Download Simple Loop Transmitter

Simple Loop Transmitter
The Simple Loop Transmitter provides an easy way of generating an AM radio signal which can be
used to demonstrate the principles of AM radio transmission and also provide a signal for a Simple
Radio in the absence of a long wire aerial or nearby AM radio transmitter. The output power is
approximately 1W, but since no external aerial is attached, much of the energy is radiated through
the magnetic field of the loop coil, and so is very unlikely to cause interference to any other radio
station. The prototype was set to transmit on a frequency of approximately 700kHz.
Simple loop transmitter circuit diagram
100nF 1k
10 F
+
100pF
TIP31A
100 F
2N3904
10k
2N3906
100nF
22k
16
1 15
2
6
4046B
+12V
+
100nF
10k
10mH
9
4
7
1k
11
10
3 8 5 14 12 13
3.3k
loop
1M
2N3906
10k
–
+
100nF
1M
+
47pF
100nF
©IPK0402
10k
VN66AFD
500pF
100 F
0V
10k
The loop coil consists of 10 turns of 24swg wire wound onto a 35cm square former and tapped
every turns.
The variable capacitor is any variable capacitor of around 500pF maximum capacitance. The
prototype used a very old 500pF solid dielectric type
The 10mH inductor was extracted from an old computer power supply but a suitable device to use is
a common mode suppression choke (Rapid Electronics 26-7084) with the two windings connected
in series. (This increases the inductance by a factor of 4).
Both the TIP31A and the VN66AFD require small heat sinks.
The input is high impedance and so a microphone, signal generator, portable music system etc can
be used to provide an audio input.
Adjustment.
Using the values shown in the circuit diagram, the frequency will be around 700kHz. To change the
frequency, either the 100pF capacitor or the 22k resistor can be changed.
The output is set by connecting an ammeter in series with the power supply and adjusting the
variable capacitor to give the maximum current reading (around 0.2A on the prototype). It is also
worth monitoring the output on an oscilloscope in order to ensure that the transmitter is tuned to the
fundamental frequency and not a harmonic. A suitable way of obtaining a signal for the
oscilloscope is to clip the earth connection to the probe, so forming a small loop and then placing it
near to the loop coil.
Construction.
It is important to ensure that the rf section is well away from the af section in order to avoid any
feedback which may make the circuit unstable.