Hello everybody and welcome back to Everything Hamradio! Today we are going to continue our series on the Technician Class License Question Pool! Today we will be talking about Antennas.
As always, the correct answers will be in bold. I also recommend that you only read the correct answers when studying for your test. If you do this, when you take your test and you see a question, there will be a better chance that the correct answer will jump out at you easier. Some say that this approach is kind of like cheating, but the way that I see it, you will always be learning something in this hobby and you don’t have to know everything there is to know about the hobby to get your license.
If you would like to purchase a copy of the Technician Class study book, written by Gordon West, WB6NOA with Eric P. Nichols, KL7AJ, that this series is loosely based around, below is a link to it.
Let’s move on to the good stuff, shall we?
- A ground wave antenna
- A horizontally polarized antenna
- A rhombic antenna
- A vertically polarized antenna
If the conductor of an antenna is mounted so that is it parallel to the Earth’s surface, the waves that are transmitted fro it will be horizontally polarized. This is typically used in HF applications. Most VHF or higher frequencies are vertically polarized.
- Equally in all directions
- Off the ends of the antenna
- Broadside to the antenna
- In the direction of the feed line
Most of the radiated energy will come from the broadside of the antenna. Let’s say for example that you have an antenna that is strung up in a north to south orientation. Most of the radiated energy will go in an east and west direction and you will get very little from the north or south. As a more practical example, grab a pen or pencil or some kind of writing utensil and hold it up flat in front of you with the writing part of it facing left. You can see the full pen/pencil in all it’s glory. Now rotate it to where you are looking at the writing part of it. Can you still see the whole pen/pencil? No, you can only see a small part of it. The same principle applies to radio wave, if the waves can only “see” a small part of your antenna, your antenna is not very likely to hear them.
T9A09 What is the approximate length, in inches, of a 6 meter 1/2-wavelength wire dipole antenna?
To calculate the length of a half-wave 6 m dipole, we first have to know the frequency. Two figure the frequency we use the formula 300 divided by meters to get an approximate frequency. So using this formula, 300 divided by six equals approximately 50 MHz. Now that we know the frequency we can use the formula 468 divided by 50 equals 9.36 feet. Now we need to convert feet to inches so multiply 9.36 times 12 and you get 112.3 inches.
T9A05 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
In order to change a dipole antenna to make the arrests made at a higher frequency you would shorten the length of it. Because the higher frequency wavelength in a shorter than a lower frequency wavelength.
T9A02 Which of the following is true regarding vertical antennas?
- The magnetic field is perpendicular to the Earth
- The electric field is perpendicular to the Earth
- The phase is inverted
- The phase is reversed
If you see an antenna that is vertical or perpendicular to the earth this is called a vertically polarized antenna. This also means that the electric field that is radiated from the antenna is also perpendicular to the earth’s surface. This is commonly use in VHF frequencies or above.
T9A08 What is the approximate length, in inches, of a quarter-wavelength vertical antenna for 146 MHz?
To find the approximate length of a quarter-wave antenna and 146 MHz we would use the formula 234 divided by 146 equals the length. If you figure this out it comes to 1.6 feet. Now we need to convert the 1.6 feet into inches by multiplying 1.6 times 12 which will give you the answer of approximately 19 inches.
- Non-resonant antennas
- Loop antennas
- Directional antennas
- Isotropic antennas
A quad, yagi, and microwave dish antenna are directional antennas. They take your signal and focused its in a specific direction. Check out my post on antennas for further reading.
T9A01 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
A beam antenna is an antenna that concentrates a signal in one direction. It has a lot like the old TV antennas that you use to see a lot of several years ago. You would see a boom width several elements ranging from long too short. Maybe even you have to use one of these before. If you have you might remember that’s the way you pointed to the antenna changed the picture quality on your TV.
T8C01 Which of the following methods is used to locate sources of noise interference or jamming?
- Doppler radar
- Radio direction finding
- Phase locking
You would use a radio direction finding to help locate source of noise interference or jamming, in not only the ham bands but in any band. Several years ago as we had some interference on our local sheriff’s office channel. I was able to take a directional antenna and after driving around and taking readings at several different locations I was able to locate the source of the interference. It just so happened that the source of the interference was a another ham radio operator who was also a deputy for the sheriff’s office. He had a simplex repeater that he used for personal communications that somehow got changed to the sheriff’s frequency.
- Calibrated SWR meter
- A directional antenna
- A calibrated noise bridge
- All of these choices are correct
One of the many aspects in the amateur radio hobby is called fox hunting. In fox hunting, someone or something is set up as a transmitter and others use directional antennas or other methods to locate the transmitter. This can be done in close range like on VHF or UHF frequencies or it can be done over long distances using in H.F. radio. I had even heard stories of people putting a transmitter in a sprinkler head or using a directional antenna and bouncing the signal off a water tower.
T9A11 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
- 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
The gain of an antenna is the increase in signal strength in a specified direction when compared to a reference antenna. The reference antenna is a zero gain antenna called a isotropic radiator. While an isotropic radiator cannot exist in real life, it is an extremely useful measuring stick or ideal. An antenna with gain is one that takes energy from somewhere within the antenna pattern and radiates that part of the signal in the desired direction. In your mobile VHF are UHF 2 m or 440 antennas, all of them have a little bit of omni-directional gain, taking energy that would normally go straight up and radiating at down close to forge the horizon. Think of a lightbulb, when you turn on the light switch, light is radiated from the bulb in all directions. If you take a reflective surface and put it on one side of the bulb, the light that would normally be wasted is redirected in a direction that you want it to go.
T3A03 What antenna polarization is normally used for long-distance weak-signal CW and SSB contacts using the VHF and UHF bands?
- Right-hand circular
- Left-hand circular
In the small part of the VHF/UHF bands that is for CW or SSB “weak signal”, you would use horizontal polarized antennas. With horizontally polarized signals, they tend to bounce off the ionosphere better than vertically polarized signals do, therefore giving your greater distances to talk.
T3A04 What can happen if the antennas at opposite ends of a VHF or UHF line of sight radio link are not using the same polarization?
- The modulation sidebands might become inverted
- Signals could be significantly weaker
- Signals have an echo effect on voices
- Nothing significant will happen
Overall the signals could be significantly weaker. Here is a practical example, take two pieces of paper or maybe cardboard so it will be easier to hold stiff. Put one piece in each hand and hold that where one hand you are holding it flat, parallel with the ground and in the other hand hold it vertical, where it is perpendicular to the ground and hold them together. As you hold them together, how much of the two pieces are touching? Not very much, huh?
T9A12 What is a reason to use a properly mounted 5/8 wavelength antenna for VHF or UHF mobile service?
- It offers a lower angle of radiation and more gain than a 1/4 wavelength antenna and usually provides improved coverage
- It features a very high angle of radiation and is better for communicating via a repeater
- The 5/8 wavelength antenna completely eliminates distortion caused by reflected signals
- The 5/8 wavelength antenna offers a 10-times power gain over a 1/4 wavelength design
For the most part, a 5/8 wavelength antenna will give you more gain and usually provides improved coverage than a 1/4 wavelength antenna does. A 5/8 wavelength antenna has more of a “squashed” pattern and a lower angle of radiation. This will normally provide for a better user experience. However, that is not always the case. Because of the lower radiation level, if you a talking to another station at a higher altitude, a 5/8 wavelength antenna may tend to undershoot who you are talking to. Just like with any antenna installation, some experimentation is usually a good idea.
T9A13 Why are VHF or UHF mobile antennas often mounted in the center of the vehicle roof?
- Roof mounts have the lowest possible SWR of any mounting configuration
- Only roof mounting can guarantee a vertically polarized signal
- A roof mounted antenna normally provides the most uniform radiation pattern
- Roof mounted antennas are always the easiest to install
If you mount your antenna as close to the center as possible, you will have the best ground plane possible. Especially, if you are using a 1/4 wavelength antenna. The ground plane tends to extend approximately a quarter wavelength in any direction from your antenna when using a 1/4 wavelength antenna. If you mount it near the edge, your will only have a good ground plane on part of your antenna so the transmission lobe of your radiation pattern will be bigger towards the roof area. Mounting the antenna near the center normally provides the most uniform radiation pattern.
T9A14 Which of the following terms describes a type of loading when referring to an antenna?
- 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 at the base of the antenna to absorb the effects of collisions with other objects
- Making the antenna heavier so it will resist wind effects when in motion
An antenna that’s shorter than it’s resonant length can be made resonant by adding inductance in series with the antenna. Most antennas are shorter than they need to be to provide the best performance. Take an HF antenna for example. Let’s say that you want a 1/2 wavelength antenna for 20 meters. Remember the formula we used earlier? First off, lets find the frequency. 300 divided by 20(meters) equals 15(ish MHz). Now lets take that and figure out the length of the antenna. For a 1/2 wavelength antenna take 468 divided by 15(Mhz) equals 31.2 feet or 374.4 inches long!! That is a little bit long to mount on a vehicle. In this case you would add a loading coil to the antenna to make the antenna shorter but still allowing the antenna to resonate at that frequency. There is a loss in performance but that is minimal when compared to a full-sized antenna, especially when that antenna is WAY to long to mount on a vehicle.
So that brings us to the end of this section. Next week we will be talking about Coaxes! Please share my blog with your friends and if you have not done so already, please subscribe to my email list to get emails when I publish a new post. Please Like me on Facebook, and follow me on Twitter, Google+, LinkedIn and StumbledUpon. Links to all of these can be found under social on the menu.
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Until next time…
73 de Curtis, K5CLM
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