On Thu, 19 Jan 2006 22:19:04 -0800, Richard Clark
wrote:


These three paragraphs reveal arguments that vary by application,
rather than by degree. It seems to me that most AM stations' ground
fields are shallow buried in gravel simply to permit foot traffic. The
HAARP site uses a grid that is elevated sufficiently to allow
vehicular traffic. Neither really attend lightning as they are more
ground screens and principally constructed for RF.

Yes, I understand that a ground system may be called upon to perform a
role as the other terminal of a Marconi for instance, and as the drain
for lightning or other EMP. Hence my earlier comment that a rule of
thumb that buried radials should be insulated seems to deny fullest
lightning protection to mitigate a small risk of corrosion.


I found a much more compelling report in:
UNITED STATES DEPARTMENT OF AGRICULTURE
Rural Electrification Administration
REA BULLETIN 1751F-802
SUBJECT: Electrical Protection Grounding Fundamentals
Which is vastly more comprehensive and directly answers these
questions when viewed in the terms of the resistivity of the earth
connection.

OK, I found it and it is substantial. It will be an interesting read,
thank you for the pointer. It appears to be focused more on power /
lighting protection that an antenna ground system.

For others, the URL is
http://www.usda.gov/rus/telecom/publ...s/1751f802.pdf
..

One of the things that intrigues me is the common "expert" advice to
cut radials for 7MHz to 33' long and bury them. It seems to me that
when buried and considering the wire as a transmission line, the
velocity factor will be somewhere between 0.3 and 0.8 depending on the
soil type, so that 33' is likely to be closer to a half wave
electrically, and present a relatively high and reactive impedance at
the antenna base if it were not for the attenuation of the wave on the
radial. It would seem a length more like 17' to 20' would be a better
estimate by the SWAG method (Scientific Wild Arsed Guess), although if
ground attenuation is high enough, it could be cut shorter and the
extra wire used for another radial for a more effective solution.

More when I digest some of the article.

Thanks again... Owen
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