Single band dipole antenna - KORPI - · PDF fileSingle band dipole antenna CC-BY OH1AYR Rev...

Coaxial Dipole, Double Bazooka Antenna Single band dipole antenna CC-BY OH1AYR Rev 1.1 Date 16.02.2015 Antenna Coaxial Dipole, or Double Bazooka, is a single band RX|TX antenna with fair band width. It tunes and radiates like normal half-wave dipole; it works as flat-top, V, inverted-V, sloper, NVIS... The stub structure eliminates most of the wide band urban noise and harmonic radiation. Antenna works without tuner... easy field trip antenna. Antenna is heavy; on low bands it may need nylon rope support, especially with permanent installations. Theory W8TV (QST) writes: "Antenna consists of a half-wavelength section of coaxial line L2 with the outer conductor, screen, opened at the center and the feed applied to the open ends. The outside of the coaxial thus operates as a half-wave dipole. The inside sections, which do not radiate, are quarter-wave shorted stubs, which present a very high resistive impedance to the feed point at resonance." The L2 section is cut according the velocity factor of the coaxial. However, the velocity factor does not apply to the outside of the cable. So it is necessary to build out each end of the antenna to the necessary overall length L1. These L3 sections may be the same type coaxial or open feed line... we try to get low Q-value for the radiator. Tune the resonance symmetrically with both L3 sections. Typical SWR on 40m test antenna Material RG-58C/U coaxial, style inverted-V. Top height 10 m, wire ends at 3 m. Measured with MiniVNA Pro analyzer. SWR < 1.5 @ 290 kHz SWR < 2.0 @ 580 kHz Dimensions With velocity factor of 0.66, RG-58C/U, RG-213/U, we get following metric dimensions for antenna elements: L1[m] = ((300/f[MHz])/2)*0.95 L2[m] = ((300/f[MHz])/2)*0.66 L3[m] = (L1-L2)/2 Formulas: Karl Rothammel Y21BK Tune symmetrically with both L3 sections. MB MHz L1 L2 L2/2 L3 80m 3.700 38.51 26.76 13.38 5.87 40m 7.050 20.21 14.04 7.02 3.09 30m 10.140 14.19 9.86 4.93 2.17 20m 14.100 10.11 7.02 3.51 1.55 17m 18.100 7.87 5.47 2.74 1.20 15m 21.100 6.75 4.69 2.35 1.03 12m 24.900 5.72 3.98 1.99 0.87 10m 28.200 5.05 3.51 1.76 0.77 CC-BY OH1AYR Auvo Korpi www.korpi.biz [ ] PDF file may include certificate, root: www.cacert.org HC 16.02.2015 Page 1|2

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Transcript of Single band dipole antenna - KORPI - · PDF fileSingle band dipole antenna CC-BY OH1AYR Rev...

Coaxial Dipole, Double Bazooka Antenna

Single band dipole antenna

CC-BY OH1AYR Rev 1.1 Date 16.02.2015

Antenna

Coaxial Dipole, or Double Bazooka, is a single band RX|TX antennawith fair band width. It tunes and radiates like normal half-wavedipole; it works as flat-top, V, inverted-V, sloper, NVIS... The stub structure eliminates most of the wide band urban noiseand harmonic radiation. Antenna works without tuner... easy fieldtrip antenna. Antenna is heavy; on low bands it may need nylonrope support, especially with permanent installations.

Theory

W8TV (QST) writes:"Antenna consists of a half-wavelength section of coaxial line L2with the outer conductor, screen, opened at the center and thefeed applied to the open ends. The outside of the coaxial thusoperates as a half-wave dipole. The inside sections, which do notradiate, are quarter-wave shorted stubs, which present a very highresistive impedance to the feed point at resonance."

The L2 section is cut according the velocity factor of the coaxial.However, the velocity factor does not apply to the outside of thecable. So it is necessary to build out each end of the antenna to thenecessary overall length L1. These L3 sections may be the sametype coaxial or open feed line... we try to get low Q-value for theradiator. Tune the resonance symmetrically with both L3 sections.

Typical SWR on 40m test antenna

Material RG-58C/U coaxial, style inverted-V. Top height 10 m, wire ends at 3 m. Measured with MiniVNA Pro analyzer.

SWR < 1.5 @ 290 kHz

SWR < 2.0 @ 580 kHz

Dimensions

With velocity factor of 0.66, RG-58C/U, RG-213/U, we get followingmetric dimensions for antenna elements: L1[m] = ((300/f[MHz])/2)*0.95

L2[m] = ((300/f[MHz])/2)*0.66

L3[m] = (L1-L2)/2

Formulas: Karl Rothammel Y21BKTune symmetrically with both L3 sections.

MB MHz L1 L2 L2/2 L3

80m 3.700 38.51 26.76 13.38 5.87

40m 7.050 20.21 14.04 7.02 3.09