Tom wrote:
> The fact remains unequal currents in equal length radials
> proves each radial is located in a greatly varied electrical
> environment. Perhaps the radials are at differing electrical
> heights because of soil conditions below the radial, perhaps
> some radials are near other unintentional re-radiators, or
> perhaps the earth below some radials is substantially
> different in loss than earth under other radials.
>
> If we balance currents when radial surroundings have been
> proven to be unequal, equal currents will guarantee some
> radials will radiate more than others!
Tom,
I think I see your point. From the limited experimentation I have done so far,
I can assert that the ground topology at the site is very obviously varied.
For example, one radial (the one that seems to consistently attract the most
current) is suspended over (and down stream from) my sewer drain field. This
is an extremely moist, loamy area that has been supersaturated with nutrient
and mineral rich liquid for the past 15 years or so (the whole family deposits
contributions to the conductivity several times each day...). The other radial
(of the three) is suspended through bushes, small trees, and the like. Anybody
that has dug very deep in trees knows that the soils in trees are very dry by
comparison. The third radial is much more in the clear, but is not suspended
over soils that compare to either extreme. Interestingly enough, this radial
generally carries current consistently in between the other two extremes.
So what's the fix? It seems to me that the area underneath this vertical needs
to be beefed up. Even though the tower insulators are about 10 degrees above
the earth, should I consider attaching the tower legs to the elevated radials?
Then, beef up the base with a radial system the best I can? I know I cannot
run full 1/4 wave length radials in uniformity around the compass, but I bet I
can slip quite a few 50' to 70' radials into the sod and attach them to the
lightning ground rods at the base.
What haunts me with this discussion comes from a close examination of the
graphs in ON4UN's 3rd edition book. (may be in the 4th ed too but that's 450'
away right now and the 3rd ed is at my fingertips) In particular Figure 9-15
on pg 9-11. "Gain of 0.25wL 80M vertical over very good ground as a function
of radial length and number of radials." Going from 4 to 32 -- 20M long
radials on 80M provides only 0.75dBi of additional gain. Figure 9-14 is
average ground and supposes an additional 1.75 dBi of gain when going from 4 to
32 radials. On a band whose MDS is noise limited anyway, from a gain
standpoint, who cares?
But if beefing up the ground right under the antenna with a series of short
radials can prove to balance out the pattern at zenith, perhaps this main
antenna can provide a better (quieter) response at extremely narrow bandwidths
(like 80 Hz). I can hear stuff on my current antenna at narrow bandwidths that
cannot be heard on my beverage, which is -11dB to -14dB (estimated) from my
main antenna. I'm hoping for even better from this new antenna. Trying to
determine if anything I do 'improves' this aspect is likely going to be
impossible. Once again--the holy grail elludes me.
Thanks by the way for all the thoughful replies. There is no substitute for
experience. I appreciate it.
Ford-N0FP
ford@cmgate.com
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