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27 Antennas

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All about amateur radio Antennas This relates to Section 27 of the NZART Radio Syllabus and may be used to teach this section of the exam.

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27 Antennas

1. 1. Big Antennas = Big SignalsBig Antennas = Big Signals But you can still talk to the worldBut you can still talk to the world with small wire antennas.with small wire antennas. The principles of how antennas workThe principles of how antennas work antennas are all the same for allantennas are all the same for all antennasantennas  27 - Antennas27 - Antennas
2. 2. 18el 15M 21el 10M 15el 20M 80M 4SQ 8el 40M 160M Array K3LR – Antennas Contest Station AntennasContest Station Antennas
3. 3. A 4 Element 10 metre (28 Mhz) Yagi Antenna Note the “Balun” and the Coax Feedline. Which element does it feed ?(Connect to) Which Direction does the Signal Go?
4. 4. Wavelength, Frequency and Light • The speed of light is c = 3 x 108 metres per second, Knowing our operating frequency, we can derive the wavelength of a radio wave as follows: • Wavelength (in metres) = 300 divided by the frequency in MHz.. • A simple way to remember this is to remember 10 metres and 30 MHz, (to get the value of the constant, 300 !). • That gives a wavelength! The half-wavelength of a wave is half of the wavelength figure you obtain! • So a half-wavelength at 10 metres (30 MHz) will be 5 metres. The amateur 10 metre band is 28 to 29.7 MHz so a half-wavelength for that band will be a little longer than 5 metres. • Pick a frequency and calculate it!
5. 5. Dipole Antennas
6. 6. Half Wave Dipole Versions A version of the simple dipole is the “Folded Dipole” The impedance at the feed point is about 4 times that of a simple dipole (About 300 ohms) Another version of the dipole is the “Trap Dipole” The inside of the antenna to the feedpoint and the traps are at the higher frequency and the total length of the dipole including the traps is at the lowest frequency. You can therefore operate on two separate bands such as 80M and 40M and the feedpoint still looks like 72 ohms
7. 7. Vertical Antennas The simplest vertical is the Marconi - a quarter-wave radiator above a ground-plane. It has a feedpoint impedance over a perfect ground of 36 ohms. Above real ground it is usually between 50 and 75 ohm, a good match for 50 ohm cable. It is the smallest antenna with reasonable efficiency and is used for mobile communication. Vertical antennas require a good highly conductive ground. If the natural ground conductivity is poor, quarter-wave copper wire radials can be laid out from the base of the vertical to form a virtual ground. Vertical antennas provide an omni-directional pattern in the horizontal plane.
8. 8. “Yagi” Beam Antennas The Yagi-Uda antenna is designed to radiate in one direction with more gain than a dipole. The dipole “Driven element has a Reflector element 5% larger behind and several Parasitic Director elements in front of the dipole. These antennas have considerable gain (about 2 to 4 times the power) in one direction and usually are rotated to the direction of interest. Forward gain of 6 dB to 12 dBs are possible and the Front to back ratios often runs 25 dB to 35 dB. Very useful indeed!
9. 9. Height Above Ground The higher, the better is the word for antennas. But there is another advantage. The antenna tends to reflect some weird feedpoint impedances when it is positioned less than a half wave length above ground which often has an impact on your transmitter matching power into the antenna.
10. 10. Some Antenna Terminology The Dipole on the right shows maximum radiation of RF energy broadside to the axis of the wire. (The E-Plane pattern) The radiation of RF energy is like a doughnut broadside to the axis of the wire and very little off the end of the wire. (The H-Plane pattern which is perpendicular to the E-Plane)
11. 11. Some Antenna Terminology Isotropic Radiator Is a theoretical point-source of radio energy. It is a hypothetical antenna that will radiate equally well in all directions in all planes Horizontal Antennas radiate well in the broadside direction and also tend to radiate equally well vertically. Vertical Antennas radiate equally well in the horizontal direction and poorly in the vertical direction