Typical values of field intensity in receiver

[buffallo]

Hi,

I am sort of a newby in RF and as for beginning I want to test a simple Tx-Rx circuits at (let's say 27MHz). For this purpose I am designing a transistor-driven oscillator with a fixed frequency and I use a λ/4 monopole antenna to emmit the non-modulated signal. However, I have a 'problem' with the detection of this signal.

Even worse - how could I know if I am actually generating and emmiting any signal, if I can't detect it? (assuming I don't have a Hi-Freq oscilloscope or spectrum analyzer).

For now I am reading and deciding on simpe Shottky diode AM detector, consisting of a capacitor and resistor. The receiver antenna is connected to the resonant tank circuit and the signal for the detector is obtained from it.

Running simulations with LTSpice (modelling the antenna as a voltage source with fixed frequency, voltage of 20uV an impedance of 36 ohms) the output of the detector does not show any signs of detecting anything.

My question is what are the typical values of the intensity of the field (i.e. voltages across inductance/capacitance in LC tank circuit)? What are the minimum signal voltages a typical shottky diode can detect and can biasing the diode help in any way to improve sensibility of the circuit?

I hope this thred would turn into a constructive discussion.

Thanks,
Dimitar

 

[biff44]

you should publish your exact schematic. it is hard to comment from the written description

 

[chuckey]

For 27 MHz you must use an oscillator followed by another transistor and NOT "transistor-driven oscillator with a fixed frequency". This is to stop your aerial from detuning your oscillator.
If you take your transmit aerial as being a 200 resistive and 200 ohm capacitive impedance you will be in the right ball-court. An oscillator should deliver 1V into 1k then followed by another transistor, you should be able to get 1V into the resistive part of your aerial, BUT you must use an inductance of 200 ohms reactance to tune out the capacitive bit.
Frank

 

[vfone]

20uV would not really open a simple Schottky diode detector.
You need an RF amplifier in front of your diode detector, and probably a simple DC amplifier after the diode.
After that, the circuit might need a calibration for showing the right level of voltage/power at the input.