tag:blogger.com,1999:blog-11282959.post4727492691592079593..comments2009-03-31T18:37:40.005-04:00Martin Ewinghttps://profiles.google.com/106339184393867367097noreply@blogger.comBlogger4125tag:blogger.com,1999:blog-11282959.post-46949629142019069852009-03-31T18:37:00.000-04:002009-03-31T18:37:00.000-04:00Oops. By "relatively short" I mean that the bursts happen for a fairly small fraction of the time on a given path and frequency, perhaps 10% or less.<BR/><BR/>Since propagation details would be strongly dependent on frequency, it would be interesting to use a swept frequency transmitter and receiver. The pattern over frequency may resemble the pattern over time.Martin Ewinghttp://www.blogger.com/profile/14329680325036851280noreply@blogger.comtag:blogger.com,1999:blog-11282959.post-69475062169808879902009-03-31T18:28:00.000-04:002009-03-31T18:28:00.000-04:00N8SRE: Not exactly. A deep null caused by two scattering centers that interfere can attenuate your signal by &gt;30 dB (signal goes almost to zero but only for a very short time), while if they add, they will only double your power, or +3 dB.<BR/><BR/>What we do seem to see are relatively short bursts of enhancement, maybe 20 dB, like those cusps on the bottom of the pool, but they can last for minutes.<BR/><BR/>73 AA6EMartin Ewinghttp://www.blogger.com/profile/14329680325036851280noreply@blogger.comtag:blogger.com,1999:blog-11282959.post-24463632490865289542009-03-31T18:06:00.000-04:002009-03-31T18:06:00.000-04:00If you have deep nulls when multipath signals arrive out of phase, you'll also have the opposite effect -- strong peaks when they're in phase.<BR/><BR/>SWLs are very familiar with this "selective fading" effect. On AM this causes cyclical fading and sometimes distortion, the latter because the fading is sometimes so wavelength-dependent that the carrier fades and one of the sidebands doesn't, making the signal appear over-modulated at the detector. With a narrow signal like CW you'll only perceive it as a strengthening and weakening of the signal.<BR/><BR/>I'm not sure if this explains what KB6NU is seeing, though. Selective fading usually has a period of seconds to a few minutes, and what he's describing seems like a longer-term phenomenon.N8SREnoreply@blogger.comtag:blogger.com,1999:blog-11282959.post-60423829243338052802009-03-31T11:32:00.000-04:002009-03-31T11:32:00.000-04:00I don't think the selection effect accounts for this phenomenon because the signals are much stronger than usual. The other idea has merit, though, and I'll have to think about this more.<BR/><BR/>73, Dan KB6NUKB6NUhttp://www.blogger.com/profile/00044061786299982977noreply@blogger.com