Good morning, Dick.
Good question. Since it relates to horizontal antennas (Yagis), the
following addresses horizontal antennas, not verticals. Hope it helps
understand a bit more.
FORCE 12 GAIN REFERENCE
The gain reference for Force 12 products that are horizontally polarized is
the full size, half wave dipole. The two page section in the standard
brochure entitled, "Antenna Specifications" contains specific information on
this and other subjects. The following information can be found in this two
page section, as well as in footnotes in the brochure (actually any Force 12
brochure).
FORCE 12 GAIN SPECIFICATIONS
We asked many people how they would like to see the specifications. The
number one request was true specifications. After that was for a typical
installation and then how the antenna compares to a dipole. This is exactly
what is presented. There is nothing hidden in any figures. Several people
have independently verified many of them. Some of them are on this
Reflector.
The Force 12 gain specifications are provided in two ways
and the column headers are:
GAIN This is the antenna's forward gain in dBd as installed over real
ground a height of 74'. It is the forward gain figure you will see
on the screen if running the common modeling software of a typical
installation on a 72' crank-up and short mast. This figure includes
ground reflection gain. To clarify ground reflection gain, there is
a section and a chart included in the Antenna Specification pages.
This GAIN number is higher than the NET GAIN number by the ground
reflection gain.
NET GAIN This is the antenna's forward gain in dBd as compared to a
full size dipole at the same height. It is an apples to apples
comparison of the selected antenna to a full size dipole installed
in the same location.
If an antenna has a NET GAIN figure of 5.7dBd, the antenna will have
5.7dB more energy in the forward direction than a full size dipole
at the same height above ground. The measurements being made at the
front and point of maximum energy of the main lobe of the antenna.
(A dipole has a figure 8 pattern, so it is equal at the front and
rear.)
GAIN
Gain is achieved by the redistribution of energy. So, to achieve "forward
gain", energy must be redistributed from other directions and "pushed" out
the front, which is the desired direction. A simple way to demonstrate this
is to begin with a round water balloon. This would represent an isotropic
radiator, with energy being equal in all directions being emitted from a
point source right at the exact middle of the balloon. The water within the
balloon is all the available energy. If you now squeeze the balloon at the
center, it makes a figure 8 pattern, like a dipole. There is more energy in
two directions (the ends of the "8") and there is less energy in the sides
of the "8". The dipole has "gain" over the isotropic radiator, 2.15dB worth,
because the energy has been redistributed. You can now simulate a
directional Yagi by squeezing one of the ends of the "8", while holding the
middle. More water will push to the other end, creating a "front to back"
ratio", as there is now more water at one end than the other. There will
never be more energy than the original, BUT if the balloon has a leak, the
pattern can be maintained while the balloon is losing water (hopefully not
getting you wet!). The shrinking balloon represents an antenna with loss.
In a Yagi antenna, gain is redistributed by the various relationships
(spacing, tuning) of the elements. Energy is caused to move in a desired
direction at the expense of energy in other directions. The other directions
have less energy than the desired direction and the relative measure between
these directions and the main (front) lobe is given in dB. These relative
measurements are commonly referred to as "front to rear" ratio, or "front to
back" ratio.
An antenna exhibiting a good front to rear, or front to back ratio has the
implication of forward gain; however, depending upon the losses in the
design and components used, it might have little or even no forward gain at
all.
COMPUTER MODEL, REAL TIME CHECK
Each Force 12 gain specification is provided from the computer model and is
validated in real-time using a full size dipole at the same height. The
real-time data correlates within a reasonable measurement to the computed
values.
The majority of antennas are calculated at a height of 74'. The 6 mtr Yagis
are calculated at 45'. The exception to this is the 160 rotatable dipole and
the 80 meter antennas, as the physical aspects of making full size
comparisons has not been available!
GAIN FIGURES ARE ABOVE REAL GROUND
The figures are all for antennas above real ground, not in free space. This
is done because over real ground is where the antenna is installed. Besides
that, antennas designed in free space for HF work are not necessarily the
best design as when done over real ground.
ISOTROPIC REFERENCE
If one wants to use the isotropic radiator reference (dBi), it is simple. To
make the Force 12 specified gain figures into a dBi reference, add 2.15dB.
For example, if a NET GAIN figure (the antenna compared to a full size
dipole at the same height) is 4.5dBd, add 2.15dB and the result is 6.65dBi.
Conversely, if you have a gain figure for a horizontally polarized antenna
is specified at 7dBi, the antenna will have 7 - 2.15 = 4.85dBd. This means
the antenna will have 4.85dB forward gain as compared to a full size dipole
at the same height.
Hope this helps in understanding.
Have a good day and 73,
Tom, N6BT
Force 12 Antennas and Systems
(Home Page http://www.QTH.com/force12 )
P.S. The following references a method suggested for trapped antennas:
USING BOOM LENGTH AND "ACTIVE ELEMENTS" AS A GUIDE FOR TRAPPED ANTENNAS
For any antenna occupying a certain boom length, one can surely compute the
forward gain for a properly tuned monobander. A trapped antenna is much
different situation. If one wants to use the forward gain for a monobander
on the boom length in question, then we need think carefully.
First, the monobander model selected should be a reasonable design, meaning
moderate gain for the boom length. This would be a starting point, or the
maximum possible gain, assuming a perfect world. For a 32' boom on 20 mtrs,
it would be in the 204CA range, maybe a little more. In round numbers, 6 to
6.5dBd. From here, we subtract. The items to allocate in the reduction
include using elements not full size (e.g. 20 and 15), having elements not
in the best locations, elements not being tuned properly (variances in
traps), losses due to traps (linear loaded or otherwise), losses in the feed
system. In any event, the resulting figure is always lower than the
monobander.
The 20 mtr band will be the hardest hit, then 15 is probably midway and 10
might be the best. (On most shorter boom trapped antennas tested, 15 mtrs
was the best of the three.) Back to 10 mtrs...if we are maintaining our
rational thought, we will realize the 10 mtr portion on the longer boom
tribander shares some of the same things to be allocated as on 20 and 15:
elements not necessarily in the best position, element tuning variables
(they are at least slightly end-loaded), maybe feed system losses.
No free lunch anywhere!
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