Medium wave Transmitter - construction problems.

[squb]

I have problem with building a medium wave transmitter.


I have designed circuit and put it together on breadboard.I using Epson programmable oscillator (4,57 MHz) and johnson decade counter in order to divide signal by 10 (457 kHz).I have a complementary power amplifier as a output stage.I connecting output stage with antenna by transformer( I trying with 21 turns on ferrite rod).

Antenna is tuned to 457 kHz - according to formula f=1/sqrt(LC).

My questions:

-I need approximately 0.1W output power and 50 m range , but I don't know how to measure it?

-Another problem is with proper antenna impedance matching. I couldn't find any formula for transformer to calculate how many turns of a wire are need to be made on a ferrite rod in order to get right impedance so I tried few random numbers of turns.Do somebody know how impedance matching should be ?

- I made simulation in LTspice but results are not the same like in reality. So, if somebody could tell me-Do I biased transistors in good manner ?


Circuit in attachment.

 

[betwixt]

Driving square waves to an antenna is a bad idea, especially when you do it through an electrolytic capacitor! If it works at all you have made a bad interference generator!

The ouput power is limited by R5 to no more than 25mW but in practice it will be significantly less. The range has very little to do with the power, the radiation losses and receiver sensitivity are far more important.

You have not mentioned what the transmitters purpose is. In any case I would suggest if you really want to do it with a digital drive signal, you use lots of filtering and a suitable output matching network.

Brian.

 

[squb]

Hi Brian

Thank you for your help and fast reply.

I work on my project for studies ( Topic: " Security system for climbers and skiers. Design and build working prototype of avalanche transreciver for 457 khz ")

I try many times with different circuits layout ,but I didn't manage to get sufficient results.I affraid that I made mistakes in calculations.I made simulations in LTspice and them shows that everything is all right.

In attachment is reciver circuit. Sensitivity of my reciever is fairly small and I don't know how to increase it

I figured out that it is also possible to use delta-sigma Ic to get RMS value. I need DC voltage signal which is proportional to distance from transmitter.

What do you think. Two stage BJT amplifer and op amp integrator in line as a reciever circuit are good idea ?

Could you tell me ,what do you mean by "..suitable matching network" . I know about T-type matching network but I still don't know how to calculate ferrite rod antenna impedance in order to calculate matching network components.

Best regards

 

[betwixt]

Your receiver is also badly designed. First though, you must understand that measuring signal strength is not the same as measuring distance, many other factors, including attenuation in avalanche debris and snow will affect the received strength. It will give a rough approximation of proximity though. If you want to find direction as well as distance you need far more complicated receivers and transmitter circuits.

The ideal bandwidth of a receiver will be around 200Hz which would be difficult to achieve but your design is probably 20KHz or more, by reducing bandwidth you will increase sensitivity and also immunity to nearby MW transmtters. Bear in mind that 457KHz is bordering a band which has lots of high powered transmitters in it. The best way to improve it would be to reduce the loading on the tuned circuit by increasing the input impedance of the first stage. I would suggest a JFET, source follower then a second filter, possibly a ceramic IF filter then a high gain stage. Also consider rectifying the received signal before measuring it. At the moment it appears you are trying to digitize the RF which plays no useful purpose and makes it far more complicated. The transmited signal is pulsed, if you average the pulse levels and use that as your measurement it will be less susceptible to interference and give a more stable reading.

In the transmitter, you really want a clean sine wave at the output and as much RF delivered to the antenna as possible within the limitiations of the battery life. If you use the digital divider method, apply filtering to the signal to remove harmonics before amplifying it and use a simple keyed supply amplifier using a single transistor. Match it's output to the antenna with a pi network or tapped coil to maximize energy transfer and also minimize the chance of causing interference. Incidentally, to cover 50m (metres) range reliably you should only need about 5mW output power.

Brian.

 

[Bob60]

Wow... very interesting. You should to know that your ferrite loop antenna (21 turns on ferrite rod ) has a lot and big minuses. First of all I would to say, for example, when I was may be 12 years old I trying made some link on MW and with ferrite loops antennas. I have no understanding concerning effective lengths of the antennas and so on. And reality was not so bright. Probably, performances of such type antennas on MW, is some centimeters of the effective length and plus it depends from loaded Q and loop square. So, conclusions are, if you wants efficient distance, you should move to the loop with half- one meeter diameter, of course with tunable coupling and resonance frequency. For receiver it OK ( without errors of course, you have errors at the receiver part ), only you should have BPF for up to tens kilohertz at he beginning or right before detector. OP is not good those for these purpose. So good luck, enjoy