[BC] Hey Roy, Carl or Phil

[RSTYPE@aol.com]

In a message dated 2/12/2006 9:07:15 AM Eastern Standard Time, 
stanleybadams at yahoo.com writes:
I have noticed that elevated grounds are beginning to gain in popularity,
where they were primarily used in difficult installations, like buildings
and such.

What is the theoretical disadvantage in using the elevated over the ground
based system?

Thanks

Stanley Adams
Memphis
Stanley:

There are some engineers who are beginning to use elevated ground systems 
consisting of four or less radials per tower and they feel that they're just as 
effective as the standard 120 radial ground systems.  In my opinion, this is 
just experimentation and I'm not sold (yet) on the effectiveness of such 
nontraditional ground systems. Nor have I seen the results of adequate testing to 
document that such a ground system is as effective as a traditional ground 
system.  The FCC has also obviously not been sold on the effectiveness of these 
nontraditional ground systems, as they still require that a full proof of 
performance be conducted to document the antenna efficiency whenever such a 
nontraditional ground system is used on a nondirectional antenna system.

There is also a practical issue associated with theft and vandalism.  Based 
on Dr. Brown's work in the 30's, the ideal ground system would be 113 radials 
(which has been increased to 120 for the convenience of a nice round number) 
located on the surface of the earth.  (The optimum radial length is 0.4 
wavelengths, but 0.25 wavelengths works almost as well, except on very tall towers.)  
Unfortunately, however, radials installed on the surface are normally not 
practical, as they make it difficult to maintain the site (mow grass, etc.) and 
are prone to being stolen in most areas.  As a result, the normal practice is to 
bury the radials approximately four to six inches below grade level, just 
deep enough to provide adequate protection and security without substantially 
increasing the ground losses resulting from the earth through wihch the 
displacement currents must pass to reach them.

We have used a tuned counterpoise type ground system in rare occasions when 
short towers dictate the need for a good ground system and there is not enough 
land available at the site to accommodate one.  This involves installing both 
a standard buried short ground system and an elevated one around the tower.  
The elevated ground system is not connected directly to ground but, instead, is 
tied to a buss at the tower end of the radials and then connected to ground 
through a tuning circuit (normally a vaccuum capacitor).  This tuning network 
is then adjusted to resonate the elevated radials against the buried ones to 
maximize the current at the tower end of the radials.  I've never really dug 
into the theory behind this, but I know that Carl Smith used this technique 
successfully in the 60's for an AM antenna system in Alaska (Barrow?) where the 
site restrictions required the installation of a very short tower and prevented 
the installation of an adequate traditional ground system.  We subsequently 
used a similar installation (under similar circumstances) for the north tower of 
the three tower directional which was installed in 1988 on 660 kHz in 
Fairhope, Alabama (now WDLT).  (It appears that this may have subsequently been 
removed, as the CDBS presently indicates that this tower is taller than the other 
ones in the array, which was not the case when we built it.)

Just my thoughts (and I've been wrong before).

Roy Stype
Carl E. Smith Consulting Engineers