HF diode receivers (was VHF crystal diode receivers)

HF diode receivers (was VHF crystal diode receivers)

Post by Ed Burre » Fri, 07 Feb 1997 04:00:00

I was interested in Henry Titchens question about building a
diode detector aircraft band receiver, and had similar thoughts
about using them for HF bands.

So my question is this:
Has anyone successfully built and used a germanuim crystal receiver
on the HF bands for QSO's recently?  If so, how do you keep out the
local AM radio stations without compromising sensitivity? Is picking
up code without a side tone oscillator practical?

I know that this is not a new question, and many might wonder why on
Earth a person would waste their time on this.  I find that I still
have the same enthusiasm in the hobby that I had as a kid, and enjoy
all facets of the hobby: QRP-SSB, fast scan TV, packet, homebuilt
duplexer design, etc.  I also collect antique radios, and have thought
of operating a 1920's-1930's vintage station one of these days.
(receiving your signal well on 1st and 2nd harmonic) ;^).
Picking up a signal without a battery has always fascinated me.

Thanks,
Ed Burress
KC7GFX

 
 
 

HF diode receivers (was VHF crystal diode receivers)

Post by Jim Garv » Sat, 08 Feb 1997 04:00:00



Quote:>Has anyone successfully built and used a germanuim crystal receiver
>on the HF bands for QSO's recently?  If so, how do you keep out the
>local AM radio stations without compromising sensitivity? Is picking
>up code without a side tone oscillator practical?
>KC7GFX

Not recently, but as a kid I built a shortwave crystal set.  It only
received the stronger stations in the 31 meter band but I didn't
any trouble with local broadcast interference.  I kinda got it to
work on shortwave by accident but I remember that there was a book
at the library on building crystal sets that had all kinds of
complex, multi-coil designs to improve selectivity.  Would be a
collector item now along with the the black bakelite crystal sets
with cat whisker and Galena rock that my Dad used to bring home for
me.

Jim  K7YO

--

 
 
 

HF diode receivers (was VHF crystal diode receivers)

Post by Tom Bruh » Sat, 08 Feb 1997 04:00:00

: So my question is this:
: Has anyone successfully built and used a germanuim crystal receiver
: on the HF bands for QSO's recently?  If so, how do you keep out the
: local AM radio stations without compromising sensitivity? Is picking
: up code without a side tone oscillator practical?

Hey, you said "question."  Isn't that supposed to be singular??  I count
at least three explicit ones there!  ;-)

First, I haven't done it recently.

Second, you better plan on a good filter...if you can get a loaded Q of
300 and unloaded Q of 600, which I think is possible, though not easy,
you would have 3dB loss and a bandwidth on 4MHz of about 13kHz at the
3dB points.  That's really not too shabby.  It gives you over 40dB
attenuation a megahertz away from the resonator center frequency.  (The
actual design of a filter I'd use in such a "crystal radio" would be
more complicated than a single resonator, but I just wanted to suggest
those numbers as a starting point, that it would be conceivable to do
something like this.)

I can tell you that for extra credit in a graduate level circuits class
I took long ago, I built a TRF receiver with a filter designed to
receive an AM station 120 miles away in the presence of a local station
50kHz away; it worked, though rejection of the local station was not
perfect.  It was basically the filter, a single RF amplifier, a
detector, and an audio amplifier.  It illustrated that the paper designs we
did for the class really did work, but that there were also parasitic
resistances and reactances we might not have considered on paper.

Finally, what you'd pick up with a CW signal is a changing DC level at
the output of the detector.  Unless you amplified that and drove a
telegraph sounder with it, I don't think it would be very practical.  If
you add a sidetone oscillator, you have, essentially, a direct-conversion
receiver, where the sidetone oscillator is the local oscillator and the
diode becomes an unbalanced mixer.  I'd make it a little more complicated
and use a double-balanced diode ring mixer instead...or at least a couple
diodes in one of the single-balanced mixer circuits found in the ARRL
Handbook.  Oh, and SSB without the sidetone is probably totally
unrealistic...as if the CW wasn't.  (Though the CW is certainly possible
for stations that put more than a few millivolts of RF into your antenna.)

--
Cheers,
Tom

 
 
 

HF diode receivers (was VHF crystal diode receivers)

Post by Tom Bruh » Sat, 08 Feb 1997 04:00:00

: Is picking up code without a side tone oscillator practical?

To what I wrote in a followup posted a little while ago, I'd add
that in the days of spark transmitters, the signal was really
"MCW" -- amplitude modulated CW.  So you could pick it up on
something like a simple crystal detector OK without a sidetone
oscillator.  Perhaps you'd like to suggest a return to spark
transmitters along with crystal radios... maybe we could get the FCC and
other regulatory bodies around the world to set aside one day when
spark transmissions would be allowed for a few hours...   ;-)
(Just so long as the emissions outside the ham bands were properly
attenuated...  ;-)   ;-)

--
Cheers,
Tom

 
 
 

HF diode receivers (was VHF crystal diode receivers)

Post by Don Huf » Sat, 08 Feb 1997 04:00:00


> I was interested in Henry Titchens question about building a
> diode detector aircraft band receiver, and had similar thoughts
> about using them for HF bands.

> So my question is this:
> Has anyone successfully built and used a germanuim crystal receiver
> on the HF bands for QSO's recently?  If so, how do you keep out the
> local AM radio stations without compromising sensitivity? Is picking
> up code without a side tone oscillator practical?

> I know that this is not a new question, and many might wonder why on
> Earth a person would waste their time on this.  I find that I still
> have the same enthusiasm in the hobby that I had as a kid, and enjoy
> all facets of the hobby: QRP-SSB, fast scan TV, packet, homebuilt
> duplexer design, etc.  I also collect antique radios, and have thought
> of operating a 1920's-1930's vintage station one of these days.
> (receiving your signal well on 1st and 2nd harmonic) ;^).
> Picking up a signal without a battery has always fascinated me.

> Thanks,
> Ed Burress
> KC7GFX

  Hi Ed OM,

   Your question was really three questions.

   Firstly, any crystal receiver can work on HF to copy AM stations.
Many have been built.  Selectivity is
a problem.
   Secondly, elimination of AM radio station QRM is accomplished with
high-pass and/or bandpass
input filters ahead of, (but not loaded down (de-Q'd) by, the crystal
detector load), same as in any
other receiver.  You must be familiar with this, having designed
duplexers.
   Thirdly, you cannot conveniently "hear" an unmodulated, keyed carrier
(A1 CW) with an AM receiver.
Consequently, you cannot do it with a crystal receiver, which is merely
an AM receiver.  You can do it
if you make a locally generated signal (oscillator), and use it to beat
against the received on/off
keyed signal, generating a difference frequency in the audio range.
This local oscillator has always
been called a BFO.  (Beat Frequency Oscillator).
   See any ARRL handbooks of past several decades for more info.

   73 and GL,

        Don, W6JL

 
 
 

HF diode receivers (was VHF crystal diode receivers)

Post by Stev » Sat, 08 Feb 1997 04:00:00

Well, if you're gonna use spark tx then you should use a coherer for the
rx or a gelena and cat's`whisker.  To be authentic and all that....

A friend of mine built a crystal set for 80 meter a few years back.  It
used two tuned rf circuits.  If you're really interested, I might be
able to get the details.

S



> : Is picking up code without a side tone oscillator practical?

> To what I wrote in a followup posted a little while ago, I'd add
> that in the days of spark transmitters, the signal was really
> "MCW" -- amplitude modulated CW.  So you could pick it up on
> something like a simple crystal detector OK without a sidetone
> oscillator.  Perhaps you'd like to suggest a return to spark
> transmitters along with crystal radios... maybe we could get the FCC and
> other regulatory bodies around the world to set aside one day when
> spark transmissions would be allowed for a few hours...   ;-)
> (Just so long as the emissions outside the ham bands were properly
> attenuated...  ;-)   ;-)

> --
> Cheers,
> Tom


 
 
 

HF diode receivers (was VHF crystal diode receivers)

Post by Paul Kein?n » Sun, 09 Feb 1997 04:00:00



>: So my question is this:
>: Has anyone successfully built and used a germanuim crystal receiver
>: on the HF bands for QSO's recently?  If so, how do you keep out the
>: local AM radio stations without compromising sensitivity? Is picking
>: up code without a side tone oscillator practical?

<clip>

Quote:>Finally, what you'd pick up with a CW signal is a changing DC level at
>the output of the detector.  Unless you amplified that and drove a
>telegraph sounder with it, I don't think it would be very practical.  

If you take a low quality AM-only MF/HF receiver and tune across the
bands, you can sometimes hear very high pitched CW-signals (provided
that no resolute attempts have been made to limit the audio bandwidth)
which are most likely some intermodulation products between the CW
signal and a strong broadcast station. I do not know if this is
practical in a crystal receiver with only one stage generating
intermodulation products.

Someone from Region 2 wrote in rec.radio.amateur.* that in the
7100-7300 kHz range, you could turn off the BFO and use the strong
European broadcast carriers as BFOs to pick up local CW signals
between them.

Paul OH3LWR

 
 
 

HF diode receivers (was VHF crystal diode receivers)

Post by w8ji.. » Mon, 10 Feb 1997 04:00:00


writes:

Quote:>Perhaps you'd like to suggest a return to spark
>transmitters along with crystal radios... maybe we could get the FCC and
>other regulatory bodies around the world to set aside one day when
>spark transmissions would be allowed for a few hours...   ;-)

Cool, I'll jump in my model T and drive the petetion personally to
Washington.

That is, if we can get a few hours set aside to allow a 10 mph car on the
freeways.

73 Tom