Hi all,

I am attempting to build an AM transmitter. I have the oscillator working very nicely and I am getting a nice smooth signal the problem I am having is with my antenna. With an AC signal on a very small, when compared to wavelength, antenna I get a transmission that goes in every direction. I have two questions:

a.) Is it possible to direct the signal into an arc or beam type pattern besides dipole arrays with reflectors and directors?

b.) What would be a good material to prevent the transmission of the signal in directions that I don't want to go in without disrupting the signal itself. I have tried connecting ground plates near the antenna and that just causes massive power loss.

I would like to be able to transmit in 180 degrees of a sphere but prevent it in the other 180. Any suggestions would be welcome.

Thanks,
Greg

 

You might want to read up a bit on antenna theory. If your antenna is smaller but not a harmonic fraction of the applied signal, then not only do you lose efficiency in the antenna, but you get significant power reflected back into the output stage that causes heat losses (and sometimes the output driver).

Commercial broadcast antennas are arrays of radiators that suppress antenna radiation in the vertical plane. This increases the coverage of an area, by raising effective ratiated power.

By extension, the same might be effective for limiting power radiated into one hemisphere. I wouldn't want to guess at the number or placement of radiators to achieve this. Might be a challenge to cancel waveforms in one direction.

 

why dont you use a parabolic antenna , the radiation pattern of a parabolic curve sufrace reflector antena is highly directive and should get you the directed beam.

 

why dont you use a parabolic antenna , the radiation pattern of a parabolic curve sufrace reflector antena is highly directive and should get you the directed beam.

Probably because of the wavelengths involved.
I am of couse assuming that by AM you are meaning MF frequencies from 530Khz to 1600Khz (i.e., a half wavelength of 94 meters (308 feet) to 280 meters (920 feet)) .. ?