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A 4 Kilometer Range 100MHz FM Transmitter Circuit

Date:2020/1/4 17:32:26 Hits:

1). Here is the Specification of the Transmitter:

Frequency of operation: About 100MHz
Antenna type: Folded 300 ohms dipole.
Range obtained in free space: Up to 4km with dipole antenna 30 feet above ground level. More range with yagi antenna.

This was made to give it a little directional bias, and indeed, it transmitted very directionally, boosting its range to several kilometers in free space.

Our intention, as a small startup company, to sell this as a finished kit, or do-it-yourself kit. In those days, people were tending to copy any transmitter design and claim as their own. So, we made a tricky decision. In the PCB, I changed the dimension of the three coils so that they mimic something like our company name ‘RCL’. See the PCB design below:

For the sake of this particular transmitter, consider the special design of coil now just mearly a coil; and does in no way represent any company or company name.

This actual design was made using a very early version of ‘Protel‘, called ‘Protel 99SE’. I designed this on a 486 desktop having just 4MB RAM and a 256KB video RAM. Today when I see this, I find myself that, to create brilliant things, we may always not need the latest gadgets and software.

Because, this first design, and all subsequent improvement of this version (that I designed on the same computer using same software), was best-seller for our company back then.

So after selling many thousand units of this transmitter across the globe, the design has been retired and I decided to offer this for free to the internet community and posted that to ‘Tripod’.

2). Circuit Diagram and Brief Description:

The transmitter is built on a Printed Circuit Board. This board uses track inductor for L1, L2 and part of L3. The section built around Q1 is the oscillator section. Oscillation frequency is determined by L1, C4 & C5 which forms the tank. Actually C5 is the feedback capacitor. This is required to sustain oscillation. This also influence the operation of tank formed by L1 & C4. Modulation is directly applied to the base of Q1 via C2. A microphone is connected here to serve this purpose. You can alternately feed direct audio here after disconnecting the microphone biasing resistor R1. Q2, Q3 & Q4 gradually raises the output power up to the desired level.
A 4 Kilometer Range 100MHz FM Transmitter Circuit2(图片）

As most of the inductors are PCB etched, there is practically very little frequency drift provided you use a highly regulated and ripple free power supply.

RF output from the transmitter is taken from the junction of C11 & C12. This is unbalanced output of around 75 ohms impedance. But a folded dipole is a balanced type antenna of around 300 ohms impedance. So we need to use a ‘BALanced to UNbalanced transformer’ or ‘BALUN’. A 1:4 type BALUN is employed here for this purpose. Antenna connection is taken from this BALUN via a 300 ohms flat parallel feeder cable commonly used in television to receive terrestrial broadcast. No coaxial is used to feed antenna. This saves cost. Also a parallel feeder cable provides much less signal loss compared to a coaxial.

3). Design of BALUN:

balun

The BALUN or BALanced to UNbalanced transformer is made using a Wideband Multi-Aperture BALUN core made of 73 Ferrite material. It has two-hole binocular ferrite bead as shown below.

You need to use parallel insulated twin wire to construct this. This wire is commonly used to wind TV BALUN transformer. If you want to get rid of this, then buy a ready-made TV BALUN that is generally used at the back of your television set for interfacing with feeder wire.

If you prefer to build this yourself, the circuit diagram is given above. You need to carefully construct it keeping in mind about the ‘sense’ & ‘direction’ of turns. See there are four coils. Two coils in the upper section, which are red and blue, required to be wound on left side of the BALUN and the remaining two (blue & red) in the lower half to be wound on right side. Connection marked ‘A’ and ‘B’ at the left side of the circuit is required to be connected to the PCB at the shown point. As dipole antenna is balanced type, so you need not to worry about its connection.

PCB Design details
The transmitter is built on a single-sided PCB. See the PCB drawing once again.

As mentioned earlier, this PCB has a number of etched inductors. For this reason, you need to very carefully construct the PCB as shown in the picture above.

The above drawing is the copper side and below shown is the component mounting plan.

In the copper side view, you can see that there are three track etched inductors that resembles ‘RCL’ Every corner and track width/length are calculated and then they are drawn so that each ‘RCL’ section becomes an inductor of required value. Never play with this; otherwise optimum result could not be achieved.

Use a very good quality, preferably Double Sided 1.6mm FR4 Grade Copper Clad Board for the PCB.

You need to use a laser printer or a high-quality printer to get a printout of the drawings. First, save the picture to disk. Now try to print it from such a software which permits you to control print size. ‘Paint Shop Pro’ is such a software. Of course, you can use any other software. Print the drawing so that copper side drawing is exactly 59mm X 59mm. Few trial will give you the perfect print. Now construct the PCB using ‘Photo-etching’ method so that all the tracks becomes exactly same as you are now seeing.

You will also need good quality Ferric Chloride Solution to etch the board properly. Now drill the PCB carefully. The PCB is now ready to populate.

You can use this PCB Drilling machine for this purpose. It is a very good one for your home laboratory personal prototype PCB production.
A 4 Kilometer Range 100MHz FM Transmitter Circuit7（图片）

You will get a lot of how-to information for building such a homemade PCB for RF Projects here.

Now start populating the board according to the component mounting plan. You can also get a true size copy of this plan printed and glued to the PCB. This will help you work fast.Part of L3 is required to be constructed. This is described in parts list.

Please note that in the picture of the transmitter kit, capacitor C1 & C10 are not mounted by mistake and the kit is filmed. Please add these two capacitors. Try to keep all component leads as short as possible.

5). Detailed Parts List:

Believe it or not, a 2N2369 from Philips, used in the final power amplifier section, can give this much of range.

RESISTORS
R1 – 22K
R2 – 100K
R3, R7, R9 – 1K
R4, R8 – 100E
R5 – 390E
R6 – 330E
R10 – 15E
R11 – 10K

CAPACITORS
C1, C3, C10 – 1n
C2 – 100n
C4,C8,C9 – 47pF
C5, C11 – 10pF
C6 – 100uF/25V Electrolytic
C7 – 100pF
C12 – 3pF

TRANSISTORS
Q1, Q2, Q3 – BC548
Q4 – PN2369 (Plastic casing) or 2N2369 (Metal casing)

MISC.
L3 – 7 turns, 22SWG wire, 3mm ID, Close wound, Air core.

Wideband Multi-Aperture 73 binocular BALUN core, BALUN wire, 300 ohms TV feeder wire,
JP1 to JP5 – All jumper wires.

Copper Clad Board

Ferric Chloride Solution for etching the PCB

6). Conclusion

The Tripod original article has a link for full antenna construction and tuning details at the bottom. Unfortunately, that link is broken now. So, I am planning to write a fresh article for those, who really want to construct this transmitter. So please comment below, share this, so that we can get enough encouragement to write about the antenna details in a next article.

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