The numbers on the outer ring are degrees of a compass. The numbers in the center are in dB of gain. Negative dB of gain are shown on the chart. Positive dB gain are not shown.
Looking at the \(-3 \text{ dB}\) ring (the second largest circle), find the two points where the radiation pattern crosses the ring. The negative point is about \(25^{\circ}\) and the positive is about \(25^{\circ}\); therefore, the beam width is the sum of \(25^{\circ}\) and \(25^{\circ}\) which equals \(50^{\circ}\). - K4AGO
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Front-to-back ratio is the ratio of power gain between the front and rear lobes of a directional antenna. See Wikipedia.
In this case, the main lobe has \(0 \text{ dB}\) gain and the rear lobe has \(-18 \text{ dB}\) gain. (The rear lobe gain is midway between the \(-12 \text{ dB}\) and \(-24 \text{ dB}\) circles.) Therefore the difference between them is \(18 \text{ dB}\).
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This is simply a case of taking the difference between the value of the peak front radiation and the peak side radiation. The front is 0dB and the side is less than -12 and more than -24, much nearer the -12 value. So, the front-to-side ratio is greater than 0 - -12dB or 12dB but only a little greater so given the answers shows 14dB looks more likely than 18dB.
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Antenna gain comes from when radiofrequency energy arrives from a given direction at portions/elements of the antenna in phase; rejection happens when it arrives out of phase.
As frequency changes, wavelength changes, and these phase relations change. Thus, the antenna pattern changes with frequency.
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Notice that the reference of the propagation pattern is in a semi circle as looking at from the side. Transposing the notion that the antenna is positioned in the middle and projecting outward the numbers along the outer radius depict the various degrees of azimuth that the signal is being projected, we can visualize the various strengths at various degrees. For example the maximum lobe of this pattern is at 0db at approximately at the 18th degree of ELEVATION.
-KE0IPR
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Looking at the radiation pattern chart, the lobe that has the most gain (In this case, the bottom lobe) is considered the peak response. This occurs about halfway between 0 and 15 degrees.
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Total radiation emitted includes all directions. The directional antenna radiates more in a given direction than the isotropic antenna, but when all directions are included they both radiate the same amount.
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There's no "explanation": the correct answer to this question is merely a definition.
Beamwidth, (i.e. 3 dB beamwidth) can be used to compare different antenna.
On directional antennas, beamwidth is an important consideration. Beamwidth is always described by the angle between the two points where the signal strength is down 3 dB from the maximum signal point. The "tighter" your main lobe pattern, the narrower the beamwidth.
If we think of a piece of pie cut from its whole, we can measure the width of the pie piece is with the central angle. The correct answer contains the word "angular."
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The Method of Moments modeling program is best used to break down the antenna into various wire segments of which each is analyzed for their respective propagation properties and then combined by using accumulative algorithms to calculate the overall effectiveness of the antenna modeled.
-KE0IPR
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Method of Moments (MoM) is a computational/numerical technique for modeling antennas and other electromagnetic structures.
In its simplest form, a wire antenna is split into many small segments. Each small segment is assumed to have a uniform current which is calculated through particular computations. The results allow for many things like far-field patterns and input impedances to be determined.
Mnemonic: current moment
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First, let's understand why some answers are wrong (the sequence of answers varies by user so referencing the answers by letter is useless):
Ground conductivity - suppose you are modeling a half-wave dipole in free-space, i.e., where no ground is even present: since ground conductivity is not always relevant to antenna modeling, this answer is non-sensical.
Harmonic energy - each model computation is performed at a single frequency and assumes linearity. Since harmonics arise from nonlinearities, this answer is non-sensical.
Mechanical stability - An antenna model that measures wire segment lengths in fractions of a wavelength is inherently an electrical model, not a mechanical model. Therefore, this answer is non-sensical.
Now, let's focus on why feed point impedance is the best answer:
The accuracy of the computed feed point impedance is highly dependent on the accuracy of the computed current at the feed point. Using wire segments that are electrically too long (i.e., too few wire segments per wavelength) will not accurately model the true current, and therefore impedance, at the feed point.
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As the RF energy leaves the antenna, it generally expands into the full beam. However, this expansion only continues for a specific distance. At a certain distance from the antenna, the beam pattern no longer changes but remains relatively constant. This distance is referred to as the far field distance.
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The Numerical Electromagnetics Code (NEC) is a popular antenna modeling software package for wire and surface antennas. It is credited to Gerald J. Burke and Andrew J. Poggio, and was originally written in FORTRAN in the 1970s. The code was made publicly available for general use and has subsequently been distributed for many computer platforms from mainframes to PCs.
http://en.wikipedia.org/wiki/Numerical_Electromagnetics_Code
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Antenna modeling programs can calculate many things about a proposed antenna design; as long as the variables going into the program are accurate a good modeling program will be able to tell you the SWR at any given frequency for the proposed antenna, the gain of the antenna, and also generate plots showing the radiation patterns of the antenna, etc.
In short, this question should be pretty straightforward -- a good antenna modeling program should be able to tell you just about everything you'd want to know =]
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Front-to-Back Ratio applies to directional antennas. It is not generally used in connection with omni-directional antennas (such as verticals) or antennas with symmetrical radiation patterns (such as dipoles). Front-to-Back Ratio is generally defined as the ratio (in dB) of the power emitted in the desired direction (the "front" direction) to that emitted in a direction 180 degrees from the desired direction (the "back" direction. The radiation from an antenna is not usually found concentrated exclusively in a single direction, or even two directions, so a polar plot of radiation versus azimuth will show several "lobes" or regions of strong radiation, with nulls between the lobes.
In the polar radiation plot for this question, the strongest lobe ( which is by default the "front" of the antenna) is at zero dB (outermost curve on the plot). The lobe that is 180 degrees opposite to this strongest lobe is just a bit stronger than the -30 dB curve. Thus, the ratio of the front power to the back power is just under 30 dB or 28 dB as deduced from the multiple choices in the question (nothing else being close). If the lines on the graph were closer together (and the graph larger, it would be possible to read -28 dB directly.
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The forward direction of this elevation plot is the portion of the graph from 90 degrees to 0 degrees. In this area there are 4 major lumps, so there are 4 lobes.
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