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

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

Coaxial Dipole, Double Bazooka Antenna

Single band dipole antenna

CC-BY OH1AYR Rev 1.1 Date 16.02.2015

Test antenna details

Center box, before isolation with hot glue

Stub ends

Stub connections

End isolation

CC-BY OH1AYR Auvo Korpi www.korpi.biz [ ] PDF file may include certificate, root: www.cacert.org HC 16.02.2015 Page 2|2