Fictional "Intercom System" Over City Power Lines

You guys all read books like "The Mad Scientist's Club" when you were kids, right?

I recall that in this book they (the pre-adolescent boys in the Mad Scientists Club) devised (among many other things) an intercom system that allowed them all to communicate from house to house across a small town, using its power grid.

I know there are in-house (room to room) intercom kits that you can buy, and they work with some success, but is it possible on this scale? Not that i'd try it, mind you. I suppose there are a lot of obstacles, i.e. having to filter out a 60hz AC hum with a huge amplitude, for starters. You'd be limited to AM instead of FM, I presume, blah blah.

I realize that it is a fictional book and it has a lot of, shall we say "embellishments" for the sake of the story. For instance, they often used lots of radio equipment, and while nothing seemed to have antennas on it, nobody ever mentioned line-of-sight, signal gain, pine trees, radio control frequencies vs. ham radio frequencies, the FCC or the usual stuff, most everything had a perfect 5+ mile range. Maybe the RF situation in the sky was different in the 1960s, eh?

I had completely forgotten about this 'intercom system' i read about 20 years ago, until I overheard (yet another) discussion about the "broadband over powerlines" concept that people have been debating the last few years.

For what it's worth, I remember that book fondly in my youth, and lately I've been developing a small (but present) interest in communications using unconventional means.

-phaeton

Reply to
phaeton
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Power lines would be tough. You can use dirt instead. Just bury a pair of electrodes as far apart as possible and drive with a power amp, receive with any sensitive audio amp. Party-line commiunications are possible across goodly distances.

John

Reply to
John Larkin

Interesting. I'm going to have to try that.

What kind of wattage power amp, how far is a goodly distance, and wouldn't the dirt also be full of all kinds of other electrical 'noise' in this case? Oh yeah, and do I need decoupling caps on the output and input of the amps?

:-D

-phaeton

Reply to
phaeton

As much as you can get.

I've heard of a mile or two.

Yup, some worthless and some interesting.

No.

John

Reply to
John Larkin

It's called a field telephone. you might be able to get a pair at a surplus store (probably they've all gone to collectors).

more is better, a step-up transformer on the output may help too.

more than 10 times the electrode separation. should be possible. possibly much more.

depends on the territory.

could help. Bye. Jasen

Reply to
jasen

Ok...

Dumb question time- Need I put both + (signal) and - (ground) into the dirt for both sending and receiving end?

I found some 6 penny nails and crimpable wire connectors to use as electrodes, but it was thunderstorming pretty heavily last night, so no go. I have both some TDA2002vs and LM383 amplifiers (~7W) that I could try out for this. (I have a small yard, so....)

Thanks!

-phaeton

Reply to
phaeton

You need something like 4 foot or longer ground rods at opposite corners of your property (if you want any hope of getting the signal beyond your own yard), and a step up isolation transformer (something like a tube amplifier output transformer, or line to speaker transformer) connected backwards (to step the voltage up) to get a fair impedance match to the resistance between the rods and to eliminate any ground loop current caused by the instantaneous difference between the ground potential at the rods and that at your amplifier.

You can drive the transformer with an ordinary speaker output from any amplifier.

Reply to
John Popelish

Real field phones used two conductor cable, usually made with steel wire so it wasn't easy to cut or break. Using the ground to carry the signal would allow the enemy to monitor your communications. they had a hand crank generator to signal other phones on the line, and even portable switchboards for command locations.

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Reply to
Michael A. Terrell

Yeah. The field telephone for GI Joe (the action figure) came with a roll of wire.

I gather the system of driving rods into the earth and using them to communicate appeared frequently(?) during WWII in the hobby publications, in that it was something one could experiment and play with while radio was out of the picture. QSTs from the period likely would offer up an article or two.

Electronics Illustrated in the sixties ran an article about it, and the image I remember is of a drawing with a telephone handset. That certainly conjures up an image of a "field phone".

Michael

Reply to
Michael Black

It could have been used in a secure area for non secure communications, but not anywhere near combat zones where targeting information and troop movements were discussed. Also, there was little, or no electrical grid in those zones, so you didn't have to worry about

20/50/60 Hz hum on the receiving end. There are places that you could have enough AC ground currents to completely swamp this type of communications.
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Reply to
Michael A. Terrell

That's what I'd figure. Although, the typical field phone is usually radio, i presume. I'm sure that modern field phones use encryption and frequency hopping and stuff, but I bet the phones used in WWII probably had absolutely no scrambling/descrambling capability.

Reply to
phaeton

Field phones are used where you are in a fixed location for a while. radios are used when you have to keep moving. If you every looked at an old field phone you would see how simple, and how rugged they were.

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Reply to
Michael A. Terrell

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