Tom W8JI wrote:
>
>Once the radials are increased beyond 50-60 for 1/4 wl long wires, or 100 for
>1/2 wl long wires, there is no real change. 100 1/2 wl radials are so close
>to perfect anything beyond that is a waste of time. The exception is if you
>are interested in very low wave angles and you can install many radials many
>wavelengths long in the desired direction.
>
[stuff deleted...]
>
>The long and short of all of this is put in as many long radials as you can,
>and don't worry about all the recent gimmicks. With 60 1/4 wl radials above
>the ground, on the ground, or slightly below the surface (and no ground
>screen) the system will be within about a dB of theoretically perfect.
>Otherwise we can take a chance on unconfirmed models and theoretical analysis
>that uses a simple capacitor to replace complex induction and radiation field
>effects.
>
These comments are essentially correct if one assumes the antenna is a
standard quarter-wave vertical whose radiation resistance is theoretically
around 36 ohms. However, one can make a case for more extensive radial
systems when dealing with verticals with lower radiation resistance. There
are two cases which come to mind: (1) The first is a vertical physically
shorter than a quarter wave by a significant amount and using some form of
loading (either capacity top hat or base loading coil, which is commonly
done on 160), in which case the radiation resistance is much less than 36
ohms. For example, a base-loaded eighth-wave vertical has a radiation
resistance in the single digits (around 6 or 7 ohms if I remember correctly)
and will suffer poorer efficiency compared to the quarter-wave antenna
unless the resistive ground loss can be made much smaller than the radiation
resistance by means of lots of radials or whatever. (2) The other case
is for vertical arrays in which the radiation resistance of individual
elements can drop well below 36 ohms. For 2 quarter-wave verticals spaced
by a quarter wave and phased 90 degrees, one of the elements has a radiation
resistance around 20 ohms. I use an eighth-wave spaced array in which
the resistance of one of the elements drops under 10 ohms. The important
point is that the resistive ground loss needs to be much less than
the antenna's radiation resistance, whatever that is, to achieve high
efficiency.
73, John W1FV
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