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Subelement T9
ANTENNAS AND FEED LINES
Section T9A
Antennas: vertical and horizontal polarization, concept of antenna gain, definition and types of beam antennas, antenna loading, common portable and mobile antennas, relationships between resonant length and frequency, dipole pattern
What is a beam antenna?
  • An antenna built from aluminum I-beams
  • An omnidirectional antenna invented by Clarence Beam
  • Correct Answer
    An antenna that concentrates signals in one direction
  • An antenna that reverses the phase of received signals

The term "beam antenna" is just another name for a directional antenna; it's an antenna that concentrates signals in one direction. You can think of it as "beaming" the signals in a certain direction.

This term may be in common usage, but using it to identify an antenna is not a good practice. More accurate alternatives are "high gain", "directional", or "electrically large" antenna.

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Tags: antenna directional antenna arrl chapter 4 arrl module 9

Which of the following describes a type of antenna loading?
  • Correct Answer
    Electrically lengthening by inserting inductors in radiating elements
  • Inserting a resistor in the radiating portion of the antenna to make it resonant
  • Installing a spring in the base of a mobile vertical antenna to make it more flexible
  • Strengthening the radiating elements of a beam antenna to better resist wind damage

Inductors in series make an antenna appear electrically longer. So you'd insert an inductor into the radiating portion of the antenna to make it appear electrically longer.

Adding a resistor will reduce current flow, but it wouldn't affect the resonant frequency.

The spring at the base of an antenna would absorb the effects of collisions with other objects, but absorbing collisions has nothing to do with loading. It might make the antenna slightly longer, especially at higher frequencies, but the distractor doesn't say anything about that.

Resisting wind effects has to do with what can be known in civil engineering as "wind loading" - but that's not what they're referring to when they talk about loading.

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Which of the following describes a simple dipole oriented parallel to Earth's surface?
  • A ground-wave antenna
  • Correct Answer
    A horizontally polarized antenna
  • A travelling-wave antenna
  • A vertically polarized antenna

The orientation of the conductor of an antenna relative to the earth's surface determines its "polarization". If the polarization of the sending station's antenna does not match the polarization of the receiving station's antenna significant loss in signal can be the result.

If the antenna is vertical (perpendicular to the ground), as most antennas are thought to be, then it is "vertically polarized" and if it is horizontal (parallel to the ground), then it is "horizontally polarized".

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What is a disadvantage of the short, flexible antenna supplied with most handheld radio transceivers, compared to a full-sized quarter-wave antenna?
  • Correct Answer
    It has low efficiency
  • It transmits only circularly polarized signals
  • It is mechanically fragile
  • All these choices are correct

Smaller antennas use electrical components to maintain resonance on the target frequencies, but having less surface area they don't absorb (or emit) as much power. Therefore they do not transmit or receive as effectively as a regular full-sized antenna.

As a general rule of thumb, the shorter the antenna on a given band the worse the performance will be and the longer the better. Of course, other factors such as the resonance of the antenna on the frequencies used can also affect this!

The only reasons to use rubber duck type antennas are that they take up less space and are usually more durable than longer antennas, along with generally being cheaper. This makes it easier to keep an HT on your belt compared to a possibly much longer antenna. These sorts of tradeoffs are more typically worthwhile for commercial users such as security guards who will be near a repeater or other HT users almost all the time, so they aren't nearly as concerned as much about gain as most amateur operators.

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Tags: troubleshooting antenna arrl chapter 4 arrl module 9

Which of the following increases the resonant frequency of a dipole antenna?
  • Lengthening it
  • Inserting coils in series with radiating wires
  • Correct Answer
    Shortening it
  • Adding capacitive loading to the ends of the radiating wires

Antenna length is inversely related to frequency. The higher the frequency, the SHORTER the wavelength. Cannot be longer!

Another way to remember it: Antenna length is directly related to the wavelength. Recalling that wavelength and frequency are inversely related, we must shorten the antenna length.

Also note that coils are inductors, and adding inductors to an antenna is a way to electrically lengthen the antenna, therefore adding a coil and lengthening the antenna both achieve the same result. Since there is no 'All of the above' answer, you can immediately eliminate those two answers.

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Which of the following types of antenna offers the greatest gain?
  • 5/8 wave vertical
  • Isotropic
  • J pole
  • Correct Answer
    Yagi

The Isotropic Antenna is a theoretical antenna with equal radiation in all directions, and its gain is assumed to be 1.0dB.

The 5/8 vertical and J-pole are (mostly) omnidirectional with gains of ~4dB and ~2dB respectively.

Since the Yagi is a directional antenna, its gain may increase as the number of elements increase, with an average gain of 6-20+dB, depending on the number of elements.

[Exam Tip] - The answer (Yagi) is the only directional antenna found in the available choices.

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What is a disadvantage of using a handheld VHF transceiver with a flexible antenna inside a vehicle?
  • Correct Answer
    Signal strength is reduced due to the shielding effect of the vehicle
  • The bandwidth of the antenna will decrease, increasing SWR
  • The SWR might decrease, decreasing the signal strength
  • All these choices are correct

A rubber duck antenna is a sub-performant antenna to start out with, but when you're inside your car you are surrounded by a metal shield that impedes the RF energy to and from your radio, which means that the signal will often be significantly weaker than if you were outside of your vehicle. In addition some rubber duck antennas are too long to be held vertically which changes the polarization of the signal and causes additional loss.

One easy solution to this is to get a cheap magnetic mount antenna that can be placed on top of your car and then connected to your handheld radio. These can be found for under $20 on ebay.

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What is the approximate length, in inches, of a quarter-wavelength vertical antenna for 146 MHz?
  • 112
  • 50
  • Correct Answer
    19
  • 12

146 MHz is in the 2 meter band. \(\frac{300}{146}\approx2\text{ m}\)

2 meters is almost 80 inches.

\(\frac{1}{4}\times80\text{ in} = 20\text { in}\).

19 inches is the closest answer.

OR

\begin{align} \frac{300}{146\text{ MHz}} = 2.0547 \text{ m}\\ 2.0547 \text{ m} \times 39.37 \text{ in/m} = 80.89\text{ in} \end{align}

Then to determine size of antenna from the question, \(80.89 \times \frac{1}{4}[\lambda] = 20.22 \text{ in}\) rounded to nearest whole number is \(20\text{ in}\).

OR

The speed of light may be expressed as \(11.8 \text{ GHz}\times\text{inches}\). To get the length of the wave divide by the frequency in GHz.

\begin{align} \frac{11.8 \text{ GHz}\times\text{in}}{0.146\text{ GHz}} \approx 80\text{ in} \end{align} \[80 \times \frac{1}{4}[\lambda] = 20 \text{ in}\]

Since the question asked for quarter wave we must divide by four. For more information Google: "speed of light in gigahertz inches".

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Tags: antenna 2 meter math arrl chapter 4 arrl module 9

What is the approximate length, in inches, of a half-wavelength 6 meter dipole antenna?
  • 6
  • 50
  • Correct Answer
    112
  • 236

There are many factors that will affect the amount of length needed for the 1/2 wave dipole antenna, such as the physical characteristics of the wire or nearby conductive sources. But the easiest way to solve this problem is to remember that a meter is a little longer than a yard, or approximately 39 inches. To calculate this, half of the 6 meter wavelength would be 3 meters. To convert that to inches, multiply by 39 inches per meter:

\begin{align} 3\text{ m} \times 39\text{ in/m} = 117\text{ in} \end{align}

112 inches is the closest to this.

For those interested in the formula to get the closest answer:

To get half-wavelength dipole antenna lengths in feet, divide 468 by the frequency in megahertz:

  1. First convert the wavelength to the frequency in megahertz. Approximate speed of light divided by length in meters: \[\frac{300}{6} = 50\text{ MHz}\]
  2. Then divide 468 by that number: \[\frac{468}{50} = 9.36 \text{ feet}\]
  3. With 12 inches in a foot, you get \[9.36\text{ ft}\times12\text{ in} = 112.32\text{ in}\]

Quarter wavelength dipole is the same, but divide 234 by the frequency in megahertz. This is easier to remember (since 234's digits are sequential), so just remember that one and convert up when needed!

ALTERNATE METHOD

  1. We are looking for a half wavelength antenna length, so we can begin by dividing the 6 meter wavelength by 2, which gives us a 3 meter wavelength

  2. Convert 3 meter wavelength to frequency in MHz \[\frac{300}{3} = 100\text{ MHz}\]

  3. Then divide that by 1000 to convert it to GHz \[\frac{100}{1000} = .1\text{ GHz}\]

  4. Now that you have converted to GHz, you can divide the speed of light expressed as 11.8 GHzĂ—inches (gigahertz inches) by the frequency in GHz to arrive at a length in Inches \[\frac{11.8}{.1} = 118\text{in}\]

112 inches is the closest to this.

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In which direction does a half-wave dipole antenna radiate the strongest signal?
  • Equally in all directions
  • Off the ends of the antenna
  • In the direction of the feed line
  • Correct Answer
    Broadside to the antenna

The radio waves emit out along the length of the dipole outward, and is strongest at the middle. There is little to no RF energy coming out the ends of the antenna. So Broadside is the correct answer. A good way to think about dipoles is to picture a fluorescent tube or a glow stick. They appear brightest when looking at the side, but you don't see anything when looking at the ends.

An isotropic antenna radiates equally in all directions. This kind of antenna is only theoretical, there is no actually isotropic antenna in reality.

The feed line has no effect in the emission of a dipole antenna if properly chocked off by a baluns

see http://en.wikipedia.org/wiki/Dipole_antenna

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What is antenna gain?
  • The additional power that is added to the transmitter power
  • The additional power that is required in the antenna when transmitting on a higher frequency
  • Correct Answer
    The increase in signal strength in a specified direction compared to a reference antenna
  • The increase in impedance on receive or transmit compared to a reference antenna

Think of gain as a focusing quality of an antenna, like the reflector on a flash light.

By the geometry of the antenna we can change how the antenna emits radio waves, or RF energy. We can focus it like a spot light by using a yagi antenna, or we can let it flood out more evenly like a room shop light, by using a dipole antenna.

The higher the gain, the more focused the beam of RF energy, which results in an increased signal strength in a particular direction.

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What is an advantage of a 5/8 wavelength whip antenna for VHF or UHF mobile service?
  • Correct Answer
    It has more gain than a 1/4-wavelength antenna
  • It radiates at a very high angle
  • It eliminates distortion caused by reflected signals
  • It has 10 times the power gain of a 1/4 wavelength whip

Stations with a lower radiation-angle gain the ability to transmit further with better signal strength. Antenna gain is a measure of how omni-directional an antenna transmits it's signal. An antenna with a gain of zero would perfectly radiate in all directions, so depending on your purposes, gain could be better as a large number, or better as a small number.

In this case, a 5/8 wave is larger than a 1/4 wave. A larger wave means a lower angle and a higher gain. Since gain could be good at low or high values, it becomes irrelevant with regard to the question asked. Lower angles are better in almost every case. The 5/8 wave has a lower angle than the 1/4 wave, so the 5/8 wave has more advantages.

Lots of interesting distractors, but the simple fact is that a 5/8 wave radiates at a lower angle than a 1/4 wave antenna, which is usually a good thing. So just remember 5/8 is larger than 1/4 so 5/8 is better. Makes it easy to remember the correct answer

What about the distractors? The 5/8 wave radiates at a lower angle, which is better, than a 1/4 wave, so "It has very high angle radiation for better communicating ..." doesn't make any sense.

No matter what angle your signal is radiated, you're likely to encounter reflected signals, so a 5/8 wave antenna doesn't eliminate those. In fact, if it sends the signals into nearby mountains, you're more likely to encounter reflected signals with the 5/8 wavelength antenna!

To get "power gain" of 10-times the power, compared to a 1/4 wave antenna, you'd have to focus the signal so that it appears 10 times as strong, at the lower angles of radiation. While a 5/8 wave antenna will, indeed, exhibit an increase in signal strength at the lower angles, that increase is in the order of 3dB, (2-times the power) nowhere near the 10dB of a 10-times increase.

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