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Antenna No 66


40m/80m link dipole


I wanted to use 80m on an upcoming SOTA activation thus requiring a wire dipole antenna.  Using the 40m/60m link dipole as base I decided to make an add on section for the 80m band.  This would give me flexibility to work 40m or 80m without the need of an ATU.


For my existing 40m and 60m link dipole the wire length was about 10% shorter than the "free space" length.  I calculated the required extension for 80m and cut the wire about 10% shorter than the "free space" length.  I left about 10cm extra as I would be cutting off small sections to ensure that the dipole would be resonant in the 80m phone section.



Initial measurements.


The first antenna measurement was promising: the center frequency was 3.590MHz with an SWR of 1.23;  there was an 110kHz section with SWR < 1.5 (3.535MHz to 3.645MHz) and an 200kHz section with SWR < 2.0 (3.490MHz to 3.590MHz).



I cut 100mm off the tip and measured again, the centre frequency had shifted up 40kHz to 3.630MHz, I cut a further 50mm off the tip, the center frequency had shifted up by 20kHz to 3.650Mhz.



Final measurements.


Final measurement: the center frequency was 3.650MHz with an SWR of 1.22;  there is an 100kHz section with SWR < 1.5 (3.595MHz to 3.695MHz) and an 200kHz section with SWR < 2.0 (3.545MHz to 3.745MHz).


I decided to leave the 80m wire dipole as such, no doubt the SWR curve would be further influenced by the terrain as the extension wire comes very near to the ground.



My SOTA link dipole: center section (below) is for 40m with extensions for 60m and 80m.


The final measurements are (wire length per dipole side): 40m = 9.65m, 80m extension = 8.88m.  The latter value was only 0.04m off from my calculated extension length. 


These dimensions work for my SOTA set-up in inverted-V configuration:  the dipole apex is at 6m (fiber mast) and the wire ends are supported at 1.0m above ground. 


I am using 4mm Hirschmann plugs + contraplugs to link the wire sections.




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