QRP AM(cw) Transmitter
for the 20 meterband
RE-TX02HF20
14 MHz (20-meter band) AM/CW QRP HF transmitter
By Guy, de ON6MU
Revision v1.4 (Sept 2018, update dec 2023)

About the QRP AM/cw 20-meter band transmitter

In this project, you will make a simple low-power broadcast-type circuit, using a crystal oscillator integrated circuit and an a collector modulated AM oscillator. You can connect the circuit to the an electrec microphone (pointed out in gray on the diagram) or amplified dynamic microphone (no amplified microphone has a to low output voltage to work. Approx. 100mv is needed). You could also add a LF preamp stage of one transistor to allow connecting a dynamic microphone directly.

You'll see that you can receive the signal through the air with almost any AM radio receiver. Although the circuits used in radio stations for AM receiving are far more complicated, this nevertheless gives a basic idea of the concept behind a principle transmitter. Plus it is a lot of fun when you actually have it working!
Remember that transmitting on 20 meter band you'll need a valid radioamateur license!

A wide range of different circuits have been used for AM, but one of the simplest circuits uses collector modulation applied via (for example) a transformer, while it is perfectly possible to create good designs using solid-state electronics as I applied here (T2 BD135).

The transmitter is build as a Colpitts Oscillator with a strong 2N2219(A) transistor. HF-output of the oscillator is 400 to 600 mW, depending on the supply voltage of 10 to 15 Volts. The transmit frequency is stabilized with the 14.3Mhz crystal which can be bought in almost any electronicparts shop. A slight detuning of approx 2kc is possible by using C11 trimmer capacitor. The oscillator signal is taken from the collector of T2 by induction and via a low-feedthrough filter and guided to the output via an L-filter circuit cleaning up the signal pretty good. The oscillator is keyed by T1 and the morse key (S). By keying the morse-key T1 is not been used for modulation and is biased, hence lets T4 freely oscillate.

AM

Amplitude Modulation (AM) is a process in which the amplitude of a radio frequency current is made to vary and modify by impressing an audio frequency current on it.
This was the first type of modulation used for communicating signals from one point to another and is still the simplest to understand.
A radio frequency current has a constant amplitude in absence of modulation and this constant amplitude RF carries no information, i.e. no audio intelligence and is of no use to radio telephone (voice communication), but has application in morse code communication.
In its basic form, amplitude modulation produces a signal with power concentrated at the carrier frequency and in two adjacent sidebands. Each sideband is equal in bandwidth to that of the modulating signal and is a mirror image of the other. Thus, most of the power output by an AM transmitter is effectively wasted: half the power is concentrated at the carrier frequency, which carries no useful information (beyond the fact that a signal is present); the remaining power is split between two identical sidebands, only one of which is needed.

CW
CW is the simplest form of modulation. The output of the transmitter is switched on and off, typically to form the characters of the Morse code.
CW transmitters are simple and inexpensive, and the transmitted CW signal doesn't occupy much frequency space (usually less than 500 Hz). However, the CW signals will be difficult to hear on a normal receiver; you'll just hear the faint quieting of the background noise as the CW signals are transmitted. To overcome this problem, shortwave and ham radio receivers include a beat frequency oscillator (BFO) circuit. The BFO circuit produces an internally-generated second carrier that "beats" against the received CW signal, producing a tone that turns on and off in step with the received CW signal. This is how Morse code signals are received on shortwave.

Although this design is primarely designed for AM, it can be used for CW by keying S.

RF Oscillator
Is been carried out by T4 (NPN 2N2219). This is the stage where the carrier frequency intended to be used is generated by means of Crystal Oscillator Circuitry or capacitance-inductance based Variable Frequency Oscillator (VFO). The RF oscillator is designed to have frequency stability (Xtal) and power delivered from it is of little bit more importance, although it delivers 600mW@12v , hence can be operated with low voltage power supply with little dissipation of heat. However, here we use the oscillator for a bit more power and so it does neet a heat sink.

You could add a switch (very short connections if using an ordinary switch) to select different Xtal's (frequencies). You could also use a more effective diode-based switch I've build here. This hasn't got the problems with longer connections at all.

Filter
RF power amplification is also done here and this stage is coupled to the antenna system through antenna impedance matching circuitry (L1/L2/L3). Care is taken at this stage so that no harmonic frequency is generated which will cause interference in adjacent band (splatter) on other bands (C17...C21). This 3-element L-type narrow bandpass filter circuit and a low-pass filter for the desired frequency cleans out any remaining harmonic signals very efficiently hence good spectral purity.

Modulator
Is done by T1 and T2. Audio information is impressed upon the carrier frequency at this stage. Do to selective components circuits (C1,R1,R2,C2,C3,C4,C8) the voice component frequencies are enhanced, whilest others are suppressed (bandwidth +- 3kc/side) keeping it between specs. The modulation depth need/can to be controled by R6.
Important: Remember that the modulation must not exceed the maximum power. This means that you need to set R6 at least 50% the maximum RF power.

Why overmodulation is not desirable?..
Overmodulation is not desirable, i.e. modulation should not exceed 100 %, because if modulation exceeds 100 % there is an interval during the audio cycle when the RF carrier is removed completely from the air thus producing distortion in the transmission.

Housing/shielding
The whole circuit needs to be mounted in an all-metal/aluminum case. If you're unable to obtain an all-metal case, then use a roll of self-sticking aluminum tape (available from your hardware store) or PVC box painted with graphite paint. Just make sure that all individual pieces of aluminum-tape (or the graphite paint) are conducting with each other. Works fine.

Use it with your receiver
If you put a relay, or better a transistor switch to mute your receiver (if equiped) you can easily make a QSO HI. A simple BC338, Bc547, 2N2222 (T3) at pin a" with the base biased with a 100k resistor, emmitor at the gnd and the collector fed to your receiver's mute input works fine. Or you can use a 12v relay... Every time you PTT the transistor (or when using a relay, the switch) is "shortened" between the ground, hence muting your receiver (again; if your receiver has mute capabilities).

Schematic for the QRP 20 meter band 14MHz HF transmitter / oscillator

Parts list 20-meterband transmitter

Ls1, Ls2

Revision 1.1
Filter unit efficiency peaked by changing C19 & C20
BIAS T4 improved by changing R9 + adding ferrite in series
Revision 1.2
Audio modulation spectrum and linearity improved (changing C1,C2,C3)
Revision 1.3
Ls1 coil removed, C9 removed (extra choke was not needed)
Revision 1.4
R6 increased to 500 Ohm to allow better control AM modulation depth

 

Note:
Always use a dummy load for testing and adjusting the transmitter!!!

Specifications

Antenna's

It's important to use a correct designed antenna according to band you would like to operate, or at least a good tuned antenna using a matcher (protecting your transmitter). Several examples can be found on my website and all across the Web. A dipole is always a good alternitive (total length = 150/freq - 5%).
The performance (distance relative to you RF power) of your transmitter/transceiver is as importent (if not more) as the RF power you transmit! A dummy load gives also a perfect 1:1 SWR, but you wont get any farther then the street you live in HI. Finally, athmospheric conditions (D-,E-,F-layers depending on the frequency you're using) is as important as all the above.

Related

Remember that transmitting on the 20 meter band needs a valid radioamateur license!

Another related project:10 meterband transmitter project


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