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Subelement T9
Antennas and feed lines
Section T9A
Antennas: vertical and horizontal polarization; concept of gain; common portable and mobile antennas; relationships between resonant length and frequency; concept of dipole antennas
What is a beam antenna?
  • An antenna built from aluminum I-beams
  • An omnidirectional antenna invented by Clarence Beam
  • 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 that it "beams" the signals in a certain direction.

This may be in common usage but is not good practice to identify an antenna. A more accurate terminology is high gain, directional, or electrically large antenna.

Last edited by lordmorgul. Click here to edit

Tags: antenna directional antenna arrl chapter 4 arrl module 10

Which of the following describes a type of antenna loading?
  • Inserting an inductor in the radiating portion of the antenna to make it electrically longer
  • 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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Tags: arrl chapter 4 arrl module 10

Which of the following describes a simple dipole oriented parallel to the Earth's surface?
  • A ground-wave antenna
  • A horizontally polarized antenna
  • A rhombic 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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Tags: antenna polorization antenna arrl chapter 4 arrl module 10

What is a disadvantage of the “rubber duck” antenna supplied with most handheld radio transceivers when compared to a full-sized quarter-wave antenna?
  • It does not transmit or receive as effectively
  • It transmits only circularly polarized signals
  • If the rubber end cap is lost, it will unravel very easily
  • All of 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. 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 10

How would you change a dipole antenna to make it resonant on a higher frequency?
  • Lengthen it
  • Insert coils in series with radiating wires
  • Shorten it
  • Add 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!

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Tags: antenna radio waves arrl chapter 4 arrl module 10

What type of antennas are the quad, Yagi, and dish?
  • Non-resonant antennas
  • Log periodic antennas
  • Directional antennas
  • Isotropic antennas

These are all examples of "beam antennas", also called Directional Antennas. Yagi are the most common type in ham radio and you've probably seen TV antennas that are yagi antennas; they have long elements in the back and short ones in the front and make a sort of V shape with their outline. Dish antennas, such as those commonly used by satellite TV systems, are another type are much more obviously directional.

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Tags: directional antenna antenna arrl chapter 4 arrl module 10

What is a disadvantage of using a handheld VHF transceiver, with its integral antenna, inside a vehicle?
  • Signals might not propagate well due to the shielding effect of the vehicle
  • It might cause the transceiver to overheat
  • The SWR might decrease, decreasing the signal strength
  • All of 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.

Last edited by chrisie56. Click here to edit

Tags: antenna troubleshooting arrl chapter 4 arrl module 10

What is the approximate length, in inches, of a quarter-wavelength vertical antenna for 146 MHz?
  • 112
  • 50
  • 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 10

What is the approximate length, in inches, of a half-wavelength 6 meter dipole antenna?
  • 6
  • 50
  • 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: \[\frac{300}{6} = 50\text{ MHz}\]
  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!

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Tags: antenna 6 meter math arrl chapter 4 arrl module 10

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

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.

An isotropic antenna radiates equally in all directions.

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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Tags: antenna polorization antenna propagation arrl chapter 4 arrl module 10

What is the gain of an antenna?
  • The additional power that is added to the transmitter power
  • The additional power that is lost in the antenna when transmitting on a higher frequency
  • 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 using a properly mounted 5/8 wavelength antenna for VHF or UHF mobile service?
  • It has a lower radiation angle and more gain than a 1/4 wavelength antenna
  • It has very high angle radiation for better communicating through a repeater
  • It eliminates distortion caused by reflected signals
  • It has 10 times the power gain of a 1/4 wavelength design

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? Well, a 1/4 wave antenna might be better for reaching repeaters, but a 5/8 wave antenna is not.

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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