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Subelement T9
Antennas, feedlines
Section T9A
Antennas; vertical and horizontal, concept of gain, common portable and mobile antennas, relationships between antenna length and frequency
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.

Last edited by rccapps. Register to edit

Tags: antenna directional antenna

Which of the following is true regarding vertical antennas?
  • The magnetic field is perpendicular to the Earth
  • Correct Answer
    The electric field is perpendicular to the Earth
  • The phase is inverted
  • The phase is reversed

Remember that with an electromagnetic (RF) signal, it is the electrical field that emanates outwards parallel to the originating antenna, which means with a vertical antenna it is the electric (not the magnetic!) field that is perpendicular to the Earth.

Using the Right Hand Rule of electromagnetic fields would tell you that if the fingers wrap in the direction of the magnetic (B) field, the thumb will point in the direction of the electric (E) field.

Last edited by themountain. Register to edit

Tags: antenna propagation antenna polorization

Which of the following describes a simple dipole mounted so the conductor is parallel to the Earth's surface?
  • A ground wave antenna
  • Correct Answer
    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

What is a disadvantage of the "rubber duck" antenna supplied with most handheld radio transceivers?
  • Correct Answer
    It does not transmit or receive as effectively as a full-sized antenna
  • It transmits a circularly polarized signal
  • If the rubber end cap is lost it will unravel very quickly
  • 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, 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

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

Last edited by old man. Register to edit

Tags: antenna radio waves

What type of antennas are the quad, Yagi, and dish?
  • Non-resonant antennas
  • Loop antennas
  • Correct Answer
    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, and are much more obviously directional.

Last edited by jmyles. Register to edit

Tags: directional antenna antenna

What is a good reason not to use a "rubber duck" antenna inside your car?
  • Correct Answer
    Signals can be significantly weaker than when it is outside of the vehicle
  • It might cause your radio 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. Register to edit

Tags: antenna troubleshooting

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".

Last edited by rjstone. Register to edit

Tags: antenna 2 meter math

What is the approximate length, in inches, of a 6 meter 1/2-wavelength wire 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.

Last edited by saalberg. Register to edit

Tags: antenna 6 meter math

In which direction is the radiation strongest from a half-wave dipole antenna in free space?
  • Equally in all directions
  • Off the ends of the antenna
  • Correct Answer
    Broadside to the antenna
  • In the direction of the feedline

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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Tags: antenna polorization antenna propagation

What is meant by 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
  • Correct Answer
    The increase in signal strength in a specified direction when 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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Tags: antenna transmit power

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