The 200mW FM Transmitter

Free Web Hosting Provider - Web Hosting - E-commerce - High Speed Internet - Free Web Page

This FM transmitter is the 1st and 2nd Stage to the overall 3 stage 7 Watt FM Transmitter.

If you have never made a LPFM (low power frequency modulated) Transmitter before, it would be wise to start with the 10mW FM Transmitter...to get your feet wet. Even though the unit would seem a very simple project as some would say, I would have to inject that... it can be very difficult to get the unit working right...or for that matter...at all. Many variables come into play working in the vhf (very high frequency) arena. I have given a step-by-step procedure in making the 10mW unit. Once you have mastered that...you could then proceed to this 200mw unit and then later, the heavy-duty 7 watt. Do take a look at the 10mW transmitter...and read all about it...just click HERE. I propose this wise decision in the fact that I have made all three...and starting from the first is a good thing!

This type PCB is the same style of design used on the 7 Watt FM Transmitter. Take a look at the digital snap shot on the 7 Watt webpage. Notice on that picture...all of the components that are soldered onto the copper...are both on the same side of the PCB. I decided to use this style as it was very quick and easy to change out components...during the many months of experimentation. So, there are no holes to be drilled on the PCB, except for the four mounting holes. When looking at the 200mW PCB Template below, there are 15 copper island (shaded in orange) areas, which are surrounded by white areas. These white areas is where there is 'no copper'.That is the way you should make your PCB. I have had great results in using this style. Once you get it working, you may want to make another PCB...with holes...whatever you prefer. But my 'strong' suggestion is that you make the 'first' PCB according to my instructions. You will warrant a 'good show' in the end!

Go ahead and print out a copy of the above PCB template. It has to be 107 mm's by 57mm's. If not, send the drawing to a graphics program and squeeze and/or stretch until proper measurements are acheived. Once this is done, you may continue to make your PCB the exact way it looks like in the said template. Let me say again...there are 15 copper islands (where you see the color orange) on the above template. Surrounding these islands are areas of no copper (where you see the color white). The above template should be mimicked as closely as possible in order to acheive my said style.

When you have completed and finished your new 'exact' PCB, drill a hole where the small black square is in the template. Then place an 18 gauge solid through the hole and solder the wire to the frontside and backside of the PCB. This will continue the ground plane needed on the backside of the PCB.

Once that is done...you may continue to start and solder all of your components on the PCB. All of the components are soldered in a vertical 'standing up' position, except for the two coils. To see where the components are positioned on the PCB, just look below...

Just match up the number in 'Component Placement Guide' to the component in question in the 'Component Chart'.

1A - 5.6K 1/ Watt Carbon 1B - .001uF Ceramic Capacitor	16 - 22Ohm 1/2 Watt Carbon Resistor
2 - Electret Microphone (2 terminal)	17 - NPN 2N2222A or MPS2222A Transistor
3 - 1uF Electrolytic Capacitor	18 - Tapped Leg of the Air Core Coil
4 - 4.7K 1/2 Watt Carbon Resistor	19 - Air Core Coil
5 - 47K 1/2 Watt Carbon Resistor	20 - 4.7pF Ceramic Capacitor
6 - 5.6K 1/2 Watt Carbon Resistor	21 - 5-30pF Variable Capacitor
7 - 1.2K 1/2 Watt Carbon Resistor	22 - 1N914 Diode
8 - 100 Ohm 1/2 Watt Carbon Resistor	23 - 1uF Electrolytic Capacitor
9 - 5-30pF Variable Capacitor	24 - PNP 2N2907 or MPS2907 Transistor
10 - Positive lead to Power Supply	25 - .001 Ceramic Capacitor
11 - Antenna Terminal	26 - 4.7K 1/2 Watt Carbon Resistor
12 - Tapped Leg of Air Core Coil	27 - 1uF Electrolytic Capacitor
13 - Air Core Coil	28 - NPN 2N3904 or MPS3904 Transistor
14 - .001 Ceramic Capacitor	29 - 22K 1/2 Watt Carbon Resistor
15 - Negative lead to Power Supply

L1 and L2 are tapped air-core coils. Just click CLICK HERE to see how these two coils are constructed. Both coils are made exactly the same way.

If you are like me and have no test equipment, other than a homemade watt meter and a DVM, you can locate the main transmitting frequency by just using a typical AM/FM radio and switch it to FM...then go all the way to the lowest frequency...all the way to the left of the radio dial...which is around 87 Mhz...that is where I have adjusted the coils for. You will have to experiment with this as the transmitter will not only put out the main oscillating frequency, but also harmonics.

The Electret Microphone...Radio Shack sells this microphone as a two terminal connection and a three terminal connection. Use the two terminal connection. The side that is connected to the housing of the microphone is the negative side and that terminal goes to ground on the PCB. The other terminal is the positive side. Also, when you are at Radio Shack for this item and other items listed below, it would be wise to pick up about one foot of 75 ohm coax cable. With this cable you can 'extend' the length of the electret microphone so it won't be so close to the circuitry. I have mine right at 3 inches, which out and away from the circuitry and seems to work excellent as regards to clarity; not to mention your head staying a little ways away from the unit so as to not affect the frequency (see 'tank circuit' below).

2N3904 or MPS3904 Transistor...This transistor may be purchased at Radio Shack. Refer to the 'Component Placement Guide' for proper orientation of each leg of the transistor.

2N2907 or MPS2907 PNP Transistor...This transistor may be purchased at Radio Shack. Refer to the 'Component Placement Guide' for proper orientation of each leg of the transistor.

5-30pF Variable Capacitor...Radio Shack does sell a 5-30pF variable capacitor (Part Number RSU 11919081), but it must be ordered through the store. It sells for around $2.00 This is one of two devices that make up the oscillating circuit (commonly known as the 'tank circuit'). A variable capacitor is needed in order to adjust for a specific transmitting frequency. It would be wise to understand how the tank circuit operates...since it is this variable capacitor, together with the air-core coil that you will experiment with...in order to capture the specific transmitting frequency you want. It will be left up to you to tune-in your transmitting signal to a receiver. Just CLICK HERE to understand how to tune your transmitter once you have made it and ready for the first 'turn-on'.

The 'tapped' Air-Core Coil...This is a home-made device and must be made by you. Just CLICK HERE for construction of the coils. This is one of two devices that make up the oscillating circuit (commonly known as the 'tank circuit'). One coil is needed in order to adjust for a specific transmitting frequency. The second coil is for amplification of the first coil. It would be wise to understand how the tank circuit operates...since it is this 'Tapped' Air-Core Coil, together with the 5-30pF Variable Capacitor, that you will experiment with...in order to capture the specific transmitting frequency you want. It will be left up to you to tune-in your transmitting signal to a receiver. Just CLICK HERE to understand how to tune your transmitter once you have made it and ready for the first 'turn-on'.

1N914 Diode...This device may be purchased at Radio Shack. Observe polarities on diode. The cathode (that's the negative side of the diode) goes to ground.

4.7pF Fixed Disk Capacitor...This is a non-polarized capacitor, meaning that is does not matter which leg is used for placement. Just make sure one leg goes to the emitter of the MPS2907 and the other leg goes to the collector of the MPS2907. Keep the distance of the legs no further than 1/8 of an inch to the said transistor.

27K Resistor...This particular resistor can not be found at Radio Shack. That is why I use a 5.6K and a 22K resistor in series. Although the ohmage will be slightly above 27K, this will still be sufficient.

The Antenna...I have used a four to five foot antenna on this unit and it radiated out well with it. You can purchase a four or five foot telescopic antenna from Radio Shack.

At 12 volts, this little jem should give you a good 200mW's of output power. When you have finished the project, and would like to see for yourself how much power the unit is giving out, just click HERE to make a Home-made RF Probe/Watt Meter. It can also be used as an RF Probe to see how much the output voltage is, with no load. If you are like me and have no test equipment, other than a homemade watt meter and a DVM, you can locate the main transmitting frequency by just using a typical AM/FM radio and switch it to FM...then go all the way to the lowest frequency...all the way to the left of the radio dial...which is around 87 Mhz...that is where I have adjusted the coils for.

So there you have it my friend. Should you notice errors on this wepage or information that you are not quite sure of, do drop me a line. If this project interest you...I am here to help you through it in any way that I can.