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Subelement E9
ANTENNAS AND TRANSMISSION LINES
Section E9D
Directional antennas: gain; Yagi Antennas; losses; SWR bandwidth; antenna efficiency; shortened and mobile antennas; RF Grounding
How does the gain of an ideal parabolic dish antenna change when the operating frequency is doubled?
  • Gain does not change
  • Gain is multiplied by 0.707
  • Correct Answer
    Gain increases by 6 dB
  • Gain increases by 3 dB

Note that the gain of a parabolic antenna is governed by the following:

\[G = \frac{ 4\pi{A} }{ \lambda^2 }e_A\]

Where:

  • \(A\) is the area of the antenna aperture (the mouth of the parabolic reflector)
  • \(λ\) (lambda) is the wavelength of the radio waves
  • \(e_A\) is a dimensionless "aperture efficiency" parameter between \(0\) and \(1\)

It is clear that by doubling the frequency, the wavelength is halved. Using proportional reasoning, we see that substituting \(\frac{λ}{2}\) for \(λ\) results in a change in \(G\) by a factor of \(4\).

In decibels, \(10\log_{10}(4)\) is equal to \(6.02\text{ dB}\). Hence, the correct answer is "Gain is increased by \(6\text{ dB}\)". -kevinruggles

-KE0IPR

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How can linearly polarized Yagi antennas be used to produce circular polarization?
  • Stack two Yagis fed 90 degrees out of phase to form an array with the respective elements in parallel planes
  • Stack two Yagis fed in phase to form an array with the respective elements in parallel planes
  • Correct Answer
    Arrange two Yagis perpendicular to each other with the driven elements at the same point on the boom fed 90 degrees out of phase
  • Arrange two Yagis collinear to each other with the driven elements fed 180 degrees out of phase

Hint: Only one answer has the word perpendicular in it.

-KE0IPR

Imagine the capital letter L is made up of 2 yagis pointed in different directions. Looking down on the L from above, the yagis are perpendicular to each other and the feed points would be at the corner of the L

The distractors talk about arranging the antennas in parallel or linearly. Neither of those things make for good circles.

Check out this great video from Khan Academy that explains linear and circular polarization of electromagnetic waves.

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Where should a high Q loading coil be placed to minimize losses in a shortened vertical antenna?
  • Correct Answer
    Near the center of the vertical radiator
  • As low as possible on the vertical radiator
  • As close to the transmitter as possible
  • At a voltage node

Due to the fact that short verticals have a low radiation resistance, they are naturally ineffective so you will need to do whatever you can to make them as efficient as possible.

An HF mobile antenna loading coil should have a high ratio of reactance to resistance to minimize losses.

A high-Q loading coil should be placed near the center of the vertical radiator to minimize losses in a shortened vertical antenna.

-KE0IPR

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Why should an HF mobile antenna loading coil have a high ratio of reactance to resistance?
  • To swamp out harmonics
  • To maximize losses
  • Correct Answer
    To minimize losses
  • To minimize the Q

A small loading coil simply inserts a series inductive reactance that cancels capacitive antenna reactance. By using a mobile antenna loading coil you will minimize ground related losses. - KM4ARR

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What is a disadvantage of using a multiband trapped antenna?
  • Correct Answer
    It might radiate harmonics
  • It radiates the harmonics and fundamental equally well
  • It is too sharply directional at lower frequencies
  • It must be neutralized

Multiband antennas are good radiators on many different frequencies.

A single band antenna might be a poor radiator at multiples of the desired transmission frequency, and thus reject them relatively well. However, a multiband antenna will allow those higher frequencies through the trap and not reject radiating them.

This is exacerbated in the amateur service because many bands are harmonically related. For example, the 15M band overlaps with most of the 40M band tripled. So an antenna which is a good radiator on 40M and 15M will easily radiate the harmonics of the 40M signal.

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What happens to the bandwidth of an antenna as it is shortened through the use of loading coils?
  • It is increased
  • Correct Answer
    It is decreased
  • No change occurs
  • It becomes flat

Bandwidth is inversely proportional to quality factor Q, and \[Q = \frac{\text{reactance}}{\text{resistance}}\] Thus, adding reactance reduces (decreases) the bandwidth.

-robotoloco

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What is an advantage of using top loading in a shortened HF vertical antenna?
  • Lower Q
  • Greater structural strength
  • Higher losses
  • Correct Answer
    Improved radiation efficiency

Eliminate Distractors:

Lower Q - Actually raises Q by tuning antenna to be resonant

Higher Losses - Not an advantage

Greater structural strength - Nonsense

Only real answer is Improved radiation efficiency


Top loading is a methodology which increases radiation resistance, hence efficiency, even if the ground plane is substandard; seemingly a ubiquitous vertical antenna shortcoming. A top loaded vertical antenna has several advantages over the conventional vertical, but the biggest advantage is that it's shorter in length.

Source: Antenna 013: 20 Meter Top Loaded Vertical

Maximizing Efficiency in HF Mobile Antennas

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What happens as the Q of an antenna increases?
  • SWR bandwidth increases
  • Correct Answer
    SWR bandwidth decreases
  • Gain is reduced
  • More common-mode current is present on the feed line

The Q or Q-Factor, when it relates to antennas, is simply an inverse measure of the bandwidth that the antenna is useable in.

Q is defined as the center frequency divided by the bandwidth. So something with a higher Q would have a smaller bandwidth around its designed center frequency.

Example: You have a dipole which is made for 14.2 MHz and has a bandwidth (acceptable VSWR) of \(\pm 250\) kHz (0.5 MHz bandwidth)

The Q factor would be: \[Q=\frac{14.2}{0.5}=28.4\]

If the bandwidth were to be cut in half, the Q factor would double accordingly.

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What is the function of a loading coil used as part of an HF mobile antenna?
  • To increase the SWR bandwidth
  • To lower the losses
  • To lower the Q
  • Correct Answer
    To cancel capacitive reactance

The coil (inductor) is added to cancel out the capacitance already present in the circuit to try to achieve resonance.

It also facilitates a method to electrically shorten an antenna to "tune" to lower frequencies than the intended "designed" antenna length.

-W8RBJ

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What happens to feed point impedance at the base of a fixed length HF mobile antenna as the frequency of operation is lowered?
  • The radiation resistance decreases and the capacitive reactance decreases
  • Correct Answer
    The radiation resistance decreases and the capacitive reactance increases
  • The radiation resistance increases and the capacitive reactance decreases
  • The radiation resistance increases and the capacitive reactance increases

The way I remember this is that the radiation resistance moves opposite to the capacitive reactance.

\[X_C=\frac{1}{2\pi fC}\]

As \(f\) decreases, the quantity \(2\pi fC\) decreases in value which in turn increases \(X_C\). Stated another way, as frequency decreases, capacitive reactance increases. So the only choice that fits is:

The radiation resistance decreases and the capacitive reactance increases

ericthughes

-KE0IPR

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Which of the following types of conductors would be best for minimizing losses in a station's RF ground system?
  • A resistive wire, such as spark plug wire
  • Correct Answer
    A wide flat copper strap
  • A cable with six or seven 18 gauge conductors in parallel
  • A single 12 gauge or 10 gauge stainless steel wire

A wide, flat strap (preferably NOT braided, same reason as NOT stranded) has more surface area than an equivalent gauge wire. RF energy travels on the surface of the conductor. Greater surface area = greater conductor. Commercial broadcast stations have solid 00 ga. RF ground cables.

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Which of the following would provide the best RF ground for your station?
  • A 50 ohm resistor connected to ground
  • An electrically short connection to a metal water pipe
  • Correct Answer
    An electrically short connection to 3 or 4 interconnected ground rods driven into the Earth
  • An electrically short connection to 3 or 4 interconnected ground rods via a series RF choke

The best ground is a low-resistance connection to earth. While a water pipe might accomplish that, ground rods is the answer they want on the test.

The reason why the water pipe is not the best answer is that usually the condition of the pipe and its connection to the ground is unknown. For example, it is likely to be corroded. ground rods are clad in copper and designed such that they don't have the corrosion problems of the average water pipe.

Another reason is that the metal section of a water pipe may be very short because someone at some point replaced the water main with PVC pipe but you're unable to see it since it is buried.

The other answers are wrong because: a resistor doesn't improve your ground, it only makes it worse, and a series RF choke either does nothing or makes things worse.

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What usually occurs if a Yagi antenna is designed solely for maximum forward gain?
  • The front-to-back ratio increases
  • Correct Answer
    The front-to-back ratio decreases
  • The frequency response is widened over the whole frequency band
  • The SWR is reduced

Essentially, there is a point of diminishing returns when you're trying to maximize forward gain, after which the front lobe spreads out more and front to back ratio starts to fall off again.

There's a great explanation on the ARRL forum: http://www.arrl.org/forum/topics/view/118

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