Kids' "crystal radio", making an amplifier thereof

[Rich Grise]

Would, say, 10 meters of vertical wire work for an antenna?

Yes, since AM broadcast is generally vertically polarized.

But the "good ground" can't be overemphasized. If you have metal water
pipes, then wherever the city water comes into the house, clamp to the
pipe. That's an amazing ground plane - all that metal pipe all over
town! ;-)

Theoretically, you could use the green wire at the outlet box for
ground, but depending on how much wire you've got between the box
and "real" ground, the conduit could be serving as an antenna.

And for extra credit, you could make an "antenna tuner" - another tuned
circuit between the antenna and the set, to match the impedance - that
should improve the signal too.

Have Fun!
Rich

 

[Rich Grise]

Oh that's mean. That's a bit like powering a wind turbine alternator
coil with power from the coal power plant down the street...

What you need is two crystal sets alongside each other, with one tuned
to the most powerful station in the area, but instead of a speaker, you
have big capacitors so you just rectify its signal and filter it to DC,
then use that to power the set with the amp. ;-)

My first "crystal set" was a kit that came with a transistor, that
detected and amplified, but it also used 2 D cells. My older brother
had already built the simple one, which was really easy, since we were
across the street from WCCO's 50 KW antenna. ;-) (well, about a half-
mile away, actually.)

Cheers!
Rich

 

[jasen]

My 5 year old son is building (with my help) a "crystal radio", which
is a air core tuning coil, a diode, and a earphone. It is a cheap
chinese kit.

some of those kits are absolute crap. does this one atleast function?

As the instructions allude to, it is kinda hard to get out of this
thing enough power to really hear anything. They talk about trying to
touch "cold water faucet" with one end for better pickup, etc etc etc.

Not the skylab brand one is it? I encountered that one a few (~10) years
back and had no luck with that one at all (I seem to recall it had no
capacitor at all... could have the brand wrong too)

The cold water faucett business is alluding to needing a good earth connection
but with plastic water pipes becoming increasingly popular I don't see that
that's necessarily the best earth connection.

for good results from a crystal set a good earth connection and an long
antenna wire are important.

you're going to need to isolate the audio section from the RF in the tuned
circuit or otherwise provide a high impedance input or the thing is not
going to work.

if your apliifier has low impedance (less than 100K) you probably benefit
from a radio-freqency choke in series with the audio signal, if you've got
a junk AM radio there one of those little square tin tunable transformers
could probably be used for that purpose

So, I think, it would be great to make an amplifier that would amplify
the sound enough to hear it well.

easiest solution is powered speakers (like for mp3 player or pc)....

I'm not certain that they have enough sensitivity though, and they'll be low
impedance input

see you in "sci.electronics.basics"

bye.
Jasen

 

[jasen]

My dad tried to build a crystal set with me, and it was an abysmal failure.

My dad knew less than I did, and the guy at the electronics store told my
dad that the TV aerial would do the job. Which it won't because there is
only a short length of metal picking up anything. It might work for an
amplified radio, but not a crystal set.

they work great at 100Mhz (for the FM band) but yeah absolutely useless
at around 1Mhz for the AM band. silly store-person.

 

[Michael A. Terrell]

they work great at 100Mhz (for the FM band) but yeah absolutely useless
at around 1Mhz for the AM band. silly store-person.

You disconnected it from the TV and used one side of the 300 ohm TV
antenna lead to a roof mounted TV antenna as a long wire antenna. They
were usually 50 feet or more and did work, but a longer wire was
definitely better. Silly you.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Kryten]

You disconnected it from the TV and used one side of the 300 ohm TV
antenna lead to a roof mounted TV antenna as a long wire antenna. They
were usually 50 feet or more and did work, but a longer wire was
definitely better. Silly you.

We tried inner core, shield, or both together.

No combination worked.

 

[Michael A. Terrell]

We tried inner core, shield, or both together.

No combination worked.

It worked for me over 40 years ago.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Michael A. Terrell]

We tried inner core, shield, or both together.

No combination worked.

Does the coax go through a ground block? If it does, it is shunting
the RF to ground. I did it when 300 ohm twinlead was common.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Kryten]

Does the coax go through a ground block? If it does, it is shunting
the RF to ground. I did it when 300 ohm twinlead was common.

I recall it was just co-ax cable.

My intuition at the time told me the inner was the aerial signal and the
outer was the ground reference.

Years later I did feel it to be pretty lame.

While it might work under good circumstances (as it did for you), the father
and son should not be disheartened if it does not. As an engineer I would
recommend an aerial that was a decent fraction of the radio wave desired.
IIRC, a quarter wave antenna is respectable. At 300 kHz a wavelength would
be 299,792,458 / 300,000 = about 100 m, so a quarter wave would be 25m or
about 75 feet.

 

[Michael A. Terrell]

I recall it was just co-ax cable.

My intuition at the time told me the inner was the aerial signal and the
outer was the ground reference.

Years later I did feel it to be pretty lame.

While it might work under good circumstances (as it did for you), the father
and son should not be disheartened if it does not. As an engineer I would
recommend an aerial that was a decent fraction of the radio wave desired.
IIRC, a quarter wave antenna is respectable. At 300 kHz a wavelength would
be 299,792,458 / 300,000 = about 100 m, so a quarter wave would be 25m or
about 75 feet.

300 MHz = 1 meter, so your 300 KHz reference would be 1000 meters for
a full wave, but people rarely use an antenna anywhere near even a
quarter wave. Its like a car radio antenna on the HF bands is more of a
probe coupled to the tuned input so it isn't frequency selective.

If you are using coax for a TV antenna you would have to connect to
the braid, because it blocks the signal from reaching the center
conductor. There is a matching transformer at the antenna, or a matching
stub, neither of which have good HF characteristics. Also, most of the
coax is connected to the antenna frame for grounding which attenuates a
lot of the HF signal.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Rich Grise]

I recall it was just co-ax cable.

My intuition at the time told me the inner was the aerial signal and the
outer was the ground reference.

The problem is, that the cable is parallel to the vertical mast, which
is very probably grounded. So, they both get the same voltage induced in
them, so there's no differential signal at the bottom end. Using the
core doesn't work, because it's shielded.

But 300 ohm twin-lead doesn't have a rounded shield, and so one lead
of it - or even both in parallel - make a dandy vertical random-wire
antenna.

Cheers!
Rich