Monday, June 11, 2018

SIRIO 5/8 ground plane antenna for 40 MHz

The Italian firm SIRIO manufacture vertical antennas that cover 36 to 60 MHz. The TORNADO 35-42 model covers 35 MHz to 42 MHz which would tune nicely to the new 40 MHz (8 metre) band.

While SIRIO also make quarter-wave ground plane antennas which are roughly 1.8m long at 40 MHz, the TORNADO is a five-eight version and is quite a bit longer at roughly 5 metres.

While the 5/8 is generally a better performer with slightly more gain, the extra length may be an issue in a windy location where the antenna is more likely to get damaged (5m vs 2m in length). For general use and listening to the low VHF spectrum, the quarter wave may be a better choice.

The VSWR plot for the 42-50 model is shown below but it should be similar to the 36-42 model at 42 MHz.

The 1:1.5 VSWR bandwidth seems to be roughly 1.4 MHz
The 1:2.0 VSWR bandwidth seems to be roughly 2.5 MHz

Note that the quarter wave model is 2.5 MHz (1:1.5) and 4 MHz (1:2.0).

The one-eight loading coil is shown below to give the antenna a 50 ohm impedance at 40 MHz. Each of the four horizontal radials are 1.117m in length.

1)  SIRIO TORNADO 5/8 Ground Plane

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