On 8/30/20 8:10 AM, Jeff Blaine wrote:
There is a chart in the N6LF articles which shows the impact of radial
length on field strength. Figure 19. 60% would be 0.15 WL which is on
his chart. Rudy's numbers say there would be roughly a 0.4 dB drop in
overall gain vs. 0.3 WL which is about the optimal length in the case of
4 radials.
Note this assumes a certain soil, consistent soil, 8' height and 80m
frequency - some other charts show the interaction of these variables
with gain so some interpolation of the various charts may be needed. In
my builds I had a bit of slope, drainage varied across the site and in
one antenna, there were two bands involved.
You can literally look at these things until your eyes are crossed
trying to nail down what they are saying to the 1/10 of a dB but that's
probably not necessary. I think a reasonable case can be made that more
radials is better (which N6LF states outright) - more meaning 8+, and
that current balance can be improved by moving away from exact 1/4 WL -
either longer or shorter. I've never seen any other data on measuring
radials individually for the same X0 - so that may or may not have value.
I say longer or shorter - both push the radial off it's minimum Z which
is really what you are going for. For a higher count of radials (8+),
the optimal length is a bit longer (around 0.35 WL). For my 80/160
elevated, I think those were about 100' long which was a compromise
value on the two bands.
This is an interesting idea - the variation in X with length is MUCH
bigger than the variation in R - and length for "minimum X" is highly
dependent on surroundings.
So what you're doing is pushing the radials to be deliberately
non-resonant, so they're reactive, and the large reactive impedance
swamps the real impedance. Then, you tune out the cumumulative
reactance at the feedpoint.
I'd have to think about what this does to the phase of the current in
the radials. In a resonant circuit, adding extra reactance (shortening
or lengthening the radials) is generally bad, because the reactive
current flow has IR losses. (why small mag loops are inefficient). And,
of course, the pattern might be affected if the current in the radials
is seriously "wrong" vs the current the vertical.
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