Topband: K2AV Counterpoise

[Dan Maguire]

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