The purpose of this 1 watt am/cw transmitter circuit is to develop a 3 stage broadcast pattern circuit. The primary components you need to build this simple tool are: a) Collector modulated AM oscillator with an amplifier and b) Crystal oscillator integrated circuit.
The circuit can be connected with an amplified dynamic microphone or to an electret microphone. Also there is possibility to enable connection directly with a dynamic microphone to the LF pre-amp stage of a transistor.
This way you can build a small system that would receive signal from an AM wave receiver.
Though the circuits used in radio stations are more complex, this simple design will at least give a clear concept behind the understanding of designing such system.
However, when you design the circuit, do keep in mind not to transmit 10-meter band, as you will radio broadcasting license to do so!
While there are a range of circuits available to design and use for AM, it is better to start with a simple circuit, like what we have used here, T1 BC557.The transmitter further uses Colpitts oscillator with a transistor [BSX20].
The HF-output of the oscillator is measured to 50 mW. However, the measurement entirely varies based upon voltage supply [8 – 16 volts]. The output is then further amplified using BD135 to bring the power to 1 watt [approx.] at the rate of 14 volts keeping modulation to 100%. The transmit frequency rate is managed by 28Mhz crystal.
If you want to detune at any point you can do it to 1kc approx. by using 120pF trimmer capacitor [C8]. Also, to clean up the signal and ensure clear output the signal of the oscillator is fetched from T2 collector, passed to T3 and further passed via L-filter and PII low pass circuit. The oscillator is also connected to morse key (S) and T1.
While the T1 morse key is biased and is not used for modulation, this leads T2 to oscillate freely, without obstruction. The oscillator, which is made up of crystal and single coil, when tuned for the output frequency will generate harmonic or crystal frequency.
AMPLITUDE MODULATION [AM]: Amplitude modulation is one of the most common electronic standard used to transmit information via carrier waves. Since the radio frequency generates amplitude at all time for no modulation, therefore, it is unable to carry any audio information.
AM is widely used during communication via morse code. Simply put the AM produces signal on two adjustment sidebands and power stationed at the carrier frequency level. It is for this reason the majority of the power output generated by the transmitter gets wasted.
CW: It is a very basic modulation form where a transmitter output when switched ON and OFF state helps to build the characters in the Morse Code. CW transmitter is cheap and widely available in the market. Furthermore, the signal carried by the CW have its frequency less than 500Hz.
Because of low frequency the sound generated remains somewhat inaudible only a faint noise can be heard] on a general receiver. However, to make up the crisis ham radio receivers and shortwave has a beat frequency oscillator [BFO] attached to its circuit.
The BFO helps to generate second carrier internally. The internal carrier beats the signal received by CW transmitter and further generates a tone that goes in ON/OFF state upon the receipt of CW signal. This is actually the process the shortwave receives the Morse code signals.
RF Oscillator: It is on the T2 [NPN BSX20]. AT this stage the carrier frequency is generated by Crystal Oscillatory Circuitry.
Alternatively, the frequency can be generated through capacitance-inductance Variable Frequency Oscillator [VFO]. While the RF oscillator is developed with an intent to manage frequency stability [Xtal] and power delivered to 50mW@14v, it is equipped to work on low voltage power supply. Moreover, it generates very less heat.
Filter: Here the power amplification of RF is taken care. The RF is connected to the antenna via antenna impedance matching circuitry [L1 / L3, C16, C18].
Also it is important to take care so as not to generate any harmonic frequency. Generating the frequency will end up interference in the splatter and other adjacent bands [L3 / L4, C16, C17, C18, C19, C20].
Furthermore, the L-type narrow band-pass filter circuit [3-element] along with a low-pass filter used to generate the frequency efficiently removes the harmonic signals.
Modulator: This is managed in T1 [PNP BC557] where the audio is taken care of. Here, the voice gets enhanced because of some component circuits that [R10, R11, C25, C3, C4, C5, C6, C7], whereas the others get suppressed [+- 3kc/side bandwidth] by managing the HAM radio specs along with the use of collector modulation. However, it is important to consider not to overdo the modulation [don’t exceed above 100%] as that may lead to distortion.
Housing / Shielding: Care should be taken to mount the whole system onto a metal case [aluminum based]. And if you are unable to manage steel-made cases an alternative would be to arrange graphite-painted PVC box or self-sticking aluminum tape. Also ensure that the aluminum tape pieces are conducting each other.
However, following are the specifications to design the proposed 1watt AM/CW 10-meter band transmitter RE-TX1HF10:
- RF PEP Power Output: 1W@14v, modulation rate 85% [approx.]
- Frequency range at peak: 28Mc – 30Mc
- AM Modulated [if it is keyed then CW]
- Output impedance adjustable [till 50 ohms]
- Low-pass PII filter + harmonic L-filter [band-pass type]
- Voltage: vcc 10 – 16 volts
- Average current: I= 120mA
- XTal Oscillator: 28.xxx
- Frequency adjustment of 2Kc
- LF Input: +/- 100mV@1K
- Connecting with external oscillator
Parts list 10-meterband AM / CW transmitter
T1 =BC557 (modulator)
T2 =BSX20 oscillator (2N2219. BC109 works also, but little less power)
T3 =BD135 amplifier (with heat sink isolated from the transistor)
T4 =2N2222, BC338 mute
C1 = 100nF
C2 = 47uF/16v (tantal)
C3 = 2.2 uF/50v (changed in rev v1.5)
C4 = 33nF (polyester) (changed in rev1.5)
C5 = 10nF (polyester)
C6 = 47nF (changed in rev1.5 )
C7 = 4.7uF/50v
C8 = 10nF
C9 = 0…22pF (60pf for 27Mc)
C10 = 120pF
C11 = 56pF
C12 = 470uF/16v
C13 = 100nF
C14 = 47nF
C15 = 470pF
C16 = 6…40pF
C17 = 12pF
C18 = 120pF
C19 = 56pF
C20 = 100pF
C21 = 470pF
C22 = 100nF
C23 = 10pF*(added in revision v1.2)
C24 = 33nF (changed in rev1.5)
C25 = 0,47uF (polyester, added in rev1.5)
R9= 4k7* (added in revision 1.4)
R10= 270 (added in rev1.5)
R11= 390 (added in rev1.5)
Ls1, Ls2 = 470 1/2 watt carbon, 0,2 Cul turned 3 times over the entire length of the resistor (or 2.7uH inductor)
L1 = 0.8 mm insulated copper wire, 8.5 turns close together, 7mm inner diameter
L2 = 0.8 mm insulated copper wire, 12 turns close together, 6mm inner diameter
L3 = 0.8 mm insulated copper wire, 13 turns close together, 7mm inner diameter
L4 = 0.8 mm insulated copper wire, 7 turns close together, 7mm inner diameter
L5 = 100 uH inductor (*added in revision 1.3)
L6 = 100 uH inductor (*changed in revision 1.4)
Crystal fundamental frequency or overtone for your desired frequency (28…30Mc)
C4, C5, C6, C25 polyester film capacitors