Sunday, April 8, 2012

Antenna Specifications

Antenna Specifications

Force 12 provides you with 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 performing tribander per foot of boom length and Mr. Morris stated that the C-3 factory specifications are the only ones that tracked the actual results

The following definitions will provide the information necessary to understand Force 12 product specifications. This is not intended to be an exhaustive discussion on antenna theory. All Force 12 antennas are designed and optimized for 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.

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 highest energy in the preferred lobe of the antenna pattern. Gain is expressed in a relative term, “dB something” (“something” is the reference -what it is compared to). Without a defined reference, the gain figure is meaningless. The following demonstration is one that Tom, N6BT uses many times each year during forums to show where gain “comes from.”
Begin with a water balloon. Imagine there is a point at the center of the balloon that is emitting energy equally in all directions. The energy ends at the surface of the balloon. This image is the “isotropic radiator.” It is located in free space – not anywhere near ground. 
Grab the balloon around the center. The energy inside the balloon is forced into a different shape. This is a dipole in free space. It is longer in two directions and narrower around the middle. The amount of energy it has in the long directions is 2.14dB more than it had originally as the round balloon. This energy comes at the expense of the sides, where there is now less energy. Properly stated, this dipole in free space has 2.14dBi gain: 2.14dB compared to the isotropic radiator. Let’s redistribute the energy some more and add a parasitic reflector element to the left, making a 2 element Yagi. If the reflector is tuned and spaced properly, it will redistribute the energy in a favored direction. Squeeze the energy in the left side and it moves to the right. We now have more “gain” in the favored direction and we also have a “front to back ratio.” We have not added energy to get gain. We only moved it around to favor a particular direction. We cannot add more energy, but we can lose it through components in the antenna system that have loss. 

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 (see the demo above). The second is from “ground reflection gain,” which is the enhancement for a horizontal antenna, due to the effect of the ground. This applies only to horizontal antennas and its height above real ground. The average ground reflection gain will be 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.

FRONT-TO-BACK (F/B or F/B ratio) is the difference between the maximum forward gain and the rearward pattern.
There are two terms that apply:
1) peak, which is at the exact center of the rearward lobe; and,
2) average, which is taken from the 90° off the side around to 180° directly at the rear.
FORCE 12 designs its antennas for overall average rejection, normally providing the best 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.

VSWR specifications is given in two ways:
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,
2) 2:1 VSWR points means the bandwidth between the 2:1 VSWR indications. 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:

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