Topband: Multiple Ground Systems

[Jim Brown]

On Sun, 19 Aug 2007 13:15:20 -0400, Brian Machesney wrote:

>Between jobs last year, I spent nine months, full-time, simulating antennas
>with a NEC2 engine in an attempt to figure out how they actually work. Like
>so many others down the years, I focused my energies on electrically small
>antennas for low frequencies. Even though I live on several acres in the
>country, I don't particularly want to visibly dot the property with big
>antennas, spend all my money on wire or mow over a field of radials.

One of the major shortcomings of the NEC2 engine is how it deals with the 
earth. The more expensive NEC4 engine is regarded as more accurate. And as the 
real pros (like W8JI and N6LF) will tell you, real world earth is far too 
complex to model with good accuracy with any of these engines, so the best way 
to know is to build it and make well controlled field strength measurements. 

Walt Maxwell's website (W2DU) has pdf's of several chapters from his book 
"Reflections 2, one of which includes an excellent discussion of how vertical 
antennas and radials interact with each other and the earth. Google on W2DU to 
find it. 

Early this past week, I posed a question to the NCCC email reflector with 
respect to what I should expect by going from a top loaded vertical with 40 70 
ft radials to a full quarter wave vertical over those same radials, and to a 
top loaded quarter wave (100 ft horizontal on top of 130 ft vertical) over 
those radials. I had done some modeling using EZNEC5 (NEC2 engine) and 
theorized that I might pick up several dB. N6BV is a member (and past 
president) of NCCC, and ran NEC4 models of the three conditions, as well as a 
few more suggested by other members. Below is the executive summary of his 
results. B is my current vertical, A is the vertical another member uses. By 
going from B to E, his model suggests that I might expect to pick up about 1 
dB. 

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Let me summarize the combinations of 160-meter verticals that I modeled for 
K9YC over "Average" ground (conductivity = 5 mS/m; dielectric constant = 13) 
using #12 wire, arranged in order of increasing gain:

A. 131' high vertical; no T-Top wires; only two 70' long radials, buried 1'
deep
   Gain = -1.82 dBi at 21 degrees TOA
   Zfeed = 76.49 + j 27.2 ohms

B. 50' high vertical; two 50' long T-Top wires; 40 each 70' long radials,
buried 1' deep (original K9YC combination)
   Gain = -0.31 dBi at 23 degrees TOA (take off angle)
   Zfeed = 18.09 - j 47.97 ohms

C. 131' high vertical; two 50' long T-Top wires; only two 70' long radials,
buried 1' deep
   Gain = -0.05 dBi at 21 degrees TOA
   Zfeed = 199.4 + j 695.8 ohms

D. 131' high vertical; no T-Top wires; 40 each 70' long radials, buried 1'
deep
   Gain = 0.13 dBi at 23 degrees TOA
   Zfeed = 47.16 - j 2.556 ohms

E. 131' high vertical; two 50' long T-Top wires; 40 each 70' long radials,
buried 1' deep
   Gain = 0.72 dBi at 21 degrees TOA
   Zfeed = 165.6 + j 673 ohms

73, Dean, N6BV 

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In a subsequent post, Dean noted that there should be no difference between 
radials that are buried and those that are laying on the ground (as mine are). 

73,

Jim K9YC