For some of my other instructables you may want to build this to make your job easier.
This project has 3 features.
1. It's a Field Strength Meter,
2. A Frequency Meter and,
3. An aid for testing detuned transmitters.
Its uses will become clear in a moment but firstly let's go over the background of a Field Strength Meter.
A Field Strength Meter is essential when designing and building transmitters. It provides signal strength values and allows us to compare and estimate the efficiency of a transmitter and its expected range.
Obviously the most accurate way of getting these results is to make a field test but this sometimes requires travelling long distances, so the next best thing is to get results on the bench by using a piece of test equipment such as an RF POWER METER.
An RF power meter is similar to a field strength meter, however the two are used slightly differently.
An RF Power Meter is generally connected directly to the antenna of a transmitter whereas a Field Strength Meter is placed NEAR the antenna without physically touching it.
When you only have 5 - 50 milliwatts available, it is very difficult to place a measuring device (such as a Power Meter) in the antenna circuit without it absorbing and upsetting the energy being radiated.
When you are dealing with frequencies in the 100MHz range, the signal flows over and through any device you place in the antenna circuit. Some of the signal is absorbed in the measuring device so that the reading may not be a true indication of the output. At the same time the performance of the transmitter is reduced so you don't know how to interpret the results.
A much more accurate way of detecting the energy is to place a device NEAR the radiating source (the antenna) so that it does not interfere with the transmission.
This is the advantage of our FSM. It is placed near the radiating source and detects the energy AT A DISTANCE so that the output is not upset.
This project differs from our Field Strength Meter MkI in that it is a stand-alone unit and does not require connection to a multimeter.
It contains a set of 3 LEDs, wired in a staircase arrangement, so that they light up progressively as the strength of the signal increases.
A trimmer capacitor at the front end tunes the exact frequency of the transmission and as the FSM is brought closer to the antenna of the transmitter, more LEDs will turn on.
We have already commented on the effectiveness of FM transmission in our many transmitter articles and shown that the range is a result of good design. The efficiency of a transmitter has a lot to do with the design of the output stage and this can be improved by adding features such as a TANK CIRCUIT and a RADIO FREQUENCY CHOKE. These are truly amazing additions as they increase the range of the transmitter without consuming any more current because they concentrate the signal into a narrow band.
One of the most-often asked questions is "How much power is a particular transmitter producing?" This is very difficult to answer but a simple rule of thumb is to allow 30% of consumption from the supply as the output power.
One of our designs consumes 7mA @ 3v has an output of about 7 milliwatts. Another design
has the same consumption and yet the range is only one quarter, so you can see that efficiency plays a big part in getting the range.
Its output would be less than 1 milliwatt and this is shown by the fact that the output is barely detectable on the LEDs.
The output difference between our highest and lowest transmitter is more than 100:1 and this has made it difficult for us to produce a project that will cover the whole range.
To measure the output of the weakest transmitter you will have to wind up the antenna and push the probe into the centre of the coil.
All the other transmitters have sufficient output to detect the radiation when the antenna is outstretched.
With some of the transmitters, the tank circuit must be adjusted so that the output is a maximum.
If you have a radio with a signal strength meter, you won't need this project, but if you don't, it's what you need.
Most Field Strength Meters are designed for connection to transmitters with an output of 1 to 1000 watts and are not capable of detecting outputs in the milliwatt range.
For low outputs we need a Field Strength Meter that will detect 1 - 50 milliwatts and that's why we designed this project.
As we have said, it is an adaptation of Field Strength Meter MkI and in place of the meter in the output we have used a series of 3 LEDs. This makes it self-contained and "frees-up" your multimeter for other uses.
The third feature mentioned in the introduction enables you to determine the frequency of detuned transmitters. It is able to detect frequencies as low as 75MHz.
This is very handy when designing transmitters for operation below the 88MHz band.
When working with a transmitter in this range it is important to keep the frequency just below 88MHz as many radios can only be detuned a few MHz before the stations at the top of the dial start to appear at the bottom.
If a bug is below this limit it will be impossible to find, even on a detuned radio.
There are two methods of detuning a radio.
One is to move the turns of the air coil near the tuning gang and see if the stations move up or down the dial.
To produce a space at the bottom of the band, the stations must be moved up and if you squash the turns too much, the top stations will wrap around and appear at the bottom.
The other method is to adjust the trimmers on the back of the tuning gang. This has proven to be the easiest and best method. Simply turn the trimmers until a space is created at the bottom of the dial and your transmitter can be fitted into the space thus created.
When you try to pick up the transmitter on a normal radio, it will be invisible!