I'm trying to prepare small demo for school lecture on radio history.
sorry for confusion, MW here means medium wave, not megawatt :)
And I want to create small and very simple transmitter which could be "heard" with old factory built receiver. There are two points in this demo:
- to show that AM is really simple kind of modulation and explain construction as best as I could (my pupils have some basic understanding of physics and electronics);
- to show that old receivers are not "broken" (as I was asked several times), but rather that there are almost no stations broadcasting.
I'm working from the idea expressed in the picture below (it refers to "Electronics World, August 2004" - but I couldn't find original) - simply to have logic generator on desired frequency (here 555 timer) driving antenna with its push-pull output:
I'm not sure that author of this design really succeeded as he explains in making it work for distances up to 25 meters. It's said "antenna is just 6-10 feet of wire".
I've used spare Arduino to drive pin with 1.5 MHz (most 555s I've seen are not working well at the frequencies above 100-200 kHz) and apply some trivial modulation. Attaching 2-3 feet telescopic antenna for test I've found that the signal is "heard" with portable radio in a wide range of frequencies (say, from 1.8 to 0.8 MHz - I believe because output is not sine wave) and only in very close range.
I've googled a bit and found the "rule of thumb" that if your antenna is short (compared to wavelength) it should be loaded with series inductance (and with capacitance if long). I've tried several which I had at hand and found the best result is with 22uHn. Now it is heard only when receiver is precisely tuned and at about 1-1.5 meters.
But I'm curious to improve it to range of 10 meters (at least it would be good to have range larger than antenna).
So I have questions:
- what can be done (besides trying longer antenna) to improve range (if this is possible at all - I'm not sure I've ever seen working MW transmitter of small scale);
- is there any guide to load antenna with inductance (choosing proper value etc.)
- can some different type of output for driving antenna make it work better (e.g. transistor with inductance in collector?) - are there any general recommendations
- may I benefit from trying loop antenna? As I understood from wikipedia, small antennas for medium/long waves are not good for transmitters...
Thanks in advance!