Tuesday, August 2, 2011

The Ancients Have Stolen Our Inventions

In his August, 2001 QST Magazine article, "The WBR Receiver," Dan Wissell, N1BYT, writes

"The future of Major Armstrong’s namesake may be more open-ended, however, because a simple and effective solution to the coupling problem has been found. The method of coupling the antenna to the tank circuit described below is reminiscent of a Wheatstone Bridge circuit, and thus the receiver name,“Wheatstone Bridge Regenerative(WBR) Receiver.” I’m reluctant to claim that this is a “new” detector design, even though an extensive search hasn’t yielded anything similar. But with nearly 90 years of use, I’m sure every method of detector-antenna coupling has been tried at one time or another!"

It turns out that Dan was correct in assuming that our grandfathers must have first hit upon his imaginative technique for improving the reverse-isolation between the antenna and an oscillating regenerative detector.



A portion of N1BYT's 2001 circuitry appears at the left-hand side of the above image. On the right hand side Jackson H. Pressley's 1924 oscillating mixer is shown (the antenna connects between terminals 1 and 2-3). N1BYT's bridge-isolation circuitry is employed as part of a Q-multiplier -> detector type regenerative receiver, whereas Mr. Pressley's nearly identical circuit appears in the front-end of his superheterodyne receiver.

In fact, I thought that I was being clever when I adapted N1BYT's idea to a vacuum-tube regenerative receiver some years ago. Please click here to view my design for a 40m regenerative detector employing a 6AK5 pentode. Notice there is precious little difference between Mr. Pressley's circuit and mine!

Update 24 December 2011: Today I came across an excellent example of an early WBR regenerative receiver by the well-known, Frank C. Jones. This circuit appeared on pages 24 and 25 of the May, 1927 edition of Radio Broadcast magazine. Please click here to read the original article.


Chester Rice, the co-inventor of the moving-coil loudspeaker once exclaimed in exacerbation, "The ancients have stolen our inventions!" 

My feelings are quite the reverse. I find it positively charming that dwelling on some particular problem, human minds far removed in time, place and culture so often seize upon nearly identical solutions. 

Once upon a time I took up solving Sangaku problems as a past-time. The more difficult proofs would often require a week or so of intense concentration. Eventually arriving at what I surmised was a novel proof; only then I would peek at the given historical proof. Time after time I was pleasantly surprised to find that some ancient Japanese court mathematician, or perhaps a farmer of noble mind had hit upon precisely the same solution. 

I am a man: little do I last
and the night is enormous.
But I look up:
the stars write.
Unknowing I understand:
I too am written,
and at this very moment
someone spells me out.
 
Octavio Paz, Brotherhood   
   

6 comments:

  1. Very interesting Michael. That bridge circuit idea was too good to have not been thought of before. I love and agree with your thoughts about different people in different times, places and cultures arriving at the same idea.

    I e-mailed N1BYT to let him know about this; I figured he might be interested.

    By the way, I always love reading of your exploits with radio - especially your quest to make a complete transmitter using no active devices. Great stuff!

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  2. Thank you Dave,

    I hope you'll also point Dan, N1BYT to the great video that you made of your WBR. I'm pretty sure he'll enjoy watching it!

    http://aa7ee.wordpress.com/2011/08/13/video-of-wbr-regen/

    Thanks again, and keep up the good work!
    Mike, AA1TJ

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  3. Hi Mike - It's difficult to see how the bridge in the WBR cannot be unbalanced by whatever antenna is attached to it. Your design has that differential cap to null out unbalancing reactances. Ken GI4FLG

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  4. Hi Ken,

    At resonance the bridge behaves as an impedance transformer. With no load on the top and bottom legs of the bridge (i.e., at the anodes of the twin varicaps), and assuming an infinite Qu for all the reactances, the impedance looking into the bridge from the antenna input side approaches zero.

    In which case, whether you disconnect the input entirely or short this node to ground makes no difference; it effectively remains at virtual ground.

    It's a bit more complicated than that. For instance; a network analysis indicates the resonant frequency depends upon which node you are examining.

    Still, the practical isolation that can be achieved with this setup is impressive. With a properly balanced WBR I've found that it's possible to short the antenna terminal to ground without noticeably disturbing the detector oscillating frequency.

    A common-grid (gate, base) RF amplifier preceding the detector can provide roughly the same isolation. The WBR is just a clever way of producing some measure of unilateral transmission without an added active stage.

    Kind Regards,
    Mike, AA1TJ

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  5. Mike,
    You might want to look at the Old Army BC-148, BC-151, and BC-156. You can find them at http://www.fernblatt.net/m7.html#a1218.
    Bill Higdon (willard561ataol.com

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  6. Hello Bill,

    Wow...what a nice find, thank you! Did you notice in my link concerning the 1924 Pressley Superheterodyne that Jackson H. Pressley was Chief Engineer at the Signal Corps radio laboratories? Hmmm...I wonder if he had a hand in the design of those BC-148, 151 and 156 radios dating from the early 1930's?

    I poked around a bit on Google for more information on these early radios. I didn't find much, however this was interesting

    http://www.antiqueradio.com/vinmilequip_Oct-00.html

    "the receiving station operator would hear the oscillating detector signal when the the key was up and the transmitter signal when the key was down. The key-up signal was loud and clear and almost as strong as the transmitter signal."

    If the bridge circuit located at the BC-148 receiver input were properly balanced there wouldn't have been much in the way of backwave radiation from the oscillating regenerative detector. In fact, I adjust the bridge in my receivers for a backwave null at the antenna node. The null drops off on either side of the balance set-up frequency but I should think the BC-148 manual would have warned the operator to recheck the backwave null when he QSY'd.

    The bridge reverse isolation technique would have been a natural for short-range work with those loop antennas. I'm really tickled to see this one Bill. Many thanks, OM!

    73/72,
    Mike, AA1TJ

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