dB is a ratio of two gains or other quantities, so gain expressed in dB
is meaningless unless both antennas being compared are specified. dBi is
a common way of expressing the gain of an antenna, and its meaning is
universally accepted and unambiguous -- it's the gain in dB relative to
that of an isotropic antenna.

Let's begin by assuming that all antennas are perfectly efficient. That
is, all power applied to them is radiated and none turned to heat. This
assumption is a valid one for most antennas of a reasonable size in
terms of wavelength. For other antennas, the assumption is useful
because it separates loss, which can be modified by construction,
materials, and other factors, from more fundamental phenomena which can't.

Given that assumption, the first important point is that no antenna can
have a gain lower than 0 dBi (the gain of an isotropic antenna) in its
most favored direction. If it did have a gain lower than 0 dBi in some
direction, it would have to have a gain higher than 0 dBi in some other,
because the only way gain can be increased in a perfectly efficient
antenna is by robbing radiation in one direction in order to increase it
in another.

The next important point is that a thin half wave dipole in free space
has a gain of about 2.15 dBi. If you make the dipole shorter, the gain
decreases slightly due to a change in the current distribution. When the
dipole gets vanishingly short, the gain has dropped to 1.76 dBi. In
practice, the loss increases dramatically when a dipole is made very
short in terms of wavelength, so this gain can't be achieved in real
life. However, the loss depends on construction, materials, and other
factors which can be controlled to minimize loss when needed. The gain
of a lossless quarter wave dipole in free space is about 1.84 dBi. The
loss of a real quarter wave dipole can usually be made negligible,
although attention also has to be paid to matching network components to
keep the system loss low.

Over a perfect ground plane of infinite extent, the gains of antennas
typically increase by about 3 dB compared to free space. In the case of
a free space dipole vs. a quarter wavelength antenna over a perfect
ground plane, the difference is exactly 3 dB. This is because they have
exactly the same radiation pattern (except of course, that the half
pattern of the vertical below the ground plane is missing), but the same
amount of power is being concentrated in half the volume. Consequently,
the gain of a 1/4 wave vertical over a perfect and infinite ground plane
is about 5.15 dBi. This also can't be achieved in practice because there
isn't any such thing as a perfect ground plane of infinite extent. You
can come close if working over salt water, but nowhere near if the
ground plane is dirt.

Roy Lewallen, W7EL

Spajky wrote:
Hi guys,
hope that someone will clarify me certain doubts!

The problem: I have 2 different sources (some book, something DL-ed
from net) stating that approx.:

-isotrophic antenna (teorethical all around ball space HF radiation of
a single round point) has only 2,15dB less gain than half wave di-pole
(=2nd source)

- the book instead says that 1/4 lambda antenna has -3dB worse gain
that same dipole as before, [5/8 one just -1dB] & so my thought is,
that 1/4 antenna must be better than isotrophic one, since even if it
radiates 360° horizontally (if vertical polarized-mounted); vertically
it does not more than 120° both sides (all around).

I believe that on second source there is a mistake & was ment
that isotropic is 2 dB worse than 1/4 antenna & not 1/2 dipole.

so, it would be so IMHO : 0dB isotropic one
+2dBi 1/4 one
+5dBi 1/2 dipole (3dB more than 1/4 one)
Am I write or wrong?
--
Seasons Greetings & Regards , SPAJKY ®
mail addr. @ my site @ http://www.spajky.vze.com
more than 3y - "Tualatin OC-ed / BX-Slot1 / inaudible setup!"