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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Tags: arrl chapter 9 arrl module 9a

Be extra careful with this question! There are two questions which reference this diagram and some answers are shared, so it'd be easy to mix them up if you're reading too quickly.

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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Be extra careful with this question! There are two questions which reference this diagram and some answers are shared, so it'd be easy to mix them up if you're reading too quickly.

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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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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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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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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