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Subelement E9
ANTENNAS AND TRANSMISSION LINES
Section E9C
Wire and phased array antennas: rhombic antennas; effects of ground reflections; e-off angles; Practical wire antennas: Zepps, OCFD, loops
What is the radiation pattern of two 1/4-wavelength vertical antennas spaced 1/2-wavelength apart and fed 180 degrees out of phase?
  • Cardioid
  • Omni-directional
  • A figure-8 broadside to the axis of the array
  • A figure-8 oriented along the axis of the array

The most significant factors here are that the elements are fed 180 degrees out of phase and are spaced 1/2-wavelength apart. In a half-wavelength a radio frequency signal differs by 180 degrees from the source. So, you can also say that the elements are spaced 180 degrees apart. Therefore, when a signal leaving one element reaches the other it is in phase with the radiation from the element it is reaching and thus the output of each element re-enforces the output from the other.

That means the pattern is essentially two overlapping circular outputs from the two elements and strengthened in the direction away from the antenna in each direction in line with the axis through the two antenna elements. In other words: a figure-8 oriented along the axis of the array.

See also: http://www.bellscb.com/cb_radio_hobby/antennas/antarray.html, last Fig

Memory Tip (for Test answers): "180 degrees" has an 8, so it's a figure eight answer. The signal is "O"ut of phase, so the pattern is "O"riented along the axis. Choose the answer with 180 and an "O" word. (See other easily confused similar question, E9C3, whose answer is a broadside figure 8.)

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What is the radiation pattern of two 1/4 wavelength vertical antennas spaced 1/4 wavelength apart and fed 90 degrees out of phase?
  • Cardioid
  • A figure-8 end-fire along the axis of the array
  • A figure-8 broadside to the axis of the array
  • Omni-directional

See Figure 3 at: http://www.bellscb.com/cb_radio_hobby/antennas/antarray.html

Memory Tip: The "9" in 90 degrees looks a bit like a drawing of a cardioid shape.

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What is the radiation pattern of two 1/4 wavelength vertical antennas spaced a 1/2 wavelength apart and fed in phase?
  • Omni-directional
  • Cardioid
  • A Figure-8 broadside to the axis of the array
  • A Figure-8 end-fire along the axis of the array

There are many ways to put up antennas that are directional. One way to get directionality without a tower is to use phased vertical arrays.

In general, the phased vertical array consists of two or more quarter-wave vertical antennas. The radiation pattern that the array will have depends on how you feed the vertical antennas.

So, for example, the radiation pattern of two 1/4-wavelength vertical antennas spaced 1/2-wavelength apart and fed 180 degrees out of phase is a figure-8 oriented along the axis of the array. (E9C01) The radiation pattern of two 1/4-wavelength vertical antennas spaced 1/4-wavelength apart and fed 90 degrees out of phase is a cardioid. (E9C02) The radiation pattern of two 1/4-wavelength vertical antennas spaced 1/2-wavelength apart and fed in phase is a Figure-8 broadside to the axis of the array. (E9C03)

Source kb6nu.com - Extra Class question of the day: Wire and phased vertical antennas

-mvs90


Memory Tip for Test Answer: "In phase" is pretty standard, and a dipole is pretty standard. So the pattern is roughly the same as a dipole: a figure-8 broadside to the antenna.

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What happens to the radiation pattern of an unterminated long wire antenna as the wire length is increased?
  • The lobes become more perpendicular to the wire
  • The lobes align more in the direction of the wire
  • The vertical angle increases
  • The front-to-back ratio decreases

The main lobe of the radiation pattern starts to follow along the direction of the wire when the length is increased past 1 to 2 wavelengths. The general rule of thumb is that the longer the wire the more constricted the main lobes.

-KE0IPR

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What is an OCFD antenna?
  • A dipole feed approximately 1/3 the way from one end with a 4:1 balun to provide multiband operation
  • A remotely tunable dipole antenna using orthogonally controlled frequency diversity
  • An eight band dipole antenna using octophase filters
  • A multiband dipole antenna using one-way circular polarization for frequency diversity

OCFD stands for Off Center Fed Dipole. This specific antenna is fed away from the center (usually around 1/3 the way). At this point the input impedance is closer to 200 Ohms so a 4:1 balun is used to match the impedance from 50 Ohms to 200 Ohms.

Due to this configuration, the antenna will present good matching on multiple frequencies to allow for multiband operation.

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What is the effect of a terminating resistor on a rhombic antenna?
  • It reflects the standing waves on the antenna elements back to the transmitter
  • It changes the radiation pattern from bidirectional to unidirectional
  • It changes the radiation pattern from horizontal to vertical polarization
  • It decreases the ground loss

Normally, a rhombic antenna is bi-directional along the longer axis; adding a terminating resistor, usually in the 600 ohm range, at the vertex opposite to the feedpoint makes the rhombic unidirectional in the direction from feedpoint to resistor.

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What is the approximate feed point impedance at the center of a two-wire folded dipole antenna?
  • 300 ohms
  • 72 ohms
  • 50 ohms
  • 450 ohms

Think of those flat twin ribbon TV antenna wire with an impedance of 300 ohms.

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What is a folded dipole antenna?
  • A dipole one-quarter wavelength long
  • A type of ground-plane antenna
  • A dipole consisting of one wavelength of wire forming a very thin loop
  • A dipole configured to provide forward gain

The key is to remember that this antenna is one wavelength long "folded" in to a thin loop one-half wavelength long.

For more information see wikipedia: Folded Dipole Antenna

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What is a G5RV antenna?
  • A multi-band dipole antenna fed with coax and a balun through a selected length of open wire transmission line
  • A multi-band trap antenna
  • A phased array antenna consisting of multiple loops
  • A wide band dipole using shorted coaxial cable for the radiating elements and fed with a 4:1 balun

The G5RV antenna is a dipole with a symmetric resonant feeder line, which serves as impedance matcher for a 50 Ohm coax cable to the transceiver.

Reference: https://en.wikipedia.org/wiki/G5RV_antenna

-kc7nyr

Those of you who would like to know; G5RV is the call sign of the inventor of the antenna. Louis Varney. (Not the Opera dude)

More info on the legend here:
http://www.radioworks.com/Varney/G5RVmain.htm

-KE0IPR

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Which of the following describes a Zepp antenna?
  • A dipole constructed from zip cord
  • An end fed dipole antenna
  • An omni-directional antenna commonly used for satellite communications
  • A vertical array capable of quickly changing the direction of maximum radiation by changing phasing lines

Zepplins used to have this type of antenna, a long wire connected to the transmitter in the zepplin and hanging down. It's also called an end-fed zepp (picture the zepplin with a wire hanging down - it has to be end-fed).

Perhaps think of the "pp" in zepp and think of two letters, "di" means two. So, that should remind you of a di-pole.

The origin is the antenna used in zepplins, and the length of the antenna was a half-wave, hence the answer, "An end-fed dipole antenna"

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How is the far-field elevation pattern of a vertically polarized antenna affected by being mounted over seawater versus rocky ground?
  • The low-angle radiation decreases
  • The high-angle radiation increases
  • Both the high-angle and low-angle radiation decrease
  • The low-angle radiation increases

The main effect of placing a vertical antenna over an imperfect ground is that it reduces low-angle radiation. Mounting it over seawater (a very even and highly conductive surface) will increase the low-angle radiation.

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Which of the following describes an extended double Zepp antenna?
  • A wideband vertical antenna constructed from precisely tapered aluminum tubing
  • A portable antenna erected using two push support poles
  • A center fed 1.25 wavelength antenna (two 5/8 wave elements in phase)
  • An end fed folded dipole antenna

The Zepp, short for Zeppelin is any resonant antenna end-fed by ladder line.

A Double Zepp is a center-fed 1 wavelength antenna.

The double extended Zepp is a dipole type Antenna consisting of two collinear 0.64 wave length elements fed in phase. The "extension" being an extra 1/8 wavelength added to the Zepp (on each side).

This double extended version provides 3 db gain over a dipole on the band it is designed for and each side or leg is about 5/8 wavelength long.

The antenna is constructed much like an ordinary Dipole antenna but with 5/8 Wavelength Elements matched with an added Impedance Matching Section of balanced feed line.

For calculations pertaining to the construction of a Zepp Antenna see: Zepp Antenna Calculator

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What is the main effect of placing a vertical antenna over an imperfect ground?
  • It causes increased SWR
  • It changes the impedance angle of the matching network
  • It reduces low-angle radiation
  • It reduces losses in the radiating portion of the antenna

How and where you install an antenna affects its radiation pattern. For example, the far-field elevation pattern of a vertically polarized antenna is affected when it is mounted over seawater versus rocky ground. What happens is that the low-angle radiation increases. The main effect of placing a vertical antenna over an imperfect ground is that it reduces low-angle radiation.

Hint: Think of low angle pointing to the ground...

-KE0IPR

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How does the performance of a horizontally polarized antenna mounted on the side of a hill compare with the same antenna mounted on flat ground?
  • The main lobe takeoff angle increases in the downhill direction
  • The main lobe takeoff angle decreases in the downhill direction
  • The horizontal beam width decreases in the downhill direction
  • The horizontal beam width increases in the uphill direction

The performance of a horizontally polarized antenna mounted on the side of a hill will be different from the performance of same antenna mounted on flat ground. Specifically, the main lobe takeoff angle decreases in the downhill direction. (E3C10)

Hint: It Decreases in the Downhill Direction.

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How does the radiation pattern of a horizontally polarized 3-element beam antenna vary with its height above ground?
  • The main lobe takeoff angle increases with increasing height
  • The main lobe takeoff angle decreases with increasing height
  • The horizontal beam width increases with height
  • The horizontal beam width decreases with height

The further the antenna is above ground, the less the ground will affect it's radiation pattern. The same is true for vertical omni-directional antennas. The main lobe takeoff angle decreases with increasing height.

If their ground radials are brought up they will raise the radiation pattern and main lobe, or increase the main lobe takeoff angle.

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