It had only been eleven years since Guglielmo Marconi sent the first "wireless" transmission with his new invention, and only five since Marconi sent signals across the Atlantic. Making use of the high-frequency alternator, Canadian-born physicist Reginald A. Fessenden made his historic Christmas Eve broadcast, in which he transmitted music as well as human speech.

Another early broadcast took place in 1910 when Lee de Forest, inventor of a type of vacuum tube called a triode, aired programs from New York's Metropolitan Opera House.

But it was not until 1916, when a Westinghouse engineer named Frank Conrad played records for his friends over the air, that the idea of radio as a public medium took shape.

An executive at Westinghouse heard about Conrad's broadcast and realized its potential. Here was a medium available to the masses -- a huge potential audience. An audience that would listen to radio broadcasts... with radios made and sold by Westinghouse.
In 1920, Westinghouse's KDKA began regular broadcasts. That same year it aired the results of the 1920 presidential election before the results could be read in the papers. This caused a sensation and is considered the beginning of professional broadcasting.

FM signals have a great advantage over AM signals. Both signals are susceptible to slight changes in amplitude. With an AM broadcast, these changes result in static. With an FM broadcast, slight changes in amplitude don't matter -- since the audio signal is conveyed through changes in frequency, the FM receiver can just ignore changes in amplitude. The result: no static at all.

为了展示我国电子工业水平，为了使来我国大饭店居住的国际友人能够及时地收听到来自自己国家的消息和声音，北京无线电厂立即组成了北京饭店高级半导体收音机设计小组，确定了“牡丹”2241调频调幅全波段台式一级收音机的设计方案。

经过一段时期的研制，“牡丹” 2241傲然开放。73岁的北京无线电厂前总工程师严毅回忆说，测试接收效果时，将“牡丹”2241收音机与日本索尼（SONY）的一台同类的收音机进行了比较，接收效果毫不逊色。那台索尼的机器在日本被称作收音机之王，“牡丹”2241应该是中国的收音机王！”

“牡丹”2241是北京无线电厂牡丹系列晶体管台式机的经典作品，但北京无线电厂值得骄傲的却不仅仅是2241。实际上，上个世纪50年代至90年代，北京无线电厂的“牡丹”牌收音机在北京乃至全国一直占据着主导和领先地位，它曾被誉为我国收音机的“四大名旦”之一，名扬海内外。

1956年3月11日大声无线电商行、中国无线电业商行等27家私营企业成立了公私合营广播器材厂（北京无线电厂前身）

1958年，牡丹商标审定书

郭沫若题写的牡丹商标标牌

早期的牡丹矿石收音机

1960年，刘少奇将牡丹收音机当成国礼送给外宾

陈毅为外宾演示牡丹收音机

1963年牡丹8402收音机研发大会战

牡丹8402收音机，它是1963年北京市组织的半导体收音机的试制大会战的成果

文化大革命期间，牡丹被改名红旗

文化大革命期间的红旗收音机643型

鉴于大量出口产品的需求和用户的要求建议恢复使用“牡丹”商标，1971年9月首次恢复使用“牡丹”牌商标出口644型半导体收音机。

1974年何起蛰、俞锡良设计的大型台式牡224l型22管全波段半导体收音机。

牡丹收音机厂生产车间

当年的北京无线电厂大门

北京无线电厂外景

北京无线电厂后改称益泰集团。这是益泰集团的员工参与鸟巢奥运监控设备的建设

OK. So we have a signal that is sent to a radiosmitter's antenna. How does that signal get from the antenna to the air?

Let's first take a look at the signal. The signal is an electic current, and every electric current is actually electrons moving in a wire.

Wire is made of metal, usually copper. All of the atoms that make up the wire have something in common -- each has one or two electrons in its outer-most shell.

These electrons do not have a strong bond with the rest of the atom. In fact, it takes just a slight amount of energy to push the electron away from its atom.

But if you have enough energy, the outer electrons from all of the atoms will move at once. They will each travel from one atom to the next atom, and so on.

Back to our radio signal. The electrons in our wire are moving, but not in one direction. These electrons are moving back and forth.

Actually, the wave displayed in the activity is a representation of the back and forth movement of electrons. If the wave has a frequency of 200,000 Hz (cycles per second), the electrons in the wire are moving back and forth 200,000 times a second.

When electrons move in a wire, an electromagnetic field is created around that wire. There's no magic behind this; it's just the way things work.

Just as the electrons move in the wire, they move in the transmitter's antenna. And just as an electromagnetic field is created around the wire, a field is created around the antenna.

But there is a difference between the wire and the antenna. The wire is shielded (surrounded by another wire) to keep the electromagnetic field in. The antenna, on the other hand, is designed to radiate the electromagnetic field.

The electromagnetic field travels from the antenna in all directions and at the speed of light. It travels until it hits your radio's antenna as well as hundreds of other receiving antennas.

And what happens at the receiving antenna? Just as a current in a wire produces an electromagnetic field, an electromagnetic field produces current in a wire (or antenna). This current is then amplified and processed by the radio.

Circuit Description:
The schematic diagram for the Telephone Transmitter is shown in Fig. 1.
That circuit connects in series with the either the Tip or Ring, (green or red) leads of the telephone line. Power for the circuit is full-wave bridge rectified by diodes D1 through D4.
Transistor Q1, capacitors C1 and C8, and inductor L3 form an FM oscillator that operates at a frequency of around 93 MHz. Variable capacitor C8 allows the oscillator frequency to be adjusted between 90 and 95 MHz. To move the tuning area up to the 98 to 105 MHz range, C1 must be replaced with a 10-pF capacitor.
Audio from the phone line is coupled through R3 and C2 to the base of Q1 where it frequency modulates the oscillator.
Transistor Q1, inductor L1, and capacitor C6 form a power amplifier circuit. The signal tapped off L3 in the oscillator circuit is fed to the base of transistor Q2 and the FM signal is transmitted from Q2's collector. Inductor L2 is a radio-frequency shunt that decouples power and audio from the amplifier circuit.

Construction:
The circuit is simple enough to build on perforated construction board, but the tight design of the PCBoard shown in Fig. 2 is more desirable. If you would like to use a PCB, you can either use the foil pattern provided in that figure to make your own PCB, or order a pre-etched and drilled board as part of a complete parts kit available from the source given in the Parts List.
When mounting parts to the PCB, use the parts-placement diagram shown in Fig. 3 as a guide. Begin by installing the resistors and diodes; the board is so thight that those components must be mounted vertically. The printed circuit board measures 20x50mm (1x2"), so it is small. Resistors have to be mounted 'standing up'. Depending on the size of C8, the trimmer capacitor, you may have to solder two pieces of wire to the legs of C8 and mount it that way. Either way, it is a tight space.
The next step is to install the inductors. Coils L1 and L2 are six and eight turns, respectively, of enameled copper wire. If you are winding your own coils, use approximately 22-gauge wire and a 1/8 inch drill bit as your winding form. Any enamel on the leads where the coils are to be soldered must be sanded, scraped, or burned off with a soldering iron before solder will adhere to them properly. Coil L3 is six turns of tinned copper wire in which the coils must be spread out with about 1 millimeter between each turn; none of the turns are allowed to touch each other, or the printed circuit board itself! After you are done spreading out the windings of this coil, the total width will measure approximately 3/8" (9mm). Again, this is approximately and depending on your skill. Don't worry too much about it. You can always spread it out more or squeeze it a bit when the coil is mounted.
A tap is connected between the top of the first turn of L3 and the PC board (see picture).
Go on to install the fixed capacitors and the variable one (C8). Again, C8 may need some bending. Also, this cap may have 3 legs, you can cut 1 off and leave one side+walker in place. Safes on space on the PCB.
Then you can solder the two transistors to their appropriate places.
Be certain to inspect the board for errors before connecting it to the phone line. The range of the Transmitter is less than 100 feet. That distance can be increased, however, by soldering a wire antenna (about 5 feet (150cm) of 26 gauge wire) to the collector of Q2.

Test and Tune:
Connect the Transmitter to the phone line using whatever method you prefer. Just make sure it is in SERIES.
This means cutting either the RED(Ring) or GREEN(Tip) wire and hook the circuit up in between those two wires you just cut.
Turn on a nearby FM radio and tune to a quiet spot (no station using that frequency) on the dial somewhere between 85 and 95 MHz. Pick up the phone; you should here the dial tone right away on the FM radio. If that is not the case, adjust C8 until you do hear the dial tone. First adjust C8 for the best reception, and then fine tune the radio.
Should you have trouble finding a spot on the dial that is quiet enough, remember that the tuning area can be moved up to the 98- to 105 MHz range by replacing C1 with a 10pF capacitor. Using a 22pF cap will tune the 90 to 95MHz range. These ranges are also dependible on the coil-spacing.
The picture of C8 shows a blue color which was used for the prototype. The actual color of this trimmer capacitor is red. Since the printed circuit board is so small, some components may have to be 'bend' in place, like C8 and the three coils, L1, L2, and L3. (Don't forget to scrape the coating of before soldering!). All other components mount best 'straight-up'.

Parts List:

Semiconductors:

   Q1 = NTE123A, BC107, PN100, 2N4401, BC547 NPN transistor (or equivalent)

   Q2 = 2N3563, ZTX320, or NTE108 - NPN transistor (or equivalent)

D1-D4 = 1N4148, signal diode

Resistors, 1/4W, 5% tolerance:

R1 = 100 ohm (brown-black-brown)

R2 = 33K (orange-orange-orange)

R3 = 10K (brown-black-orange)

R4 = 47K (yellow-violet-orange)

R5 = 390 ohm (orange-white-brown)

Capacitors:

   C1 = 22pF or 27pF, ceramic disc

   C2 = 0.1uF (100nF, or 104), ceramic disc

   C3 = 0.022uF, (22nF, or 223) ceramic disc, 100V type

   C4 = 0.001uF, (1nF, or 102) ceramic disc

   C5 = 5.6pF, ceramic disc

C6,C7 = 47pF, ceramic disc

   C8 = 5 to 20pF trimmer cap, (red cap)

Additional Parts and Materials:

  L1 = 15nH (nanoHenry), 6 turns of enameled copper wire (see text)

  L2 = 30nH, 8 turns of enameled copper wire (see text)

  L3 = 8nH, 6 turns of tinned copper wire (see text)

 PCB = Printed Circuit Board

Optional: miniature aligator clips

Article More Info:http://www.uoguelph.ca/~antoon/circ/FMTelTx/telephon.html

This is the schematic for an FM transmitter with 3 to 3.5 W output power that can be used between 90 and 110 MHz.

The circuit has been tested on a normal RF-testing breadboard (with one side copper). Make some connections between the two sides. Build the transmitter in a RF-proof casing, use good connectors and cable, make a shielding between the different stages, and be aware of all the other RF rules of building.

Q1 and Q5 should be cooled with a heat sink. The case-pin of Q4 should be grounded.

C24 is for the frequency adjustment. The other trimmers must be adjusted to maximum output power with minimum SWR and input current.

Introduction

I have had a few inquiries about a low power FM transmitter, and this article should satisfy those who might want to build one.  It is designed to use an input from another sound source (such as a guitar or microphone), and transmits on the commercial FM band - it is actually quite powerful, so make sure that you select an unused position on the dial!

The FM band is 88 to 108MHz, but is getting fairly crowded nearly everywhere, but you should be able to find a blank spot on the dial somewhere.

NOTE: A few people have had trouble with this circuit.  The biggest problem is not knowing if it is even oscillating, since the frequency is outside the range of most simple oscilloscopes.  See Project 74 for a simple RF probe that will (or should) tell you that you have a useful signal at the antenna.  If so, then you know it oscillates, and just have to find out at what frequency.  This may require the use of an RF frequency counter if you just cannot locate the FM band.

The circuit of the transmitter is shown in Figure 1, and as you can see it is quite simple.  The first stage is the oscillator, and is tuned with the variable capacitor.  Select an unused frequency, and carefully adjust C3 until the background noise stops (you have to disable the FM receiver's mute circuit to hear this).

Figure 1 - Low Power FM Transmitter

Because the trimmer cap is very sensitive, make the final frequency adjustment on the receiver.  When assembling the circuit, make sure the rotor of C3 is connected to the +9V supply.  This ensures that there will be minimal frequency disturbance when the screwdriver touches the adjustment shaft.  You can use a small piece of non copper-clad circuit board to make a screwdriver - this will not alter the frequency.

Note:  A reader has suggested that the frequency stability is improved considerably by adding a capacitor from the base of Q1 to ground.  This ensures that the transistor operates in true common base at RF.  A value of 1nF (ceramic) as shown is suitable, and will also limit the HF response to 15 kHz - this is a benefit for a simple circuit like this.

Capacitors
All capacitors must be ceramic (with the exception of C1, see below), with C2 and C6 preferably being N750 (Negative temperature coefficient, 750 parts per million per degree Celcius).  The others should be NPO types, since temperature correction is not needed (nor is it desirable).  If you cannot get N750 caps, don't worry too much, the frequency stability of the circuit is not that good anyway.

How It Works
Q1 is the oscillator, and is a conventional Colpitts design.  L1 and C3 (in parallel with C2) tunes the circuit to the desired frequency, and the output (from the emitter of Q1) is fed to the buffer and amplifier Q2.  This isolates the antenna from the oscillator giving much better frequency stability, as well as providing considerable extra gain. L2 and C6 form a tuned collector load, and C7 helps to further isolate the circuit from the antenna, as well as preventing any possibility of short circuits should the antenna contact the grounded metal case that would normally be used for the complete transmitter.

The audio signal applied to the base of Q1 causes the frequency to change, as the transistor's collector current is modulated by the audio.  This provides the frequency modulation (FM) that can be received on any standard FM band receiver.  The audio input must be kept to a maximum of about 100mV, although this will vary somewhat from one unit to the next.

With the value shown for C1, this limits the lower frequency response to about 50Hz (based only on R1, which is somewhat pessimistic) - if you need to go lower than this, then use a 1uF cap instead, which will allow a response down to at least 15Hz.  C1 may be polyester or mylar, or a 1uF electrolytic may be used, either bipolar or polarised.  If polarised, the positive terminal must connect to the 10k resistor.

Inductors
The inductors are nominally 10 turns (actually 9.5) of 1mm diameter enamelled copper wire.  They are close wound on a 3mm diameter former, which is removed after the coils are wound.  Carefully scrape away the enamel where the coil ends will go through the board - all the enamel must be removed to ensure good contact.  Figure 2 shows a detail drawing of a coil.  The coils should be mounted about 2mm above the board.

For those still stuck in the dark ages with imperial measurements (grin), 1mm is about 0.04" (0.0394") or 5/127 inch (chuckle) - you will have to work out what gauge that is, depending on which wire gauge system you use (there are several).  You can see the benefits of metric already, can't you?  To work out the other measurements, 1" = 25.4mm

NOTE: The inductors are critical, and must be wound exactly as described, or the frequency will be wrong.

Figure 2 - Detail Of L1 And L2

The nominal (and very approximate) inductance for the coils is about 130nH. This is calculated according to the formula ...
L = N² * r² / (228r + 254l)
... where L = inductance in microhenries (uH), N = number of turns, r = average coil radius, and l = coil length.  All dimensions are in millimetres.

It is normal with FM transmission that "pre-emphasis" is used, and there is a corresponding amount of de-emphasis at the receiver.  There are two standards (of course) - most of the world uses a 50us time constant, and the US uses 75us.  These time constants represent a frequency of 3183Hz and 2122Hz respectively.  This is the 3dB point of a simple filter that boosts the high frequencies on transmission and cuts the same highs again on reception, restoring the frequency response to normal, and reducing noise.

The simple transmitter above does not have this built in, so it can be added to the microphone preamp or line stage buffer circuit.  These are both shown in Figure 3, and are of much higher quality than the standard offerings in most other designs.

Figure 3 - Mic And Line Preamps

Rather than a simple single transistor amp, using a TL071 opamp gives much better distortion figures, and a more predictable output impedance to the transmitter.  If you want to use a dynamic microphone, leave out R1 (5.6k) since this is only needed to power an electret mic insert.  The gain control (for either circuit) can be an internal preset, or a normal pot to allow adjustment to the maximum level without distortion with different signal sources.  The 100nF bypass capacitors must be ceramic types, because of the frequency.

The mic preamp has a maximum gain of 22, giving a microphone sensitivity of around 5mV.  The line preamp has a gain of unity, so maximum input sensitivity is 100mV.

Select the appropriate capacitor value for pre-emphasis as shown in Figure 3 depending on where you live.  The pre-emphasis is not especially accurate, but will be quite good enough for the sorts of uses that a low power FM transmitter will be put to.  Needless to say, this does not include "bugging" of rooms, as this is illegal almost everywhere.

I would advise that the preamp be in its own small sub-enclosure to prevent RF from entering the opamp input.  This does not need to be anything fancy, and you could even just wrap some insulation around the preamp then just wrap the entire preamp unit in aluminium foil.  Remember to make a good earth connection to the foil, or the shielding will serve no purpose.

Details of the coils:
L1, L2, L9, L10, L11 = coil windings of 6 by 6 hole ferrite bead.
L5, L6, L7 = 5 coil windings, pop. 6mm diameter, 1mm wire: L6 a winding mass from a branch.
L3, L4 = 4 coil windings, pop. 5mm diameter, 1mm thread.
L8 = 1 coil winding, pop. 5mm diameter, 1mm thread.
If you have the circuit with a PLL may also want to send. Remove R3 and R8 then. Connect the output of the PLL filter and between R7, C7, C8 to a resistance of 390k.
Again CAUTION! for the Ministry V & W division telekom (RDR). You are solely responsible for the consequences of using this channel. The creator of this site is not responsible for any damage or whatever.
This transmitter is designed on the basis of many FM transmitter diagrams on the internet can be found.