Summary of 1750 Meter Lowfer Band amplitude modulated RF source using ATtiny2313 microcontroller
This article details a low-power 187 kHz RF signal source built for receiver testing under FCC Part 15 regulations. The project utilizes a 74HC4060 oscillator to generate a frequency, which is then modulated by a microcontroller and amplified via a gated current source driving a resonant loop antenna. The design emphasizes strict power limits to ensure compliance with license-free operation rules while acknowledging the inefficiency of short antennas at this frequency.
Parts used in the Low Power Signal Source:
- 74HC4060 oscillator/divider IC
- Crystal oscillator (6.000 MHz)
- IC socket (modified)
- Microcontroller (ATtiny2313)
- 2N4401 transistor
- 2NSC2458Y transistor
- 9V transistor radio battery
- Resonant loop antenna
- 10 uF capacitor
- LED with internal resistor or external 120 Ohm resistor
- 330 Ohm resistor
- 10 Ohm resistor
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..
CHAPTER I–FEDERAL COMMUNICATIONS COMMISSION
PART 15–RADIO FREQUENCY DEVICES–Table of Contents
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:
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.
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.
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What regulations allow unlicensed operation on the 160-190 kHz band?
Federal Communications Commission Part 15 rules permit operation without a license if total input power does not exceed one watt and the antenna length does not exceed 15 meters. -
How is the 187.5 kHz frequency generated in this circuit?
The 74HC4060 oscillates at 6.000 MHz and divides the signal by 32 to produce 187.5 kHz. -
Can I use an LED without an internal resistor in this circuit?
If using an LED without an internal resistor, you must use a 120 Ohm resistor in series instead of the 10 Ohm resistor shown in the original design. -
What type of modulation is transmitted in this project?
The project transmits tone modulated AM (Mode A2) using Morse code modulated by an audio frequency square wave. -
Why is the antenna considered a poor radiator?
The maximum antenna length of 15 meters is electrically very short compared to the 1600 meter wavelength of the 160 kHz band. -
What is the peak emitter current of the 2N4401 transistor?
The peak emitter current is approximately 1.8 milliamps, determined by a 0.6 volt drop across the 330 Ohm resistor. -
How much power does the output stage supply?
The signal source supplies only a few milliwatts to the output stage when powered by a 9V battery.
