FM Crystal set??
rgp
Like most of you, I seen a zillion diagrams for AM crystal sets but it
has struck me that I have never ever ever seen an FM crystal set. Do
any of you know if such an animal exists!!?? That is a n FM crystal set
without tubes and/or in stereo. Or am I dreaming in technicolor!! :_)
CommQuart
In article <3491D8...@qbc.clic.net>, rgp <pel...@qbc.clic.net> writes:
>Subject: FM Crystal set??
>From: rgp <pel...@qbc.clic.net>
>Date: Fri, 12 Dec 1997 17:37:08 -0700
Crystal sets work well on AM, the detection process yields recovered
audio and a DC level. FM modulation involves swinging the transmitter
frequency. Receiving a FM signal on a crystal set yields the DC level,
but no recovered audio. The output from the detector is constant, since
it cannot discriminate small changes in the transmitter frequency.
If stations in the lower portion the AM broadcast band were FM,
it might be possible to recover some audio using a crystal set
that used a good hi-Q coil and capacitor for tuning. Signal recovery
would be provided by "Slope Detection" . As the carrier is modulated,
it would be sweeping across the slope of the tuning curve of the LC
circuit, resulting in an amplitude variation that could be detected.
This technique would not work well on the current FM bands. While
you could build a crystal set to receive signals for the 88 to 108
MHz band, the deviation is only 75 kHz and is very small percentage
of the operating frequency. The circuit would not offer enough selectivity
to allow slope detection. This effect is called "arithmetical selectivity" --
that is why the lower IF frequencies yield better selectivity; and why a
single tuned circuit can give excellent image rejection in the low HF
ranges, while being entirely inadequate much above 12 MHz or so.
Pete
.
Peter Bertini K1ZJH
Senior Technical Editor, Communications Quarterly Magazine
"The Radio Connection," Popular Communications Magazine
-- proud members of the CQ family --
Bill Sheppard
Hi Pelley In theory at least, an FM crystal set would be next to
impossible due to the nature of FM detection. The detector, or
discriminator, has to "see" a very narrow bandwidth in order to read the
carrier's excursions from center frequency. A crystal set simply has
'way too wide a bandwidth to allow detection of anything other than
amplitude swings.
As far as stereo, that can only be done (in current practice) by
"multiplexing" a subcarrier, which requires a full-blown superheterodyne
receiver.
Happy Holidays!!
Bill
Bill Sheppard
Just an afterthought on the FM crystal set (and kudos to commquart's
analysis). If you wanted to go one step beyond a crystal set, you could
build a superregenerative set using a VHF mosfet. As the regeneration
control is advanced to just under the brink of oscillation, the "Q" of
the tuned circuit goes sky-high, and slope-detection of FM should be
possible (as well as of AM, of course). The output could drive
headphones directly, giving you the next closest thing to a crystal set.
But forget the stereo Har!
Ho4bart
> If you wanted to go one step beyond a crystal set, you could
>build a superregenerative set using a VHF mosfet. As the regeneration
>control is advanced to just under the brink of oscillation, the "Q" of
>the tuned circuit goes sky-high, and slope-detection of FM should be
>possible (as well as of AM, of course).
well, surprise, gang. how do you think superregen radios
receive FM? without staying at the pre-oscillation ( i.e.
regenerative ) point? by slope detection, without the
supposedly q-multiplying effect. you can verify this
inexpensively, look at Radio Shack's kits. For wideband
broadcast FM, you need no heroic measures in the circuitry.
the hi-q state like in any regenerative receiver, is only necessary
to demodulate narrowband FM via slope detection.
( RSGB, UK's equivalent of ARRL, has a QRP projects
book with a circuit for a regenerative 2 meter band ( 144
MHz ) receiver for NBFM 5 kHz deviation ).
as for 'FM crystal set', the newsletter of the 'XTAL Set
Society" within the last couple years published a schematic
for an FM crystal circuit. ( not simple, but workable. i
barely remember it, but it used a discriminator plus non
LC selectivity, i believe......cavity?? )
"not possible" ?????
is that what "theory" really is telling you???
hue miller
CommQuart
In article <19971213112...@ladder02.news.aol.com>, ho4...@aol.com
(Ho4bart) writes:
>
>"not possible" ?????
>is that what "theory" really is telling you???
>hue miller
>
>
More like not "practical"?
Pete
Jim Cross
Ho4bart wrote:
>
> well, surprise, gang. how do you think superregen radios
> receive FM? without staying at the pre-oscillation ( i.e.
> regenerative ) point? by slope detection, without the
> supposedly q-multiplying effect. you can verify this
> inexpensively, look at Radio Shack's kits. For wideband
> broadcast FM, you need no heroic measures in the circuitry.
> the hi-q state like in any regenerative receiver, is only necessary
> to demodulate narrowband FM via slope detection.
> ( RSGB, UK's equivalent of ARRL, has a QRP projects
> book with a circuit for a regenerative 2 meter band ( 144
> MHz ) receiver for NBFM 5 kHz deviation ).
>
A friend of mine made an FM crystal set that worked essentially exactly
that way, by slope detection. The things looked kind of unusual.
Essentially thay were copper tubes about 4 inches in diameter and 15
inches tall with a tiny bit of circuitry and headphones. I can get the
schematic for anyone who is interested.
JIm Cross.
CommQuart
In article <34929B...@cinternet.net>, Jim Cross <vact...@cinternet.net>
writes:
>
>A friend of mine made an FM crystal set that worked essentially exactly
>that way, by slope detection. The things looked kind of unusual.
>Essentially thay were copper tubes about 4 inches in diameter and 15
>inches tall with a tiny bit of circuitry and headphones. I can get the
>schematic for anyone who is interested.
>
>JIm Cross.
>
>
He probably made tuned coaxial lines, hi-Q resonant cavities,
to achieve any sort of a reasonable steep tuning slope at 88 MHz.
A neat idea. Still, the amount of recovered audio is going
to be rather limited as
compared to AM signals. I wonder if it might be possible
to use a high-overtone crystal as a discriminator in a single-
frequency FM receiver. The cyrstal should yield much a
much steeper slope and better audio recovery.
William P Turner
In my youth I built a crystal set for FM taken (as i remember) from
Science and Mechanics Magazine. This would have been about about 1950.
If memory serves it had just a loop of heavy gauge wire resonant to the
FM band with an apc cap at the junction. The 1N23 or 34 and 'phones
were attached at that point. It worked quite well as I recall.
Interesting to read comments about how it could't work etc.
BILL TURNE
Bill Sheppard
Hey by golly let's just build an FM crystal set now that there's
verification it's been done. Any metro area with lots of strong stations
should give it a good run. Then maybe QSLing the outback with a hi-gain
antenna. HMMM
CommQuart
In article <66ua1q$oh4$1...@newsd-113.bryant.webtv.net>, dial...@webtv.net
(William P Turner) writes:
> It worked quite well as I recall.
>Interesting to read comments about how it could't work etc.
>
>BILL TURNE
>
>
No one said "it wouldn't work". The comment was made that
as you higher in frequency, the selectivity of a LC circuit broadens,
requiring a proportionally higher deviation swing for same amount
of recovered audio.... FM demodulation in a normal
crystal set design is based on slope detection.
If the slope rolloff is wider a higher frequencies,
the detection will occur over a smaller section of the slope curve,
yielding less recovered audio.
This is based on Arithmetic Selectivity -- not conjecture, not guessing,
but scientific fact.
Using higher Q circuits with steeper slopes will improve the situation.
A loop of copper wire with a APC (ceramic variable) obviously
makes a high Q circuit. Light antenna and/or detector
loading (coupling) will only improve the situation.
BUT, if the FM station increased
their deviation from 75 to say 300 or 400 kc, you would notice
a tremendous increase in recovered audio as more of the tuning
slope is used for detection. Or, if you used a very hi-Q coaxial
tuned cavity, the tuning skirts would again be sharper, giving better
recovered audio.
Most early crystals sets employed what was called the NBS
circuit, a simple dual tapped coil with a detector and headphones.
The simplest crystal set is simply a germanium diode across the
headphones. Darn good AM detection, but I defy you to prove to
me its performance would be the same for an FM signal.
I would like to see someone offer some experiments
based on using a ratio detector or discrimator circuit
at the receiving frequency for FM reception..
Does this clarify my earlier statements?
Ho4bart
>No one said "it wouldn't work". The comment was made that
>as you higher in frequency, the selectivity of a LC circuit broadens,
>requiring a proportionally higher deviation swing for same amount
--the gist of the earliest postings was that due to such technical
difficulties, you could just forget this as a workable idea.
>.... FM demodulation in a normal
>crystal set design is based on slope detection.
>If the slope rolloff is wider a higher frequencies,
>the detection will occur over a smaller section of the slope curve, yielding
less recovered audio. This is based on Arithmetic Selectivity -- not
conjecture, not guessing, but scientific fact.
>Using higher Q circuits with steeper slopes will improve the situation.
--is this news? i think everyone already has this down. don't
forget there in the urban environ there is a lot of field strength.
even if you work with less than ideal circuit parameters, you
stil likelyl have a signal in the 0.05-0.5 volt range, which is enuff
to have a workable circuit.
>
>A loop of copper wire with a APC (ceramic variable) obviously
>makes a high Q circuit.
--at vhf where your coil will be just a several turns of wire,
you can use good heavy gauge wire easily, and Q is not the
problem to achieve that it is at the AM broadcast band.
thanks to Bill Turne for his comment on the 'tuned loop' radio,
i remember seeing the original article and wondering if the
loop would recover enuff signal strength to work. combining the
antenna and tuned circuit sounds like a good idea.
>The simplest crystal set is simply a germanium diode across the
>headphones. Darn good AM detection, but I defy you to prove to
>me its performance would be the same for an FM signal.
--this is the circuit Marconi used, and it didn't last much longer.
not "darned good AM detection" if you consider that adding
a tuned circuit raises the voltage to the detector considerably.
why would anyone want to prove that this circuit - without any
selectivity slope - could somehow detect FM? "straw man" issue.
>
>I would like to see someone offer some experiments
>based on using a ratio detector or discrimator circuit
>at the receiving frequency for FM reception..
--see if you can get back issue of the XTAL Set Society
newsletter. i think they have a website maybe under
"midnightscience", is that right? i have not visited it.
>
>Does this clarify my earlier statements?
--definitely, maybe.
--let me add that one of the neat things about working with
crystal radios on the "dc" ( low frequency AM ) is that you
can just have a bunch of RS alligator jumpers and some diodes
coils and transistors and lash em together on your desk-- no
danger of shorts, shock, feedback, leads too long etc. - while
you try different parts and circuits.
tnx- hue miller
CommQuart
In article <19971214015...@ladder01.news.aol.com>, ho4...@aol.com
(Ho4bart) writes:
>
>--this is the circuit Marconi used, and it didn't last much longer.
>not "darned good AM detection" if you consider that adding
>a tuned circuit raises the voltage to the detector considerably.
>why would anyone want to prove that this circuit - without any
>selectivity slope - could somehow detect FM? "straw man" issue.
Hue, this one I have to question. I don't see how adding a tuned
circuit (and let me clairfy this as being a parallel resonant
LC circuit, and NOT an L, PI net or matching network) would in any
way raise the detected voltage. Adding a tuned circuit would
improve selectivity, but not increase detector voltage. The diode
current will be the same -- regardless if a tuned circuit or RF
choke is used. Also, in many designs, the diode is positioned
at a lower impedance tap on the coil to limit loading -- a tradeoff
between selectivity and sensitivity (i.e. the Heath CR1 design)
My own design goals are for maximum power transfer. Voltage
levels are meaningless.
I suspose we could continue to endlessly argue the point
regarding VHF FM detection.
(Probably more fun doing so than reading the endless Ebay
debacles!)
IMHO it will never work as well as it
would on an VHF AM signal. You can't take the stance that
everyone is well aware of the laws of arithmetic selectivity,
and then one the other hand ignore them and claim that VHF FM
crystal sets are equally practical devices. I want someone
to show me a crystal set design that offers the same recovered
audio for a FM signal of given bandwidth at both 1 MHz and
100 MHz.
>>
>>I would like to see someone offer some experiments
>>based on using a ratio detector or discrimator circuit
>>at the receiving frequency for FM reception..
>
>--see if you can get back issue of the XTAL Set Society
>newsletter. i think they have a website maybe under
>"midnightscience", is that right? i have not visited it.
Hee. Midnitescience would be a proper website for FM
crystal sets. Opps, sorry. Cheapshot. :) Last time I saw
the website/newsletter, they were playing with the
Ferranti ZN414 TRF chip. Cheating, cheating cheating!!!!!!
(A sad note: Ferranti is discontinuing the ZN414 device.)
>--let me add that one of the neat things about working with
>crystal radios on the "dc" ( low frequency AM ) is that you
>can just have a bunch of RS alligator jumpers and some diodes
>coils and transistors and lash em together on your desk-- no
>danger of shorts, shock, feedback, leads too long etc. - while
>you try different parts and circuits.
>
My pet crystal sets all start that way. My best set can easily separate
WJR at 760kc from CKLW at 800kc; WCBS at 880 from CHML
at 900kc. The strongest local station at nite is WTIC running
50kw on 1080kc, yet I can easily copy WBZ on 1030kc and
WBBR on 1130kc. This is no small feat for a crystal set. It also
easily logs stations such as WWVA, WTOP, WBAL, WOR, WTP,
WABC, WGY, etc. About 50 stations were logged in a three week
period. Best DX was only about 800 miles, which I thought was
rather disappointing given the DX claims made by others using
much simplier sets.
It took three very hi-Q tuned stages with
variable coupling to achieve equal results across the band, and also
a bank of loading coils for the 150 longwire to improve low frequency
antenna performance. The neverending quest for the ultimate
crystal set is never quite meet.....
>tnx- hue miller
>
regards, Pete
>-
Ho4bart
>Hue, this one I have to question. I don't see how adding a tuned
>circuit (and let me clairfy this as being a parallel resonant
>LC circuit, and NOT an L, PI net or matching network) would in any
>way raise the detected voltage. Adding a tuned circuit would
>improve selectivity, but not increase detector voltage. The diode
>current will be the same -- regardless if a tuned circuit or RF
>choke is used.
--the tuned ckt maximizes power transfer. your antenna is typically low
resistance hi capacitive reactance. an inductance in series with this maximizes
current. even if you parallel the inductance with a tuning capacitor for a less
than perfect match you still have a hi impedance circuit. compare this to an RF
choke, which due to it's own capacitance will not present as high an impedance.
try it!
>My own design goals are for maximum power transfer. Voltage
>levels are meaningless.
--only if you have a perfect diode. i couldn't find one. what you
do find is at these signal levels is that the diode itself has a
*resistance* of up into 10k+ ohms. then you absolutely want
to maximize the voltage level/ ckt impedances thru the diode,
then after demodulation worry about converting to headhphone
impedance levels. this maximizes power transfer.
>
>I suspose we could continue to endlessly argue the point
>regarding VHF FM detection. IMHO it will never work as well as it would on an
VHF AM signal.
--this is absolutely true and i don't recall this being an issue
> I want someone to show me a crystal set design that offers the same
recovered audio for a FM signal of given bandwidth at both 1 MHz and 100 MHz.
--you'll probably have to keep looking. i don't really recall this as
being the original issue, which was more like "why FM crystal
set can't work".
>>--see if you can get back issue of the XTAL Set Society
>>newsletter. i think they have a website maybe under
>>"midnightscience", is that right? i have not visited it.
>Hee. Midnitescience would be a proper website for FM
>crystal sets. Opps, sorry. Cheapshot. :
--i may have the name wrong. i think he picked it cuz the
group likes oddball radio projects and power sources, like
a small transistor radio powered by earth currents.
> Last time I saw the website/newsletter, they were playing with the Ferranti
ZN414 TRF chip. Cheating, cheating cheating!!!!!!
--well, unless we find better components ( and the search is
always on ) there's only so many ways you can hookup x
number of components. so they have been wandering into
simple active-component sets like simple transistor sets.
> My best set can easily separate WJR at 760kc from CKLW at 800kc; WCBS at
880 from CHML at 900kc.
--20 kHz separation is *excellent* !
> Best DX was only about 800 miles, which I thought was
>rather disappointing given the DX claims made by others using
>much simplier sets.
--it probably helps greatly to be out in the farmland flat country with a wire
stretched out to the barn like on the olde crystal radio instructions.
my best with 20 foot indoor antenna in Seattle was spokane on
1550, using the Heath CR-1. ( this is a pretty good circuit, 2 tuned
circuits with attention to antenna matching, a good little radio ). also using
a germanium hi beta transistor in a 'self powered' circuit, VOA california on
the 6 MHz band, same antenna.
> The neverending quest for the ultimate crystal set is never quite meet.....
--yes, and it's fun to keep trying this game. "the ultimate....is out
there.....somewhere..."
actually, we're maybe in the last golden years of crystal radio
fun, do you think? with digital broadcasting modes on the
horizon??
hue
CommQuart
In article <19971214051...@ladder02.news.aol.com>, ho4...@aol.com
(Ho4bart) writes:
>
>--the tuned ckt maximizes power transfer. your antenna is typically low
>resistance hi capacitive reactance. an inductance in series with this
>maximizes
>current. even if you parallel the inductance with a tuning capacitor for a
>less
>than perfect match you still have a hi impedance circuit. compare this to an
>RF
>choke, which due to it's own capacitance will not present as high an
>impedance.
>try it!
>
I will cede the point on this one. I was thinking more of a situation
in a lab, with a RF signal generator feeding a milliwatt meter.
Adding a parallel resonant circuit across the matched line
would not normally have any effect on the voltage level.
>>My own design goals are for maximum power transfer. Voltage
>>levels are meaningless.
>
>--only if you have a perfect diode. i couldn't find one. what you
>do find is at these signal levels is that the diode itself has a
>*resistance* of up into 10k+ ohms. then you absolutely want
>to maximize the voltage level/ ckt impedances thru the diode,
>then after demodulation worry about converting to headhphone
>impedance levels. this maximizes power transfer.
High impedance = higher voltages. Are we arguing or
agreeing here? Have you ever experimented with DC biasing
on germanium or silicon diodes? I was told it does work, but
I have never been able to duplicate verifiable resutls.
>>
>> I want someone to show me a crystal set design that offers the same
>recovered audio for a FM signal of given bandwidth at both 1 MHz and 100 MHz.
>
>
>--you'll probably have to keep looking. i don't really recall this as
>being the original issue, which was more like "why FM crystal
>set can't work".
>
Well, I am motivated enuff to breadboard one on the workbench.
I doubt I have an antenna feed with ample signal strength
on the FM BCB to give the circuit a good workout. I do have
two WaveTek 3000-200 generators that will do both AM and
WBFM for evaluation....
>
>> Best DX was only about 800 miles, which I thought was
>>rather disappointing given the DX claims made by others using
>>much simplier sets.
>
>--it probably helps greatly to be out in the farmland flat country with a
>wire
>stretched out to the barn like on the olde crystal radio instructions.
Well, this is farmland country, and the antenna is over 100 feet
long. Probably problems with interaction with towers and other
HF wire antennas......
>my best with 20 foot indoor antenna in Seattle was spokane on
>1550, using the Heath CR-1. ( this is a pretty good circuit, 2 tuned
>circuits with attention to antenna matching, a good little radio ).
I have two of those sets. (Came from an estate as a pair.)
I never had much luck with the CR1, signal levels always seemed
to be rather low and my hearing is not what it was 40 years ago.....
>a germanium hi beta transistor in a 'self powered' circuit, VOA california on
>the 6 MHz band, same antenna.
>
I tried a similar scheme, using a seperate tuned circuit just for
1080kc WTIC on the same antenna. Used a voltage doubler and
some off the shelf early germanium xistor. It did work, but not as
well as I had hoped on the weaker stations. It is nice to see
the wasted power put to some use, and to have that monster
station knocked down a few dB as another benefit. I'll sometimes use
one of my small Clemens SG-83 signal generators as a "spotter,"
to hetrodyne a weak signal I am receiving to find the exact frequency.
Its interesting to find a hetrodyne on virtually every 10kc AM channel
doing so... Oh, for a few more dB in the antenna....
>
>
>--yes, and it's fun to keep trying this game. "the ultimate....is out
>there.....somewhere..."
>actually, we're maybe in the last golden years of crystal radio
>fun, do you think? with digital broadcasting modes on the
>horizon??
I am waiting for that shoe to drop. I shudder to think what will
happen when the last AM station goes dark. We love the sound
of QSB at nite, the public demands CD player quality.....
There was some talk that stations would have to use a
compatible format a few years back, but that seems to
have died off. If I am not mistaken,
the clock is already ticking for Canadian AM and FM broadcasters.
The really scary thing is you never hear the term "public airwaves"
used these days. Most folks don't realize, or care, that in a few
years all existing VHF television frequencies are going to be
auctioned off and taken from the public domain. We are slowly
being trained to pay for what once free, whether it be receiving
local stations via cable tv, or paying monthly fees to our
satelite, PCS or cellphone providers. The radio spectrum is being
gobbled up by corporate interests with the blessing of a money
hungry congress. The FCC has been deballed since the Regan era
by the same folks.
They haven't decided how the proposed American FM digital
service will be financed -- tradional sponsors buying airtime
or a "subscriber" service like cable. Enjoy free radio on the
public airwaves while it still exists....
Pete
>
>hue
>
>
>
>------
Ho4bart
> capacitance will not present as high an
>>impedance.
>>try it!
>>
>I will cede the point on this one. I was thinking more of a situation
>in a lab, with a RF signal generator feeding a milliwatt meter.
>Adding a parallel resonant circuit across the matched line
>would not normally have any effect on the voltage level.
>
>>>My own design goals are for maximum power transfer. Voltage
>>>levels are meaningless.
>>
>>--only if you have a perfect diode. i couldn't find one. what you
>>do find is at these signal levels is that the diode itself has a
>>*resistance* of up into 10k+ ohms. then you absolutely want
>>to maximize the voltage level/ ckt impedances thru the diode,
>High impedance = higher voltages. Are we arguing or
>agreeing here? Have you ever experimented with DC biasing
>on germanium or silicon diodes? I was told it does work, but
>I have never been able to duplicate verifiable resutls.
>>>
what i used for the shortwave crystal set was a very high
beta germanium transistor, some special one with nonstandard
number but it had a beta in the hundreds but still with a high
frequency cutoff unlike the usual audio transistors. i used the
emitter-base in the diode postion and fed the base-emitter
with out of phase signal from a secondary winding, thru a
capacitor. the bias on the e-b junction because of the rectifying
action is actually *reverse* of normal transistor action, but
honest it did detect better that way. i'd like to find a good
source of germaniums still, i think maybe NTE or some such
sells them but probably 10x the $.25 they should be worth.
Popular Electronics maybe 10 years ago showed a couple
crystal set circuits that used silicon transistors and rectified
"noise" at the antenna to power the thing, 'noise' was
undefined but i suppose it means 60 Hz power and all the
FM, TV etc stations, everything you're not listening to. i
never could understand why they picked silicon transistors
since they require higher operationg voltage. the special
germaniums i found for the shortwave set would actually
amplify ( voltage gain of 2 ) down to 0.05 volts on the collector.
i also read years ago in QST about some GE ( brand name,
not germanium "gold bonded " diodes with lower "knee "
voltage, this would seem to me to be a good idea for a crystal
set. i think the article said they cost $20 each ( then, maybe
15 years ago ?? ) but none ever popped up in front of my
face while turning catalog pages. after thinking about
crystal radio lore again, i am gonna have to dig out my old classic DeVry
schools breadboard and RS jumpers and have at it again!
have fun! hue
William P Turner
".....I HAVE NEVER SEEN AN FM CRYSTAL SET. DO ANY OF YOU KNOW IF SUCH
AN ANIMAL EXISTS? THAT IS AN FM CRYSTAL SET WITHOUT TUBES.....THE
ANSWER IS YES, NO MATTER HOW MUCH GARBAGE YOU HAVE TO READ TO GET THERE.
THAT INCLUDES TRANSISTORS TOO.
BILL TURNER. WAOABI
CommQuart
In article <670uah$mvv$1...@newsd-112.bryant.webtv.net>, dial...@webtv.net
(William P Turner) writes:
>...THE
>ANSWER IS YES, NO MATTER HOW MUCH GARBAGE YOU HAVE TO READ TO GET THERE.
Bill, you're shouting again.
I assume by "garbage" you might be referring to earlier posts..
I don't consider "garbage" to be much of a technical rebutal to
what I posted. There are a lot of newcomers on this newsgroup
who are unfamiliar with the principles of arithmetic selectivity
and how it affects radio circuits. The principle hasn't been
discussed in technical manuals for decades.
My replies were intended to show the limitations placed on
FM detection using slope detection methods in a crystal
receiver. To give a reply
such as yours, without giving consideration to the basics and
limitations of such a device is not of benefit to anyone. I guess
replies to newcomers should be flippant and not based
on technical issues. That seems to be the direction this
newsgroup is heading towards. You certainly haven't laid
any foundation for showing anyone how to "get there".
Myself and Hue have some basic dissagreements on this
subject. But at least Hue offered some technical data
to back his claims. I respect Hue's views since he obviously
has some knowledge of subject and in depth
first hand experience. Hue's opinions have at least motivated
me to experiment a bit with a FM cyrstal receiver. Yours tend
to make me believe time spent answering questions in this
forum is a complete waste of time.
Have a nice day.
Pete
Bill Sheppard
Just a wild idea for possibly raising the Q enuff for FM slope
detection. "A picture is worth a thousand words" they say. But since
this webtv rig won't send pictures, will have to do the "words" route.
A center-feed dipole (not a folded dipole) with both arms ADJUSTABLE TO
WAVELENGTH, with the inductance connected directly across the center of
the dipole. The capacitance of the dipole itself supplies the C of the
LC circuit. Turn spacing of the coil is adjusted for tuning, in
conjunction with adjusting the length of the dipole arms. So you have to
fiddle with multiple adjustments –- but that's not unlike many of the
earliest radios.
To maintain the highest Q, there is no direct connection to the LC
circuit. Rather, a secondary coil is placed within or near the primary
(tuned) coil for inductive coupling, to feed the detector diode.
A couple of ordinary "rabbit ear" rods could serve as the adjustable
component of the dipole, with the whole thing mounted on a 10' 2X4
(being sure the wood is dry and everything well insulated from the
wood). Or maybe a PVC pipe. Such a rig would be directional for max.
signal strength. Caveats would be the room required for swinging it
around, and a sympathetic OL. Maybe ½ or ¼-wave versions could be
tried if she squawks too loud.
Bill
Ho4bart
>Just a wild idea for possibly raising the Q enuff for FM slope
>detection.
>A center-feed dipole (not a folded dipole) with both arms ADJUSTABLE TO
>WAVELENGTH, with the inductance connected directly across the center of
>the dipole........
>To maintain the highest Q, there is no direct connection to the LC circuit....
--i don't want to discourage you from trying out ideas in
experimenting with selfpowered receivers.
but---problem with the selectivity in these is not so much in the
antenna loading the precioius tuned circuit, as it is the diode +
earphone load on the other side. this load is anavoidable since
you are getting power ( to the earphone ) out of the tuned
circuit.
>A couple of ordinary "rabbit ear" rods could serve as the adjustable component
of the dipole....... Such a rig would be directional for max signal
strength.....
--here's where i think this idea would be good: since the
crystal radio at these frequencies is going to have rotten
selectivity, being able to turn the whole antenna would help
"tune out" station you didn't want. however i'm thinking that
maybe a simple dipole would not be able to "null" an
interfering station that much?? in the city there are a lot of
"reflection paths" so the signal is not just arriving from one
angle. the way to go if you want to be fanatic about being the
world's record holder for FM crystal set would be an outdoor
rotatable FM antenna with multiple elements. maybe the "loop
FM set " Bill Turne mentioned would be an easy way to
experiment with the directional idea.
another silly thot occurred to me, in that if you built a crystal
radio to tune 118-130 MHz, like in a little plastic box, with a
whip antenna of maybe 30 ins., and a piezo earpiece, you
could use this at the "viewing deck" of an airport to listen to
tower-plane communications. the signal strength there would
probably be strong enuff that this would work fine. there were
some projects in the old days for such tuners but they always
followed them with a transistor or 2. i don't think you would need
the transistor amp absolutely. especially if you were in the plane.
also, it occurred to me you could use the headphone wire as the
antenna, in this environment, by isolating it from the diode part
by RF chokes.
have fun with the "thought experiments" !
hue
John W. Burgoon
Peter,
When you wrote
>There are a lot of newcomers on this newsgroup
>who are unfamiliar with the principles of arithmetic selectivity
>and how it affects radio circuits. The principle hasn't been
>discussed in technical manuals for decades.....
I had to cheer! Thank you thank you thank you! Among all the crotchety
old geezers with nothing to do but complain, in between the vast waves
of argument over what to collect, buried 'neath negative attitudes and
politics and other crap... occasionally I find something of value that
helps me out.
You keep right on with the professional attitude and the good help! Some
of the others are great, too, and I thank them all as I continue to
home-brew and curse and shock myself and annoy my wife....
When I was a boy with no money at all I would go around on Saturday
morning and pick up used paperbacks from people's trash and then use
them at the bookstore, digging for hours to come up with a few precious
volumes. Digging through the crap and crud that fills the newsgroups is
the same, and gems like Peter are worth every minute. I thumb my nose at
the Naysayers!
John Burgoon, Editor, BloomingtonTODAY.com
KQT-7428 (how's that for useless!)
George R. Gonzalez
CommQuart wrote in message
<19971215004...@ladder01.news.aol.com>...
>
>I think the best solution would be to use a full size cavity, but
>that is rather big and awkward. A high Q helical resonator
>should be practical to build, and reasonably sized. Using
>adjustable link coupling to the reasonator cavity should allow
>one to set the coupling for a good tradeoff of signal level and
>selectivity. The diode tap-off point would be critical. Actually,
>two helical resonators with adjustable apperture coupling would
>be a practical excercise... Does anyone have some cookbook
>formulas for helical coil resonators??
>
>Peter
Sorry to burst the bubble, but...
You're still going to have real problems picking the signals apart.
VHF channels are on narrow spacing (15 or 10KC?). Plus you have a lot
of continuous VOR beacons and megawatt-sounding pager xmitters squeaking
away. A super-regen receiver was just barely adequate 30 years ago. Now
with maybe 10 times the number of signals and much more power, plus the
problems with re-radiating receivers, super-regens are definitely
unusable, crystal sets probably even more so.
Crystal sets can be fun, but may only be practical on the old AM BC band.
Then again, I opened up one of the original "Fuzz-busters" and it seemed
to be just a microwave horn antenna feeding a crystal detector. So
maybe you can use crystal sets at ridiculous frequencies.
Bill Sheppard
A crystal earpiece would be a foregone conclusion in any case, to
minimixe loading.
I'm a little rusty in HF theory, having never done design work in that
area, but wouldn't a resonant dipole be the functional equivalent of
cavity resonance or helical resonators in terms of Q value? This is
assuming that all inductors, diode etc. are physically located in the
center of the dipole. I know there's a wrinkle somewhere in my reasoning
here.
We can all learn something from the experts.
Bill
William P Turner
A. I HAVE IMPAIRED VISION, I'M NOT SHOUTING (YOU WILL NOTE THAT I AM
NOT A BIT LOUDER THAN ANYONE ELSE) B. IF YOU WILL READ THE QUESTION,
YOU WILL SEE THAT THE MAN DID NOT ASK YOU ( OR ANYONE ELSE ) FOR A
TECHNOLOGICAL EXPLAINATION. A SIMPLE EXPLAINATION WOULD HAVE BEEN
SUFFICIENT. IF SOMEONE ASKED FOR MORE GIVE HIM AS MUCH AS HE NEEDS BUT
TO RUN THE ORIGINAL MAN OFF SMACKS OF GARBAGE TO ME. KEEP IT SHORT,
KEEP IT SIMPLE, AND DON'T FIND FAULT WITH THOSE WHO EXPRESS THEMSELVES
COUNTER TO YOUR BELIEFS.
BILL TURNER. WAOABI
William P Turner
HAVE YOU EVER WORKED AROUND AN AIRPORT? I HAVE, BELIEVE ME LAMBERT ST.
LOUIS IS NO PLACE FOR A CRYSTAL SET. THERE ARE HUNDREDS OF TRANSMITTERS
IN PLANES, IN THE TOWER ,IN THE COMPANY FACILITIES, PLUS NAVAGATIONAL
TRANSMITTERS ETC ITS BEDLAM EVEN FOR A GOOD FRONT END.
BILL TURNER. WAOABI
Bill Sheppard
Thanks Pete for your PROFESSIONAL hands-on approach to answering an
ostensibly-innocuous electronics question.
But a thought just popped in concerning the resonant-dipole idea I
mentioned earlier. Is BCB FM transmitted in vertical or horizontal
polarization? If vertical, then the receiving antenna should also be
vertical. This would eliminate the directional feature of the receiving
antenna. Combination TV/FM roof antennas all have horizontal
elements, as do FM-only roof antennas. So what IS the polarization
practice of BCB FM? It could have major signifigance in design of an FM
crystal set.
Several years ago, a bizarre-looking catamaran-style airplane called
"Challenger" made the first round-the-world flight without refueling.
The plane served absolutely no practical purpose other than to prove it
could be done. The quest for an FM crystal set that'll actually work
is about on the same par. So why do it? Because it's FUN !!!
Bill
CommQuart
In article <19971214233...@ladder02.news.aol.com>, ho4...@aol.com
(Ho4bart) writes:
>another silly thot occurred to me, in that if you built a crystal
>radio to tune 118-130 MHz, like in a little plastic box, with a
>whip antenna of maybe 30 ins., and a piezo earpiece, you
>could use this at the "viewing deck" of an airport to listen to
>tower-plane communications. the signal strength there would
>probably be strong enuff that this would work fine. there were
>some projects in the old days for such tuners but they always
>followed them with a transistor or 2. i don't think you would need
>the transistor amp absolutely. especially if you were in the plane.
>also, it occurred to me you could use the headphone wire as the
>antenna, in this environment, by isolating it from the diode part
>by RF chokes.
>have fun with the "thought experiments" !
>hue
>
>
Since aircraft transmssions are old fashioned AM, this is not
a silly idea. The best thing is that since crystal sets don't
radiate, using one aboard an aircraft should be perfectly
legal and safe!
Peter
CommQuart
>>detection.
>>A center-feed dipole (not a folded dipole) with both arms ADJUSTABLE TO
>>WAVELENGTH, with the inductance connected directly across the center of
>>the dipole........
>>To maintain the highest Q, there is no direct connection to the LC
>circuit....
>
>--i don't want to discourage you from trying out ideas in
>experimenting with selfpowered receivers.
>but---problem with the selectivity in these is not so much in the
>antenna loading the precioius tuned circuit, as it is the diode +
>earphone load on the other side. this load is anavoidable since
>you are getting power ( to the earphone ) out of the tuned
>circuit.
>
>
I would think regardless of the coupling method, once you
begin loading the antenna you will be will lowering the
Q of the circuit. I have pretty much given up on single-coil
AM BCB sets because of the dectector loading. Using a
separate coil/cap on the antenna side (series resonant)
with the detector coil mounted so the coupling can be adjusted
manually seems to work the best. The antenna tuning is always
the shapest.
I think the best solution would be to use a full size cavity, but
that is rather big and awkward. A high Q helical resonator
should be practical to build, and reasonably sized. Using
adjustable link coupling to the reasonator cavity should allow
one to set the coupling for a good tradeoff of signal level and
selectivity. The diode tap-off point would be critical. Actually,
two helical resonators with adjustable apperture coupling would
be a practical excercise... Does anyone have some cookbook
formulas for helical coil resonators??
Peter
CommQuart
I promised I would try a simple FM crystal set before passing
judgement.
I found a nice military surplus inductor in my junque box -- made of
1/8" heavy silver plate wire. The coil is airwound, slightly over 1" ID,
and is five turns.
It measured about .43 uH on my digital inductance
meter. If anyone insists, I can provide a Q reading from
my Boonton Q meter, but I think the coil choice is about
as reasonable as can be expected.
This was mated with a ceramic insulated military surplus
VHF quality tuning capacitor.
The antenna was coupled by a tap at a very low impedance
point on the coil. The diode was connected at the high impedance
point of the coil. The headphones used for the test were Baldwins.
Priliminary tests using a WaveTek 3000-200 signal generator
showed that a 700 uV AM signal was copiable over a 60 to
120 MHz tuning range. The generator was switched to wideband
FM (75 kHz deviation). No recovered audio could be hear until
the signal was increased to 7000 uV (7 millivolts) or about 20
dB above that needed for AM reception.
The most interesting finding was that the FM signals "dissappeared"
as the tuning was centered on the signal generator frequency.
At center tuning, audio recovery is equal on both sides of the
tuning slopes, 180 degrees out of phase, and effectively cancelled.
The FM signal could be heard for several MHz either side of
center frequency due to the wide tuning slope curve of this particular
LC combination. On the other hand, AM tuning was very sharp.
The only antenna available for actual on the air testing is my
my Channel Master vertical dipole scanner antenna.
Using the Channel Master antenna, the AM video carrier
for WTIC channel 3 tv was clearly audible. WTIC is about
35 airline miles from this location. Several 50,000 kW FM
stations are located to the north, south and west of this
location, at the same or closer distances. I could hear audio
from one station -- this may have been the aural carrier for
WTIC judging by its tuning position. I could not verify what
I was hearing.
I will try using my tower mounted Channel Master VHF-tvFM
band antenna as soon as I can arrange a feed to the workbench
that bypasses the distribution amplifiers and FM traps.
I suspect the scanner antenna performance is far from optimal
at 88 MHz.
A calibrated spectrum analyzer showed the antenna is delivering
a -40 dBm signal for the WTIC visual carrier. The strongest
FM signals (about 8 of them!) are all clustered at the -50 dBm mark, or
1000 uV. Had these FM stations been AM modulated, they
could have been easily monitored. About 20 dB more signal
is needed for FM detection.
I couldn't find my 20-uA meter movement, used in series with
the headphones it would probably have the detected the presence
of FM station carriers even though the audio was not recoverable.
Conclusions to date:
1. VHF FM reception using a simple crystal set is possible, BUT
with considerably less sensitivity than for VHF AM signals..
2. The broad tuning slopes using a simple design would make
single signal reception very difficult in an urban environment.
However, the nulling effect observed when a FM signal was
tuned dead-on channel should provide a nifty means to eliminate
the strongest unwanted station.
3. A discrimator or ratio detector circuit looks like it would be the
way to go. I will definately try this approach at a later date, and
also try using high Q tuning, probably with a helical resonator..
4. AM aircraft crystal sets would appear to be very practical and
easy to build. AM sensitivity was >>>remarkable<<<.
I am going to park on some of the active air channels for
Bradley field and see what happens. I am near some
approach lanes, but then again aircraft run fairly low power...
5. Folks living in an urban setting, with FM stations located
with a few miles, will probably have far greater success
than I am.
6. If you disagree with my findings, don't bother posting
innane comments. Instead, whip up a few designs,
and post your own observations..
Regards
Pete
Ho4bart
--good point. altho few of these transmitters would be going at
the same time, one strong beacon would ruin the tower-plane
communication.
btw, the front end problem is different than what we're talking
about here, front end problems mean your receiver is overloaded and producing
strange mixing products from all
the powerful signals.
i owned a Martian Big-4 crystal radio once, it had absolutely
no tuning, no controls of any kind. it amazes me people could
use these to listen back when 500 watts for a radio station was
big power.
hue
RGang5562
>Like most of you, I seen a zillion diagrams for AM crystal sets but it
>has struck me that I have never ever ever seen an FM crystal set. Do
>any of you know if such an animal exists!!?? That is a n FM crystal set
>without tubes and/or in stereo. Or am I dreaming in technicolor!! :_)
>
>
Can be done, I think there was a schematic in one of Bob Brown's "XXX easy
transistor projects" books. You can design it as any other crystal set, but
with real Hi-Q tank circuit - tap the germanium or shottkey diode halfway down.
.. If your'e not within several thousand microvolts of an FM station, you'll
have to forward bias the detector on to get make it work (a few microamps from
1.5v thru 1 meg, heavily de-coupled) - and u still need the ears of a bat to
hear anything without gain somewhere. (also needs to be tuned off-carrier to
slope demodulate- hence, the Hi-Q tank)
As regards stereo, stop eating what you're making with that chemistry set.
CommQuart
Below Bill extolls the virtues of FM crystal receivers:
>".....I HAVE NEVER SEEN AN FM CRYSTAL SET. DO ANY OF YOU KNOW IF SUCH
>AN ANIMAL EXISTS? THAT IS AN FM CRYSTAL SET WITHOUT TUBES.....THE
>ANSWER IS YES, NO MATTER HOW MUCH GARBAGE YOU HAVE TO READ TO GET THERE.
>THAT INCLUDES TRANSISTORS TOO.
>BILL TURNER. WAOABI
Bill, having an apparent change of heart, decides perhaps VHF crystal sets
aren't such a hot idea afterall??
------------------- Headers ----------
>HAVE YOU EVER WORKED AROUND AN AIRPORT? I HAVE, BELIEVE ME LAMBERT ST.
>LOUIS IS NO PLACE FOR A CRYSTAL SET. THERE ARE HUNDREDS OF TRANSMITTERS
>IN PLANES, IN THE TOWER ,IN THE COMPANY FACILITIES, PLUS NAVAGATIONAL
>TRANSMITTERS ETC ITS BEDLAM EVEN FOR A GOOD FRONT END.
>BILL TURNER. WAOABI
In article <672a2c$29d$1...@newsd-113.bryant.webtv.net>, dial...@webtv.net
(William P Turner) writes:
Bill, I don't care. Get a bigger monitor, or buy some reading glasses.
The question was asked if anyone had heard of crystal sets being used for VHF
reception.
I replied it was impractical to do so -- and gave the gentleman several reasons
backing that
opinion. Despite your feelings, what I gave the gentleman was a "simple
explanation".
I feel the gentleman deserved an expanation rather than an off the cuff reply.
Apparently,
based on the e-mail I have received, most others feel what I said was
informative. May I
remind you that you were the one to find fault with my post. A few who
disagreed with some of what
I said choose to engage me in debate rather than expressing themselves in the
ungentlemanly
manner you elected. This usegroup is a discussion forum, unless someone has
rewritten
the Charter since I last read it. Bill, what is it about "discussion" that
you have a problem
understanding?
Now that the topic has shifted to using crystal receivers for monitoring AM
aircraft traffic
in the 120 to 140 MHz VHF region, you have again reversed course and have
decided
that crystal receivers are not suited for this application. Let me make sure I
have this
correct: Crystal sets make swell FM receivers (despite the fact my spectrum
analyzer
shows a myriad of strong stations clustered in a relatively small spectrum).
However,
despite being about 20 dB more sensitive in AM reception, they are not suited
for
monitoring aircraft comms, which are lowpowered and of short duration?
I think I have pretty well covered the technical basics backing my opinions,
including
spending most of an evening on the bench giving the idea the benefit of a doubt
-- and posting
those findings in this forum.
If you can't offer any technical or empirical substance in your posts to
rebute what I said,
I am afraid the garbage is eminating from YOUR end. Isn't it a bit obvious to
even you, Bill,
that reqardless whatever I say or post you will disagree with? Even to the
extent of contradicting
yourself?
Regards
Peter
>
>------------------- Headers --------------------
>
>Path:
>lobby01.news.aol.com!newstf02.news.aol.com!portc03.blue.aol.com!news-out.int
CommQuart
In article <672j8i$k3f$1...@newsd-143.iap.bryant.webtv.net>, old...@webtv.net
(Bill Sheppard) writes:
>
>But a thought just popped in concerning the resonant-dipole idea I
>mentioned earlier. Is BCB FM transmitted in vertical or horizontal
>polarization? If vertical, then the receiving antenna should also be
>vertical. This would eliminate the directional feature of the receiving
>antenna. Combination TV/FM roof antennas all have horizontal
>elements, as do FM-only roof antennas. So what IS the polarization
>practice of BCB FM?
Bill
Most FM stations now run both horizontal and vertical radiation
patterns. The horizontal polarization is standard for most home
installations. However, when FM car stereos became increasingly
popular, the stations had to accomodate the vertically polarized
whip antennas used on most autos. There is a theorectical
20 dB loss for using the wrong polarazition.
Peter
CommQuart
Bill,
I find no fault in your reasoning -- but I am not an antenna person.
My location precludes using an indoor antenna and still having
enough signal to work with. That is why I am going to try a
helical resonator (hopefully with a ratio detector) on the workbench.
Why limit yourself to simple dipoles for experimenting? You might
want to play around with some full size loop antennas made from
copper tubing. I believe these run a full wavelength in diameter
and might offer better Q and slightly improved gain.
Pete
John Byrns
Here's another idea for a detector for a crystal FM radio. This idea is
only for those people who are using a beverage antenna on the AM band, and
have a lot of real-estate available. This scheme does not provide any
selectivity, it only provides FM detection, so selectivity must be
provided by other means before this detector. Selectivity could be
provided by several shorted quarter wave stubs tuned to the desired
station and connected across the antenna feed line at suitable intervals.
It will be necessary to provide adequate selectivity in front of this
detector, because signals to which it is not "tuned" may be demodulated
with severe distortion, depending on the exact frequencies involved.
The basic idea of the detector, is to feed the signal to be demodulated
into a 2460 foot piece of 6.125 inch transmission line, of the sort used
for feeding broadcast transmitter antennas. The large diameter line is
needed to minimize the losses in the long length. The length is chosen to
be a quarter wavelength at 100 kHz, neglecting the velocity factor of the
transmission line, which I don't have at hand. The 6.125 inch line feeds
an adjustable length line segment which can be adjusted between a full
wavelength at 88.1 MHz, and zero length. The inputs of a high speed two
input diode OR gate are connected to the antenna end of the 6.125 inch
line, and the output of the adjustable length segment at the far end of
the 6.125 inch line. The output of the diode OR gate feeds the headphones
for listening. There are some details that would need to be worked out in
the design of the OR gate for lowest distortion.
The basic idea here is to create a quadrature detector operating at the
station frequency, and designed to accommodate a +-100 kHz frequency swing
around the nominal carrier frequency. The adjustable line segment is used
to set the proper quadrate relationship at the desired carrier frequency.
The audio output should be nearly as great as an AM carrier of the same
strength.
Regards,
John Byrns
Bill Sheppard
Wow. John's idea sounds nifty if we just had real estate for
something as big as a particle accelerator.
Question for Pete –- What type diode are you using, and are you going
to forward-bias it? Probably a little bias cell wouldn't be cheating
the spirit of the the thing, huh?
CommQuart
In article <jbyrns-1512...@kebob-19.d.enteract.com>,
jby...@enteract.com (John Byrns) writes:
> The 6.125 inch line feeds
>an adjustable length line segment which can be adjusted between a full
>wavelength at 88.1 MHz, and zero length. The inputs of a high speed two
>input diode OR gate are connected to the antenna end of the 6.125 inch
>line, and the output of the adjustable length segment at the far end of
>the 6.125 inch line. The output of the diode OR gate feeds the headphones
>for listening. There are some details that would need to be worked out in
>the design of the OR gate for lowest distortion.
>
>
Anyone have a few extra sections of GR line stretchers?
Peter
Ho4bart
c'mon pete, at an airport viewing deck the tower signal is going
to be weak?? how far from the field xmtrs would you be?
this might be an idea i might actually try.
on my shortwave crystal set i regularly heard CBers at ranges
up to about a half mile, maybe more. mobile units.
hue
CommQuart
In article <19971216091...@ladder02.news.aol.com>, ho4...@aol.com
(Ho4bart) writes:
>Subject: Re: FM Crystal set??
>From: ho4...@aol.com (Ho4bart)
>Date: 16 Dec 1997 09:12:49 GMT
>--
Hello Hue
I am not quite sure what you are referring to. The only comment
I made earlier was that aircraft comms are lowpower and
sporadic in nature. Is this what you are drawing conclusions
from? If so, the point being made was a broad
tuning device would be practical in such a situation.. Please
quote from what I wrote next time. I NEVER said tower comunications
would be weak, and you know it..
Below are two comments I made -- directly quoted from early
posts -- that express my views on VHF am crystal sets.
Peter Bertini wrote:
>4. AM aircraft crystal sets would appear to be very practical and
>easy to build. AM sensitivity was >>>remarkable<<<.
>I am going to park on some of the active air channels for
>Bradley field and see what happens. I am near some
>approach lanes, but then again aircraft run fairly low power...
(Yes, and I am twenty miles from the airfield, Hue.. A LONG
reach to to observation tower!)
Peter Bertini wrote (in response to your saying VHFAM crystal
sets might be "a silly idea"):
>Since aircraft transmssions are old fashioned AM, this is not
>a silly idea. The best thing is that since crystal sets don't
>radiate, using one aboard an aircraft should be perfectly
>legal and safe!
>Peter
However, below is quote you made regarding the use of crystal
sets for monitoring local aircomms....
Hue wrote in response to a post by Mr. Turner:
>--good point. altho few of these transmitters would be going at
>the same time, one strong beacon would ruin the tower-plane
>communication.
Maybe I missed something, but the only quotes showing any
lack of enthusiasm for this application were posted by you
and Mr. Turner. I fail to find any inconsistencies in my earlier
statements.
This thread continues to degenerate. I offered my opinions,
based on known scientific principles and have been taken to
task for it. I have done several bench experiments which largely
back my statements, posted the results here, and have YET to
see anyone refute those findings. Now it seems the only
avenue left open to a few folks is to continually change their
positions, and try to twist whatever I say into an argument.
I refuse to play this game any further and this will be my last
response to posts of this nature.
Regards
Peter
.
CommQuart
In an earlier post I replied to a gentleman that
bias cells could be found in 1920 vintage radios.
While I was referring to C bias batteries, which serve
the same purpose as bias cells, Bill Turner has
suggested that I clarify my statement to mention that
the term BIAS CELLS should only be used in reference
to 1930 and later radios.
In this intrepretation, I assume that BIAS CELLS maybe
classified as very small-sized cells of very limited
current handling ability that were installed underchassis in
1930s or later radios.
If someone can offer a more technical distinction between
BIAS CELLS and a C Bias Battery/Cell, I would like to
see that information...
Regards
Peter
Jim Mueller
I've always seen the term "bias cell" used in reference to the little
button style cells. C batteries are larger, usually rectangular,
batteries made up of cylindrical cells. As you pointed out, they all
serve the same purpose.
The Meissner "How to Build" Instruction Manual 53-1 from 1952 has 2
paragraphs and an illustration on page 5. Here is the second
paragraph:
"They are actually batteries of essentially constant voltage but very
high resistance. As a consequence if a voltmeter is connected across
the cell, the meter will read a voltage far lower than the
open-circuit or no-load voltage of the cell. It is not a good idea to
measure the voltage of a bias cell or to permit it to become
short-circuited. If, through accident, the bias cell is
short-circuited for some period of time it will probably resume its
normal operating characteristics shortly after the short-circuit is
removed. If it is desired to determine whether the bias cell is
operating properly, a single flash-light dry cell may be substituted
for the bias cell to check for similarity of action, The outside
containers of both the flash-light battery and the bias cell are the
negative terminals."
A couple of comments seem to be in order. First, as mentioned, this
is from 1952 when the common voltmeter was 1000 Ohms per Volt. Modern
meters draw far less current so would give a more accurate reading.
Second, the polarity of the cell is case negative. This is opposite
to the watch batteries that are likely to be used to replace them now.
They weren't used only in radios but also test equipment. The
Meissner book shows one being used in the Analyst, a newer version of
the Ryder Chanalyst (page 130).
As for replacing them with flashlight cells, I would consider using
zinc-carbon "standard" or "classic" cells rather than alkaline cells.
My experience has been that the open circuit voltage of an alkaline
cell falls off as the cell is exhausted, In fact, it is a good
indication of the amount of life left in the cell. Not so with a
zinc-carbon cell. Its open circuit voltage remains essentially 1.5V
long after it is dead for all normal uses. Since a bias cell doesn't
need to supply current, the higher capacity of an alkaline cell is of
no advantage and the zinc carbon cell might give longer life. I don't
know for sure since I have never compared the open circuit life of the
two types.
As for electrolyte leakage from the cells, I have never seen any type
of cell that "for sure" won't leak. The best thing to do is to mount
it so that they only thing the crud will damage is the battery holder
which should be easy to replace.
Have fun,
Jim Mueller
dea...@stairnot.com
The stated return address is incorrect to reduce the amount of junk e-mail I
receive. To get the valid return address, replace "deadmen" with "dadoman"
and "stairnot" with "azstarnet".