This page, and the links to a couple of N6LF documents that are included there,
might help:
http://ac6la.com/aecollection3.html
Not covered on that page are a) how the efficiency changes as the radials are
placed closer to the ground, and b) whether there is any difference in best
radial length for a straight vertical vs an inverted L.
To address those points I ran some additional test cases at 1.85 MHz. Instead
of the radial height (and base of the antenna) at 10 ft, I put the radials at a
height of 2 inches. On 160m that is very close to "on ground" radials. Then I
ran scenarios for a straight vertical 129.6 ft high followed by an inverted L
with vertical section 60 ft and horizontal section 73 ft. Both scenarios were
with just 4 radials, average ground, bare #12 copper wire.
https://s1.postimg.org/3f3wirjedb/N6_LF.png
Two things become obvious:
1) As the radial height is lowered the "length to avoid" gets shorter. When
the radials are at 10 ft the large drop in efficiency happens at a radial
length of ~0.47 WL (~250 ft at 1.85 MHz). When the radials are at 2 inches the
efficiency dip happens at a radial length of ~0.36 WL (~191 ft).
2) For both the straight vertical and the inverted L, the highest efficiency
happens at a radial length of ~0.2 WL (~106 ft). But if you make them shorter,
say the length of the vertical portion of the inverted L (60 ft, ~0.11 WL),
you're really not giving up very much.
For AutoEZ users, also see the 3rd section on this page:
http://ac6la.com/aecollection8.html
Model "Vertical with Tapered Seg Radials.weq" uses tapered segment lengths
which greatly reduces the number of segments and hence the calculation times.
If you'd like a copy of the model used to produce the above chart, drop me a
note off-list.
Dan, AC6LA
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