Topband: Short radials?

[Richard Fry]

Guy Olinger wrote:

>Can you pass along your source of information that BL&E was done over 4 
>mS/m soil, ... Or are you using the FCC map for typical soil conductivities 
>and presuming a common New Jersey value and no variation at the site?

It is my presumption that for their cost and logistics, and with my 15 years 
of insight as an RCA Broadcast field engineer (1965-1980), the BL&E 
measurements were made near Princeton, NJ.  I'll try to confirm that, and 
advise. Princeton was the corporate facility containing the 
office/laboratory of George H. Brown.

If those tests had been made at some physical location where earth 
conductivity at/near the test site was significantly better than in New 
Jersey, this would have been evident in their measured data.

For an example of this, if earth conductivity at/near the test site was 30 
mS/m, then even relatively few/relatively short buried radials in contact 
with that earth would enable higher radiated fields than shown in the BL&E 
data for those radial numbers and lengths. This is illustrated by the NEC4 
study using a short monopole on 1.85 MHz at this link: 
http://i62.photobucket.com/albums/h85/rfry-100/10m_Vert32Buried_Radials.jpg 
.

Even for an earth conductivity of 4 mS/m, the BL&E data show that monopoles 
ranging from about 45 to at least 90 degrees in physical height, driven 
against an r-f ground system consisting of at least 113 x 0.412-wave buried 
radials, produces a groundwave field at 3/10 of a mile that is within 
several percent of the maximum theoretical value possible for a perfect 
monopole driven against a perfect ground plane, for that applied power.

The bottom line in all of this is that the worse the earth conductivity 
within 1/2-wavelength of the base of a monopole (especially a short 
monopole) while driving that monopole against a set of buried radials, the 
more important it becomes to use a large number of such radials of lengths 
approaching 1/2 of a free space wavelength.