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?

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. (I'm making the assumption here that the graph is for the far
field and you stay in the far field for your observations.) dB is a
relative measure, so all you can immediately tell from the graph is how
strong the signal is in one direction compared to what it is in another,
at the same distance.

There is an absolute reference, however, the dBi. That's the field
strength an antenna will produce compared to the strength of an
isotropic antenna -- one with a perfectly spherical pattern -- with the
same power applied. So if you know the gain in any direction in dBi, you
can find the field strength for a given power input in V/m or A/m or the
power density in W/m^2 at any distance in that direction, and in other
directions by using the graph. When in the far field, the field strength
decreases in inverse proportion to the distance, so if you know the
field strength at one distance, you can find the field strength at other
distances in the same direction. The field strength is also directly
proportional to the square root of the applied power, so if you know the
field strength for one power level, you can calculate what it will be
for others.

In short, what you need in order to find the field strength at any
distance and direction a

1. The graph showing the gain in that direction compared to some reference.
2. The dBi or field strength value which that reference represents.
3. The power being applied to the antenna.

Roy Lewallen, W7EL