• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

AM Transmitter

Status
Not open for further replies.

darkatsuki12

New Member
I am planning in creating an AM Transmitter for a range of 10-20 meters using a desired carrier frequency and modulating frequency ranging from 535-1605 KHz with a Bandwith of 10 KHz. I know it might sound dumb but I am confuse to how should i proceed in making this and where should i begin.
 

darkatsuki12

New Member
We have created our own design of an AM Transmitter with reference to the transistor modulator we learned in class. Due to trials and error with the circuit, It is working somehow but output is pretty much distorted. The range is not that long as well. We use a 200pF Variable Capacitor due to availability issue with other values. How can I improve this. The Transistor is actually 2n3904, mistype it.
 

Attachments

Last edited:

audioguru

Well-Known Member
Most Helpful Member
You do not show what type of microphone. Since it feeds the emitter and its resistor then it must be a low impedance, maybe a dynamic type (coil and magnet like a little speaker). The emitter voltage is causing the mic coil to be pressed against its case causing distortion. Fix it by adding a coupling capacitor in series with the mic to block the DC:
 

Attachments

audioguru

Well-Known Member
Most Helpful Member
I forgot to say the range is low because either the radio has poor sensitivity or the antenna length is too short (the frequency is low so the antenna must be fairly long).
 

MikeMl

Well-Known Member
Most Helpful Member
I'm guessing that you are using a carbon microphone, in which case, it will not work with the capacitor suggested by AudioGuru.

Carbon mics are very distorted (raspy) under the best of conditions.

Your transmitter has a very low power input, and an antenna that is at best about 1% efficient, so I would expect the range to be only a few meters.
 

darkatsuki12

New Member
Because of the distorted output. It makes it hard for me to hear from the microphone input so I used an mono Audio input instead. How will I know the right length for the antenna.
 

atferrari

Well-Known Member
Learning what wavelength is, maybe?
 

MikeMl

Well-Known Member
Most Helpful Member

audioguru

Well-Known Member
Most Helpful Member
Haven't you seen the huge antenna used for an AM radio station? Your transmitter needs a very long antenna and also needs a good earth ground. The AM broadcast band old and is at low radio frequencies.
I never made an old AM transmitter. My first transmitter was FM and its antenna is less than 1 meter long. FM sounds much better than AM.

Are you going to tell us what type of microphone you used? Dynamic, carbon, electret, piezo or a speaker? A photo, datasheet and description are needed.
 

Rich D.

Active Member
The antenna can be made into a coil to shorten the length. It's done all the time, called a loading coil I think. Really you can just use a very long wire and experiment with winding it up in different ways to get it to work reasonably well. You can get away with a lot because it is low power. If it were high-power, a lot of calculations and tuning would be recommended.
One neat cylindrical coil may not be best, because that would make it very directional, so coil it in various orientations.

Now if you WANT it directional to improve transmission distance, try to find an antenna coil from a cheap old AM radio.
Those have a ceramic iron core and are very directional. Don't bother using a iron bolt as a core, it has eddy losses a ceramic core doesn't have.
 

dr pepper

Well-Known Member
Most Helpful Member
Your circuit is very basic and might well generate distortion at the best of times.
To be honest if you want more range and better quality you'd better ditch the circuit, or at least modify it to one that uses 2 or 3 transistors.
The length of wire for the aerial needs to be below a certain length to be legal, I think about 3m, to be effective on am it would need to be really long 50m or more, as said you can load the aerial with an inductance, tuning it however is not straight forward without special gear.

I occasionally restore tube radios, I have an am transmitter, its basically a loop of wire about 1m across, its tuned with a capacitor and is fed one end with a crystal oscillator made from a logic ic and the other end is fed from a audio amp ic normally used to drive a speaker, because the loop/capacitor is resonant range is good and even though square waves are involved harmonics which would otherwise be bad are acceptable.
Hams still dis it though.
 

darkatsuki12

New Member
Your circuit is very basic and might well generate distortion at the best of times.
To be honest if you want more range and better quality you'd better ditch the circuit, or at least modify it to one that uses 2 or 3 transistors.
How can I modify this? Should I add a pre amp before the input part?
 

audioguru

Well-Known Member
Most Helpful Member
An electret mic has a fairly high output impedance. The emitter of the transistor and its 1k resistor is a fairly low impedance. Then the low output from the mic is loaded down to almost nothing.
The mic needs a preamp with a gain of 20 to 100 times and a low output impedance. An opamp with an output coupling capacitor would make a perfect preamp.
 

audioguru

Well-Known Member
Most Helpful Member
The student was probably instructed to make a Hartley RF oscillator that he showed. Then its modulation input is the low impedance emitter so it needs a low output impedance of the preamp for the electret mic.
 

BobW

Active Member
At normal AM medium wave broadcast frequencies, a range of 20 meters is fairly easily accomplished with a 100 mW transmitter and a 3 meter long antenna as long as the antenna is properly matched to the transmitter. Proper matching is the key.

Because the antenna and the tank are connected in parallel to the transistor collector, the antenna will be inherently matched at whatever frequency the transmitter is tuned to. But that certainly doesn't mean that the circuit is optimum or the antenna is optimum. It merely means that for that particular circuit and that particular antenna, the antenna is carrying as much RF as it can at any given operating frequency.

And, there is another issue at play. At these frequencies, the antenna will have an extremely low radiation resistance, and at the same time a very high negative reactance. A 3 meter antenna will have a capacitance of about 50 pF while the radiation resistance is only a few milliohms. To get maximum range, you need to send as much RF current through the antenna as possible. The RF current is split between the antenna and the tank capacitor. The total tank capacitance is equal to the antenna capacitance in parallel with the variable capacitor. At resonance, maximum RF current passes through the total tank capacitance, but at the lowest resonant frequency, only 1/5 of the total RF current will flow in the antenna because the variable capacitor is set to its maximum value. At the highest frequency, the variable capacitor will be at minimum capacitance and considerably more RF current will go to the antenna. To maximize the range, the transmitter should be operated with the minimum amount of tank capacitance. Therefore, to operate more efficiently at lower frequencies, a larger value should be chosen for the inductor, so that the resonating capacitance can be reduced.

Of course, this causes other problems. Since the antenna is part of the tuned circuit, movement of the antenna, or moving objects near the antenna will cause detuning of the transmitter. And if the antenna capacitance contributes a larger fraction of the tank capacitance, the detuning effect will be worse. There's no free lunch. In this case, it's a trade-off between frequency stability and efficiency.
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top