Mulletron:



In regards to your sense antenna port on the copper frustum, when not being used for taking VNA S21 frequency sweep plots I either leave our sense antenna as an open circuit, AKA in a high-Z state, or use it as a feedback control signal for our phase locked loop (PLL) circuit. If the latter, I still have to make the E-field sense antenna in the frustum very small, (like just the solder cup on an SMA bulkhead connector), so as to not overload the PLL's mixer input. Even with this small of an antenna, I still have to add at least 40 dB of 2W each 5-to-10 dB attenuator pads to the sense antenna's output to keep the PLL's mixer circuit from overloading. And the higher the Q-factor of the mode under study, the more sense antenna padding isolation that is required.



Best, Paul M.



That's good advice about making the sense antenna small. What I have right now, the sense antenna is as good at coupling energy out of the cavity as the feed antenna is at getting energy into the cavity. They are both the same and interchangeable.During my testing I noted that the terminated sense antenna port played a huge role on how much power was reflected back into the directional bridge connected to the frustum input port.Based on what I learned today from testing and from you, the sense antenna port should be impedance matched to whatever is driving the frustum and what is attached to the sense port, but it need not be (and shouldn't be) an efficient coupler of energy out of the cavity for the sake of the cavity storing as much energy as possible and for the sake of the test equipment attached. This lesson learned applies to during force testing with higher power levels.During low power VSWR testing and other tests, a matched input/sense port is ideal for an accurate test. In the next post, I'll show how I can efficiently couple energy into and out of the cavity (from sweep gen to power meter/spectrum analyzer) at certain frequencies and only lose a couple db of power in the round trip. Exact figures will be in that post.Based on what you're saying, I'm going to make another (hobbled) sense antenna, for now it will be just a solder cup. Then I will repeat the same VSWR test to see how things change.I will also do a VSWR test with NO sense antenna, the hole will be covered in conductive copper tape on both sides similar to what's in this shot here:I won't be working with much power so I have to be certain I don't waste even a milliwatt of power, so that's why I'm digging into the sense antenna and also making sure that I get the very lowest VSWR as possible on the input.