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Subelement E9

ANTENNAS AND TRANSMISSION LINES

Section E9E

Impedance matching: matching antennas to feed lines; phasing lines; power dividers

Which matching system for Yagi antennas requires the driven element to be insulated from the boom?

  • Gamma
  • Correct Answer
    Beta or hairpin
  • Shunt-fed
  • T-match
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What antenna matching system matches coaxial cable to an antenna by connecting the shield to the center of the antenna and the conductor a fraction of a wavelength to one side?

  • Correct Answer
    Gamma match
  • Delta match
  • T-match
  • Stub match

A Gamma-match can match impedance below 50 ohms right up to that 50 ohms which your transceiver wants to see. A Yagi antenna almost never has an impedance of 50 ohms.

REF: http://www.dx-antennas.com/Gamma_match.htm

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What matching system uses a short length of transmission line connected in parallel with the feed line at or near the feed point?

  • Gamma match
  • Delta match
  • T-match
  • Correct Answer
    Stub match

Memorization trick: a SECTION of transmission line = a STUB (like a movie ticket stub is a section of the ticket)


In microwave and radio-frequency engineering, a stub is a length of transmission line or waveguide that is connected at one end only. The free end of the stub is either left open-circuit or (especially in the case of waveguides) short-circuited. Neglecting transmission line losses, the input impedance of the stub is purely reactive; either capacitive or inductive, depending on the electrical length of the stub, and on whether it is open or short circuit. Stubs may thus be considered to be frequency-dependent capacitors and frequency-dependent inductors. Because stubs take on reactive properties as a function of their electrical length, stubs are most common in UHF or microwave circuits where the line lengths are more manageable. Stubs are commonly used in antenna impedance matching circuits and frequency selective filters.

http://en.wikipedia.org/wiki/Stub_(electronics)

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What is the purpose of the series capacitor in a gamma match?

  • To provide DC isolation between the feed line and the antenna
  • Correct Answer
    To cancel unwanted inductive reactance
  • To provide a rejection notch that prevents the radiation of harmonics
  • To transform the antenna impedance to a higher value

The PVC insulation that encases the center conductor of a coaxial cable is within the outer tube that provides a series capacitance of some gamma-type matching networks.
The inductive reactance is cancelled by the series capacitance within the matching network. This will lead to a defined resonance.

Hint: "Matching Network" is in the answer.

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What Yagi driven element feed point impedance is required to use a beta or hairpin matching system?

  • Correct Answer
    Capacitive (driven element electrically shorter than 1/2 wavelength)
  • Inductive (driven element electrically longer than 1/2 wavelength)
  • Purely resistive
  • Purely reactive
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Which of these transmission line impedances would be suitable for constructing a quarter-wave Q-section for matching a 100-ohm feed point impedance to a 50-ohm transmission line?

  • 50 ohms
  • 62 ohms
  • Correct Answer
    75 ohms
  • 90 ohms

When implementing a quarter-wave Q-section (also called a quarter wave impedance transformer), the impedance \(Z_o\) of the quarter wave section should be equal to \(\sqrt{Z_{in} Z_L}\). In this case: \(Z_o = \sqrt{50 * 100} = 70.71 \Omega\), which is closest to 75 ohms.

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What parameter describes the interaction of a load and transmission line?

  • Characteristic impedance
  • Correct Answer
    Reflection coefficient
  • Velocity factor
  • Dielectric constant

The reflection coefficient is defined as

\[\Gamma = {(Z_L - Z_0) \over (Z_L + Z_0)}\]

where \(Z_L\) is the impedance of the load and \(Z_0\) is the characteristic impedance of the transmission line. If \(Z_0\) and \(Z_L\) are the same, the reflection coefficient (\(\Gamma\)) is zero. And the reflection coefficient increases the more \(Z_L\) and \(Z_0\) differ, indicating stronger mismatch between the load and transmission line.

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What is a use for a Wilkinson divider?

  • To divide the operating frequency of a transmitter signal so it can be used on a lower frequency band
  • To feed high-impedance antennas from a low-impedance source
  • Correct Answer
    To divide power equally between two 50-ohm loads while maintaining 50-ohm input impedance
  • To divide the frequency of the input to a counter to increase its frequency range

A WIlkinson Power Divider is used in microwave circuitry to split the power coming into the divider's input port into two output ports. Each port has the same impedance and the two output ports are of equal power and are isolated from each other, which minimizes crosstalk between ports. The Wilkinson Divider is passive, so it works equally well as a power combiner due to reciprocity.

Hint: it divides the power "50/50" between two 50 ohm loads.

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Which of the following is used to shunt feed a grounded tower at its base?

  • Double-bazooka match
  • Beta or hairpin match
  • Correct Answer
    Gamma match
  • All these choices are correct

The gamma match is an effective method of connecting a 50-ohm coaxial cable feed line to a grounded tower so it can be used as a vertical antenna.

The gamma match is the name of an antenna matching system that matches an unbalanced feed line to an antenna by feeding the driven element both at the center of the element and at a fraction of a wavelength to one side of center.

Mnemonic: A Grounded tower needs a Gamma match.

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What is the purpose of using multiple driven elements connected through phasing lines?

  • Correct Answer
    To control the antenna’s radiation pattern
  • To prevent harmonic radiation from the transmitter
  • To allow single-band antennas to operate on other bands
  • To create a low-angle radiation pattern
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