RadioBanter - View Single Post

July 15th 05, 06:42 AM

wrote:

Before I start my rant, let me excuse myself to Len (who I highly
regard) for taking some issues to point on the actually-having-done-it
level.

"Gain" of a crystal radio depends on the bigness of the antenna.

If you are talking about a loop antenna on an AM [BC band]
radio, then it's a different story. The loop antenna on an
AM receiver is small/tiny/micro-stuff relative to the 200+
meters of AM BC wavelengths. The received signal VOLTAGE
is directly dependent on the number of turns in that loop and
the physical size of the loop.

Loop antennas do suck on a xtal set for that reason but "bigness" can
suck equally if not done right.

A humungous-long wire is going to supply the greatest amount of
POWER to a crystal receiver. POWER drives the headphones. But,
the amount of power coupled in involves IMPEDANCE and that, right
away, gets into a complicated mess of more electrical rules.

You know how you read all those old texts about using nice glass
insulators and keeping the wire away from anything? In everyday
practice you can do pretty well what you please with wire antennas and
powered radios and notice very little difference. Once you get into the
realm of truly hi-z installations those tree leaves brushing against the
wire become noticeable.

Simple crystal receivers want to keep impedances very high at
both input, middle, and output. ["crystal" or piezo-electric
headphones are the best for that, next best is the highest
impedance magnetic headphones (2000 Ohms or higher) you can get]

Acutally the best is a good matching xfmr and sound powered phones.
Those old 1920's 2k headsets are relatively deaf. But you did specify
"simple crystal receivers" and it that case it doesn't really matter.

For the typical parallel-tuned L-C input to a crystal set, the
inductor Q will make a difference. It must be as high as is
practical; Qs of 200 to 300 have been done. But, the Q of the
coil is dependent on a LOT of different factors which I noted
in the other message.

Q in excess of 1000 is readily achievable. 200-300 is a starting point
on a decent dx set.

How come the coils on many of the CR schematics

Mostly, that is just old-time tradition! :-) [I kid you not]

The formula for resonance is: F^2 = 1 / (39.478 * L * C)

.....snipping here....

The maximum to minimum variable capacitance ratio is equal to
the square of the maximum to minimum frequency tuning ratio
desired. That's about IT.

Most of these old ckts relied on a 17 or 21 plate cap which was in the
400-500 pf range. They would tune the entire band at full tapping. The
tapping allowed a guy to really nitpick his tuning for two
reasons...ease of fine tuning and higher Q at the best combination.

"Taps" on a coil can be to select different inductance values
for resonance with limited-range variable tuning capacitors.
Note: Back in the prehistory of radio, like around the 1920s,
variable capacitors were expensive and not so easy to get. A
few old-time crystal sets "tuned" via lots of coil taps using
a fixed parallel capacitor. I had a Philmore crystal radio kit
back in 1946 that did that. Very cheap kit. It worked, so-so.

I think you described the Philmore kit well. That was a toy.

My 4 cents worth.

-Bill