Field Strength Meter

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This fun project is a home-made 'Tunable Field Strength Meter', tunable for FM Transmitters operating in the FM Broadcast Band. Below is the detailed plans on how to make this project a success! The layout of the circuitry is small enough for the finished unit to be placed directly on top of your DVM, in order to keep output leads of the FSM as short as possible. I had made mine on a 2 inch by 2 inch piece of PCB...which is MORE then enough room to place the entire circuitry on.

Below is the overall look of the project. There are just 5 components in the project...excluding the antenna and the DVM. Making of the homemade air-core coil is explained. Once you get the unit operating, you may then start using different antenna lengths...to find out what length will give you the highest readings. Strive to get the highest voltage readings possible.

Placement of the germanium diode...the anode side (the positive side) will go toward the antenna. The cathode side (the negative side) will go toward the 10K ohm resistor.

The air-core coil is made with 18 gauge solid un-insulated copper wire. Get a 3/8" standard threaded bolt from a hardware store (such as Lowe's), and wrap 4 turns of the wire in the grooves of the bolt. Then gently unscrew the formed-coil from the bolt. DO NOT squeeze or stretch the coil, once it is formed. The two legs to be soldered, should be no longer than 1/4 of an inch. Next, it is ready to solder to the PCB.

Once you have made the unit, turn your transmitter to ON and place the FSM on some sort of a stand or pedestal...within 50 feet of the antenna. Turn the DVM to ON, and start adjusting the variable capacitor, until you get the highest voltage reading possible on the DVM. Set your DVM to the lowest possible DC scale.

This home-made FSM is not accurate at all. Commerical bought FSM's are indeed accurate. But accuracy is not what we are looking for when taking a reading from our homemade FSM. What is more important is the increase or decrease in readings from a prior reading.

Let's take an example...Let's say that you were 15 feet from your antenna, with your FM transmitter to ON...with your power supply set at 9 volts dc. And then you placed your FSM on some sort of stand or pedestal...then you backed yourself away from the FSM some 5 feet and recorded the voltage reading.

Body capacitance will affect the voltage on the DVM. So to get a constant and non-fluctiating reading on the DVM, stand about 5 feet away from the DVM, and record your voltage reading.

Let's say the reading was around 25mV's. OK...then you increased the power supply of the transmitter from your original setting (9v) to 12 volts dc. Then you took another FSM reading...and the reading it gave you was now 40mV's.

It tells us that you have increase your output voltage some 60%...from the two known readings...that is...the 25mV's and the 40mV's. The FSM does not tell you how much output power you have, nor does it tell you your exact output voltage. Although with my RF Probe/Watt Meter, you can find out a close approximation of the output voltage/wattage.

Again, what it DOES tell you is that you have just increased your output voltage by 60%. Which in turn means that you have just increased your output wattage.

(1) It will let you know if you have increased or decreased your output voltage (therefore, your output wattage) from you prior reading. Therefore, when tuning your transmitter for maximum ouput wattage, the FSM would come in handy for see for yourself, just what the electro-magnetic field is doing.

(2) It will also let you know in what areas, walking around your antenna, is the most intense electro-magnetic field. The highest reading you get...would be the strongest output wattage in that particular area.

(3) It is also good for 'pruning' your antenna. Pruning is just a fine tuning of the antenna, once it is up and running. After you have determined that proper lengths for each rod on the dipole, you can then start cutting pieces off the antenna (that is, equal amounts on both rods)...and find out if the signal has increase or decrease...from your FSM. And you may have to 'add' more length to the each rods...should the FSM give a lower reading when trimmed.

What is so great about the FSM, is that there are no wire connections between the device and the transmitter/antenna. Plus the meter does not need a battery...the only source that feeds it, is the electro-magnetic wave coming from the antenna....yes...a wireless hook-up !

In fact, if you put the FSM close enough to the antenna, and hook up an LED directly to the output of the FSM, that is, taking away the DVM...you can actually light up the LED, just with the signal coming directly from the antenna...and that in itself...is truly amazing ! ! !

So in essense, this particular home-made Field Strength Meter is geared to pickup electro-magnetic fields in different areas of the FM broadcast range...by adjusting your variable capacitor.

If our eyes could see the electro-magnetic wave coming off of the antenna, that would surely be nice....but we can't. So it is left for the 'eyes' of the home-made FSM to do this for us.

When your FM transmitter is complete, together with your antenna system...you can then start experimenting with your transmitter/antenna system...to see how it affects the radiating signal coming out of you antenna. If the effect had a decrease in output wattage, the FSM Meter would show a decrease in voltage. If you altered the transmitter/antenna system so that you increased the radiating signal...again...the FSM would show an increase in voltage. The meter will not give you an accurate account of the voltage reading...it will only let you know that there was either a decrease or an increase in relation to the previous reading taken. But that is what we are stiving for anyway !

Yes, you can always find out what your output wattage is...by building the 'Switchable RF Probe/Watt Meter...also on this website. This device will give a more reliable account of what your FM transmitter output wattage is.

So my friend, output wattage and the voltage reading on your DVM is directly proportional to each other. When one increases, the other has to increase...and visa versa.

Do let me know how things turn out, as I might learn a little more from you...concerning your own FSM!