Dave VanHorn wrote:
"Butch" wrote in message
...

No gain in a pure isotropic source?


If there was one, it wouldn't have any.
That's part of the definition, since it radiates in all directions equally.
You get gain by adding directivity.

Those statements illustrate several misconceptions about gain and
directivity.

The first is that gain is an absolute figure -- that a particular
antenna, like an isotropic radiator, has an immutable, single, value of
gain. That isn't true. Gain is always a relative term. It's the ratio of
two numbers. One of those two numbers is the field strength from the
antenna, the other is the field strength from the reference antenna in
the same direction. You can get just about any gain you want from an
antenna simply by changing the reference. For example, a dipole in free
space has 0 dB gain relative to a dipole in free space in its most
favored direction. It also has 2.15 dB gain relative to an isotropic
source, something like -6 dB gain relative to a 3 element Yagi in free
space, and about -5 dB relative to the most favorable direction of a
dipole mounted over ground. So the same dipole has a gain of 0, 2.15,
-5, and -6 dB in its most favored direction. It also has an infinite
number of other gains. This hasn't been lost on antenna manufacturers,
who are often very creative in their choice of reference. People who
believe that gain is an absoulte value independent of the reference are
their rightful prey.

The second misconception is that the gain of an antenna is a single
value, even if the reference is given. The gain of a dipole in free
space varies from 2.15 dBi (dB relative to an isotropic source) to -
infinity, depending on the direction. It makes sense to look only at the
maximum gain if you're able to rotate or construct the antenna so you
can point the most favored direction at the station you want to
communicate with. Otherwise, it's a meaningless number. Who cares how
strong a signal the antenna radiates straight up (unless you're using it
for NVIS propagation) or at some azimuth or elevation angle other than
the one you're using to communicate? An extended double Zepp (EDZ) has
gain over a dipole -- but only over a rather narrow range of angles. At
all other angles, the gain is negative. If the station you're working is
at one of those other angles, you're better off with a dipole -- because
it has more gain than the EDZ at that angle. And if you're equally
likely to work stations in any direction, you'd do better with a dipole
most of the time. Whenever the antennas have different pattern shapes,
their gains will be different in different directions when compared to
the same reference or to each other.

The third misconception is that the gain and the directivity are the
same. If two antennas are equally efficient, then the one with the
greater directivity will have the greatest gain (by the amount of the
directivity) in its most favored direction. But there's more to gain
than just directivity, and that added something is efficiency. Two
antennas can have equal directivities but different gains relative to
the same reference. For example, a free space dipole with a 73 ohm
resistor at the feedpoint will have a gain of -3dB -- in all directions
-- compared to one without the resistor, even though both have the same
directivity. (Here, I've used the antenna without the resistor as a
reference. I could also use the other as a reference and say that the
antenna without the resistor has a gain of 3 dB relative to the one
having it. Or I could have said that the one without the resistor has a
gain of 2.15 dB relative to isotropic in its most favored direction, and
the one with the resistor has a gain of -0.85 dB relative to the same
reference. All are equally valid.) A Beverage antenna typically has high
directivity but considerably lower gain than antennas with lower
directivity such as a Yagi of a few elements, or even a dipole. An
inefficient antenna with a perfectly isotropic pattern has a negative
gain (in dB) in all directions relative to the theoretical, perfectly
efficient isotropic source.

There is one unambigous way of stating gain without describing the
reference, and that's to give the gain in dBi. If you do this, it's
understood that the gain is relative to a 100% efficient isotropic
source. Of course, you still have to say whether that's in the antenna's
most favored direction or in some other direction unless it's obvious
from the context. If you don't use the isotropic reference, you need to
clearly describe the reference, or any gain figure you quote is
meaningless. "dBd" is a popular term among hams, and a windfall for less
scrupulous antenna manufacturers. Often defined as gain relative to a
dipole in free space, it's just as often defined, understood, or
misunderstood to mean gain relative to a dipole over ground at the same
height as the test antenna. The 5 or so dB difference between these two
meanings of the same catchy term offers ample opportunity to confuse the
consumer and make an antenna look much better than it really is. So be
very wary if you see this term, and don't make any assumptions about
what it might mean.

Roy Lewallen, W7EL