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To have a groundmounted quarter wave vertical with a decent efficiency you need at least 16 radials.

So normally you would spread them out on the ground evenly around the base. This would give an angle of 360/16 = 22,5 degrees between two radials.

Would it make a great difference in efficiency to use the same amount of radials of the same length but to spread them out like this: 4 radials 5 degrees apart, then 75 degrees then again 4 radials 5 degrees apart and so on? Or would this only be as efficient as 4 radials?

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The purpose of the radials is to increase the effective ground conductivity, thus reducing losses. As you propose to have 16 radials but in four groups, I'd expect the efficiency to be somewhere between 4 radials and 16 radials evenly spaced.

That said, if you need to do this to work around some obstacle, then by all means do it. You can also make the radials shorter or slightly bent if it gets around your obstacle. Putting more copper in the ground can only make things better, regardless of how its done.

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  • $\begingroup$ I agree 100%. Ideal is one thing, but available real estate can limit that. I would only add that it is better to have full-length radials that go around an obstacle than to cut them shorter, when possible. $\endgroup$ – SDsolar Oct 23 '17 at 8:34
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The effects of using unequal length buried radials on the radiation patterns of a monopole are not very significant, as shown by the NEC4.2 analysis below. enter image description here

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As long as the radials are symetrical it will make no noticable difference. A single radial opposite the other half of a dipole or 2 radials opposite each other will yield a reasonable pattern. The key is symetry so that the radiation pattern is not distorted. Any number of radials will do, even odd numbers, if kept symetrical. Most people agree that more radials are much better, but some argue that, especially at lower frequencies, too much time and money is spent for too little of an improvement. Note also that elevated radials need only 2 - 4 to be effective according to most people.

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