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Registered Member #3925
Joined: Fri Jun 03 2011, 10:50AM
Location:
Posts: 121
hi,
a single wire can act as an antenna for radio waves right? so how come its not possible to detect light with it? and also, it there any particular standard type of antenna which is most efficient? and doesnt the size of the antenna depend on the wavelength of the "radio waves" you wanna detect?
because obviously if you connect an led to a diode (or whatever it is that the guy on svbxlabs.com did), and put it infront of a magnetron from a microwave oven, the led will glow, but this cannot be efficient, surely making an appropriate antenna and putting that infront of the magnetron, and connecting it to the led, would be most efficient? im confused! lol
Registered Member #2919
Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652
The antenna needs to be matched to whatever wavelength you are receiving; this is difficult to do for light with its sort wavelength (orders of magnitude smaller than RF). If you had a nice antenna on the LED, you could couple more energy to it, but why bother?
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
You can buy these cheap enough:
The wavelength of 2.45 GHz microwaves in air is 12.5 cm. I think a half wavelength or quarter wavelength antenna will be ok but I'm no expert.
I know Steve Connor made one by twisting together an LED and a diode in anti-parallel (I think!) and it worked.
Some of the cheaper meters show 5 mW/cm^2 and 10 mW/cm^2, so you can work out roughly, by moving the meter closer to, and farther away from, the source, but this will only give a rough indication, especially as the field is unlikely to be uniform (if you are planning what I assume you are planning!)
I'd recommend getting one of the proffessional meters, but again, while these come up on Ebay cheap sometimes, there is no guarantee of accuracy with a second-hand meter (it may not even work at all!)
Too strong a field will undoutably 'blow' a cheap meter, rendering it useless, leading to a situation where you may think you are safe, but end up with fried gonads or worse.
microwaves decrease in intensity according to the inverse square law, but the field can be directional.
I'm looking at doing some magnetron experiments as well, but without a known good (ie 'new') proffessional meter, it's pretty much a 'non starter' (unless anyone knows different!)
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ash Small wrote ...
I'm looking at doing some magnetron experiments as well, but without a known good (ie 'new') proffessional meter, it's pretty much a 'non starter' (unless anyone knows different!)
You could make a useful microwave calorimeter to give a fair estimate of magnetron output with not a lot more than a known quantity of water, a thermometer, and some thermal lagging like glass wool.
Googling microwave calorimeter and microwave bolometer* will throw up enough material for you to work out how to do it for yourself.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
That's quite a good idea, PM.
I've just done a 'rough and ready test', though.
250 cc water, 80 watts power for 30 seconds.
temperature rise around 0.5 degrees C.
Does that mean it is safe for me to put my hand in a microwave oven at 80 watts (defrost setting) for 30 seconds?
(I've tried five minutes 5 minutes, and the temperature rise, is around 3 degrees C, )
Obviously, the way a microwave oven restricts the power is be turning the magnetron on and off, using the inbuilt timer mechanism. operating a magnetron CW, as one would do for any experimentation, would require further tests, but it should be possible to set up a test with a magnetron removed from the oven, with the waveguide pointing at, and close to, an insulated container of water.(with appropriate distance/sheilding between the experiment and anything that might be affected (myself, neighbour's cat, etc.) Looks like I might have a starting point for some experiments. Thanks, PM
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ash Small wrote ...
That's quite a good idea, PM.
I've just done a 'rough and ready test', though.
250 cc water, 80 watts power for 30 seconds.
temperature rise around 0.5 degrees C.
Does that mean it is safe for me to put my hand in a microwave oven at 80 watts (defrost setting) for 30 seconds?
(I've tried five minutes 5 minutes, and the temperature rise, is around 3 degrees C, )
Obviously, the way a microwave oven restricts the power is be turning the magnetron on and off, using the inbuilt timer mechanism. operating a magnetron CW, as one would do for any experimentation, would require further tests, but it should be possible to set up a test with a magnetron removed from the oven, with the waveguide pointing at, and close to, an insulated container of water.(with appropriate distance/sheilding between the experiment and anything that might be affected (myself, neighbour's cat, etc.) Looks like I might have a starting point for some experiments. Thanks, PM
Designing the matching sections for coupling the magnetron output to the H2O load might well prove challenging. Mode instability and spurious emission could result from a bad match.
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antenna construction?
