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Author Topic: Antenna lengths...  (Read 312 times)
KC7JBV
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Posts: 11




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« on: April 21, 2001, 10:43:19 PM »

Can anyone explain to me the benefits (other than space) of using quarter or half wave antennas rather than full wave antennas?  Obviously there is a length issue here, a full wave 2 meter antenna will be roughly six feet long, a 6 meter antenna will be roughly 18 feet long and so on.  But what are the real benefits of using half wave and quarter wave antennas?  What are the downsides?  Would signal reception be compromised by anything less than a full wave?    Thanks for you help!  Jon
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W4BQF
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Posts: 73


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« Reply #1 on: April 21, 2001, 11:22:19 PM »

Take a look at the ARRL antenna books. You will see that the radiation pattern from a wire changes as you change the length. If you have a half wave dipole, the RF radiation is out the side(s) of the wire, in a figure 8 pattern. As you lengthen the dipole, the radiation pattern changes..a full wave length and the radiation pattern starts shifting towards the ends of the wire, to more like a clover leaf pattern. As you keep increasing the length of the wire, to 2 or 3 or 4 wavelengths, the radiation pattern begins to generate many more smaller radiation lobes, along with several narrow beamwidth, larger gain lobes. You can use this to your advantage when you know how the higher gain lobes radiate from longer wires. For instance, my 80 mtr dipole, whose wire runs north/south, puts out major gain lobes into the nw/ne/sw/se. An inexpensive, but effective means of obtaining gain with a wire antenna.
73  Tom/W4BQF
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AC5UP
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Posts: 3927




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« Reply #2 on: April 21, 2001, 11:32:30 PM »

Odd multiples of a half wavelength offer a low impedance match point at the center of an antenna. Even multiples of a half wave have a high impedance at their center, but this can be overcome by a 1/4 wave matching section or by moving the feed point off center. All resonant antennas are essentially some multiple of a half wave, the design considerations are usually chosen for mechanical practicality and/or matching to the feed line. Multiple-wave antennas do perform better, but on the lower bands they often become impractical.

If you're interested in learning more, visit http://www.cebik.com and check out the antenna primers and evaluation files. The first antenna you build will likely be your worst... They (and your understanding) will get better with practice!
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