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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:

ne555 as transmitter

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!

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    \$\begingroup\$ What about ground for your short antenna? It is not mentioned. \$\endgroup\$ – glen_geek Feb 24 '18 at 16:43
  • \$\begingroup\$ "What about ground..." - really I've missed this. I believe in my installation power cable works as ground, but I now understand I should have dedicated one... \$\endgroup\$ – Rodion Gorkovenko Feb 24 '18 at 19:30
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I think you are using the 555 as an FM modulator. This describes the function of pin 5 (the pin you are applying your signal into): -

Control Voltage, This pin controls the timing of the 555 by overriding the 2/3Vcc level of the voltage divider network. By applying a voltage to this pin the width of the output signal can be varied independently of the RC timing network. When not used it is connected to ground via a 10nF capacitor to eliminate any noise.

Hence what you get is FM and not AM but, with virtually any form of modulation on pin 5 you will get something detectable on an AM radio tuned to one of the harmonics. However, if you want to demonstrate something discernible and believable to your pupils, then use AM for sure. You can make a simple AM modulator from an oscillator, a diode and an LC filter and it can be made to work very well. Applying an amplifier at the output can boost signals considerably but may be illegal in your area of the world.

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)

A single monopole antenna needs a decent ground for it to work so that is one point.

is there any guide to load antenna with inductance (choosing proper value etc.)

For a short monopole (i.e. electrically short compared to a quarter wavelength), then the antenna looks "capacitive" so please look at this graph and use the same reactance inductor in series: -

enter image description here

can some different type of output for driving antenna make it work better (e.g. transistor with inductance in collector?)

There is no great substitute for making a "short" monopole longer and using an earth plane. Using an inductor to cancel the capacitance gets a third of the job done - the radiating resistive impedance that remains after the cancellation is still very low (just a few ohms for a short antenna) so another improvement is to use a step-down transformer to project the low radiation resistance of 1 or 2 ohms up to something manageable with low power semiconductor outputs.

may I benefit from trying loop antenna?

At circa 1 MHz the loop is virtually purely inductive hence it generates a magnetic field and very little electric field. Given that the impedance of free space is 377 ohms, it is preferable to generate an electric field strength to magnetic field strength ratio of 377:1.

Small loop antennas at this frequency therefore only produce a magnetic field and this decays with distance squared (rapidly becoming a distance-cubed at about one diameter from the loop).

Compare this with a proper EM wave - both fields sustain each other and the decay in strength is proportional only to distance. The is the real beauty of an EM wave.

So, going to a magnetic loop is not going to give a benefit. They work greate at picking up the magnetic content of an EM transmission but, when they are small relative to wavelength they are crappy as transmitters.

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  • \$\begingroup\$ "I think you are using the 555 as an FM modulator" - as I told this was the basis of idea. And yes, the original schematic works as FM due to Mic on pin 5. However it may still give change for AM receiver - when frequency deviates, amplitude lowers... However, as I told, I used arduino which yields properly modulated signal. Thanks for explanations about antennas, I now have something to learn :) \$\endgroup\$ – Rodion Gorkovenko Feb 24 '18 at 19:28

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