On Mon,8/3/2015 1:31 PM, Bob McGraw - K4TAX wrote:
I tend to agree with Brian.
I do as well. It's important to realize that a large percentage of fading is the result of signals arriving from multiple paths, resulting in slightly different travel times from the source. This time difference results in phase shift directly proportional to the time difference and to the frequency.
When heard on VHF/UHF, we call this "picket fencing," and the fading is much more rapid because of the higher frequency/shorter wavelength. On the much lower bands (AM broadcast, 160M) we call it selective fading, and the time between peaks and nulls is much greater (proportional to wavelength differences). Diversity reception for this kind of fading depends on antennas at different locations -- the time differences, and thus the phase shifts, will be different at the two locations, so a fade (strong cancellation) at one location will usually not be a strong cancellation at the other.
The most critical thing about diversity is that separation, and keeping the two receivers close enough in frequency so that the ear/brain is not bothered by the difference in the note.
Another point -- It is generally a bad idea to COMBINE (sum) the outputs of the two receivers into a single electrical channel. Two reasons. First, as Bob has noted, the ear/brain is really good at choosing signals presented to two ears -- that's how we hear directionality, and it's how stereo works. Indeed, stereo works on the TIME difference of the sound reaching the two ears. Second, if the travel times to the two antennas are sufficiently different, the AUDIO signals can be enough different in phase and equal in amplitude that they cancel in the electrical channel. When the electrical signals are directed separately to L and R loudspeakers, there is relatively little cancellation, and there is none in headphones with one radio per ear.
I've summarized this from the result of decades of experience with sound systems, an extensive study of phychoacoustics (the science of how humans hear), as well as from 60 years in ham radio.
73, Jim K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
