K8BHZ wrote:
>>> The length to avoid is nothing more than a half wavelength, which
>>> translates the same impedance from end to end
>>> i.e., the high Z open end translates to a high Z antenna base end. This
>>> results in minimum radial current.
I'm not so sure I buy that and I don't think N6LF does either. If you look at
the section "An Explanation for the Dips in Ga" (Part 1, QEX pg 40) in Rudy's
document
http://rudys.typepad.com/files/qex-mar-apr-2012.pdf
you'll find this: ["L" is the variable for radial length]
<quote N6LF>
Why do we see these large dips in Ga for some values of L? We can investigate
this by looking at the current distributions on the radials and the associated
E and H-field intensities close to ground under the radials. ... For the same
current at the feed point, with longer radials the currents are much higher as
we go out from the base. We would expect these higher currents to increase both
E and H-field intensities at ground level under the radials. ... Since the
power dissipation in the soil will vary with the square of the field intensity,
it’s pretty clear why the efficiency takes such a large dip when the radials
are too long.
</quote>
So Rudy seems to be saying that the increased loss is due to higher radial
currents, not minimum radial currents.
Here's an animation showing how the currents on the vertical and on 1 of 4
identical radials change as the radial length changes. Segments 1-15 are the
vertical with segment 1 being the base (constant 1 amp). Segments 16-37 are
the radial with segment 16 being the inner end connected to the base of the
vertical. Each radial always has 0.25 amps at the inner end but the current
distribution across the rest of the radial changes as the length changes.
The animation frames start with a radial length 0f 0.10 WL, step by 0.02 WL,
and finish at 0.60 WL. The frame where the radial current in segment #29 peaks
at ~1.02A corresponds to the sharp dip in efficiency. Radial length for that
frame is 0.36 WL.
https://s1.postimg.org/7a7f6pvu4f/N6_LFani.gif
Above is at 1.85 MHz with 4 radials at 2" above ground. Below is a different
animation, this time showing E-field intensity as the radial length changes.
For this animation the radial height was 10 ft so the dip occurs at ~0.45 WL
(frame 9). That corresponds to the highest radial current and the max E-field.
https://s1.postimg.org/45ipxxhp73/MCVert_NF3_D.gif
Dan, AC6LA
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