QRP
AM(cw) Transmitter
for the 20 meterband
RE-TX02HF20
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).
Parts list 20-meterband transmitter
T1 BC338, BC337
T2 BD135 (with heatsink)
T3 2N2222, BC338
T4 2N2219A (with heatsink)
C1 220nF (polyester) rev1.2
C2 1500 pF (polyester) rev1.2
C3 10nF (polyester) rev1.2
C4 47uF/25v
C5 100nF
C6 100uF/25v
C7 100nF
C8 100nF
C10 12pF
C11 120pF (frequency offset +- 2kc)
C12 0...18pF (peek at design frequency Xtal)
C13,C14 330pF
C15 470pF
C16 47pF
C17= 6...40pF (white) set at half position and tune to max power
C18 22pF
C19 180pF
C20 220pF
C21 150pF
R1 1k
R2 3k3
R3 120
R4 560k
R5 560
R6 500 (trimpot. to set power/modulation ratio: important for AM Set it at between 25% up to 50% of the maximum carrier power)
R7 100k
R8 4k7
R9 560
L1 = 0.8mm Cul (insulated copper wire), 15.5 turns close together, 7mm inside, tap at 6.5 turns
L2 = 0.6mm Cul (insulated copper wire), 24 turns close together, 8mm inside diameter
L2 = 0.6mm Cul (insulated copper wire), 8 turns close together, 8mm inside diameter
Ls2 = 470 1/2 watt carbon, 0,2 Cul turned 5 times over the entire length of the resistor (+/- 10uH)
Dr = small ferrite core with a few turns of 0,2 Cu (or the spare wire of R5 turned a few times through the core)
14.310Mc Xtal (or other for your desired frequency) +- 2kc with C11
C1, C2, C3, C8 polyester film capacitor
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
Peak Frequency range: 14Mc...14.5Mc
Output RF power: 600mW pep @ 13,8v (with max modulation)
AM modulated +/- 85% or better (CW if keyed, modulator can be left out if you only use CW)
Adjustable output impedance to 50 Ohms
Band-pass type harmonic L-filter + lowpass PII
2nd harmonic at -55dBm
Usable voltages: Vcc 9 - 15 volts
Average current I: 140mA@12v
Xtal oscillator,
Adjustable frequency of 2Kc
Modulation
depth (and therefor also the power ratio) can be set (R6)
Important: to get good modulation depth: you need to set
R6 to at least halve the maximum power!
LF input +/-
100mV @ +/- 4k
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