Resonant cavities have such good selectivity that are routinely used to
consolidate antennas for concurrent use with multiple transceivers on
same-band frequencies.

One can take a few interesting shortcuts to making them, both for ham
radio
( http://web.telia.com/~u85920178/use/cavity.htm )
and professional installation (e.g.
http://www.raeng.org.uk/news/publica...ue18/David.pdf
= http://tinyurl.com/cenqb - Not a joke, you can really use beer
barrels!).


Does anyone have experience in using positive feedback to turn, say, a
VHF fixed-frequency resonator into a regenerative IF filter for HF use?


Expected pros
- no images even with a modest front end low-pass filter
- single peak bandpass
- solid floor, possibly with ferocious immunity from strong nearby
signals
- may provide direct detection
- for isofrequency transceive operation, a PLL or huff-puff oscillator
could be converted back to receive frequency with good spectral purity
with little filtering

Expected cons:
- possible thermal frequency instability
- tricky to feed to and pick up from properly, compounded by having to
provide path for regeneration

Frequency would be rather unimportant, all tuning would happen in the
local oscillator.


Did anyone ever try that? In particular, I am curious about the
following:

1) Better a 1/4-wave resonator referred to ground or a ground-insulated
loop? (The latter may be easier to fith with multiple mutually
decoupled I/Os referred to ground).

2) Better single I/O or perhaps one each for signal path and regen amp?
(The former easier to fit on a standard 1/4 lambda resonator worked
against ground. Also, this could be done with a preexisting surplus
resonator.)

3a) Suggestions for ready made metal containers - such as coffee cans,
compressed gas canisters, large movie-type film spool cases, or
gold-flashed gas-inflated caramelized yak rumens?

3b) Is silver flashing a good idea, on either resonator or all over the
inside of a cavity

3c) Or, what ready made surplus cavity would you suggest?


73 es tnx de n1jpr/i2

Additional cons:
- Increased noise from the regeneration
- Potential for distortion introducted by the regeneration
- Non-flattopped response because of the single resonator
(so you need to make the resonance broad enough that
you can do a decent job with another filter, likely at a lower
frequency)
- To get high Q with reasonable regeneration still requires a
BIG resonator (at VHF). High regeneration is difficult to
make stable.

I'd enjoy reading about the results of your experiments with this, but
I honestly don't have high expectations for its being practical.

A long time ago, I worked on a receiver that applied gated regeneration
to the energy introduced into a cavity by an antenna, up in the several
GHz region, and it was a sensitive design for a very specific purpose
that worked well with few parts. But that design was optimized to
accomplish a special task, and the design approach was appropriate for
that task.

Cheers,
Tom

Hmmm, all very interesting...

K7ITM wrote:

Additional cons:
- Increased noise from the regeneration

As in all regens, it comes with the territory!


- Potential for distortion introducted by the regeneration

Which may be also a plus, if detection is to be accomplished in the
same stage. In normal concentrated-LC circuits up to HF or low VHF
rejection of signals at f/2, f/3 (which could be picked up and
amplified as a harmonics at f) tends to be good enough. This always
looked a bit counterintuitive to me, but it works. In cavities I think
one can also expect spurious resonances - no idea if regeneration makes
the whole contraption lock on the main resonance (that of the resonator
at fundamental) and kill all spurs.


- Non-flattopped response because of the single resonator

I wouldn't be too surprised if it turned out to be too tight for voice
(I can't do the math w/o a lot of additional learning though, any help
is welcome) but it would still be OK for CW. Regens are used for SSB
as well, and LCs have been used for ultimate selectivity until recently
(see Drake R8), but clearly curvy is a distant second best to flat.


- To get high Q with reasonable regeneration still requires a
BIG resonator (at VHF).

YESSS! The resonators I've seen were rather thick brass stubs, and
even w/o regeneration could kill off a transmitter signal off frequency
by 10%. As a first step I'd play with a ground-insulated loop made
with either a ~ 0.5-1 in. wide strip or pipe. I/O with coax-fed links
worked off ground, and a 50 ohm broadband amp as regen. I feel mired in
my deep ignorance - as in antennas, what should be avoided to avoid
broad resonances?


High regeneration is difficult to make stable.

This would definitely be a hands-on radio, like any regen.


A long time ago, I worked on a receiver that applied gated regeneration to the energy introduced into a cavity
by an antenna,
But that design was optimized to accomplish a special task,

By any wild chance was it an ECM? I remember that such a solution was
used in radar jamming. Cavity was powered, kept right below
oscillation. Pulse comes in, big punch goes out on the same frequency.
(You don't have to answer this one! ;-) ).

Or was it a self-activating oven klystron for genetically engineered
microwave-emitting poultry?

Just a couple of thoughts in reply--I'm more interested in getting you
to go try the thing and let us know how it works for you! Though I see
problems with it and would personally go other directions, I can see it
could be quite a bit of fun to play with if you're interested in it.

So, the Q of a 1/4 wave coaxial resonator (and as it turns out, also
1/2 wave and 3/4 wave and...) built from smooth copper conductors,
paying attention to making very good electrical contact especially
where the current maxima are, and making the structure be about 77 ohms
(D/d about 3.6) should allow an unloaded Q of about 107*D*sqrt(f),
where D is outer conductor inside diameter in inches and f is the
frequency in MHz. So a 6" diameter copper tube with a "1.5-inch"
copper pipe center conductor, at 100MHz, should give you a Qu of 6000,
roughly. That would be a bandwidth (unloaded) of 1/60th of a MHz, or
about 16kHz. Maybe that will help a bit. In general, for moderate to
high Q, bandwidth to 3dB points is equal to center frequency divided by
Q. But that's loaded Q, which will be less than unloaded. Of course,
your regeneration raises the effective Q--it is after all a Q
multiplier.

Oh, and don't bother trying to silver-plate the resonator. You're
better off just making sure it's smooth, high conductivity copper
(probably NOT that hard copper water pipe I mentioned above!) with very
good contact everywhere, protected with a thin coating of clear acrylic
lacquer to keep it from tarnishing. There have been controlled
expreiments about silver plating that show it actually can make things
worse, unless you get all the plating parameters just right.

The regenerative cavity thing was/is a receiver for pulsed radar, where
the time of the return pulse is known, roughly, and tracked, so the
regeneration is gated on at the expected time of the return pulse.

Cheers,
Tom