RF from my 300 watt class D transmitter 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 I could see that this type of matching was producing promising results as the current was in excess of 2 Amps. Looking back at this situation I feel that in light of the increase to higher power and antenna current the heating of the core was probably due to unsuitable core material and would not tolerate higher levels.
May 2004 It was suggested by Mal Hamilton - G3KEV that I should try adding a smaller diameter coil in place of the transformer, connect its bottom end to ground and it’s top end to the base of the main loading coil. I chose a PVC tube of 100mm diameter and first used a length of about 200mm but found that I was not able to obtain a match, I finally achieved best results by using the long smaller diameter coil as seen in the picture, you can also see the extra inductance added to the main coil after making “Spiral Top Mk2”. By tapping the feed on the small coil for matching, up the main coil for course resonance adjustment and using the variometer for fine adjustment, I was able to approach a much better match, the antenna current at this time was much lower (1.4 Amps). As the tapping points (matching & resonance) were adjusted and the results tested I was able to increase the antenna current to 1.8 Amps whilst improving the SWR along with increasing field strength shown by my simple Field Strength meter.
November 2004 Disaster struck due to my under-rated power supply mains transformer failing. Eventually the power supply was repaired and upgraded, a 10 Amp Variac auto-transformer was added to replace the under-rated 2 Amp type originally used to vary the 240 volt AC input to the 48 volt DC power supply which is used to adjust the transmitter PA output.