Ham Radio Testing Explained, Part Five

Keep in mind that the following topics can be very technical and that these posts are just providing general knowledge, as it pertains to the Technician Class exam.

This is Part 5 of a 5 part series.  All of the posts in this series can be found by clicking  HERE.

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Every amateur radio station needs an antenna and Ham operators need  to understand the antenna  basics. Most operators start out in the hobby with a handheld radio that has a “rubber duck” antenna”. These antennas are adequate for close range but are generally inefficient. We described the term “wavelength” in a previous article. Each frequency has a specific wavelength and the radio’s antenna system must be matched to that wavelength in order to be efficient.  An antenna’s proper length will decrease as the transmitter’s frequency increases. For example, an antenna designed for 144 MHz is approximately 19″ and will be longer than an antenna designed for 450 MHz which is approximately 6″

There are many types of antennas, they all have different characteristics that make them desirable for different uses.

Horizontal Dipole Antennas are antennas with elements that are parallel to the ground or horizon. They are most often used by Ham operators that communicate long distances on the HF ham bands by “bouncing” the radio’s signal off the earth’s  ionosphere. Horizontal antennas are rarely used in the VHF/UHF   ham bands because ionospheric conditions are not usually favorable for these signals to be used in long distance communications.

Vertical Antennas usually have a single element that is perpendicular to the ground or horizon. These antennas are designed as “omni-directional” which means they radiate equally in all directions. VHF/UHF communications in the FM mode most often use vertical antennas.

Beam Antennas have multiple elements that concentrate the radio’s signal in one direction. These are “directional” antennas. Some types of beam antennas are called Yagi or Quad antennas.

Antenna “gain” is a term that is often misunderstood. Gain refers to how an antenna radiates the radio’s signal as compared to another antenna. An antenna cannot increase a radio’s power, it can only direct it. Also keep in mind that VHF and UHF signals are not going to travel much farther than the horizon. The higher the antenna, the farther away the signal will travel.

Coax cable is another important part of any radio station. It is  actually a part of the antenna system. Most coax used for VHF/UHF amatuer radio is approximately 50 ohms impedance. When an antenna and coax cable is attached to a transmitter, the system must be checked for SWR or Standing Wave Ratio. SWR is simply a measure of how efficient your antenna system is operating. An SWR meter is placed in line and the radio’s mic is keyed. The meter will measure how much energy is leaving  the antenna and how much energy is “reflected” or sent back to the radio. A perfect match is a ratio of 1:1. Too much reflected power will cause the radio to overheat.  This is especially critical if you are using a low-powered transceiver. Most radios will automatically reduce power if this ratio exceeds 2:1.

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SAFETY

There are some very important safety precautions that need to be understood when working with radio equipment. There will be questions on the test about this as well.

VHF and UHF energy is called “non-ionized radiation” and can be potentially dangerous. Just like your microwave oven cooks moist foods, the energy from a ham radio can heat the moist tissue in your body. In fact, I personally think this is why some of the old-timer hams that I know are nearly blind. Damage to the human body will occur if the frequency and signal strength combine to cause RF energy to be absorbed. The human body is vulnerable to some frequencies more than others. Other factors to consider are distance from the antenna and the antenna’s signal pattern.

RF (Radio Frequency) Exposure must be kept to a minimum. The FCC requires that the RF exposure must be measured at a VHF/UHF radio station if the station is emitting more than 50 watts of power at the antenna. A field strength meter is used to determine the exposure. There are computer modeling programs and charts available from the FCC to help determine safe RF levels as well. Radio transmitting power and frequency, antenna height and distance must all be considered when determining RF exposure.

Antenna and tower safety precautions are extremely important. Never install an antenna around power lines and be sure that it will be farther than 10 feet away from a power line if it should happen to fall.

Safe and practical installation procedures should be followed when installing your other electrical equipment too. Your equipment should be grounded to a common ground. Lightning protection should be used to prevent fires and to protect your equipment.

Some questions that you may find on the test:

Which of the following is a common use of coaxial cable?

  • A. Carry dc power from a vehicle battery to a mobile radio
  • B. Carry RF signals between a radio and antenna
  • C. Secure masts, tubing, and other cylindrical objects on towers
  • D. Connect data signals from a TNC to a computer

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What is a disadvantage of the “rubber duck” antenna supplied withmost handheld radio transceivers?

  • A. It does not transmit or receive as effectively as a full-sized antenna
  • B. It transmits a circularly polarized signal
  • C. If the rubber end cap is lost it will unravel very quickly
  • D. All of these choices are correct

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Why is it important to have a low SWR in an antenna system that uses coaxial cable feedline?

  • A. To reduce television interference
  • B. To allow the efficient transfer of power and reduce losses
  • C. To prolong antenna life
  • D. All of these choices are correct

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How does current flowing through the body cause a health hazard?

  • A. By heating tissue
  • B. It disrupts the electrical functions of cells
  • C. It causes involuntary muscle contractions
  • D. All of these choices are correct

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This series covers most of the information required to pass the Technician Class Exam. There are some basic electrical questions that may be on the test that we didn’t cover. I recommend that you take the practice exams at the links below and Google any electrical term that you don’t understand.

Practice Exams can be found at these sites:

You should take the actual test once you are consistently passing the practice exams. The sites above can also help you locate a club that can give you the test.

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