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Signals are remarkably stable for 99.99% of the time at my home QTH IO80KX
Since the refit last year at ZZ I can access the repeater with around 500- 1000mW
On a couple the odd occasion the pathloss goes up by up to 13dB, requiring over 10 watts to get in.
Today when this happened, I could see clearly the Mendip hills which are the first obstacle at about 30 miles
The weather in Bristol was not so good with heavy rain .....However viewing the ZZ site webcam I could see that it was not raining.
I checked for the next hour or so and the pathloss was the same ...An hour after that I checked again and I was in with <400mW
I would be interested in any thoughts on the mechanism which might be in operation causing this short term effect.
Look at my web pages, http://www.mike-willis.com/Tutorial/propagation.html for a description of potential modes. Sub refraction comes to mind or multipath fading. Also look at ITU-R Recommendation P.452 on the ITU website. It is a free download and has a lot to say about the reliability of point to point microwave links.