Crystal radio question

[djf]

FReeper Do-it-yourselfers:

I bought a kit that has some parts and instructions to build your own crystal radio.

It has a diode, earphone, and coil wire. Instructions are included for building a variable capacitor out of a paper towel tube and sheets of aluminum foil.

I wound the coil. I successfully built the variable tuning capacitor. I can tune it and very faintly hear stations. I am running about 40 feet of antenna wire.

My question is this: will more turns on the coil increase the sensitivity and the output? I know it will change the inductance and thus the tuning points on the capacitor, but will it boost the output?

Anybody out there ever play with this kind of stuff?

Thanks!!

[Hawthorn]

>> AM stations now have to reduce power from sunset to sunrise <<

Many must reduce power, but certainly not all of them. There are still plenty of 50 KW stations to light up the night-time dial, all across the USA, Canada and Mexico -- even though I'll grant you that it's not as good as it was 30 years ago, before the FCC adopted its unwise policy of "breaking down the clears."

>> AM radio becomes a ghost town after sunset <<

Not where I live, which is in the suburbs of a very large metro area.

>> even in the city many stations become unlistenable <<

Again, "many" does not equal "all" -- far from it. My home locale is a very good environment for hearing a lot of strong AM signals at night, from Canada all the way down to Cuba and the Turks/Caicos Islands.

[It occurs to me either (a) that you have a very poor receiver, (b) that you live in a steel-framed building, or (c) that you live in a part of town that's drenched with man-made electrical noise on the 530 to 1710 kHz band.]

[Psalm 73]

"If your bong is copper, and is embedded at least 8 feet deep into the earth..."

You've GOT to be a Bob Newhart fan - I bet you didn't even crack a smile when you wrote that! ; )

[steve86]

Thanks, I do solder connections when possible.

[VAarea]

’m using the ground off of my home electric wiring.

In addition to safety issues, a grounding connection to electrical wiring is likely to carry significant AM-band noise from loads on the various circuits (including the new CFL lamps).

[Erasmus]

You say “I can tune it..” Does that mean that your variable capacitor is not at the end of the range but rather somewhere in the middle?

If so, then adding turns to the coil won’t help, and might even put your tuning out of range of the capacitor.

How close are you to any stations?

One more thing: find a ground, like an entrance water pipe, ground rod, or electrical service (be sure you connect to its ground, such as the conduit!). Your circuit should indicate a place to connect a ground.

Output may be enhanced by a more sensitive earphone. I think, the higher impedance the better. They used to have crystal earphones for this kind of set, but I don’t think you can get those any more.

If all the above fail, more cowbell—er, I mean, more antenna.

One final advice: These sets tune very broad, and may not be able to separate two stations that are as much as 50 KHz apart.

[steve86]

The AM environment in my area has really deteriorated due to noise sources, but when I took a DE1103 up in the remote mountains a few weeks ago, was amazed the the AM dial was packed with stations, almost no dead air, and a great many of them listenable. Didn’t have a lot of time to identify transmitters but was hearing from all over the continent at least. I live by a high-tech National Laboratory and think they have many noise sources judging from the Flash Gordon stuff connected to really big AC transmission lines.

[Erasmus]

By the shade of the great Marconi -- 65 messages!

We're telling you everything we know, and then some.

≤}B^)

[Erasmus]

The incident wave from a commercial AM broadcast is strongly vertically polarized, meaning the electric field varies in the vertical direction.

As an historical side note, early broadcast stations used "flat-top" antennas. They looked like numerous clotheslines splayed out in parallel by spacers, hung high above the ground between two towers. The signals from these were horizontally (e-field) polarized, and they radiated either direction broadside from the antenna but little or not at all off the ends.

Around 1930 they got the idea of doing away with the horizontal wires and one of the towers. The remaining tower had to be metal (conductive) and at least 1/4 wavelength tall. Then, for the first time, the tower itself became the radiator; and sent vertically polarized energy out roughly equally to all points of the compass.

Soon, the band became crowded enough to require directional antennas again, so multiple towers (2 or more) returned, but this time the towers would all be radiators. Each tower would be given a certain placement, percentage of total power, and phase, so as to produce the directional characteristics needed to minimize radiation in the direction(s) that would interfere with other stations on the same or nearby frequency. The math works out such that the more directions you need to protect, the more towers you need in order to produce the required directional nulls.

Certain stations early on acquired priority over a substantial territory on their frequency, and thus use a single large omnidirectional radiator; they transmit 24/7. The limiting case of this type of station is the "clear channel," where a station has priority over a large part of the continent; it won't have any other high-powered station on the air at night within at least a thousand miles. In recent years, other stations have crept in on these frequencies, but they are restricted to some combination of large removal from the clear channel station; low power; daytime-only; and/or directional antenna.

[meyer]

You've GOT to be a Bob Newhart fan - I bet you didn't even crack a smile when you wrote that! ; )

I am a Newhart fan, though I admit that I giggled as I wrote it. I have some electrician tendencies. :)

[Erasmus]

Neal Boortz has an absolutely hilarious comment to some addlepated lefty caller who is incensed by what he's been saying. Goes something like this:

"Ma'am, if you'll just turn your radio set around, you'll find a serial number on the back. Just give that to Bo Snerdly, and he'll make sure that we disconnect this radio station from your radio. Plus, it will allow us to connect our transmitter to some other worthy listener who is on the waiting list to receive us. Thank you in advance for your co-operation."

[Erasmus]

What type of wire would you suggest for a 15-ft run to a ground rod?

For mine, I have obtained about 15 feet of copper flashing, about 5 inches wide and maybe 10 thousandths thick. (Check out Skycraft Surplus, Orlando, FL.)

One nice thing is that it will fit under my window, which can close on it.

Another nice thing is that it will have lower series inductance than any wire.

I'm preparing to solder an electrical service lug to each end, which will accept #6 AWG wire for termination at the ground rod, and the radio ground distribution bus, respectively.

[Lonesome in Massachussets]

WBZ Boston is a clear channel station (first broadcast in 1921) and they have a directional array to keep from sending power out over the Atlantic. They changed frequencies about 40 years ago but never reconfigured their transmit array, so it’s a little sub-optimal.

I have a US road atlas with a page containing a list of all clear channel stations in North America. There are AM DXers and WBZ has gotten DX cards from New Zealand(!). People in England can tune it in pretty routinely.

[djf]

That’s part of what I was wondering. How “noisy” a house ground might be. I mean, that’s alot of wire hanging around!

I’ve tried an outside ground and it didn’t make a huge difference (but it was a tad stringer). But as others have mentioned, the big payback might not happen till after dark.

[djf]

The cap is made as follows:

You take a tube from a paper towel roll. Divide it (logically) in half.

Over this half, glue a sheet of aluminum foil.

Take a sheet of paper slightly bigger, To this sheet, glue anothe sheet of aluminum foil.

Then, roll the sheet with the paper and foil around the tube and tape it to itself.

If done right, you can slide the newly placed sheet/foil up or down the tube, so you have the two sheets of foil very close together, but always separated by the paper. Also, by sliding it, the area of “overlap” of the foil sheets can be increased or decreased.

The unit tunes when the sheets are almost out of the area of overlap. In that range, there is about 2 to 2.5 inches of range where as I tune it, I distinctly pick up different stations.

So yes, it is near the end of the range.

[jbc]

Two years later and I ran into your post. I’m researching foxhole and other improvised radios built by U.S. servicemen and have been looking for that legendary Stars and Stripes article for years. Do you happen to still have it?

[FrankR]

Unfortunately, I don’t have it. I remember when I was a kid that my Dad had a copy of it, and I used it to actually make a razor-blade radio, but I don’t know what happened to it over the years.

I think if you google “razor blade radio” or “foxhole radio” you can find directions on making one, but I’m not sure about the original S&S article.