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APRS ANTENNAS

APRS ANTENNAS. & TRANSMISSION LINES. SIMPLIFIED. by ED LAWRENCE. WA5SWD. Simple VHF or UHF Vertical Antenna. Typically 1/4 wavelength tall. Has an input impedance of about 36 . Normally fed with 50  Coax with ground conductor connected to earth ground. Best SWR 1.5:1

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APRS ANTENNAS

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  1. APRS ANTENNAS & TRANSMISSION LINES SIMPLIFIED by ED LAWRENCE WA5SWD

  2. Simple VHF or UHF Vertical Antenna • Typically 1/4 wavelength tall. • Has an input impedance of about 36 . • Normally fed with 50  Coax with ground conductor connected to earth ground. Best SWR 1.5:1 • Usually requires an array of radials to develop a better ground plane. (Marconi antenna) • For APRS uses it should be vertically polarized

  3. Monopole Antenna • ¼ wavelength fed at one end • Fed with unbalanced feedline with ground conductor connected to earth ground. • In practice it usually requires an array of radials to develop a better ground plane. (Marconi antenna) • When used at low frequencies the field should be vertically polarized and antenna could be a tower. • The tower is ground insulated and fed at a point above ground with a Gamma match. Z increases upward.

  4. APRS Ground Plane Antenna • Can use a COUNTERPOISE system of radials cut to ¼ wavelength to develop ground plane elevated above earth. • If used in a mobile application the roof of the vehicle can serve as a ground plane.

  5. CLASSIC COAX FED 1/4 WAVELENGTH MONOPOLE NOTE THE RADIALS AT RIGHT ANGLES EXPECT THE INPUT IMPEDANCE TO BE ABOUT 36 OHMS

  6. 5/8th wavelength Antenna • Application as a mobile or base station antenna.. • Omnidirectional response in horizontal plane. • Advantage is realized in the concentration of low angle radiation in horizontal direction. • Does not require as good a ground plane because feedpoint Z at 5/8th wavelength is higher therefore lower current. • Z is lowered to match 50 ohm feedline by matching section.

  7. Collinear Array • Two or more half wavelength sections. • A broadside array because the axes of the elements are along same line. • Half wave sections are linked by ¼ wave transmission lines. They develop a phase reversal to keep all dipoles in phase. • Usually vertical with an omnidirectional pattern in the horizontal plane with a narrow angle of radiation in the vertical. • What would be a good application for this system?

  8. THIS EXAMPLE IS A DIPOLE, NOT A MONOPOLE

  9. NOT your usual APRS antenna!

  10. 50  COAX USED AT 144.39 MHz • RG 58/U 6.8 dB loss/100 ft. • RG 58/U foam 4.7 dB loss/100 ft. • RG 8/U 3.2 dB loss/100 ft. • RG 8/U foam 2.2 dB loss/100 ft. • RG 8X 3.7 dB loss/100 ft. • RG 213 USE RG 8 VALUES

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