Hi,
I have read that some of you are into shortwave broadcasting - in the 13.56Mhz frequency. Also some of you have bought a ready to use transmitter from turkey.
A few of the transmitters seemed to have trouble, also they are relatively expeinsve - but therefore ready to use.
However, for those of you who want to built their own shortwave transmitter I have a little project here.
It is a simple transmitter that I designed - made it as simple as possible. It consists of all parts needed for making a small shortwave station
- Oscillator and pA (both done by one transistor)
- Lowpass filter and match network for 50Ohms on the output
- AM modulator with audio amplifier for low level (headphone) levels
- Audio deemphasis to lower higher AF frequencies to keep the transmitters
bandwidth small.
The transmitter does generate a relatively good modulation - altough - you can tell by the simplicity of the design it is not comparable to a "full" transmitter with an own final amplifier and a proper modulation stage. I have made critical modulation tests and it does go very loud (means that you can
already hear the modulation as soon as parts of the carrier are beeing received) but over all the sound is kinda tiny. Hard to describe. It definately
sounds OK for a small shortwave station, speech and music are transmitted undistorted and clear (if you don´t push the modulation depth to it´s limits) but over all it does not sound as ... "strong" as a transmitter with a proper modulation stage and the capability to make >100% AM. I have very high test
intentions, so for a DIY circuit it´s allright, but not for a wannabe professionall transmitter.
I have neither tested the range nor measured the output power. All I know is that it is probably within the 10mW or less power range - since I orientated me on the knowledge that it seemed to be allowed to transmitt with 5mW into a lambda 1/2 dipole. With a 5 ft long wire antenna connected to the output from the transmitter without any additional matching (eg PI filter) it does generate a strong enough signal for every position in my room even when using a short antenna on the receiver and attentuator activated.
The transmitter´s output power can be regulated by varying the input voltage. It will work also with 9V - providing less output power - and it will also work with 24V (at least I tested it) but the modulation is getting rather quiet / eventually distorted. It has been optimized for 12V operation as you
may have a 12V power supply handy.
The frequency can be adjusted exactly to 13.56Mhz using a trimmer capacitor instead of the fixed 18pF capacitor. The capacitor 1nF in the modulator stage
between collector and base reduces higher audio frequencies - making the bandwidth that the transmitter requires smaller. 1nF is still a littlbe bit low, and the transmitter can still transmitt rather high audio frequencies relatively well. If you increase the capacitors value, higher frequencies will be supressed better.
If you power this transmitter with a "wall wart" power supply and have problems with hum you can try to increase the 220µF capacitor to 1000µF - altough 220µF should by far more than enough considering the power requirements of the circuit. If the replaced capacitor doesn´t fix the hum issue, add some 22µH inductors in series with the power supply (+ / -) and the audio source to the transmitter. Then add a wire as "ground" for the antenna to the transmitter - it will increase range aggain then.
Powering the transmitter with a DIY power supply AC to DC power supply it will work if you ad .01µF (10nF) capacitors parallel to the diodes that convert the AC voltage to DC.
The parts can be bought in most electronic shops. As circuit board a board with 5 times 10cm was used. Parts of it got cut out as seen on the picture and glued on top of the circuit board. There´s even lots of space left for adding a battery or using the space for own additional projects.
This is not a perfect instruction how to re built the transmitter - I´ve done some where I´ve put ways more effort into. The only thing is, I sit in front of my laptop, create drawings / pictures for hours and sadly had often the experience that it did not get worshipped by people in the way I wanted it to be. On a vintage radio collector forum they asked about AM modulator - can anyone make an AM modulator circuit that uses simple parts... I made one, does use some transistors covers all am band, and I showed it... They didn´t really pay attention to that.
I do not know - have low expections - how circuits are rebuilt here, so I just gave it an attempt...
(I am not sure if there is a picture because when I click on upload nothing happens, no feedback wheter the transmitter picture is uploaded or not)
The picture is there - thanks for the post and explanation.