"Jim, K7JEB wrote"
Roy Lewallen wrote:
An antenna pattern is a graph of relative signal strength versus angle.
It's not a graph relating anything to distance, nor is it a graph of
absolute field strength. What it tells you is how strong the signal is
in each direction compared to some reference (0 dB). If you stay the
same distance away from the antenna and walk around it in a circle,
you'll see the field strengths vary as shown on the graph in the various
azimuth directions. This will be true regardless of the distance you
choose.....

Good explanation, Roy. (I didn't want to quote the whole article)

Just an additional comment. In the case of a plot of *electric field*
intensity (in volts/meter) vs azimuth angle, usually applied to standard-
broadcast AM stations, the radial distance on that plot does map directly
into the distance at which a particular field strength can be received.
....
An interesting program, called BCmap, illustrates this by overlaying
a map of North America with the patterns of AM stations on a particular
frequency, or set of frequencies. It may be downloaded free of charge
_____________

To both posters above, note that the relative field, h-plane azimuth
patterns for AM broadcast stations such as published on the FCC website and
in the BCmap program are acceptably accurate only very close to the
radiator(s) -- about 1 km or so. These are the net radiation patterns of
such stations just beyond the near-field radius, where for such short paths,
frequency and earth conductivity have relatively little affect on groundwave
field intensity.

Earth conductivity, earth curvature and frequency become very important for
long groundwave paths, and those perfect relative field patterns no longer
are a very good indicator of relative field intensities for such
groundwaves.

There are other websites that do take the applied r-f power and frequency,
earth conductivity and earth curvature into account when calculating the
distance to a given groundwave field intensity resulting from the h-plane
ERP in each direction that is "launched" by an AM broadcast station.

The link next below leads to a graphic showing three groundwave
field-intensity contours for WJR, Detroit, which is a 50 kW,
non-directional, fulltime station on 760 kHz, using a single, 195-degree
monopole radiator.

These groundwave field intensity pattern shapes would be perfect circles for
values plotted ~ 1 km from the antenna site.

But the more distant contours shown on this site are far from circular, as a
function of the varying groundwave propagation conditions in various compass
directions from the transmit antenna location.

http://www.radio-locator.com/cgi-bin...atus=L&hours=U

RF (WJR staff engineer, mid-1960s)
http://rfry.org