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First, is there any substantials improvement if i install 4 radials of the same length as of the vertical radiator? My broadcast goal is not the skywave propagation but the groundwave.
Using 1/4-wave-long elevated radials is best. Elevated radials only 3-m long are better than no r-f ground at all, but such a system is not very efficient.
Second, in order to get some groundwave improvements, is it a better idea if i ‘open’ my radials around 135 degrees from the vertical radiator instead of 90 degrees?
If the elevated radials are not installed in the horizontal plane, they are adding to the electrical length of the radiator. If that doesn’t matter, then you might experiment.
Finally, is there any improvement if i put 8 or 10 radials instead of 4 radials? How many radials would be enough on 20 foot support?
No. The length of the elevated radials is important, but any number of them greater than two does not improve system performance. Note this is not true for buried radials.
You might also re-read this thread link for a lot of discussion about this. http://part15.us/node/582