wanna make fun of my prof

[ahmed277]

my lectures hall has a wireless microphone (a transmitter in prof's pocket sends the voice to a receiver under the table) i think it uses fm (not sure).
Is there a way to know the frequency it uses and transmit mp3's from my own laptop to the hall speakers?
any software- hardware? where to start?
:idea: please help ( any idea will be useful)

 

[KJ6EAD]

It's almost certainly FM, but probably much higher frequency than broadcast radio. Get a look at the brand and model of the transmitter or receiver. Failing that, the length of the receiver antenna would be useful to determine the frequency band, then try scanning for the transmitter with a radio scanner. I don't suppose you have a spectrum analyzer handy?:-| Is there a chance of borrowing a second transmitter?

I suppose I should mention the potential downside of success at this point, but I think you can guess.:evil:

 

[ahmed277]

i can'y borrow a transmitter i shouldn't get caught
tell me more about antenna lengthand how can it be usefull
tell me about spectrum analyzer and radio scanner
i'll try to see the receiver
thanks for your reply

 

[KJ6EAD]

There is a mathematical relationship between frequency and wavelength. Antenna's are designed for the wavelength of signal they will receive/transmit. There are ways of altering or disguising the length of an antenna but assuming a ¼ wavelength dipole antenna and no loading coil is a reasonable start.

The wavelength (λ) = the speed of light (c) in meters ÷ the frequency (f) in Hz.

For example, an FM broadcast receiver has the center of it's band at around 100 MHz, so 300,000,000 ms ÷ 100,000,000 = 3 m, then λ ÷ 4 gives ¼ wavelength of 0.75 m. So when you see an antenna that's 75 cm long and work the formula backward, you get 100 MHz.

If I see one of those telescoping metal antennas on a box next to the P.A. amplifier and it's 44 cm long, I can do the math and find that it's probably for 170 MHz, a common low-cost wireless microphone frequency in the U.S.

For information on radio scanners, see: Scanner (radio) - Wikipedia, the free encyclopedia) and spectrum analyzers, see: Spectrum analyzer - Wikipedia, the free encyclopedia.

 

[ahmed277]

thanks alot it was useful information but why the transmitter antenna isn't the same length?
should i expect encryption or anti-jamming or something like that?

 

[KJ6EAD]

As I mentioned, there are ways of altering or disguising the length. The antenna on the transmitter may be a metal loop inside the transmitter housing, a short length of wire protruding or hanging from it or it may be included in the microphone cable. It may be electrically lengthened by the inclusion of an inductor in the transmitter housing or some combination of these techniques. It is therefore, not useful to measure the transmitter antenna.

Whether any attempt has been made to secure the channel depends on the budget of the purchasing agent, their previous experience with hooliganism like you propose, and other variables we can't know. I wouldn't expect it at a small educational institution, but you never know.

You must realize by now that if you put this kind of effort into your studies, you would be getting a better kind of attention from your professor.:razz: