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Subelement E9

ANTENNAS AND TRANSMISSION LINES

Section E9B

Antenna patterns and designs: azimuth and elevation patterns; gain as a function of pattern; antenna modeling

What is the 3 dB beamwidth of the antenna radiation pattern shown in Figure E9-1?

  • 75 degrees
  • Correct Answer
    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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What is the front-to-back ratio of the antenna radiation pattern shown in Figure E9-1?

  • 36 dB
  • 14 dB
  • 24 dB
  • Correct Answer
    18 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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What is the front-to-side ratio of the antenna radiation pattern shown in Figure E9-1?

  • 12 dB
  • 24 dB
  • 18 dB
  • Correct Answer
    14 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.

A Silly HINT: All distractors are multiples of 6 (12, 18, 24) except for the correct answer, 14

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What is the front-to-back ratio of the radiation pattern shown in Figure E9-2?

  • 15 dB
  • Correct Answer
    28 dB
  • 3 dB
  • 38 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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What type of antenna pattern is shown in Figure E9-2?

  • Correct Answer
    Elevation
  • Azimuth
  • Near field
  • 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 7.5 degrees of ELEVATION.

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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
  • Correct Answer
    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.

Hint: Figure E9-2 is asking for elevation. Using the figure number as a hint you can subtract two from nine, resulting in seven. 7.5 is closest to seven and is the right answer.

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What is the difference in radiated power between a lossless antenna with gain and an isotropic radiator driven by the same power?

  • The power radiated from the directional antenna is increased by the gain of the antenna
  • The power radiated from the directional antenna is stronger by its front-to-back ratio
  • Correct Answer
    They are the same
  • The power radiated from the isotropic radiator is 2.15 dB greater 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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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 constant
  • Correct Answer
    The region where the shape of the radiation pattern no longer varies with 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.

The far field radiation pattern is typically what we are most concerned with in radio communication as practically every receiving antenna is going to be in the far field under real conditions.

Hint: Antenna patterns usually have a perverbial shape, only answer with 'shape'

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What type of analysis is commonly used for modeling antennas?

  • Graphical analysis
  • Correct Answer
    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.

Stupid Test Hint: One of the more popular antenna modeling programs is MMANA-GAL (Method of Moments).

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What is the principle of a Method of Moments analysis?

  • Correct Answer
    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 single sine-wave voltage source
  • 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.

The more segments are used, the more precise the simulation but the longer it takes to run.


Mnemonic: current moment

Hint: Mom makes me wear my uniform.

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What is a disadvantage of decreasing the number of wire segments in an antenna model below 10 segments per half-wavelength?

  • Ground conductivity will not be accurately modeled
  • The resulting design will favor radiation of harmonic energy
  • Correct Answer
    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.

Hint: In this question, you may want to pick the "incorrect" answer (the only question that ends in "incorrect")

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