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(Improved) 187 KHz RF Source
An oscillator, a small power stage, some modulation, and a tiny loop antenna make RF for experiments on at 187.5 kHz on the United States' FCC Part 15 Lowfer  band (1600-1750 meters).

Notice: Before operating a radio transmitter, find out what kind of transmitter operation, if any, is permitted in your locality. Radio transmitter operation is a serious legal matter. In the United States, operation of unlicensed intentional radiators is covered by Part 15 of Title 47 of the Code of Federal Regulations. This design can be readily adapted to different frequencies and different power levels. If you choose to build and operate the transmitter described here, you do so at your own risk. I'm only publishing this as an example of what can be done.

Download the firmware: mor040220BBeacona.asm


The oscillator board is not much more than the 74HC4060 oscillator/divider. The crystal is in a socket. made by cutting down an IC socket.

This is a low power signal source I put together one evening to provide 187 KHz RF signals for an anticipated receiver investigation. Under Federal Communications Commission rules inside the United States, one is allowed to operate a transmitter without a license under certain conditions. Here, I have copied the relevant section of the most recent version of the Code of Federal Regulations that I could find..

Subpart C--Intentional Radiators
Sec.  15.217  Operation in the band 160-190 kHz.

(a) The total input power to the final radio frequency stage
(exclusive of filament or heater power) shall not exceed one watt.

(b) The total length of the transmission line, antenna, and ground
lead (if used) shall not exceed 15 meters.

(c) All emissions below 160 kHz or above 190 kHz shall be attenuated
at least 20 dB below the level of the unmodulated carrier. Determination of compliance with the 20 dB attenuation specification may be based on measurements at the intentional radiator's antenna output terminal unless the intentional radiator uses a permanently attached antenna, in which case compliance shall be demonstrated by measuring the radiated emissions.

If you are thinking seriously about building a Part 15 transmitter, I suggest you read through Part 15 to make sure you are fully compliant. The FCC provides for very heavy penalties for those who step out of line.

Here is where I found this copy of Part 15:


I have known about the provision that allows 1 watt license free operation on this band for a long time, but didn't really take it seriously. The maximum antenna length is only 15 meters. This  is electrically very short compared to the 1600 meter wavelength and is therefore be a very poor radiator.

However poor the efficency of the antennat some people using sophisticated coding and dedecoding techniques have managed to communicate over thousands of miles in this band (160 to 190 kHz), all operating within the scope of Part 15. Amazing.

In order to get started tinkering with circuits on this band, I put together a low power signal source.

This signal source only supplies a few milliwatts to the output stage when the +8V input is supplied by a 9V transistor radio battery. The low power in the output stage results from the collector load being a parallel tuned stage, thus it is a high impedance at resonance.


The charging current for the 10 uf capacitor in the output stage immediately after switching on the power can damage the LED. The LED in this circuit has an internal 100 Ohm resistor. If you use an LED without an internal resistor, use a 120 Ohm resistor in series with the LED instead of the 10 Ohm shown here. Most LEDs don't have internal resistors.

Circuit Description

Oscillator and divider
The 74HC4060 oscillates at 6.000 MHz, and is divided by 32 to get 187.5 kHz.  Pin 5 of the  CD4060 is a 732 Hz square wave which can be used to modulate the output stage rather than using a micro controller, but in this instance, the modulation is supplied by a micro controller that I programmed to send Morse code to the modulation input, with the Morse code modulated by an audio frequency square wave, resulting in tone modulated AM (Mode A2) being transmitted.

Output Sage
The output stage is a gated current source driving a resonant loop antenna. The emitter current of the 2N4401 causes a voltage drop across the 330 Ohm resistor. The 2NSC2458Y reduces the amplitude of the 187.5 kHz carrier signal on the base of the 2N4401 so that the maximum voltage across the 330 Ohm resistor is 0.6 volts, the base-emitter voltage of the 2NSC2458Y. The 0.6 volt drop across the 330 Ohm resistor means that peak emitter current for the 2N4401 is about 1.8 milliamps peak emitter current. Since the collector current is approximately equal to emitter current, this results in the resonant circuit on the collector of the 2N4401 being driven by a 1.8 milliamp square wave.

On each cycle, the 2N4401 puts some energy into the resonant circuit in the form of collector current. Some of the energy is dissipated in the resistance of the coil and a very tiny bit of it is radiated away, but most of the energy continues to slosh back and forth between the magnetic field around the coil and the electric field in the capacitor's dielectric. In the circuit I built, I measured about 30 milliamps peak-to-peak in the coil. The collector wave form is a nice sine wave. Using a Tektronix TDS-2002 in the FFT spectrum analyzer mode to look at the collector voltage, the second harmonic was in the noise and the third harmonic was 32 db below the carrier.

Proof that the 2N4401 is acting as a gated current source is that the collector voltage is 12 volts peak-to-peak when power from either an 8.4 volt battery or a 15 volt power supply. The collector current and therefore the voltage across the tank circuit, is independent of power supply voltage.

The current from the output stage is modulated by shutting of the 2N4401 by injecting current into the base of the 2NSC2458Y through the 4.7 k resistor and the 1N4148 diode. The 1N4148 is not necessarily required. I used it so that I wouldn't have to take the drive capability of the output pin of the AT90S2313 (or ATtiny13) micro controller into account when calculating the emitter current. A theoretical advantage of the 1N4148 is that it provides some isolation between the gating signal and the output stage since the diode is reverse biased, or at least nor forward biased, during the time the 2N4401 is sourcing current.

The transistor in the output is a 2N4401 because it has pretty clean switching characteristics and has plenty of current gain and a moderate cutoff frequency. In other words, it makes a pretty good current source and cuts off cleanly. The current feedback transistor, the 2NSC2458Y, was selected because I have thousands of them and am looking for a way to work them into my projects. It could also be a 2N4401 or a 2N2222, for that matter.


By request, a link to the AVR Studio assembly lanugage source code for the firmware for the AT90S2313 was placed at the top of this page.  The firmware can also run on an ATTINY2313 without modification. The source code was only commented for my own use, and the code itself is not pretty.

While you can use the AT90S2313 firmware included on this page as a Morse code generator, you might prefer to use the more finished version, the Low Cost Telemtery Device, which was written for the ATMEGA8, and is basically a plug-and-play implimentation. Just load the code into and ATMEGA8 and program the output strings and baud rate with an ASCII terminal. The basic operation of the Morse code generation is the same for both projects.  Here is a link to the Low Cost Telemetry Device.


Earlier versions

One reader of this page has performed an extensive investigation of the earlier circuit, the one in which the 2N4401 is saturated rather than driven as a current source, to show that since the base drive for the 2N4401 is a square wave, care must be taken to assure that the transistor does not saturate if a very clean sine wave is desired in the inductor.  After a couple of years, I got around to changing the circuit to being current driven so that a low harmonic sine wave can be produced without hand tweaking the base drive.

Remember, read that Part 15!  Read the Liability Disclaimer at the bottom of this page, too.


Contents 2003, 2004, 2005, 2007 Richard Cappels All Rights Reserved. http://www.projects.cappels.org/
Dick Cappels' web version first posted in December, 200; updated February, March 2004, October, 2004, and November, 2004, April and November, 2005, and May, 2007.

You can send  email to me at projects(at)cappels.org. Replace "(at)" with "@" before mailing.

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(Summary: No warranties, use these pages at your own risk. You may use the information provided here for personal and educational purposes but you may not republish or use this information for any commercial purpose without explicit permission.)
I neither express nor imply any warranty for the quality, fitness for any particular purpose or  user, or freedom from patents or other restrictions on the rights of use of any software, firmware, hardware, design, service,information, or advice provided, mentioned,or made reference to in these pages. By utilizing or relying on software, firmware, hardware, design, service,information, or advice provided, mentioned, or made reference to in these pages, the user takes responsibility to assume all risk and associated with said activity and hold Richard Cappels harmless in the event of any loss or expense associated with said activity. The contents of this web site, unless otherwise noted, is copyrighted by Richard Cappels. Use of information presented on this site for personal, nonprofit educational and noncommercial use is encouraged, but unless explicitly stated with respect to particular material, the material itself may not be republished or used directly for commercial purposes. For the purposes of this notice, copying binary data resulting from program files, including assembly source code and object (hex) files into semiconductor memories for personal, nonprofit educational or other noncommercial use is not considered republishing. Entities desiring to use any material published in this pages for commercial purposes should contact the respective copyright holder(s).