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[TowerTalk] Re: radials for shunt fed tower

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Subject: [TowerTalk] Re: radials for shunt fed tower
From: W2RU - Bud Hippisley <>
Date: Wed, 15 Dec 2004 17:31:34 +0000
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At 17:57 2004-12-05, wrote:
>Thanks for the Heads Up. Found it 

In a cursory reading of K3LC's radial article, I found two data points that 
seem counter-intuitive to me.

In general throughout Tables I - IX, for a fixed amount of wire (8000 feet, 
say), the "peak gain" determined by EZNEC4 increases monotonically as the soil 
quality is varied from "very poor" to "average" to "very good".  This seems 
reasonable to me, since the soil is effectively in parallel with the sparse 
radial screen.

However, this does not appear to be true on 40 meters, where the "peak gain" 
figures for 4000 and 8000 feet of wire are worse for the case of average soil 
than they are for either very good or very poor soil.  According to Tables I - 
III,  both "very poor" and "very good" soils provide higher peak gain (1.68 and 
2.10 dBi, respectively) at 7.15 MHz than does average soil (1.4 dBi) when using 
8000 feet of wire.  A similar "dip" is reported for the case of 4000 feet of 
wire (1.02 and 1.88 versus 0.9 dBi).

I wonder if these are typographical errors or anomalies in the modeling.  

Other comments inspired by K3LC's article:  

1.  One thing that jumped out at me as I scanned Tables I - IX was that, 
despite scaling the total wire lengths for the different bands, equivalent 
progressions from minimum total wire length to maximum total wire length 
resulted in less improvement at lower frequencies.  On 40 meters, for instance, 
the total variation in calculated peak gain for average soil was 2.89 dB (from 
-1.49 dBi for 125 feet of wire to +1.40 dBi for 8000 feet of wire).  On 80, the 
corresponding total gain variation was 2.37 dB, and on 160, it was only 1.86 

2.  Two other data points I would like to have seen are the peak gain (in dBi) 
for the same antenna model over "perfect" ground and over sea water, so I could 
get a sense of how far down in gain EZNEC4 says the article's finite radial 
fields are, compared to those two idealized situations.

3.  The charts in the article make an excellent case for choosing well when 
siting your future vertical, and for determining how "good" your soil is 
*before* engaging in a major radial project.  Consider 160 meters, for example. 
 With "very good" soil (Table IX), each doubling of total wire used in the 
radial field results in only 0.1 dB improvement in peak gain.  In "very poor" 
soil (Table VIII), by contrast, each doubling of wire results in about a 2/3 dB 
improvement -- more than 6 times as much!  But in "very poor" soil, you're 
starting from a far worse loss, so it's going to require a whole lot more wire 
(and effort) to get comparable peak gain with "very poor" soil.  Again 
comparing Tables VIII and IX, and extrapolating additional 2/3-dB increments of 
improvement for each doubling of radial wire in "very poor" soil, it's going to 
take more than 128,000 feet of radial wire for a vertical over "very poor" soil 
to have the same peak gain as one with only 500 feet of radial wire over "very 
good" soil!  Forget the wire cost -- the labor differential is more than 256 

Bud, W2RU  


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