Monday, April 28, 2014

The Guerrilla AM Transmitter

Q1 - MPS2907 RS p/n 276-2023, or a 2N3906   I used one from the 15 pack RS p/n 276-1604
Q2 - IRF510   RS p/n 276-2072   - drill a hole, mount with a bolt and nut to
    the copper board, or to a little heatsink on the board, or your chassis.
    And use an insulator and thermal grease. This part gets warm.
R1 - 47,000 ohms .125 or .25 W   anything from 27K to 100K here is OK.
R2 - 10 Ohms, .5 or 1 Watt
R3 - 100 Ohms, .5 or 1 Watt
Low-cost ceramic disc capacitors are used.   But if you have some better silver-mica
or polystyrene types, go ahead and put them in this circuit.   And for C6,
a variable capacitor with a range of 500 to 1500 picoFarad is nice.
Or a 500 picoFarad or so compression trimmer, and then add a 470 pico-Farad
capacitor in parallel to it.

C1 - 100 picoFarad   RS p/n 272-0123
C2, C3, C4, C7 - .1 microFarad RS p/n 272-135.   For C2, C3, C4 this is a RF bypass
    capacitor, so anything .1, .47, .82 microFarad is good here.   C7 is a RF coupling
    capacitor, so anything from .01 to .47 or .82 is OK
C5 - 500 picoFarad   I used two .001 microFarad RS p/n 272-0126 in series.
    but anything from 470 to 680 picoFarad is OK here.
C6 - 1000 picoFarad (.001 microFarad) RS p/n 272-126

The inductors are made from Radio Shack p/n 273-102.   Un-wind the wire off,
then break the inductor core into two pieces.   Use two plyers and grab
it then snap it in half.   Make 4 inductors out of 2 cores.
L1 - 17 turns of the wire taken off of new inductor, wound in the center
    of this piece of core, and glued down.
L2 - Flyback inductor.   10 turns of the wire removed from the new inductor,
    wound in the center of this piece of inductor core, and you can glue it down.
    But putting AWG 18 or 16 would be a little bit better.
    And not critical on the turns count. 5 to 15 turns or more OK.
L3 - 7 turns of the wire that was removed. But better to use some AWG 18 or 16.
    Put this winding in the center of the piece of inductor core.
    Do not glue down yet.   This inductor with capacitor C6 determines
    the output power.
L4 - take the wire that was removed and wind it on the whole length of this
    piece of inductor core.   Or you can use a whole new p/n 273-102 here.
    Or any inductor with over 10 microHenries, and a low resistance,
    say less than a few Ohms.     So any of them oddball inductors
    on them toroid cores will be OK here.   L4 just establishes a zero
    point for the output RF waveform, after capacitor C7.
- - To test and tune up:   connect "RF out" to a 50 Ohm dummy load, or your tuned-up
radiating antenna system.   Connect the "modulating voltage" to the 13.8 V supply
along with the oscillator section.   Spread out the turns on inductor L3 to tune up,
for 6 to 8 Watts of power.   Then cut some of the wire off, so that you can wind it
in the center of the core, to get this 6 to 8 Watts of RF output.   On the 2 of these
transmitters that I have made so far, I have used 5 turns of 18 and 16 AWG,
close wound, in the center of this piece of the inductor core.

The "Talking" Pixie2 QRP AM Transmitter

The Pixie2 is a simple QRP CW transmitter that dozens of ham radio operators have successfully built. (QRP is ham jargon for low-power operations, and CW is the simplest method of sending Morse code merely by turning a carrier-wave on and off.) The Pixie2 is usually built for the 40 meter band but it will work on frequencies from 1000 kHz up to at least 15 MHz. It is said to output a couple hundred milliwatts of RF.
The circuit can be amplitude modulated quite easily. A small audio amplifier feeds audio current into the 8-ohm side of a transformer. The 1k ohm side of the transformer is inserted in the V+ supply going to the Pixie's output transistor.
This modified Pixie2 is called the Talking Pixie. It has 18 components (not counting circuit board, jacks, power supply and external audio amp). Building it on a prototyping board only takes a few minutes if all the parts are available.
The level of the audio fed to the transformer is adjusted until the best sound quality is achieved. The Talking Pixie will not sound as loud as commercial stations but the user must avoid the temptation to over-modulate; nobody will listen to an over-modulated signal.

parts list

C1: 100 pF
C2: 220 pF
C3: 82 pF
C4: .01
C5: .01
L1: 150 uH
L2: 22 uH
Q1: 2N2222 or 2N3904
Q2: 2N2222A (metal can type) or 2N3866
R1: 47K
R2: 1200
R3: 33K
R4: 10 or 15 ohms (experiment!)
T1: 1000 ohm to 8 ohm audio transformer
The frequency is crystal-controlled. A crystal for the frequency you're interested in will have to be ordered if you don't have one handy. The transformer must be rated to handle at least half a watt of audio; a very tiny transformer will not sound good and will have too much resistance on the 1K winding. L3, C6 and C7 form a low-pass filter to attenuate the harmonics generated by the circuit. Specific values for various frequencies can be found on the Medium Wave Alliance's filters page. L1 and L2 are factory-made axial molded chokes.


6W RF Power Amplifier

This amplifier is based on the transistor 2SC1970 and 2N4427.The output power is about 1.3W and the input driving power is 30-50mW.You can use other transistor as 2SC1971 and get much more output power.1.3W will still get your RF signal quit far and I advice you to use a good 50 ohm resistor as dummy load.Make sure it can take up to 5-10W, else it will be a hot resistor. You MUST use an antenna or 50 ohm dummy resistor while testing else you burn up the transistor.
RF Power Amplifier 1.3W to 6W by 2SC1970

In all RF system and specially in RF amplifiers, it is very important to have a stable power supply and making sure you won’t get any RF out on the power line. The Capacitor C12 and C13 will stabilise the DC power supply. L1, C10, C11 and L3 with C8, C9 will also prevent RF from leaking out to the powerline and cause oscillation or disturbances. L1 and L3 should be ferrite chokes or inductance’s about 1 to 10 uH.
Transistor Q1 will act as a buffer amplifier, because I don’t want to load the previous stage to much.The input RF signal is passin C1 and F1 which is a small ferrite pearl where the wire just passing through.F1 with C2 will act as an impedance matching for Q1.F1 can be substituted with a coil as L4, but in my test I found that the ferrite pearls gave best performances.L2 is nit a critical component and any coil from 2-10uH will do the job. Q1 will amplify the input signal from 50mW to about 200mW.Q1 can amplify much more, but It doesn’t need to do that because 200mW is good for the final transistor.If you want higher power you can decrease the resistor R2.
If you look at Q2 you will also find a ferrite pearl F2 at the base to emitter. This ferrite pearls is to set the DC voltage to zero and be a high impedance for RF signals. I wounded the wire 4 times around this small ferrite pearl. You can substitute it with a coil of 1uH or more.C4, C5 and L4 forms an input matching unit for the transistor. Not much we can do about that…At the output of the final transistor Q2 you will find 2 coils L5 and L6. Together with C6 and C7, they form an impedance unit for the antenna and also for the transistor.


The circuit of a 7MHz C W / A M QRP transmitter described here can be used to transmit either CW or audio frequency Modulated signal over a 7MHz carrier.


The carrier frequency oscillator is crystal controlled using 7MHz crystal in its fundamental mode. The tank circuit comprises a shortwave oscillator coil which can be tuned to 7MHz frequency with the help of ½J gang capacitor VC1. Transistor T2 (with identical tank circuit connected at its collector as in case of transistor T1) serves as a power amplifier.

The RF output from oscillator stage is inductively coupled to the power amplifier stage. The output from power amplifier is routed via capacitor C3 and inductor L3 to a half-wave dipole using a 75-ohm coaxial cable. ½J gang capacitor VC3 along with inductor L3 forms an antenna tuning and matching network between the output of power amplifier stage and coaxial transmission line for maximum power transfer. Suitable heat sink should be used for transistor T2.

Tuning adjustments may be accomplished using a 6-volt torch bulb. Connect the bulb to the collector of transistor T1 first through a coupling capacitor and tune ½J gang VC1 for maximum brilliance. (Note: the bulb would light according to intensity of RF energy.) Same procedure may be repeated for power amplifier stage and antenna tuning network for ensuring maximum power transfer. For CW operation, switch S1 is to be kept on for bypassing the audio driver transformer and Morse key is used for on/off-type modulation. CW would be generated during key depressions. For AF modulation, Morse key points should be closed and switch S1 should be flipped to ‘off ’ position. Any suitable mic. amplifier may be used to feed audio input to the audio driver transformer X1. (For transformer X1 you may use the transistor-radio type AF driver transformer.)

I would like to say that the transistor T1 is BF495. Power output of this circuit is about 150mW. It can be further increased by using separate power supply for the power-amplifier stage (24V, 1A). The coil details are as follows— L1 is short-wave oscillator coil; L2:14 turns on 1cm-diameter air-core tube using 26 SWG wire; L3 has 12 turns on 1.5cm-diameter air-core tube using 26 SWG wire.

The frequency allotted for amateur radio operators is 7.0 MHz to 7.1 MHz. Hence, any crystal available within this frequency can be used. Range of this QRP transmitter depends on propagation conditions. If conditions are good, the range is about 500 kms in the CW mode and 100 kms in the AM mode. It is possible to convert this transmitter to 20-meter HAM band. Any crystal available from 14 MHz to 14.350 MHz range can be used for the purpose. However, this conversion needs following modifications on coils L1, L2 and L3—L1: shortwave oscillator coil; L2: 11 turns on 1cm-diameter air-core tube using 26 SWG wire; L3: 9½ turns on 1cm-diameter aircore tube using 28 SWG wire.

An ammeter with a range 0-250mA or a multimeter with 0-250mA can be connected in-between the positive of the supply and the modulation transformer. Adjust VC1, VC2 and VC3 for maximum current through ammeter (CW-200mA, AM-125mA). The power input in CW and AM mode is calculated as shown below: DC power input (CW mode) = 24V x 250mA = 6watt (the power amplifier draws 250mA current). DC power input (AM mode) = 24V x 120mA = 2.8watt (the power amplifier draws 120mA current).

LM386N MK-XI BCB Transmitter

Sounds impressive enough eh? Well for the parts count(11)it is, I've spent more time in development with this little guy than most of the tube units... Sound quality is very good and the current drain at 9v is only 11ma, so can be easily powered by a std 9v battery... The coil coupling the audio to the osc is very important, without it current draw is increased by 60%, is essential for battery life... The LM386L I first worked with howled terribly without the coil in place but the "N" version performs approx same with or without... 


Astable multivibrator AM Transmitter


Q1 and Q2 make up an astable multivibrator (flip-flop) that drives the base of Q3 at a frequency somewhere in the AM band. The signal is a square wave. Frequency determined by C1/R2 and C2/R3.

It would be fun to try that circuit. It probably won't produce buzz because the multi-vibrator is oscillating at the fundamental broadcast frequency. But it will be rich in harmonics above the broadcast band. A tuned antenna circuit would help reduce the harmonics.

AM radio transmitter

am transmitter circuit schematic

The AM radio transmitter does not use any modulator transformer. The audio output from condenser MIC is preamplified by transistor T3 (BC548). The audio output from T3 is further amplified by transistor T2 (BD139), which modulates the RF amplifier built around transistor T1 by varying the current through it in accordance with the audio signal’s amplitude. RFC1 is used to block the carrier RF signal from transistor T2 and the power supply.For antenna, one can use a 0.5m long telescopic aerial.  Details of RF choke, inductor L1 and coupling transformer:
  • RFC1 = 30 turns of 24 SWG copper wire on 6 mm diam, air core
  • X1 (transformer) = 15 turns primary, 3 turns secondary using an IFT core with tuning slug
  • L1 = 4 turns of 22 SWG copper wire on 15 mm diam air core

Long range AM transmitter


Here is the circuit diagram of an easy to build long range AM transmitter circuit based on three transistors.With correct tuning and a matching antenna, the transmitter can deliver signals up to a distance of 2 kilometers.
The audio signal to be transmitted is given to the base of the transistor Q1 via the audio driver transformer T1.The modulated signal is developed at the collector of transistor Q1.The frequency of the transmitted signal can be tuned adjusting the gang condenser C7.The required amplification of the modulated signal is done by the two transistors Q2 and Q3.The  two  transistor amplifier stages are connected in parallel for obtaining maximum power. The signal to be transmitted is coupled to the antenna via the capacitor C4.
  • The inductor L1 can be made by making 56 turns of 365 WG enameled copper wire on a 1 cm former.
  • The transformer T1 can be a general purpose audio driver transformer seen in transistor radios.
  • Inductor L2 can be a 10 mH general purpose radio frequency choke.
  • Use a 1 meter insulated copper wire as antenna.
  • The circuit can be powered from a 9V PP3 battery.
  • Adjust the value of C7 for maximum range.
  • Also, you can experiment with the length of antenna for maximum range.
  • The resonator F1 used here is a two terminal type without a ground pin. If you are using a three terminal one, the ground pin must be connected to the circuit’s ground.