>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.
...
>73, John W1FV
>
>
Actually, my choice of the 2 quarter-wave 90-degree phased verticals in the
last statement above was a bad one for this example because this array has the
interesting property that the gain over 1 vertical is independent of ground
loss as long as that ground loss is the same for the 1 and 2-vertical arrays.
The performance will still be poor in either case if the ground is really
lossy. Also, the gain of a 4-square array over 1 vertical (about 5.5 dB)
holds up pretty well with ground loss according to the computer models.
But this isn't always true for other types of arrays over lossy ground.
73, John W1FV
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