When building 27MHz transmitters (and receivers) it is essential to have a simple piece of test equipment called a FIELD STRENGTH METER to make sure the transmitter is transmitting and to determine the frequency of transmission.
This project has both features.
It is an ESSENTIAL piece of test equipment because you need to know a transmitter is working and hen you need to know it is transmitting at the correct frequency.
If the frequency of the transmitter is slightly different to the frequency of the tuned circuit of the receiver, the two will not communicate and it will be a very difficult job to work out if the problem lies in the transmitter or the receiver.
The reason is due to the TUNED CIRCUIT in the transmitter and receiver.
This allows the transmitter to produce a signal on a very narrow band and the tuned circuit in the receiver performs the same task.
When you are trying to "matchup" the two units, you do not know if the transmitter is higher or lower than the receiver and if you don't have a STARTING POINT, you can spend hours trying to get the two to communicate.
That's why you need a Field Strength Meter.
It lets you now the transmitter is working and allows you to tune it to 27MHz.
Now, you only need to tune the receiver.
The meter included in the kit has a sensitivity of about 900uA and the only difference you will notice between the 900uA and a 500uA meter is the 900uA will deflect about half the distance "up-scale" In other words, it will have about 50% the sensitivity of a 500uA meter.
The first section we use when testing a transmitter is the BROADBAND detector. This is identified by Antenna 1. This is called an UNTUNED amplifier and simply picks up RF.
This is designed to let you know the transmitter is working.
But you don't know the frequency of operation.
This is the function of the other circuit.
It is has a TUNED front end consisting of a coil and capacitor in parallel.
When the natural resonant frequency of these two components is the same as the frequency of the transmitter, the voltage produced will be greater than 1.2v and the transistor will turn ON to move the pointer up-scale.
In this case, the two components are not called a TANK CIRCUIT but a TUNED CIRCUIT and they are not loaded AT ALL until the voltage reaches 1.2v. This means it is very easy for them to generate a voltage. You can see the circuit does not provide them with any voltage and so all the voltage produced comes from the RF energy absorbed from the surroundings.
When the antenna is connected to the PC via a pin called a machine pin, a very small signal will be picked up, consisting of all the "hash" in the surroundings. This will deliver a voltage to the coil and capacitor but the signals will be coming at all different frequencies and they will be adding and subtracting so that nothing observable will be produced.
If a transmitter is brought near the antenna, the signal will be larger than all the background "hash" and it will consist of a single waveform (single frequency). It will deliver energy to the capacitor and the capacitor will deliver energy to the coil and the coil will deliver energy back to the capacitor but at a slightly different time to that of the next wave from the transmitter.
The result will be a very small signal being passed from capacitor to coil and back again.
But by adjusting the air trimmer, a position will be reached where the waveform produced by the capacitor and coil will be reinforced by the incoming signal from the transmitter and the waveform will get larger and larger. Eventually it will increase to 1.2v. If the diode is removed, this voltage will be even higher. But the diode drop of 0.6v and the base-emitter drop of 0.6v limits this amplitude to 1.2v and the extra energy produced by the circuit is passed to the transistor to turn it ON.
This section is effectively your reference for a 27MHz transmitter.
That's why the coil must be exactly as supplied in the kit and shown in the photos. Adjusting the spacing of the turns will alter the frequency at which the tuned-circuit resonates and this change will be quite considerable.
FITTING THE ANTENNA
The antenna is firstly placed in position 1 on the board to get an indication that the transmitter is working and then it is taken to antenna point 2.
Rotate the air trimmer and the pointer on the multimeter will rise and fall as the peak of oscillation is reached and passed.
A kit of components is available from Talking Electronics.
All the components are included in the kit and everything is identified on the board.
The most critical part of construction is the tuned circuit for antenna
The Balance Meter originally had the pointer positioned in the centre of
the scale so it could deflect up-scale or down-scale. The hair spring
has been adjusted so the needle is now positioned at the zero mark on
the scale and it reads "up-scale."