the test bench, in addition to high impedance (2000 ohms) headphones, as seen in related photos. 
4. Measurements with a Rohde & Schwarz SM300 signal generator with the corresponding adapter for the impedance of a long wire antenna, shows a sensibility of around – 35 dBm over the AM band, with a decent signal to noise ratio in the headphones when the “de Forest Crosley” 2 tube amplifier is used. 

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Thank you very much, Jerry for your report.

With so many degrees of freedom in the loose coupler it is possible to achieve maximum sensitivity and selectivity. Maximum sensitivity means optimal impedance matching throughout.

The circuit looks at first somewhat redundant. However some of the function of the tapped secondary with the somewhat redundant variable capacitor is to maximize Q for the particular frequency. This circuit helps to maximize Q over a 1:3 tuning range. With just a variable inductor or just a variable capacitor, they would have to vary either of these over a 1:9 range, which would mean that Q couldn't be optimal over the entire AM band.

The variable coupling is useful to optimize for selectivity or for sensitivity or for the best compromize between them.

You might enjoy looking up on the web Ben Tongue's formal study of crytal rados. He is no longer living but the site is still hosted somewhere. He was a pioneering RF engineer in the post-war decades and his company is still around in the same RF ad TV field.

Amazing how these seemingly primitive circuits are so sophisticated!

Regards,

-Joe

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You're welcome Joe; however, this was info from Serioja Tatu on his loose coupler that I posted here for him, so Serioja gets all the credit!

I agree though, it is amazing that these simple primative circuits are still so interesting, so sophisticated, and still work after a century!

Jerry

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