Radiating Efficiency
Reg,
I have taken Frank's numbers from NEC-4 (that he sent me) and compared the
results with Radials_3. Here is what I found:
NEC-4 Parameters (in parens are the values I get with Radials_3, note the
differences are LARGE)
Height 20 m;
Radials 15 m;
24 radials at 15 degree intervals;
Radials 25 mm below ground;
All wires # 14 AWG copper; (I USED 2mm wire)
Gnd. Er = 25, Resistivity 25 ohm-m;
Resonant Frequency 3.64 MHz, and; (Radial_3 shows Resonant Freq of 3.750)
Zin = 41.32 + j 0.156466 ohms at 3.64 MHz. (34.17 ohms and j -24.6)
If I change the Freq in Radial_3 to 3.750, I get Zin = 37.41 and j -0.0.
That is 110 KHz difference for the same antenna and ground characteristics.
Not very good agreement.
=================================================
Now, on to the real question I have been asking for a week: Does the current
along the radials fall off as rapidly as Radial_3 predicts? I used the
numbers from the above antenna, (supplied by Frank from NEC-4) and compared
them to what Radial_3 says for the same length. I do not have numbers
showing a 5 meter radial for NEC-4 and a 5 meter radial for Radial_3. What I
am presenting is the current along the full radial in NEC-4, vs. the current
in a radial of specified length in Radial_3. Length is in meters.
Since we are using ratios, I only used the integer and two decimal places
for the current in my calculations. All values for current are actually what
is presented * 10^-2
Length Current NEC-4 (dB) Radial_3 (dB)
0 9.15 N/A N/A
1 8.37 0.77 6.2
2 6.98 2.35 12.4
3 5.63 4.22 18.6
4 4.55 6.07 24.7
5 3.57 8.17 30.9
6 3.00 9.69 37.1
7 2.54 11.13 43.3
8 2.16 12.54 49.5
9 1.84 13.93 55.7
10 1.60 15.15 61.8
11 1.46 15.94 68.0
12 1.41 16.24 74.2
13 1.36 16.56 80.4
14 1.04 18.89 86.6
14.75 0.26 30.93 91.2
Notice at a lenght of 4 meters, Radial_3 is showing 24.7 dB of attenuation
of the current, while NEC-4 is only showing 6 dB...not even close.
Notice throughout the lengths from 3 to 11 meters, the difference between
Radial_3 nd NEC-4 is a pretty consistent 4X, i.e, Radial_3 is showing four
times the attenuation at a given length that NEC-4 is showing. Patterns like
this in data are rarely the result of chance. Maybe this will lead to a
resolution of the problem.
If Reg's threshold of 25 dB attenuation is reasonable (at which point adding
additional length is of very little value), then NEC-4 says for this
particular set of variables that we don't attain anything near 25 dB current
attenuation along the length until we get to greater than 14 meters (and the
wire is only 15 meters long!)
How could the current in a 4 meter radial drop by 25 dB within its length?
So, I'm left with two questions:
1. Extra wire....what extra wire? It looks like one needs the entire 15
meter radial wire, not some 4 meter stubby....at least according to NEC-4.
2. Why such a discrepancy in resonant freq between NEC-4 and Radial_3?
Unless someone else can point out an error in the analysis, it appears that
the transmission line model that Reg is using to predict rate of current
drop along the radial vs. length is either the wrong model, or the wrong
application thereof. NEC-4 just doesn't seem to support the rapid current
fall-off that Radial_3 predicts.
BL&E says it ain't so.
Tom, W8JI, says his measurements say it ain't so.
NEC-4 appears to say "it ain't so".
If I were a betting person, I'd say it ain't so.
Back to the drawing board?
In the mean time, anyone putting in a radial field woud appear to be better
served by the references in this thread to "optimizing radial systems", I
think by K3LC...it's in several antenna books.
73,
p.s. How all this fits into efficiency someone else can analyze, but it
can't be good.
....hasan, N0AN
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