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?

"talkinggoat" wrote in message
...
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?


the scale from the center of the plot to the line is arbitrary just to keep
the plot on the paper. it has nothing to do with a physical distance of how
the measurement is made.

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

Roy Lewallen wrote:
snip
Roy Lewallen, W7EL

Well said Roy.

tom
K0TAR

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

"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