27MHz Field Strength Meter


To buy a kit: 
27MHz FSM kit

$6.50 plus $4.00 post


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.


27MHz Field Strength Meter

The project consists of two separate circuits with the current for each section going through a 500uA meter. When either of these circuits detects energy in the form of electromagnetic radiation, it gets rectified by the diode and passed to the transistor where it turns the transistor ON more. This causes more current to flow through the 500uA meter and the pointer moves up-scale.
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.
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 27MHz FSM PC board

27MHz Field Strength Meter fully assembled

The most critical part of construction is the tuned circuit for antenna point 2.
The coil supplied in the kit has the correct dimensions for a 27MHz transmitter.
This is very critical and the coil must not be stretched or squashed as the tuned circuit will not produce a peak amplitude for 27MHz.  Just the last two turns need to be extended to produce 27MHz as shown in the photo above.
Use fine tinned copper wire to hold the cell in place and make sure the wires do not create a short-circuit.

The PC board is connected to a multimeter via two paper clips.
They are connected to the board at the holes identified by 500uA. The paper clips are made from spring steel and are very hard. They must not be cut with delicate side cutters. Simply use a file to score a mark on the side of the wire and bend it back and forth a few times and it will break.
Squeeze the paper clip together so it fits down the holes in the multimeter and makes a good SLIDING connection.
Almost any meter can be used on the 1mA or 0.5mA range.
When you turn the project ON, the meter will show about half-scale deflection to represent the quiescent current taken by the circuit.
When the antenna detects RF, one of the circuits will turn ON more and take more current. This will show on the meter.

The kit comes with a "side reading" BALANCE METER. This is actually called a MOVEMENT as it is not calibrated, however it has been adjusted to read upscale and the centre-reading corresponds to 300uA.
Maximum deflection corresponds to 900uA, so the scale is not very linear. This does not concern us in this project. We just need the meter to give us a deflection. That's why it is FREE!

This is the Balance Meter included in the kit

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."
No SHUNT RESISTOR is needed across the terminals as the sensitivity of the movement is about 1mA. This is not a very sensitive movement but that does not matter. It still gives the exact same reading as a very sensitive movement. 
Sensitive movements are only needed for voltage readings, measuring high resistances and for currents less than 30uA.   

This 27MHz link is designed to get you into understanding the concepts of transmission - it's the place to start.
The 27MHz link consists of a 27MHz Transmitter  - 2-Channel
27MHz receiver - 2-Channel

$6.50 plus $4.00 post
Order a kit

2 - 2k2 resistors
2 - 1M resistors

1 - 5-30p air trimmer
1 - 100p ceramic
2 - 1N4148 diode
2 - BC547 transistors

1 - 16 turn coil
2 - paper clips for meter
3 - machine pins for antenna
1 - 10cm 0.5mm wire for antenna
1 - AG1 cell
  fine tinned copper wire for cell
1 - mini slide switch
20cm very fine solder

1 - 27MHz FSM PCB

FREE 0-1mA Meter included
            (see text for description)

Talking Electronics has a number of pieces of TEST EQUIPMENT to help in the design and testing of projects.
Of course you can use a multimeter for most of the testing but some of the "tricky" faults need a special piece of equipment.
You may only need a LOGIC PROBE once a month, but the project you are designing will come to a stand-still if you can't locate a problem.
We designed all these projects because we needed them ourselves.
Add one of them to each order you place with Talking Electronics and eventually you will have the whole range.

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