Topband: K2AV Counterpoise

[Dan Maguire]

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