[Lowfer] Free Energy/QST October 1999

[Ralph Hartwell]

> I wonder where the line is drawn?  Sure, if a competing AM radio
station put
> up some antenna which rendered another station inoperable but did
nothing
> else, it would seem illegal.

There are accepted formulae which determine the near field transition to
far field.

> Maybe it is decided by whether the engineers at the AM station (or
other
> system) can determine or show that their equipment is being affected.

If they can show that the structure in question is causing a change in
the antenna pattern or the meter readings at the transmitter site, the
case is closed as far as the FCC is concerned.  The "absorber" loses and
must eliminate the problem.

As a real-world example, anyone who constructs a new tower within a few
KM of an existing AM broadcast station must run before and after field
strength measurements to show that the construction of the new tower has
not caused any change in the radiation pattern or field strength of the
existing AM station.  They must then present this test data to the AM
station engineers for approval.  If a problem is found, the new tower
constructors must make changes at their expense to remedy the situation.

> In my town there is a softball complex not 300' away from an AM BCB
tower.
> The softball complex has about 9 fields, each surrounded by cyclone
fences.
> I doubt they suck up much power, but they probably have some effect.

Probably very little.  If the structures in question are short compared
to a quarter wavelength (assuming that they are grounded) and is they
are not resonant at the frequency in question, then likely they will
present no problem.  Note that large concrete and steel buildings may
cause some pattern distortion, or, in the case of VHF or microwave
transmitters, cause reflections or other pattern distortions.

Another real-world example, this time a problem in the far-field region:

In the New Orleans area, there is a 30+ story downtown building that is
constructed of the usual modern style of large, flat, reflective coated
window walls.  The building just happens to be oriented such that the
signal from one of the local TV station some 4 miles away is reflected
off the building just as light is reflected from a mirror.  There is an
area some 40 miles away from the transmitter site where the direct
line-of-sight signal from the TX and the reflected signal combine to
give a serious ghost image which is impossible to eliminate.  If you
move just a mile or so off the direct line of this reflected signal and
the problem vanishes.  The problem was so serious that the station
constructed a microwave link to the affected area so that the cable TV
systems there could obtain a usable signal.

73,

Ralph   W5JGV / WC2XSR / 13

http://home.att.net/~shmrg

http://home.att.net/~ralph.hartwell