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111Re: Idealised WBR receiver simulations

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  • qrp.gaijin
    Oct 22, 2014
      Earlier in this thread I referenced a 1920s patent by Fitch that describes a WBR-style receiver. Recently, I found a 1924 magazine article by the same gentleman that describes this and similar circuits. The article is here: http://www.americanradiohistory.com/Archive-Radio-News/20s/Radio-News-1924-10-R.pdf (see p. 496 of the magazine, i.e. p. 58 of the PDF).

      The important thing to notice is the difference in tone between the patent and the magazine article. The patent presents the WBR circuit in a positive light and fails to mention its weaknesses, which was probably necessary to convince the patent examiner that the new invention was worthy of a patent. On the other hand, the magazine article is much more honest about the circuit's shortcomings, and states that the receivers are theoretically interesting but suffer from very poor gain if perfectly balanced.

      My interpretation of this article and the corresponding patent is that perfect balance prevents the antenna from exciting the resonator and forcing the signal first to take a trip through the lossy amplifier (it's lossy because it feeds back only a tiny part of the input signal into the tank, just enough to cancel the tank losses), after which the attenuated signal reaches the tank and then is regeneratively amplified. So the Fitch patent, which presents the WBR circuit in a positive light, is correct in that the signal from the antenna does eventually reach the tank after passing through the amplifier, but the patent fails to mention the great signal loss that is incurred when the signal first passes through the lossy amplifier. The 1924 Fitch article on the other hand does mention that the perfectly-balanced, non-radiating case is very inefficient as a receiver. But what about the recent AA1TJ and N1BYT WBR circuits, which seem to work well? In the AA1TJ case (cross-coupled WBR), the circuit is very well-balanced, but AA1TJ used a full-sized antenna, which likely makes up for the signal loss in the perfectly-balanced configuration. And the N1BYT circuit is inherently unbalanced (feedback occurs into only one arm of the bridge), somewhat ill-characterized (in particular the mystery surrounding the Z1 coupling inductance), and is also intended for use with a full-sized antenna, all factors which likely contribute to better sensitivity.

      My failed attempts at making a perfectly-balanced WBR work with a whip antenna, where signal levels from the antenna are very low to start with, can also be explained by the inherent circuit limitations as described above.

      Best regards,
      qrp.gaijin@...

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