> When elevated radials are mentioned they are often full
> 90-degree radials.
> The low impedance presented by the radials makes
> decoupling the coaxial cable
> shield easy. The counterpoise current will divide between
> the radial impedance
> and the shield impedance.
Maybe easy, but decoupling is still necessary if you want a
clean and reliable installation. The notion a few 1/4 wl
radials result in a perfectly unbalanced antenna is clearly
false.
A commercial 40MHz groundplane using four 1/4 wl radials
required significant choking ( a few hundred ohms CM
impedance) of the feedline shield *and* isolation of the
radial common point to the mounting bracket. Without doing
that common mode current was so bad SWR could change from
1:1 to 2:1 just by altering the height of the supporting
mast or length of the feedline. The problem also applies to
160 meter verticals as well as UHF groundplanes.
With elevated radials near earth there can be a dB or more
FS reduction when the feedline shield is not decoupled.
This says nothing about the unwanted coupling of noise into
the antenna, or RF into other devices close to the feedline
or operated from the power line.
73, Tom
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