Agreed, PA3FWM has done a lot to benefit the community.
?
The other advantage of increasing the plate capacitance, is that the amplifier input impedance can be a lot lower. It doesn't seem sensible to me, to have a ridiculously high value of input impedance, just for the novelty of having a tiny antenna. It makes it much more susceptible to 50 / 60 Hz mains noise and interference, and also electrostatic charge build up. Many Non-Contact AC Voltage Detectors work on the same principle, to light an LED or sound a buzzer, so why use something with those characteristics as an antenna ?
?
Reducing the amplifier input impedance has two advantages (maybe more).
?
1. It rolls off the sensitivity on the LF bands, so that overload from MF broadcast stations is minimised. The Signal to Noise ratio on the VLF bands is hardly affected, as the natural noise floor is so high anyway.
?
2. It makes it easier to "choke off" unwanted common mode signals on the coax feed line. If the amplifier only has an input impedance of 100 K ohm, instead of 10M ohm, any choking impedance will be more effective. In many of my active whip designs, the amplifier only has an input impedance in the region of 1 or 2 K ohms at most, and I find this works well when using a 1m thin whip antenna, or anything that has a self capacitance of around 10 to 15pF.
?
Regards,
?
Martin
?
On Sat, Sep 28, 2024 at 05:16 AM, <biastee@...> wrote:
Thanks for the links. PA3FWM deserves accolades for summarizing this topic for us.
