The WA2EBY article states that ECG-555 PIN diodes give poor UHF performance and that ordinary 1N4148 diodes are just as good for a Doppler antenna. Is that true?
First, The ECG-555 is an older PIN diode, with unspecified RF series resistance. Newer PIN diode types listed on the Hardware Sources page of this site should perform better. Second, the switcher circuit of the WA2EBY Doppler is not optimized for PIN diodes. For best isolation, RF diodes should be reverse-biased when OFF, not just at zero DC current. That’s because the series capacitance of PIN diodes drops dramatically as OFF (reverse) bias goes from 0 to 4 volts. (Examples: HP 5082-3080 goes from 1.1 to 0.4 pF. MPN3404 goes from 2.5 to 1.4 pF.) The lower the capacitance, the less RF gets through (better isolation). The Wide-Range Roanoke Antenna at this Homing In site reverse-biases the OFF diodes, which is an important reason why it performs better than the original Roanoke antenna. The WA2EBY Doppler circuit does not reverse-bias the OFF diodes. I believe that the results of the WA2EBY’s loss/isolation tests would have been quite different if it did. I discuss PIN versus switching/
