Hello KV5R, sure. Good questions and good points. I'll try to go through your note step by step question with a response.
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Rather than constructing, would something like an inexpensive "QRP Antenna Tuner" be a useful accessory for the nano? It's just a stepped inductor and air-variable capacitor, with box and connectors, with a moderate impedance matching range.
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YES. AGREE. However, the motivation in building your own is to use the VNA in making certain that the self resonate frequency of each of the matching components are WELL above the application or TEST frequency. Otherwise you are dealing with a frequency dependent L or C. Ideally the L or C maintains a C value or an L value that over its entire variable range is spot on the same value, say at 1 MHz or at 30 MHz! Not all L's and C's behave in that manner! Particularly LARGE HV variable caps have a pretty significant parasitic inductance. Hence their C value will shift quite a bit with frequency despite, you never mechanical change their value! Now normally we don't care about this affect, but if were trying to use the matching network as an instrument, then we may need to take care about this subtle aspect.
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Understood. But I think a more typical use-case would be measuring at the end of the feed-line (typically low-loss open-wire parallel "ladder line"), as for high-Z non-resonant antennas, the feed-point impedance is rarely meaningful, and the transmission line impedance varies with length and frequency. I suppose one might measure it at the feed-point if the desire is to design a tuning stub or other Z-match device at the feed-point.
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YES! I tend to look right at the antenna feed point. Yes, the series lines and stubs will further modify this Z. But that is part of the MATCHING PROBLEM. Our first challenge is to find the accurate Z at the antenna driving point. So that's where the little high performance matchbox tuner comes in. If we adjust this tuner to obtain a super return loss at its 50 ohm port where our VNA is connected, then the TUNER contains the information we seek, namely the antenna impedance!
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I'm not understanding how to connect and use the nano to determine the L and C values of this little L-network. I assumed one would need an LC meter for that.
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WELL, you are not. However, you are using the VNA to assist in the construction of this well performing tuner whose readout of C and L values (assume a simple L network) are ACCURATE. So you need to carefully characterize the C and L's used in the tuner construction to eliminate as much parasitic from their design. Remember, we are not designing a tuner to handle any power! We are designing a tuner to have stable impedance characteristics. Hence small array of C's or piston caps and an array of tapped air wound L's would be suitable. Recall, the tuner is being driver by the nanovna which has feeble power!
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Knowing the L and C values required to achieve the good return loss permits you to calculate the antenna Z.
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WHEN the L and C's of the tuner are set to produce a high performance return loss as measured by the vna, then in essence, if the tuner were terminated (where the vna was positioned) with 50 ohms and we were to look into the TUNER where the antenna was connected, we would see the ANTENNA Z CONJUGATE. Wow, that's a mouth full. The best was to see this is to do an example problem and a simulator like LT Spice is a nice tool to learn. Or there are other SMITH GRAPHIC programs that are quite helpful to assist in this process. Standby and I will see what I can assemble.
Alan
