[TowerTalk] Re: radials for shunt fed tower

[W2RU - Bud Hippisley]

At 17:57 2004-12-05, w7kxb at comcast.net wrote:
>Thanks for the Heads Up. Found it at:http://www.ncjweb.com/k3lcmaxgainradials.pdf

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 dB. 

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 times!!!

Bud, W2RU