"Tam/WB2TT" wrote
Isn't there a zone where you tend to get cancellation
between groundwave and skywave?
______________

Yes, that is what I have been writing in my posts on this subject line. The
RF phase of the skywave varies independently from the ground wave. Some
times it reinforces the ground wave; some times reduces it.

Further, both reinforcement and reduction can occur at the same time for
different RF frequencies across the AM channel, leading to a selective
combing effect in the program audio, and/or near loss of the carrier while
most of the sidebands are still present. Of course, that produces very high
distortion in an envelope detector. AM receivers using synchronous
detectors reduce this problem.

Most full time 50kW AM broadcast stations use an antenna system with
relatively low radiation at elevation angles above about 45 degrees in
order to reduce (but not eliminate) night time interference to their own
groundwave. I made up a simple graphic comparing the elevation patterns of
vertical antennas used in AM broadcast for four different electrical
heights: 90, 180, 195, and 225 degrees. The difference in their high-angle
radiation is apparent. Please email me if you would like a copy.

RF

Visit http://rfry.org for FM broadcast RF system papers.