talkinggoat wrote:
This is probably going to seem like a stupid question to most of you,
but I'm looking at an antenna pattern, which I've seen before. It's a
representation of db over degrees. It's the typical pattern showing
the lobes of the radiation that the antenna emits. My question is, on
what scale are the db represented over what distance? Is there some
universal preset saying that -10db is 100m or something like that?

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.
Looking at such a plot, such as those found in the FCC database, does
give a (rough) idea of the "range" of a particular station in various
directions and helps explain why some stations, relatively close-by,
do not reach a particular location (their antenna pattern has a null
in it in that direction).

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
from:

http://www.tonnesoftware.com/bcmap.html

Jim Bromley, K7JEB