FAQ(frequently asked questions) on making a micro radio transmitter

  1. Unfamiliar components
  2. Transistors and polarity
  3. Tuning and Testing
  4. Problems:heated transistor, noise...
  5. Antenna
  6. Starting up
  7. How to decrease the output power?
  8. How to increase the output power?

1.Some of unfamiliar parts
Q: Do the coils have an air or ferrite core?
A---Ferrite core. If you want to use air coil, you must use a bit thcker wire like 1.0 mm and protect them from trembling that leads unstable frequency. Therefore I recommend module-type coil-->See the photo. Different from the "simplest" model, unstable conditions of every section can cause problem of transmission and interference.

Q: Can I make the choke RFC by myself?
A---Yes it is possible. Prepare 4.7K ohm register (1/8 - 1/4 watt) and 0.2-0.5 mm coated wire. Give the wire 20-25 turns around the register. Solder the wires and the leads of the register. It will become 1-3 uH RFC. A---Higher frequency characteristics will be preferable.

Q: What type of varicap diode is required?
A---"varicap diode" is called as "variable capacitance diode", "voltage controlled diode", or "tuning diode" too. I used "1SV101" (Toshiba), "ISV126", "ISV2638"(Hitachi), "1T33" (SONY) and so on. The capacity is 10-25 pF by the voltage change. There are quite a lot of replacements and the exterior is different.

Q: When I build the final stage, is the 1 microH choke above the 1970 meant to be of a bigger type than the small resistor-like ones that I'm using in the rest of the circuit?
A---the 1970 needs much ampere than the 2538 or 1907, so that it must be bigger.

2.Transistor and Porarity
Q:Can you make sure of the position for the transistors to be soldered.
A-----That's all:
             2sc1970  E - to earth plate
                      C - to 1 micro Henry RFC
                      B - to 10pF cap. and 300 ohm res. 

             2SC2538  E - to earth plate
                      C - to 10pF,3.5pF and coil
                      B - to 1K Ohm res.

             2SC1907  E - to 33pF, 33pF, 10pF, 2-3 micro H. RFC
                      B - to 33pF, 33pF, 22K Ohm, 4.7K Ohm res.
                      C - to 0.01 micro F cap.               
3.Tuning and Testing
Q:How can I start transmitting?
A-----(1)Don't supply the power (12 volts DC) before connecting the appropriate antenna (even if it is a simple coax cable antenna that I made instantly).
(2)You must decide the frequency you want to transmit. Please check a vacant frequency in your FM dial of your radio receiver. And prepare a power meter and a frequency counter. Although it is not so easy to build a frequency counter by yourself, it would be easier to build a power meter. The diagram is here.
(3)Before connecting the antenna to the final output (where 100pF is connected), you must adjust the final 3.5 coil. The second and final coil must be adjusted to maximize the strength of the power meter.

Q:What is the function of the 3.5 turn coil above the 2053? It does not seem to have much effect on the quality of the sound on my receiver when I move the core.
A-----This is for adjusting the matching with the original signal. The reason may be that the matching is not correct or the coil is not appropriate.

4.Problems:heated transistor, noise
Q:When I tested the completed circuit, the 2SC2053 gets very hot after a minute or two. I assume this might damage it if I let it get too hot. Do I need to adjust the loading on the 2053, so it passes less current? Do you have any ideas how I may get the best output - i.e. where to connect aerial and if any parts need adding or changing?
A----When you have too hot temperature at 2SC2538 (when the adjustment of the final coil is complete, it will not become hot, anyway), you may exchange 50 ohm (I suggested to change before) at 2SC1907's C for 100 ohm. This means to reduce the power voltage to the first section.

Q:Can I use an "AC adopter" for 12 volts?
A----You can, but sometimes it may cause hum nose. So get a 12 volt power supply of over 1A (1000 mA) direct current with good quality. When you use not so sophisticated "AC adapter", please add the 12 volt power regulater to the initial schimatic.

Q:What type of antenna will I need?
A----I suggest that first you buy a ready made FM antenna for 88-108MHz (not for TV).
I think you can get it at Radio Shack or equivalent. Please tell them "I want to receive week airwaves of FM radio", then they will show you some model. Every receiving antenna can be used for transmitting as well.
When you buy it, please get the cable to connect antenna with transmitter. Sometimes the cable is appendant to antenna. But the cable must be quite a thick 50 or 75 Ohm coaxial cable (type of "5D2V", "5C2V","RG62", etc.). You will fix this antenna on the top of your roof and will pull the cable into your radio room.

6.Starting up
Q:Let me know how to start the transmission up?
A----Prepare the following materials:
1. Tape recorder with mini plug cable coming from audio out.
2. 12v/over 500mA DC power supply or 12v car battery.
3. Radio antenna with enough wire to reach antenna from place of transmission.
To build the system:
1. Connect the antenna and the transmitter using the coaxial cable connector. Make sure the connection is right; if you use it without doing so, the power transistor may get damage.
2. Connect the audio cable from the headphone jack or line out to the audio in on the Black box (check that the volume is down on the tape player).
3. Everything OK? If so, connect the 12v power to the transmitter (never mix up plus and minus!).
4. Start your tape player, tune your radio to the transmitting frequency (89.1MHz) and you will find the signal. Bring up the volume little by little until you've reached the appropriate level. Now you are an independent narrowcaster.

7.How to decrease the output power?
Q:Is it possible to have much simpler and smaller powered transmitter?
A----Look at the "most simplest transmitter" page.

Q:I'm going to do a sound installation. I want to use several very low power transmitters. Is it possible to built your transmitters with lower power? I only need a radius from one to five meter.
A----My answer would be the same above. Please note that this tiny transmitter has lower quality of sounds and unstable frequency. Every cable of audio input and battery influences the output frequecy---this can be a bit solved by using thin coaxial cables for these suply. But this s an advantage that you need not special materials such as s variable capacitance diode. In stead of that, this can cover at least 10 meter radius. This should be good for your artistic installation.

8.How to increase the output power?
Q:How to increase the output power?
A----This main unit is so sophisticatedly designed that you can boost the power only when you add a boosting circuit to it. The model circuit is here. I didn't mention, but the transmitter ("1-3 watt") has harmonic signals such as the double and tripple of the original frequency and you need a lowpath or bandpath filter between the transmitter and the antenna. When you connect this booster, such a filter is indispensable. Otherwise, you may have interference against television and radio.