Subject of post: NOT the K2AV Counterpoise. :)
This is a great exercise to keep the ol' brain cells from turning to mush.
(Well, in my case it probably just jiggles the existing mush around a bit but
I'll take what I can get.)
Brian, I've been trying to parse out where we agree and where we have yet to
reach a resolution. So far I think we agree that there is a large drop in
efficiency when the radials are an *electrical* half-wave long. And we agree
that as the radials get closer to the ground the *physical* length for an
electrical half-wave gets shorter.
That leaves the following to be resolved:
>>> [From your previous post] 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.
Well, it may result in minimal radial current *at the inner end* but it also
results in a very large standing wave current peak farther out along the
radial. And I believe it's that large current peak that causes the drop in
efficiency.
>>> The error of your conclusion comes from the fact that Rudy is comparing
>>> equal currents INTO the radials, Which means that the POWER into the
>>> antenna is NOT constant.
But the metric under consideration is *efficiency*, not radiated power. For
any given antenna, whether it is driven with a kW amp or a QRP rig, the
efficiency remains the same.
I don't mean to put words in your mouth but what you seem to be saying (or at
least what I'm hearing) is that the drop in efficiency with electrical
half-wave radials is because the current in the radials (at the inner end) is
lowest under those conditions. What I am saying, and I think this was also
Rudy's conclusion, is that the drop in efficiency is due to the increased
ground loss, which in turn is due to the increased E- and H-field intensities,
that result from the large standing wave current peak.
If the efficiency drop was due to minimum radial current, and not due to the
ground loss, then one would expect to see the same efficiency drop when the
entire antenna is in Free Space. But that's not the case. In the chart below,
the wire loss has been set to zero for the vertical and the radials.
https://s1.postimg.org/1u9tsuuwkv/N6_LF3.png
Since the wire loss is zero the only remaining source of loss is the ground.
No ground loss in Free Space equals 0 dB Average Gain (100% efficient) at *all*
radial lengths, ie no matter what the current distribution is on the radials.
>>> In our real world, our transmitters/amplifiers are fixed in power output,
>>> not infinitely variable, so as the combined vertical/ground system
>>> impedance goes up, current decreases.
Absolutely agree, but a decrease in current does not equate to a decrease in
efficiency.
Here's another chart for the radials 2" above ground scenario, where an
electrical 0.5 WL is ~0.36 physical WL. Red trace is with a fixed 1 amp at the
base of the vertical. Blue trace is with a fixed 100 watts at the base of the
vertical.
https://s1.postimg.org/9te6nx9sv3/N6_LF4.png
With a ~0.25 WL vertical element and with four 0.36 WL radials at 2" above
average ground, the feedpoint Z is ~430+j163. So at 100W the feedpoint current
is Sqrt(100/430)=0.48A as is shown for the blue trace. But *in both cases* the
calculated average gain (efficiency proxy) is -11.74 dB.
For comparison, this chart has the radial length at 0.21 WL where the
efficiency is highest.
https://s1.postimg.org/9mayshmskf/N6_LF5.png
Feedpoint Z now is ~39+j1. Sqrt(100/39)=1.60A. But again, same efficiency in
both cases.
If we can now agree that the efficiency does not change with a constant current
vs constant power source, we can then compare the two constant 1 amp scenarios.
In both cases the current at the inner end of each radial is 0.25A. The only
difference is whether or not there is a large standing wave (with a large
current peak) on the radials. Large standing wave equals lower efficiency, no
standing wave equals higher efficiency. Except in Free Space where there is no
ground loss.
BTW, in the two charts above the *shape* of the current distribution curves is
a little misleading due to the use of tapered segment lengths, although the
*values* shown are correct. Here is a more realistic depiction of the curve
shapes.
https://s1.postimg.org/4098emkmvz/N6_LF6.png
All calculations above were done with the NEC-4.2 engine and GN3 ground.
Whether or not we can reach a consensus, thank you for the civil discourse.
Most refreshing in this Internet age.
Dan, AC6LA
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