7W Fm Transmitter

Q1 NPN MPS3904 Transistor

Q2 PNP MPS2907 Transistor

Q3 NPN MPS2222A Transistor

Q4 NTE342 (cross reference 2SC1971)

L1 & L2 are air-core coils…8mm diameter…3 turn wrap…1 turn tap (L1 is tapped one full turn from the diode side (D1) of the coil…L2 is tapped one full turn from the collector side of the MPS222A transistor) using un-insulated 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. Do not stretch or squeeze these coils after they have been backed out of the threaded bolt.

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%) Mouser Electronics here in the USA sells this component…it sells for less than one dolar.

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.

Above is the schematic for the 7-Watt FM Transmitter. It was originally a 200mW unit, without the universal power stage added. Together with the power amp (NTE342), it then became a 7-watt 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 finding out where my ‘main’ frequency was, the unit performed extremely well. Like I had said above, right at 10 miles, the unit was at it’s 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 undersand 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…
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.

1 Comment

  1. Jamshaid zafar November 20, 2011 7:18 am 

    is it possible to make the L4 by hand, if yes than what is the method of windind. please give me complete detail.    jamshaid.

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