a basic question about high frequency signal transmission

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vonleo

Joined: 30 Oct 2004
Posts: 35
Helped: 1

05 Nov 2004 3:27 a basic question about high frequency signal transmission

Hello, everyone, my question maybe very easy for the guys working on electromagnetic. I think transmitting a signal at a high frequency can attain a longer distance than low frequency transmission, since high frequency has a stronger field. There is also a relation about the transmission in the free space:(wavelength/distance)² to describe the loss in the space. But sometimes high frequncy transmission has more loss than the lower counterpart, and i think the lower frequency has a larger wavelength, so it can circumambulate the big barrier better... Of course very low frequecy transmission get a lot of disturbance and also limit bandwidth. But what the exact relation between the loss and transmit frequency?

Thanks.

flatulent

Joined: 19 Jul 2002
Posts: 4877
Helped: 324
Location: Middle Earth

05 Nov 2004 3:48 Re: a basic question about high frequency signal transmissio

These "free space loss" charts are based on isotropic antennas at each end. At the receiving end an isotropic antenna has an equivalent capture area of about one eighth of a square wavelength. This is what makes the graph show more loss at higher frequencies.

The electric and magnetic field of a plane wave are only related to the power and the impedance of free space. It is independent of frequency.

If you are going between two points on the earth the path gets very complex and frequency dependent because of the several modes of propagation. For ground wave the attenuation is less at lower frequencies. Sky waves going through the D ionospheric layer are attenuated less at higher frequencies. The reflection from the F layer is a low pass filter effect with a relative sharp cutoff.