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Subelement E9

ANTENNAS AND TRANSMISSION LINES

Section E9D

Yagi antennas; parabolic reflectors; feed point impedance and loading of electrically short antennas; antenna Q; RF grounding

How much does the gain of an ideal parabolic reflector antenna increase when the operating frequency is doubled?

  • 2 dB
  • 3 dB
  • 4 dB
  • Correct Answer
    6 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)
  • \(\lambda\) (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 \(\frac{\lambda}{2}\) for \(\lambda\) 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 \(6\text{ dB}\)".

Silly memory aid: "para" means fo(u)r in Spanish, and you'd need 6 dB to quadruple power

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How can two linearly polarized Yagi antennas be used to produce circular polarization?

  • Stack two Yagis to form an array with the respective elements in parallel planes fed 90 degrees out of phase
  • Stack two Yagis to form an array with the respective elements in parallel planes fed in phase
  • Correct Answer
    Arrange two Yagis on the same axis and perpendicular to each other with the driven elements at the same point on the boom and 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.

Hint: only one answer has 'BOOM' in it :)

The key here is that the two Yagis are overlaid on the same boom. The result is two sets of elements, both pointing the same direction, with one set rotated 90° along the axis of the boom at right angles to the other.

The distractors talk about arranging the antennas in parallel or linearly. Neither of those things make for good circles – you have to have perpendicular angles out of phase to make the spiral wave.

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

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What is the most efficient location for a loading coil on an electrically short whip?

  • Correct Answer
    Near the center of the vertical radiator
  • As low as possible on the vertical radiator
  • At a voltage maximum
  • At a voltage null

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.

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Why should antenna loading coils have a high ratio of reactance to resistance?

  • To swamp out harmonics
  • To lower the radiation angle
  • Correct Answer
    To maximize efficiency
  • 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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Approximately how long is a Yagi’s driven element?

  • 234 divided by frequency in MHz
  • 1005 divided by frequency in MHz
  • 1/4 wavelength
  • Correct Answer
    1/2 wavelength
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What happens to SWR bandwidth when one or more loading coils are used to resonate an electrically short antenna?

  • It is increased
  • Correct Answer
    It is decreased
  • It is unchanged if the loading coil is located at the feed point
  • It is unchanged if the loading coil is located at a voltage maximum point

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 top loading an electrically short 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 in which that antenna is useable.

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 in an electrically short antenna?

  • To increase the SWR bandwidth by increasing net reactance
  • To lower the losses
  • To lower the Q
  • Correct Answer
    To resonate the antenna by cancelling the 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.

Silly hint: coil cancels capacitive reactance

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How does radiation resistance of a base-fed whip antenna change below its resonant frequency?

  • Radiation resistance increases
  • Correct Answer
    Radiation resistance decreases
  • Radiation resistance becomes imaginary
  • Radiation resistance does not depend on frequency

This question has two parts, concerning radiation resistance and capacitive reactance respectively.

For the first part: remember that radiation resistance is the amount of power which has been successfully radiated away by the antenna, described as though it was power dissipated by a resistor — so a higher radiation resistance implies a more efficient antenna, and maximum efficiency occurs at the resonant frequency. At other frequencies, efficiency decreases, which means radiation resistance decreases.

For the second part: the definition of capacitive reactance \(X_C\) in ohms is the reciprocal of the product of \(2π\), the frequency \(f\) in hertz, and the capacitance \(C\) in farads:

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

This means the capacitive reactance \(X_C\) varies inversely with the frequency \(f\) — so if the frequency decreases, capacitive reactance increases.

Putting the two parts together, we get:

The radiation resistance decreases and the capacitive reactance increases.

For more details, see https://en.wikipedia.org/wiki/Antenna_(radio)#Current_and_voltage_distribution

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Why do most two-element Yagis with normal spacing have a reflector instead of a director?

  • Lower SWR
  • Higher receiving directivity factor
  • Greater front-to-side
  • Correct Answer
    Higher gain
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What is the purpose of making a Yagi’s parasitic elements either longer or shorter than resonance?

  • Wind torque cancellation
  • Mechanical balance
  • Correct Answer
    Control of phase shift
  • Minimize losses
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