7 Watt FM Transmitter
 
7 Watt FM Transmitter


 

7 Watt FM Transmitter

 


This 7 Watt FM Transmitter was originally a 200mW unit, without the universal power stage added. Together with the power amp 2SC1971 / MRF237 / NTE342 it then became a 7W unit. I used this transmitter with a half-wave open-end dipole in a vertical position 50 feet above ground. Together with about 70 feet of coax, this transmitter delivered great audio at a distance of 10 miles ... overall distance was 17 miles, but the audio signal was weak. I had no equipment, other than a watt meter to measure it's power and a digital FM tuner with a 5-LED Signal Strength Bargraph display to use as capturing the main oscillating frequency, which was right at 87.5 MHz. This circuit worked well for me, as I had experimented with it for nearly a year. Of course, one would be better off with more equipment than I have had to capture the main oscillating frequency. That was, by far, one of the hardest things to capture. It was thru trial and error, with the FM tuner, in finally finding out how to grab the right frequency. When I finally did get used to find out where my 'main' frequency was, the unit performed extremely well. Like I had said above, right at 10 miles, the unit was at its best giving clear audible audio into the speakers of my car. With the transmitting antenna at 50 feet above ground, I decided to see how well I could receive the transmitter signal from an overpass than is exactly 15 miles from the transmitter. When I got to the top of the overpass in my car, the audio signal came in as 'clear as a bell'. I now understand what is meant when one says FM signal travels best in a line of sight. Well, being on that overpass, if I had a strong telescope with me, I am sure I could see the 50 foot antenna in my oak tree. So with the overpass being right around 50 feet in height also, the transmitter surpassed my judgement call on its signal. I surrender this circuit to anyone who likes to experiment in things like this. Enjoy!


The project below has been a culmination of 3 stages that were put together during a course of a year. By starting out with a low wattage of output power, I had brought the unit up to a powerful 7 watts using the incredible MRF237, which is cross-referenced to an NTE342 or a 2SC1971 power transistor (I say incredible because of the remarkable service it gave me during the course of much voltage abuse during the many months of experimentation...not once, did it die), together with the 200mW unit (the 1st and 2nd stage) I have managed to get outstanding results!

I would like to give you my consultation on this particular project...if you have never made a transmitter before, I strongly suggest you start out with the 30mW unit on this website. There are so many variables that seem to show its' face in the VHF arena...that by beginning with the simplest circuitry I have, you will come to understand 'why' many new-comers to the RF field cannot get these simple, yet very challenging, devices to work. Once you have mastered the 30mW transmitter, you should then tackle the 200mW unit. The time taken' in seeing yourself thru these two projects (and making them work well), will surely aid you in many ways. Hands-on experience is by far the best teacher one could have. This 7 watt transmitter would probably NOT be categorized as a Low-Power FM transmitter...you are now starting to climb up to many watts of power. It is very tempting to say..."Wow, I think I will make this 7 watter!"...but please be informed...the unit is by far the most complicated of all the three transmitters on this website...and even working your way thru the 1st and 2nd transmitter...this 7 watter will demand your every bit of know-how (hands-on experience) in getting it to function. Although...if you are familiar with making transmitters...then this one is waiting!

TRANSISTORS
Q1 NPN MPS3904 Transistor
Q2 PNP MPS2907 Transistor
Q3 NPN MPS2222A Transistor
Q4 MRF237 (cross reference NTE342 & 2SC1971)

COILS/DIODE
L1 & L2 are air-core coils...8mm diameter...4 turn wrap...1 turn tap...18 gauge solid wire. Use may use a 5/16" standard threaded bolt to wrap the wire on. It fits perfect. then back out of bolt. For a detailed description on how I made my 'tapped' air-core coils, please CLICK HERE.
L3 & L6 are air-core coils...5mm diameter...3 turn wrap...18 gauge solid wire...use a 1/4" standard threaded bolt to wrap the wire on. It fits perfect, then back out of bolt.
L4 is a VK-200 choke (If omitted, output power is reduced by 25%)
L5 is an air-core coil...5mm diameter...11 turn wrap...18 gauge solid wire...use a 1/4" standard threaded bolt to wrap the wire on. It fits perfect, then back out of bolt. Sqeeze all the wraps real close...but make sure none are touching each other.
D2 can be either a 1N4001, 1N4002, 1N4003, 1N4004, 1N4005
NOTE: Squeeeze or stretch L3 & L6 to achieve maximum output wattage. I also took off the insulation on the 18 gauge wire to make the air-core coils.

Getting It Up & Running

The best possible help I could give one in making this unique unit function well, is to have a homemade watt meter and a homemade Field Strength Meter available. Together with a DVM, these two homemade devices can aid you in fine-tuning the unit and also giving you an rough idea of how much wattage you are putting out. If you can get hold of an FM tuner, with a bar-graph LED Signal Strength readout, this can aid in finding the 'main' transmitting frequency...

The three test devices above is what I had used in the complete makings of the transmitter. I had nothing else. A lot experimentation came in when I had to govern myself and get the unit to function well with just these three items...but it did work out well in the end.




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