November 2003. During the first of my experiments with the cane prototype “Spiral Top Mk 1” it was very promising when only attached to the rear of the garage at about 3 Metres above ground. Unfortunately the cane construction weakened in the rain & was destroyed by the gale winds, so I didn't even get it up on the mast. My next work is going to be more ambitious than my first which finished up in much the same way as Alan & Finbar's.

The new construction, “Spiral Top Mk 2” is much stronger & I believe lighter, it has 2 double wound spirals whose diagonal lengths are 2 metres (the cane one was about 3 metres). The 1 Metre supports of PVC electrical conduit are mounted on a quad loop spider centre & rotated by 90 deg so that the spiral is parallel to the ground, each spiral has 28 turns of wire (14 on top of the conduit & 14 below) spaced at 50 mm. When the 2 double spirals are assembled the inductance is measured at 1.65 mH (whilst at ground level). The spiral assembly was completed & a horizontal fibreglass standoff arm mounted so as to separate the vertical antenna section from the mast supporting the spiral.

I was surprised to find that when installed on the mast at about 10 metres (35 feet) I had to place extra inductance at the base of the antenna to resonate the it on the 136 KHz band. It seems that due to the "Umbrella" type top load being at a steep angle & coming quite close to the ground that the top load capacitance was higher than with the spiral top load. I now believe that the steepness & length of the umbrella top load was restricting the efficiency & radiation of the vertical part of that antenna.

RF from my 300 watt class D transmitter (Design & kit by David Bowman G0MRF) was fed to the antenna via a matching transformer based on an article by Steve Rawlings - GW4ALG but using a scan coil yoke core from a computer monitor. The transformer seemed to be OK at first but as the matching appeared to improve the core started to get hot at power levels exceeding 200 watts DC input. Using my LF Tuning Meter (Design by Jim Moritz - M0BMU) the transformer was producing promising monitored results as the antenna current was in excess of 2 Amps. Looking back at this situation I do feel now that in light of my further work that the higher current was probably due to a mismatch & poor core material.