# A simple way to make an oscillator+amplifier+antenna circuit for very low frequencies?

Discussion in 'General Electronics Discussion' started by Tobe, Mar 19, 2017.

1. ### Tobe

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Mar 19, 2017
I'm a newbie in electronics and I want to drive a square loop antenna with very low (between 100 Hz and maybe 20 KHz) frequencies. I stuied much of BJT configurations (common collector etc.) and oscillator design (Colpitts, RC phase shifter etc.) but still I lack the skills to build a decent osc+amp+antenna circuit.

So I thought, for such low frequencies I could go for a simple speaker. Cut one of its wires (one that goes into the speaker coil) and solder a square antenna in between. Then plug into PC and with some software I could drive the antenna with sine or square waves of audio frequencies (up to 20 kHZ?).

Would this work? Do I need to care about the speakers "ohm" value? How could I calculate the peak-to-peak voltage and current through the antenna then?

Or is there any other method you would suggest?

Any help would be appreciated.

2. ### AnalogKid

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Jun 10, 2015
Why?
How large a loop?
What power level do you want?

Yes, the ohm value is important.

ak

3. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
At 100Hz, an optimal antenna will be about 750km high. A ground plane of about 2 million square kilometers should be sufficient.

Arouse1973 likes this.
4. ### AnalogKid

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Jun 10, 2015
Steve - think inductive loop audio system.

5. ### duke37

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Jan 9, 2011
An inductive loop only works if you are inside the loop or very close to it. A lot of power is required to push lots of current through the loop. I would use thick wires and put enough turns on to get 4Ω or so. Using some power to drive a speaker complicates the calculations and wastes transmitter power.

6. ### Tobe

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Mar 19, 2017
2 million square kilometers you say... Sorry that I didn't give a clear description of the purpose. I'm not aiming for far field transmission and thus an ideal antenna is not necessary here.

The loop will be a 0.2*0.2 m^2 square. Maybe it was a misconception to call it an "antenna". I only meant that it would be transmitting circularly polarized EM waves though not efficiently (it will be "electrically small"). According to here (http://www.physicspages.com/2013/04/11/magnetic-field-of-square-current-loop/) and here (http://www.angelfire.com/planet/sft_accommodation/new_page_1.htm) , I calculated that I would be able to induce voltages of about 0.5 mV to 1 mV to an identical reciever square loop with about a 7 or 8 meters distance between them, if I could drive the loop with about 1 amps peak-to-peak AC (Wİth Faraday's law of induction and other equations from the links).

Last edited: Mar 19, 2017
7. ### Tobe

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Mar 19, 2017
I wanted to build a RC phase shift oscillator and feed the output into a common collector amp which would drive a low impedance square loop. But I was lacking the understanding and knowledge to match their impedances so I settled for an inefficient speaker driven loop instead.

8. ### Tobe

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Mar 19, 2017
Loop is a 0.2 x 0.2meters square. The power I want depends on the impedance of the loopI suppose which would be very low with a thick wire and very low frequencies. I just need to run an AC of 1 or 2 Amps peak-to-peak through a square loop with audio frequencies. Without starting a fire or blowing things, that would be all.

9. ### duke37

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Jan 9, 2011
I have old wire data so I have to convert to SI leading to possible errors.
16SWG (approx 16AWG) 0.064 inch. 7.463Ω/1000yd
or 8.16E-3Ω/m
To get 4Ω need 490m or 613 turns.
1V (RMS) will give 0.25A so get 153 ampere.turns.
Drive it with a small audio amplifier.
The inductance may be a problem, audio loops for the deaf are often driven with amplifiers with current feedback or use higher powers with a series resistor.

For a pick-up aerial a higher impedance would be easier to match so use many more turns of thinner wire. A transistor could be considered to have an input impedance of the order of 1kΩ