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Subelement E9
ANTENNAS AND TRANSMISSION LINES
Section E9B
Antenna patterns: E and H plane patterns; gain as a function of pattern; antenna design
In the antenna radiation pattern shown in Figure E9-1, what is the 3 dB beam-width?
  • 75 degrees
  • 50 degrees
  • 25 degrees
  • 30 degrees

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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In the antenna radiation pattern shown in Figure E9-1, what is the front-to-back ratio?
  • 36 dB
  • 18 dB
  • 24 dB
  • 14 dB

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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In the antenna radiation pattern shown in Figure E9-1, what is the front-to-side ratio?
  • 12 dB
  • 14 dB
  • 18 dB
  • 24 dB

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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What may occur when a directional antenna is operated at different frequencies within the band for which it was designed?
  • Feed point impedance may become negative
  • The E-field and H-field patterns may reverse
  • Element spacing limits could be exceeded
  • The gain may change depending on frequency

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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What type of antenna pattern over real ground is shown in Figure E9-2?
  • Elevation
  • Azimuth
  • Radiation resistance
  • Polarization

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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What is the elevation angle of peak response in the antenna radiation pattern shown in Figure E9-2?
  • 45 degrees
  • 75 degrees
  • 7.5 degrees
  • 25 degrees

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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How does the total amount of radiation emitted by a directional gain antenna compare with the total amount of radiation emitted from an isotropic antenna, assuming each is driven by the same amount of power?
  • The total amount of radiation from the directional antenna is increased by the gain of the antenna
  • The total amount of radiation from the directional antenna is stronger by its front-to-back ratio
  • They are the same
  • The radiation from the isotropic antenna is 2.15 dB stronger than that from the directional antenna

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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How can the approximate beam-width in a given plane of a directional antenna be determined?
  • Note the two points where the signal strength of the antenna is 3 dB less than maximum and compute the angular difference
  • Measure the ratio of the signal strengths of the radiated power lobes from the front and rear of the antenna
  • Draw two imaginary lines through the ends of the elements and measure the angle between the lines
  • Measure the ratio of the signal strengths of the radiated power lobes from the front and side of the antenna

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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What type of computer program technique is commonly used for modeling antennas?
  • Graphical analysis
  • Method of Moments
  • Mutual impedance analysis
  • Calculus differentiation with respect to physical properties

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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What is the principle of a Method of Moments analysis?
  • A wire is modeled as a series of segments, each having a uniform value of current
  • A wire is modeled as a single sine-wave current generator
  • A wire is modeled as a series of points, each having a distinct location in space
  • A wire is modeled as a series of segments, each having a distinct value of voltage across it

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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What is a disadvantage of decreasing the number of wire segments in an antenna model below the guideline of 10 segments per half-wavelength?
  • Ground conductivity will not be accurately modeled
  • The resulting design will favor radiation of harmonic energy
  • The computed feed point impedance may be incorrect
  • The antenna will become mechanically unstable

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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What is the far field of an antenna?
  • The region of the ionosphere where radiated power is not refracted
  • The region where radiated power dissipates over a specified time period
  • The region where radiated field strengths are obstructed by objects of reflection
  • The region where the shape of the antenna pattern is independent of distance

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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What does the abbreviation NEC stand for when applied to antenna modeling programs?
  • Next Element Comparison
  • Numerical Electromagnetic Code
  • National Electrical Code
  • Numeric Electrical Computation

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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What type of information can be obtained by submitting the details of a proposed new antenna to a modeling program?
  • SWR vs frequency charts
  • Polar plots of the far field elevation and azimuth patterns
  • Antenna gain
  • All of these choices are correct

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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What is the front-to-back ratio of the radiation pattern shown in Figure E9-2?
  • 15 dB
  • 28 dB
  • 3 dB
  • 24 dB

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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How many elevation lobes appear in the forward direction of the antenna radiation pattern shown in Figure E9-2?
  • 4
  • 3
  • 1
  • 7

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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