On 12/20/2024 7:36 AM, Mark Lunday wrote:
I was watching on my waterfall and DL8LAS would go from unreadable to Q5 copy
and then disappear again in 10 seconds.
This is the fading which back when AM radio was all there was was called
"selective fading, and on VHF/UHF FM is called "picket fencing" when a
mobile station is involved. No matter the frequency, it is the result of
the transmitted signal arriving at the receiver via more than one path
(usually two), and thus with different travel times. Phase is directly
related to time, so the phase difference between the two arrivals is
continuously rotating over the full 360 degrees. When the two arrivals
have close to zero phase difference, the station is loud, when close to
180 degrees the station is weak. The weaker the station becomes, the
closer the two arrivals are to the same strength. The greater the
difference in the distance of the two paths, the faster the fading.
Likewise, the higher the frequency band, the faster the fading.
73, Jim K9YC
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