100 meters in 10 feet or less?

November 12, 2009 4:29 PM

DIY antenna for a crystal radio kit. Can I do this with less than 100 feet of wire?

This is for the oatmeal crystal radio box kits. We're doing a workshop with kids in a few weeks, and I'm not sure if we're going to have access to a roof antenna.

What other options do I have for being able to wire these up and get them to work? Loop antennas? Can I just throw a wire out the window? It's been about ten years since I've thought seriously about dipoles / yagis, and I'm a little lost.

I've looked here and here, and read this previous question (I'm also planning to take a look at the ARRl handbook). Any suggestions for how to set this up?

Thanks!

posted by puckish to science & nature (5 comments total)

In our day we connected them to radiators/pipes or other extensive metal things.
posted by fire&wings at 4:49 PM on November 12, 2009

Depends. Are you in a major metro area with strong AM signals all around? Then you may not need an antenna at all, or at most 10' of wire hung out the window.

OTOH, a small loop antenna is easy enough to make (even 6~10 turns around a 12" square box or frame would be enough), and you could even use it to get the kids to determine the direction to the transmitter.

On preview: remember, you'll also want a good earth (not the mains earth please! Besides the safety aspect, it's usually too noisy anyway). In fact, it's probably more important to have a good earth than a good antenna in strong signal areas.
posted by Pinback at 4:54 PM on November 12, 2009

not the mains earth please!

Why not?
(I mean, I don't really like the idea of telling kids to play around with electrical outlets (though it may be doable after some serious instruction and reasonable precautions). But, you seem to be implying something much more fundamental)
posted by Chuckles at 7:31 PM on November 12, 2009

A few feet will work with a strong signal. A longer antenna helps pull in the weaker stations. In WWII guys built these radios and they did not have 100 meter antennas you can be sure. A loop antenna is a good idea and very old school, fitting in well with this type of radio.
posted by caddis at 8:07 PM on November 12, 2009

Chuckles: Primarily, I mentioned that for the safety angle - we have no real idea of how much the OP knows about electricity, how old the kids are, how many kids the OP is trying to supervise, etc. I have no idea about common wiring practices in the US (apart from what I read on Wikipedia, and in places like AskMe where dodgy practices in older buildings crop up with alarming regularity), but protective earth != functional earth != signal earth - even in MEN countries, there can easily be several dozen volts difference between them, at a current appreciable enough to make you aware of the fact.

That, and pointing out that for this purpose, the mains earth isn't really suitable as a signal earth - it's noisy and it's floating all over the place, due mainly to induction & variations in earth bonding. I know, I tried it as a kid with crystal sets, and at my last job spent way too much time chasing down safety and functional issues between protective/functional/signal earths.

Basically, just covering my arse so the OP didn't get the idea that sticking a bared wire into the earth contact of a 3 pin socket was a suitable substitute for the metal water pipe connection the kit instructions will almost invariably suggest.
posted by Pinback at 8:59 PM on November 12, 2009