On 2/29/16 12:07 PM, Richard (Rick) Karlquist wrote:
On 2/29/2016 10:06 AM, Jim Thomson wrote:
This is from the ARRL antenna book:
------------------------------------------------
Practical Suggestions For Vertical Ground Systems
At least 16 radials should be used if at all possible.
Experimental measurements and calculations show that with
These rules of thumb are useful on the higher bands. On
160 meters, most stations are working with less than
1/4 wave verticals. Because of the lower radiation
resistance, the effect on efficiency by the number and
length of radials is much greater. Radial systems that
would be considered "overkill" based on general guidelines
may be nothing of the kind with shortened verticals.
I would expect that "many shorter" radials would be better than "fewer
longer" radials, for a shortened radiator. The current is highest near
the base of the antenna, so that's where you want to reduce your IR losses.
Experience with broadcast stations is of limited value
because they normally use full 1/4 wave verticals.
The Brown Lewis Epstein 120 radial thing is also a "if you build this,
you will comply with the regulatory requirement, otherwise you need to
measure it and prove it" kind of thing. The 120 radials system is where
there is negligible effect on the transmitted efficiency from changes in
soil properties. That is, the radiated field strength is "close enough"
to theoretical.
What I do is model my actual system over my actual ground
using NEC-4 (via EZNEC pro). Sometimes the results are
surprising. I consider the software a good investment for
any serious 160 meter experimenter.
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