|The 18m glassfibre pole from SPIDERBEAM is used, but without the top section (too fragile to support the weight / wind load of the 80m trap coil). The antenna itself is composed of 12m of heavy flexible insulated copper wire 4mm˛ section + 7m of same but 2,5 mm˛ on the top. This (about) 19m wire is tightly wound around the pole (pitch about 1 turn per 2m). The same base as for the 80m full size GPA is used, with 96 radials of about 10m length.|
|The 80m coax trap coil. This is wound on a 80mm dia PVC light-weight pipe, about 9,8 turns required. I first made one with RG-58, but had a flash-over between the insulation and a terminal bolt (800w PEP). Second version with RG-400 (Teflon Coax) was a lot more reliable, but somewhat heavier in weight. The 'L' section is about 15m50 long, made of (light weight) 0,75mm˛ PVC insulated flexible copper wire. I affixed this to the roof of the house, about 10m high and 25m from the GPA (so it is slanted). It makes no sense to give you the exact dimensions, as these will vary in function of the trap construction and it's resonance frequency, and the possible slant of the 'L' leg.|
As a GPA has a low feed resistance (about 35R at 1/4 wave, about 20R as shortened on 160m), the antenna is fed through a 1 to 2,25 balun to feed it by 50R coax. The balun is wound (5 or 6 turns bifilar) on a large 43 material ferrite core, here with RG-58 (no problems with 800w), but better would be RG-400 (this coax is rated at 5 kW). The result is a very flat SWR curve and 1:1 at resonance. You can find details how to construct this balun on the valuable site of ON9CVD Bob This balun is normally used to step up impedance from 50R to 112.5R, but can be used to step down as well !