polarization detection of one transistor fm transmitter

hi, we have a simple one transistor fm transmitter with vcc=9v and its carrier frequency is 92 MHZ. suppose if i want to know its polarization what is the procedure?(actually i took two copper wires and one LED and then formed a simple dipole antenna but its not working)

A λ/2 dipole antenna at 92 MHz is about 1.6m long.
You need to have at least 2λ distance between antennas to see any polarization differences (greater than 6.4m in your case).
When you get all these requirements you should see about 20dB variation in the path loss (or gain) when you rotate the dipole antenna from horizontal position to vertical position.

If you are connecting the LED at the center of the dipole and expecting it to light up you will be out of luck. Firstly you wil not pick up enough current, secondly, the charge stored in the LED junction will not allow you to 'flash' it at 92MHz. (it would only try to light on half cycles)

Brian.

betwixt said:

If you are connecting the LED at the center of the dipole and expecting it to light up you will be out of luck. Firstly you wil not pick up enough current, secondly, the charge stored in the LED junction will not allow you to 'flash' it at 92MHz. (it would only try to light on half cycles)

Brian.

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thank you for ur reply, can u suggest the procedure how can i detect the em waves through this simple set up.

Connect an RF co-axial cable to the center of the diople so you can couple the signal away from the pick-up location. Then at the other end of the cable, connect a tuned circuit (92MHz) and signal diode rectifier. Measure the resulting DC current on a DVM. If you seach for "field strength meter" you will get the general idea of how to monitor signal strengths.

Brian.

If you just want to know if the transmitter is working or not you can use a normal FM radio receiver. It can span the 88-108 MHz frequency band.
Switch the transmitter on as well as the radio; place them not very far one each other. If the transmitter is working, rotating the tuning knob of the radio, when you reach the TX frequency you'll "hear" the presence of the carrier, that is the noise normally present during the tuning will dissappear and the radio will seem mute. At this point, to be sure, switch the transmitter off and the noise will appear again.
This should happen around 92 MHz, if the oscillator is well tuned. However with this homemade method you can discover the signal everywhere in the 88-108 MHz band.

The polarization has nothing to do with this, if you mean the polarization of the EM wave. It's a characteristic given by the geometry of the antenna. If you have a dipole placed parallel to ground then the polarization will be horizontal, if it is perpendicular to the ground it will be vertical.

If, instead with the term polarization you mean the working point of the transistor (f.i. in Italian "polarizzazione" means biasing) it depends from the circuit.

By the way, what should be the purpose of the LED ?

sandeepkiranv said:

hi, we have a simple one transistor fm transmitter with vcc=9v and its carrier frequency is 92 MHZ. suppose if i want to know its polarization what is the procedure?(actually i took two copper wires and one LED and then formed a simple dipole antenna but its not working)

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It is not expected to work. The principle is ok, but the setup is not.

1. Estimate how much power you are transmitting out of the simple antenna.

2. Estimate how much power is being captured by the simple antenna (is that enough to turn on a led)

3. Estimate the voltage induced at the two ends of the dipole antenna. (is it sufficient to light up a led)

4. Can the led respond at 92 MHz?

5. A simple FM receiver has a ferrite antenna which is highly directional and you can rotate the radio to find the effective plane of polarization.

c_mitra said:

5. A simple FM receiver has a ferrite antenna which is highly directional and you can rotate the radio to find the effective plane of polarization.

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FM broadcast radios (88MHz-108MHz) NEVER use ferrite antennas.
They use monopole type antennas, or simple wire antennas as in the case of mobile phones that support FM reception (the antenna is the shield wire of the headphone cable).

vfone said:

FM broadcast radios (88MHz-108MHz) NEVER use ferrite antennas. ).

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