1Watt AM(cw) Transmitter
for the 10 meterband
RE-TX1HF10
10-meter band AM CW transmitter
By Guy, de ON6MU
Revision v1.6 (Jan 2012)

About the 1watt AM/CW 10-meter band transmitter RE-TX1HF10

In this project, you will make a simple 3-stage low-power broadcast-type circuit, using a crystal oscillator integrated circuit and an a collector modulated AM oscillator with amplifier. You can connect the circuit to the an electred microphone or amplified dynamic microphone. Using an electred microphone is shown (in gray) in the diagram below. (no amplified dynamic microphone has a to low output voltage to work. at least 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 the 10 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 (T1 BC557).

The transmitter is build as a Colpitts Oscillator with a BSX20 transistor. HF-output of the oscillator is approx. 50 mW, depending on the supply voltage of 8 to 16 Volts. This is amplified by the BD135 and brings the power up to approx. 1 watt @ 14volts with maximum modulation. The transmit frequency is stabilized with the 28Mhz crystal. A slight detuning of approx 1kc is possible when using a 120pF trimmer capacitor for C8. The oscillator signal is taken from the collector of T2 and guided to the input of T3 which output is lead via an L-filter and low-pass PII filter circuit cleaning up the signal pretty good and ensuring spectral purity. 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 T2 freely oscillate.

The oscillator uses a single coil and crystal. The coil is tuned to the output frequency, which may correspond to the crystal frequency, or a harmonic.

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 and so powering the oscillator. You can remove the modulation section all together if you use it only for CW. The amplifier (T3) is always fed with 12...16 volts Vcc and doesn't need to be switch off together with the oscillator.

If you only gonna use this transmitter for CW, then you can leave out the modulater section (T1). But remember that there is a 3 volt difference between Vcc and the voltage on the oscillator. So with modulator 12 Vcc is 9 volts on T2, without T1 ofcourse 12v also.

RF Oscillator
Is been carried out by T2 (NPN BSX20). 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 importance, although it delivers 50mW@14v , hence can be operated with low voltage power supply with no dissipation of heat.

You could add a switch (not recommended, but if you do, use very short connections) 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.

Injection of signal of an external tuneable oscillator to trigger T2 to oscillate is possible by removing the Xtal and connecting C8 to your oscillator.

Filter
RF power amplification is also done here and this stage is coupled to the antenna system through antenna impedance matching circuitry (L1/L3,C16,C18). Care is taken at this stage so that no harmonic frequency is generated which will cause interference in adjacent band (splatter) on other bands (L3/L4,C16...C20). This 3-element L-type narrow bandpass filter circuit and a lowpass filter for the desired frequency cleans out any remaining harmonic signals very efficiently.

Modulator
Is done by T1 (PNP BC557). Audio information is impressed upon the carrier frequency at this stage. Do to selective components circuits (R10, R11, C25, C3, C4, C5, C6, C7) the voice component frequencies are enhanced, whilest others are suppressed (bandwidth +- 5kc/side).
Collector modulation is applied here. The efficiency isn't 100%, but it does keep the simplicity of the design intact. Modulation depth can be controled by changing R2 and R3.

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.

More power
You can connect the output to my power MOSFET based 10-meterband power amplifier wich should cranck up the power to approx. 6 watt. You'll find it here.

Mute: Use the transmitter 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, 2N2222 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). This is shown in the diagram below.

 

Specifications

Schematic 10-meter band AM transmitter: fig1

10meterband or 11meterband transmitter schematic

Parts list 10-meterband AM / CW transmitter

top view Ls1,Ls2,Ls3


Revision 1.2
C21 added to prevent the oscillator from oscillating at 2e harmonic when not connected to the amp-stage. If the oscillator is coupled/connected (via C11) with the input stage of the amplifier as designed (even if the amp stage is not powered) 2e harmonic oscillations are prevented even without C21.
To resolve this issue (in any situation) C21 has been added.
C5 was missing from the partslist
R2,R3,R4 had slight diviated values from standard available resistors (thanks Medard from Switserland!)
Revision 1.3
To improve T2 BIAS: R5 was 2k2, now 1k2. L5 added (100uH)
To improve T1 BIAS: R1 was 4k7, now 3k9
C12 changed
Revision 1.4
Ls1 (former between C6 and C7) is replaced by 100uH inductor
R9 added to improve modulation
Revision 1.5 (May 2009)
R10, R11, C25 added, and C3,C4,C6,C24 changed values: to improve linearity

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

Salvatore Polito made the the 1 watt 27/28mc transmitter together with the 5 watt amplifer:

Antenna's

It's important to use a correct designed antenna according to band you would like to operate, or at least use a good antenna tuner to match the antenna (protecting your transmitter and proventing harmonics/interference...). Several examples can be found on my website and all across the Web. A dipole is always a good alternative (total length = 150/freq - 5%).
The performance (distance relative to you RF power) of your antenna 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 equally important to be able to make DX QSO's.

Remember that transmitting on 10 meter band (or building and using the transmitter) needs a valid radioamateur license!

12 meterband AM / CW transmitter project


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