I settled on a 19.5% to 80.5% split for the short and long wire legs. I optimised for the low end of 80m because I’m mainly CW mode. I chose a bit low (3520kHz) knowing that it was easier to cut wire off if the resonant frequency was too low than add extra wire on.
As discussed in my last post, there are many factors in practise than can’t be quantified for the length determination, meaning that the length may need adjusting after SWR and X,R measurements are made once in situ (which is why I recently bought a Rig Expert AA-35 antenna analyzer).
The ½-wave dipole total length calculation gave 40.53m with the leg lengths 32.62m and 7.91m. I found it difficult to measure out the wire to better than about a few mm. In any case, the sag due to the weight of the wire (~100g for the long leg) must shorten the effective length.
Low visual impact was an important consideration.
Components:
• SOTAbeams antenna wire - PVC-covered, stranded copper wire, 0.22mm² total conductor CSA (approx. 24awg), velocity factor 0.97, weight approx 3.3g/m (colour brown – virtually invisible from the ground - mid grey would be even better against the sky)
• 4:1 current balun - Guanella, 2x FT240-43 ferrite cores, 400-W rating, waterproofed, 485g (small white box on white-painted wetdash finish of chimney stack)
• Coax feeder - 20m of 50Ω RG8X with PL259s (cable upper part spray-painted in disruptive pattern).
Antenna supports:
• Balun attached to bungalow chimney stack (approx. 7m high++)
• Short-leg wire (sloping) to nylon cord (approx. 1.5m long) to gutter bracket at corner of roof (end of antenna wire approx. 4.5m high)
• Long-leg wire to bungee cord (approx. 2m long) tied to multi-stranded nylon cord Halyard (dark green to blend with tree foliage). Halyard goes over branch of mature tree (approx. 15m high++). The halyard is used to raise or lower the bungee support cord for the far end of the long leg as one would for a flag or a sail. The bungee 1) gets the end of the long wire out of the branches and leaves and 2) prevents the wire being stressed as the tree sways about in strong winds.
++The long leg is roughly horizontal but the garden slopes downhill from the house to the large trees at the bottom of the plot, so the antenna wire gains height above the ground as it runs to the tree support.
The 20m of coax feeder runs vertically down from the balun to and briefly along the rain gutter then across to the shack (an outside building about 2m from the house) under its rain gutter and in near the door and then to the operating desk. The firm that makes the current balun says additional choking (loops of coax, ferrite rings etc) shouldn’t be necessary.