WEB PAGE ANTENNA SPECIFICATIONS

Force 12 provides TrueSpecÓ. All specifications are accurate and can be verified by computer modeling, as well as real time use and testing using a vertical track or chamber. TrueSpecÓ was validated through an independent testing process. Steve Morris, K7LXC and Ward Silver, N0AX, began this concerted effort in 1997 to measure the actual performance of amateur triband antennas. Their data was first released on May 2, 1998 at the DX Convention in Visalia, California and then at the Dayton Hamvention. One of the antennas tested was the Force 12 C-3. The C-3 was selected as the highest performance tribander per foot of boom length and Mr. Morris shared that the C-3 factory specifications are the only ones that tracked the actual results. This is what TrueSpecÓ is all about. Since day #1, Force 12 has been providing you true specifications and no other company has done the same.

The following definitions should provide information to understand the Force 12 product specifications. This is not intended to be an exhaustive discussion on antenna theory. All Force 12 antennas are designed and optimized where they will be installed, which is over real, average ground (not in free space). The specifications follow accepted theory and are always open to scrutiny. To compare with other manufacturers will require detail on how the others define their figures. You can depend on Force 12 TrueSpecÓ .

GAIN is achieved by redistributing the available energy into a preferred direction at the expense of other directions. It is usually measured at the point of most energy in the preferred lobe of the antenna pattern. Gain is expressed in a relative term, "dB something", where "something" is the important reference (what it is compared to). Without the reference (and other specifications), the gain figure is meaningless. To provide the most information, Force 12 specifies dBd and dBi figures for horizontal antennas.

1) For horizontal antennas, dBd is the gain compared to a full size, resonant reference dipole at the same height, in the same location. This is the "apples to apples" comparison and is labeled in the specifications, "NET GAIN."

2) For horizontal antennas, dBi is the computed gain of the antenna at a typical height of 74’ (over average, real ground), compared to the theoretical isotropic source (reference). This dBi figure will be familiar to those who do computer modeling and is numerically much larger (7-7.5dB) than the "apples to apples" dBd figure. This difference comes from two places. One is from the difference between the theoretical isotropic source in free space to a dipole in free space, which accounts for 2.14dB (the pattern is changed from a sphere in free space to a figure 8, which indicates how the energy is redistributed). The second is from "ground reflection gain," which is the enhancement for a horizontal antenna, due to the effect of the ground. Values for ground reflection gain on horizontal antennas (none for verticals) depend on the height in wavelengths above ground and average 4.5-5.8dB. If the 2.14 (from the isotropic source) and 5.9 (maximum ground reflection gain) are subtracted from the dBi figure, one will arrive at approximately the dBd "apples to apples" number.

				Ground Reflection Gain for	dipole at 74’ over ground on
Band	FREQ	One l	74’ in l	amateur bands, compared to	the same dipole in free space
160M	1.8 MHz	546’	.135 l	5.93 dB at 88º elevation	The gain figures to the left
80	3.8	259’	.286	3.73 dB at 52º	are subtracted from the gain
40	7.1	138’	.536	5.75 dB at 26º	figures for each antenna to
30	10.1	97’	.763	5.08 dB at 18º	provide "NET GAIN".
20	14.1	70’	1.057	5.81 dB at 14º	"NET GAIN" is the gain of
17	18.1	54’	1.370	5.12 dB at 10º	the antenna compared to a
15	21.2	46’	1.609	5.83 dB at 8º	full-size, resonant dipole at
12	24.9	39’	1.897	5.31 dB at 7º	average, real ground.
10	28.8	35’	2.177	5.82 dB at 6º

(The NET Gain number will be fairly constant for antennas installed @ 35’ and higher.)

1) Both the beam and dipole are at slightly more one wavelength (l ) above ground. This results in two lobes, since the number of lobes is two (2) times the antenna height in wavelengths (e.g. 1 wavelength = 2 lobes; ½ wavelength = 1 lobe).

2) These diagrams are for a full size C-3 (20 mtr band) on the left and a resonant dipole on the right at 74’ above ground (70’ = 1l ).

FRONT-TO-BACK (F/B or F/B ratio) is the difference between the maximum forward gain and the rearward pattern.

FORCE 12 designs for an overall average rejection, which usually provides the best overall relationship between the desired direction and the "unwanted" directions, especially to the larger areas off the sides. Front to back can be viewed on a typical receiver’s S-meter and is often taken as the best indication that an antenna is working well; however, an antenna can have a great pattern and no gain.

HALF-POWER BEAMWIDTH is a width measurement of the major front lobe. The measurement points used are the –3dB points. These –3dB points are called "half power points", since –3dB means the power was halved. The same 3dB in a positive sense means that the power has been doubled. The number specified is the bearing in degrees that it takes to go from one –3dB point to the other. The smaller the number, the more the antenna needs to be rotated to receive and transmit the most energy to a specific area. It is also a general indication of the forward gain of an antenna; however, if a beam antenna has losses, it can be directional, but not have any gain.

WIND LOAD is the worst-case wind resistance for the antenna. Using the latest structural analysis, the wind load is either the total element wind load OR the boom windload, whichever is the larger resistance to the wind. Most beams have more element than boom wind load. The figure specified is the effective area, which is the projected area of the elements or boom, multiplied by 2/3 for a cylindrical surface.

ROTATING RADIUS is the dimension taken from the mast mounting location to the farthest element tip. This is the maximum clearance needed from the support to the tip. Twice this figure is the total diameter circle that the antenna will cover on one rotation.

MAST TORQUE is calculated at 70 mph (20 pounds per square foot wind pressure). It is the amount of "twist" exerted on the tower and rotator in 70 mph winds in the worst-case wind attack angle. The antenna (or stack of antennas) might still want to align one way or the other to the wind. This is because an antenna will usually have more windload in the element or boom plane. Most antennas have more element wind load. This being the case, the additional of an 80 or 40 meter dipole parallel to the boom will minimally increase the wind load on the tower. The added dipole tends to make the entire installation more neutral in the wind, since the boom (plane) wind load has been increased and is now closer to the element load.

1) VSWR in the band means the worst case found anywhere in the band, usually at the band edges. Within this range, the VSWR will be less than the maximum (dropping to or close to 1:1); and,

VSWR is primarily important for solid state equipment and amplifiers with VSWR protection circuitry. VSWR curves are usually not symmetrical. They can rise faster on one end of the band than the other and can have more than one low portion. Two typical curves are as follows:

1. Gain is achieved by redistributing the available energy in a preferred direction at the expense of other directions. It is usually measured at the point of most energy in the preferred lobe of an antenna. Gain is expressed in a relative term, "dB something", where "something" is the important reference (what it is compared to). Horizontal antennas are specified in two ways:

Gain @ 74’ is given in dBi at the computed gain of the antenna at a height of 74’ (over average, real ground), compared to the theoretical isotropic source (reference). This dBi figure will be familiar to those who do computer modeling and is numerically much larger than the "apples to apples" dBd comparison, by roughly 7-7.5dB. This comes from two places. One is from the difference between the theoretical isotropic source in free space to a dipole in free space, which accounts for 2.14dB (the pattern is changed from a sphere in free space to a figure 8, which indicates how the energy is redistributed). The second is from "ground reflection gain," which is the enhancement for a horizontal antenna (none for verticals) installed above real ground, due to the effect of the ground. Values for ground reflection gain on horizontal antennas depend on the height in wavelengths above ground and average 4.5-5.8dB. If 2.14 (isotropic source) and 5.9 (maximum ground reflection gain) are subtracted from the dBi figure, one will arrive at approximately the dBd "apples to apples" number.

2. Net Gain is given in dBd as compared to the full size, resonant reference dipole at the same height, in the same location. This is the "apples to apples" comparison and is labeled, "NET GAIN." Computations are performed using state of the art modeling software: NEC-4, EZNEC, AO, YO and are validated using consultants. Force 12 does not use traps, so there are no assumptions needed about possible Q of the traps (coils) and associated losses.

3. F/B Ratio is the peak (minimum) difference between the forward and reward pattern. A high F/B at a single point (180 degrees to the rear) does not necessarily means the antenna has good rejection to the sides and rear quadrants (F/R). Force 12 antennas have excellent F/R.

4. VSWR (max) is the highest value (usually at the band edges) for a typical installation through an RF choke or 1:1 balun. Within the band, the VSWR will usually approach 1:1. The actual VSWR measured might be lower than these values, because coax has a small amount of loss and this acts to smooth out the VSWR curve. If the frequency of the lowest VSWR shifts with a change in coax length, the balun or RF choke is not effective. Proximity to other antennas or other objects (i.e. roof and ground), can change these values.