I'm building a kit AM transmitter an for every 1m of antenna wire I connect the frequency is lowered, how is this so?

Asked by: Scott

Answer

In radio transmission, for AM or FM waves, the wave transmitted from the station uses two components: the carrier wave, which actually 'carries' the information and the modulating signal (the information itself). The quality of reception of every antenna depends on the wavelength (in metres) of the carrier wave, which is inversely proportional to its frequency f (in Hz). The antenna has the best reception when it has a size of half the wavelength, or 3/2, 5/2 etc times the wavelength. These two quantities are related through the following equation:

c = f * wavelength ==> wavelength = c / f ,

where c is the speed of light (constant value c = 2.98 * 10⁸ m/sec). As the equation shows, the lower the frequency the bigger the wavelength (bigger antenna). In the opposite thinking process, in order to receive higher frequencies (always inside the AM, FM range) the antenna length must be decreased. With the aid of the above equation exact values of antenna sizes can be calculated for a given frequency.
Answered by: Nick Prountzos, Digital Comm. Systems Grad Student, LU, UK