Antennas

1. Antennas • Once you get your license and that first radio, you’re going to need a good antenna to get your signal on the air. Antennas need not be expensive, but you do need to take some care in selecting the right antenna for the job.

2. Beam Antennas • A “beam antenna” is an antenna that concentrates signals in one direction. It is designed to focus all of the energy produced by your transmitter in the direction you want to work. Focusing your signal power in one direction makes for a stronger signal in that direction. Beams are effective, but depending on the bands covered and type, they can be expensive.

3. Beam Antennas - Quad • The quad, Yagi, and dish are all examples of beam antennas. A quad antenna looks something like a metal frame for a box kite. If you look closely, you can see the antenna wires supported by the “X” framework.

4. Beam Antennas - Yagi • The yagi is a one dimensional beam antenna consisting of several elements. It may be mounted horizontally, as shown here, or vertically.

5. Beam Antennas - Dish • Another beam antenna is the dish or parabolic reflector. It is often used to receive UHF signals or TV signals beamed from satellites, such as Dish Network ® antennas.

6. Vertical Antennas • A “vertical antenna” is an antenna that consists of a single element mounted perpendicular to the Earth's surface. Most mobile antennas are verticals. • Verticals usually require some sort of counterpoise to work their best. In a fixed station, a vertical may either be mounted on the ground or on a mast, and it may also have several radials for counterpoise. These radials may be laid out on the ground, as in the next slide, or mounted just underneath the vertical element, as in an elevated ground plane. • In a mobile installation, the metal body of the car usually serves as the counterpoise.

7. Typical Ground-Mounted Vertical • This is a rough diagram of a ground-mounted vertical. The orange radials you see may be laid along the top of the ground or buried just beneath the surface. Multi-band verticals are sometimes a good compromise between price and performance for a new ham.

8. Ground Plane Antenna • Here is a ground plane antenna – another type of vertical. It is designed to be mounted on a mast, and it usually has three or four radials coming from the base of the antenna.

9. Vertical Antenna Design • Many vertical antennas are designed to be equal in length to one-quarter wavelength of the desired operating frequency. • For a 1/4 wave vertical: • 234 • Length (feet) = ----------------- • Frequency (MHz) • You’ll need to remember this formula!

10. Sample Problem from the Question Pool • For example, suppose you want to know the approximate length, in inches, of a quarter-wavelength vertical antenna for 146 MHz. • Using the formula in the previous slide: • 234 • Length = ------ = 1.6 feet • 146 • To get inches, multiply 1.6 times 12 (since there are 12 inches in a foot) to get 19.2 or about 19 inches.

11. 5/8 Wave Verticals • Some vertical designs call for a 5/8 wave rather than a 1/4 wave. The advantage of 5/8 wavelength over 1/4 wavelength vertical antennas is that their radiation pattern concentrates energy at lower angles. (Radiation pattern describes the shape of the radiated signal, and a lower radiation angle usually means traveling a greater distance – better DX!)

12. Magnet Mount Verticals (Mag Mounts) • As already noted, verticals are very common in mobile installations. One type of antenna that offers good efficiency when operating mobile and can be easily installed or removed is the magnet mount vertical antenna. They are also fairly inexpensive and don’t require you to drill a hole in that new vehicle!

13. Horizontal Antennas • A “horizontal antenna” is an antenna that is a simple dipole mounted so the elements are parallel to the Earth's surface. So what’s a dipole?

14. Dipole Antennas • A dipole antenna is a simple antenna designed to work best on a single band. It consists of two sections that are each approximately one-quarter of the wavelength of that band, so that the total length is equal to about one-half wavelength. The transmission line from the radio is connected to this antenna in the middle of the two sections. It looks something like what you’ll see on the next slide.

15. Dipole Antenna • This is an example of a dipole antenna. Many hams getting on HF for the first time often start with a dipole. If you have the room for one, the dipole is cheap and easy to build.

16. More on Dipoles • Dipoles may be mounted either horizontally or vertically, depending on the intended use. Also, they may be made from wire or metal tubing, and are very easy for a new ham to construct. Wire dipoles are also fairly inexpensive and simple to design. With an antenna tuner, they can also be made to work on several bands. For these reasons, they are very popular with new hams on the HF bands.

17. Dipole Design • Since dipoles are fairly easy to build, it is important to know how to determine their total length. The formula for the length of a 1/2 wave dipole is: • 468 • Length (feet) = ----------------- • Frequency (MHz) • When designing a dipole, you should choose the lowest frequency for the band you want to work! • You will need to remember this formula!

18. Sample Problem from the Question Pool • What is the approximate length, in inches, of a 6-meter 1/2 wavelength wire dipole antenna? • Now this one can be done two ways. First, you are given the band wavelength already – 6 meters. This is approximate for the band, but it will do. Half of that wavelength is 3 meters. Since there are about 39 inches in a meter, that gives you an answer of 117 inches. The only answer close to that in the question pool question is 112 inches, so that’s the one you go with. But there is another way...

19. Using the Formula • To use the formula, you have to pick a frequency. In the U.S., the six meter band is 50 to 54 MHz. The lowest frequency is 50 MHz, so that’s the one you use. When you apply the formula, • 468 • Length (feet) = ----- = 9.36 feet • 50 • When you multiply 9.36 times 12 (to get inches), you get 112.32, which is a lot closer to 112. But remember, you use the lowest frequency in the band as your design frequency, or at least the lowest frequency you want to work.

20. Dipoles – Frequency Goes Up, Length Goes Down • The physical size of half-wave dipole antenna changes with operating frequency. It becomes shorter as the frequency increases. • As you might imagine, the opposite is also true. It becomes longer as the frequency decreases.

21. Rubber Ducky – Not for the Bathtub • For years, the little antenna that comes with most handheld radios has been called a “rubber duck” or “rubber ducky.” The main advantage of a rubber duck is its size. However, when range is important, the main disadvantage of the "rubber duck" antenna supplied with most hand held radio transceivers is that it simply does not transmit or receive as effectively as a full sized antenna. • If you need to increase the range of your handheld, you’ll have to connect it to a better antenna than the ducky!

22. Rubber Ducky – Not for the Car, either! • You really don’t want to try to use your "rubber duck" antenna inside your car because signals can be 10 to 20 times weaker than when you are outside of the vehicle. Much of the RF energy leaving your antenna will be absorbed by the metal in your car.

23. Dummy Load – Dummies Don’t Use Them! • You may remember that a dummy load is used when you are testing a transmitter. It absorbs the RF energy generated by your antenna so that the signals don’t interfere with stations operating on the frequency. The primary purpose of a dummy load is to avoid radiating interfering signals when making tests.

24. Check-Up Time! Now let’s try the questions from this group. You should make a note of any that you miss for later review.

25. T9A01 • What is a beam antenna? • A. An antenna built from metal I-beams • B. An antenna that transmits and receives equally well in all • directions • C. An antenna that concentrates signals in one direction • D. An antenna that reverses the phase of received signals

26. T9A01 Answer - C • A beam antenna radiates best in only one direction, concentrating the signal power in that direction. HINT: Think flashlight beam or laser beam.

27. T9A02 • What is an antenna that consists of a single element mounted perpendicular to the Earth's surface? • A. A conical monopole • B. A horizontal antenna • C. A vertical antenna • D. A traveling wave antenna

28. T9A02 Answer - C • Math whizzes will immediately recognize that perpendicular to the earth is vertical. The rest of us have to learn it now!

29. T9A03 • What type of antenna is a simple dipole mounted so the elements are parallel to the Earth's surface? • A. A ground wave antenna • B. A horizontal antenna • C. A rhombic antenna • D. A vertical antenna

30. T9A03 Answer - B • The math whizzes get another break. If the elements are parallel to the earth's surface, they must be horizontal.

31. T9A04 • What is a disadvantage of the "rubber duck" antenna supplied with most hand held radio transceivers? • A. It does not transmit or receive as effectively as a full sized antenna • B. It is much more expensive than a standard antenna • C. If the rubber end cap is lost it will unravel very quickly • D. It transmits a circular polarized signal

32. T9A04 Answer - A • The rubber duck antenna used on most handhelds is a compromise antenna. It is OK for casual operating, but there are times when it just isn't good enough. If you need the range, you are better off disconnecting the rubber duck and connecting a full size antenna.

33. T9A05 • How does the physical size of half-wave dipole antenna change with operating frequency? • A. It becomes longer as the frequency increases • B. It must be made larger because it has to handle more power • C. It becomes shorter as the frequency increases • D. It becomes shorter as the frequency decreases

34. T9A05 Answer - C • The length of the dipole is inversely related to frequency. As the frequency increases, the dipole length decreases.

35. T9A06 • What is the advantage of 5/8 wavelength over 1/4 wavelength vertical antennas? • A. They are easier to match to the feed line than other types • B. Their radiation pattern concentrates energy at lower angles • C. They pick up less noise • D. Their radiation pattern concentrates energy at higher angles

36. T9A06 Answer - B • The 5/8 wavelength antenna (or simply, "5/8 wave") has the advantage of a lower angle radiation pattern. Radiation at a lower angle is better, because it sends the signal closer to the ground, rather than up into the sky where it is not as likely to be heard.

37. T9A07 • What is the primary purpose of a dummy load? • A. It does not radiate interfering signals when making tests • B. It will prevent over-modulation of your transmitter • C. It keeps you from making mistakes while on the air • D. It is used for close in work to prevent overloads

38. T9A07 Answer - A • A dummy load is hooked up in the place of an antenna to allow an operator to test or tune a transmitter without radiating a signal that could interfere with other stations.

39. T9A08 • What type of antennas are the quad, Yagi, and dish? • A. Antennas invented after 1985 • B. Loop antennas • C. Directional or beam antennas • D. Antennas that are not permitted for amateur radio stations

40. T9A08 Answer - C • Each of these antennas - the quad, the yagi and the dish - are directional antennas. They are designed to radiate or receive signals best in one direction only.

41. T9A09 • What is one type of antenna that offers good efficiency when operating mobile and can be easily installed or removed? • A. A microwave antenna • B. A quad antenna • C. A traveling wave antenna • D. A magnet mount vertical antenna

42. T9A09 Answer - D • A magnet mount vertical antenna (also known as a "mag mount") uses a magnet at its base to hold it to the metal roof or trunk of a car. It can easily be removed for storage or use on another vehicle.

43. T9A10 • What is a good reason not to use a "rubber duck" antenna inside your car? • A. Signals can be 10 to 20 times weaker than when you are outside of the vehicle • B. RF energy trapped inside the vehicle can distort your signal • C. You might cause a fire in the vehicle upholstery • D. The SWR might increase

44. T9A10 Answer - A • Remember that a rubber duck antenna is a compromise antenna. It is small so that it can be portable, but it is not as efficient as a full size antenna. Also, when you are inside a car using the rubber duck, much of the signal is absorbed by the metal that surrounds you. It is much better to get a full size mobile antenna outside the vehicle where it will do the most good.

45. T9A11 • What is the approximate length, in inches, of a quarter-wavelength vertical antenna for 146 MHz? • A. 112 inches • B. 50 inches • C. 19 inches • D. 12 inches

46. T9A11 Answer - C • The formula for figuring the length of a quarter wave vertical is: • 234 • Length (feet) = ---------------- • Frequency in MHz • So...

47. T9A11 Answer - C • 234 • Length = --- = 1.625 feet • 144 • To convert that to inches, multiply 1.625 times 12 (there are 12 inches in a foot) to give you 19.5 inches. That is very close to the best answer of 19 inches.

48. T9A12 • What is the approximate length, in inches, of a 6-meter 1/2 wavelength wire dipole antenna? • A. 6 inches • B. 50 inches • C. 112 inches • D. 236 inches

49. T9A12 Answer - C • Here, the wavelength is 6 meters. A half wavelength is 3 meters. Since there are approximately 39 inches in a meter, 3 times 39 will give you 117, which is close to the best answer. • There is another way that will get you a little closer...

50. T9A12 Answer - C • The formula for determining the length of a half wave dipole in feet is: • 468 • Length (feet) = ---------------- • Frequency in MHz • To use this formula, you have to know that the lowest frequency of the six meter band is 50 Mhz. • 468 • Then... Length = --- = 9.36 feet • 50 • To convert to inches, multiply 9.2 times 12 to get 112.32 inches.