Building and Testing 50 MHz Traps for Antenna Experiments

This article details the construction and testing of 50 MHz traps for a new antenna experiment. The author discusses the challenge of finding a suitable high-voltage capacitor for trap applications and explores three potential solutions: doorknob capacitors, double-sided PCB capacitors, and coaxial cable traps. Doorknob capacitors were discarded due to cost and size. The author successfully constructed a capacitor using 1.6mm thick double-sided FR-4, achieving 2.6 pF/cm2. Using the VE6YP calculator, L and C values of 30 pF and 0.31 uH were determined for a 2cm diameter coil. Both FR-4 and RG-58 coaxial cable traps were built and tuned to approximately 50 MHz using a spectrum analyzer. The coaxial cable trap exhibited a significantly deeper notch (nearly 20dB deeper) compared to the FR-4 based trap. The author plans to use the coaxial cable trap for an experimental antenna operating at no more than 100 watts.