# How to transmit a 125kHz signal?

I want to transmit a 125 kHz signal. The square wave signal is generated by the microcontroller (just transmitting no data and no receiving,) the transmitting and receiving antennas are tuned to 125kHz.

I connected the transmitter antenna as the picture below.

On the receiving antenna the oscilloscope shows clear signal and peak to peak up to 80 volts when both antennas are closest to each others, the signal decreases as the distance between the antennas increase. At a distance of about 7 cm the the signal completely disappears.

How can I design a 125khz antenna driver? (The desired distance is 2 to 3 meters.)

• Use lna+filter, direction defined antennas or higher voltage Commented Apr 1, 2020 at 7:08
• 125kHz is a very low frequency for long range transmission. The transmission nature will be inductive coupling, which has a very high decay with distance. Commented Apr 1, 2020 at 18:34
• Don't try to transmit it. Use an air core transformer. Primary is the transmit antenna and the secondary is the receive antenna. Make the coil as large in diameter as you can, and tune it (with a capacitor) so that it resonates at 125 kHz to minimize noise pickup from out of band sources. Commented Apr 2, 2020 at 0:01

First, a 125 kHz square wave signal contains significant energy in higher harmonics, e.g. 1/3 the amplitude (a) of the fundamental frequency (f) at 3 times f, a/5 at 5*f, a/7 at 7*f... This means that the output coil is radiating at multiple frequencies, at least in to the AM broadcast band for a moderate quality waveform.

The wavelength of 125 kHz is (3 * 108 m/s) / (1.25 * 105 s-1), ~2,400 m (2.4 km). A quarter-wavelength is the "standard" moderately efficient antenna, so you could use 600 m long wires on the transmitter and receiver to increase range. That would be impractical for a classroom experiment, though.

You have some other ways to increase the range though:

1. Use a parallel circuit tuned to the fundamental frequency, i.e. put the correct value capacitor across L1, calculated to resonate at 125 kHz on the basis of its inductance. Do the same for the transmitter output conductor.
2. Experiment with odd harmonics, as above, i.e. tune both circuits to 375 kHz, 625 kHz, etc. This is a trade-off: reduced power at the higher harmonic is compensated somewhat by shorter antenna design.
3. Experiment with a base-loaded 1/4 wave antenna. By inserting an inductor between the R1-L1-T1 junction and the antenna, the antenna/coil combination can be made to resonate at a much lower frequency.

You can combine a parallel-tuned "tank" circuit with a base loaded antenna and greatly increase range.

• Probably a good circuit to use is called a “class e” amplifier. There are calculators on the web to design these circuits. They are very efficient. Use Google. Commented Apr 2, 2020 at 1:20

This base-loaded whip antenna may help.

The receive antenna should be identical.