[Boatanchors] "Impossible Receiver?"

[hwhall at compuserve.com]

Well, a basic field strength meter is just an RF lead (antenna stub) and diode(s) and a meter. No physical returns to ground. This looks kinda like an RF strength meter with the headset coils replacing the meter coils.

WayneWB4OGM

-----Original Message-----
From: David Stinson <arc5 at ix.netcom.com>
To: boatanchors <boatanchors at mailman.qth.net>
Sent: Sun, Dec 16, 2018 6:01 am
Subject: [Boatanchors] "Impossible Receiver?"

Between larger projects, been fiddling-around with
Crystal receivers, just for fun.  
Saw an odd circuit it 1950s
"GE Ham News" and tried it.  
The diagram is at:
https://photos.app.goo.gl/B6jAi5ToQ61ZsjKj9

The drawing is correct- there is no ground return
for the headphones.  Not only does it work, but
the output on the phones is slightly better than
the conventional circuit and the selectivity of
the tuned-circuit is far better, since the
connection to the diodes and headphones probably
couldn't get more "Hi-Z."

So how does this thing work?  I have an idea:
On the positive RF cycle, once the RF voltage 
goes above 0.3V, the top diode is forward-biased.
This can work because the bottom diode is reverse-
biased and acts as a small capacitor, so charging
the cap provides a current path.  The headphone 
coils and diode "capacitor" act as a tuned
circuit.
When the top diode goes forward-bias during the 
peak of the positive RF waveform, its conduction
gives a pulse of energy and the whole thing
"rings,"
just like a tank in a Class C amp.  When the RF
voltage reverses for the negative half-cycle, the
diodes switch operating modes, the bottom diode 
providing the current pulse and the top being the
capacitor.  This can only work with Hi-Z
headphones,
since Lo-Z would eat the voltage pulse
immediately.

That's one theory.  I would value your thoughts
on this oddity.

TNX OM DE Dave AB5S


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