I'm attempting to tune an antenna with my NanoVNAv2.
I calibrated it using the included open/short/(50-ohm)load standards by screwing them directly onto the NanoVNA one by one and running its calibration function.
The antenna itself is a Hustler RM40, mounted to a MT-4 mount (the 24" short one), screwed into a 3/8-24 mirror mount bracket. The bracket is physically mounted to a 10 foot PVC pole, with the antenna itself passing through a (nylon) zippered opening in my patio's (fiberglass) roof screen. The metal mirror bracket itself is (conductively and electrically) connected to the aluminum pool enclosure frame (**see mechanical details below), providing both a feedpoint ground connection and radials.
Yes, I know this setup sucks. It's temporary, so I can start playing with my new radio during the month or so it's going to take me to get a proper antenna installed.
In theory, it should be mostly kosher. The screen enclosure's grounded aluminum frame provides a fairly large (albeit completely untuned) radial field. The mast, loading coil, and whip are all above and perpendicular to it. The problem I'm having is, no whip length I can discern will actually TUNE it (according to NanoVNA) for approximately 7.05MHz.
I know beyond doubt that this particular RM40 works on 40m... I used it last summer at my dad's house (in the middle of his yard, mounted to a ground rod driven into the earth, with a radial field that was asterisk-like, but still semi-random length). The SWR between 7.04MHz and 7.08MHz was well below 2.0:1, using various masts between 24 inches (the MT-4) and 17 feet (shorter masts had worse performance, but aside from requiring whip-length adjustments, didn't affect SWR nearly as much as I thought they would).
My best guess is, the untuned grid-like arrangement of my de-facto radial field (the screen enclosure frame) is still usable, but is skewing the feedpoint impedance enough to invalidate the NanoVNA's SWR readings (which blindly assume a 50-ohm feedpoint).
My assumption right now is that I might or might not have to build something to tweak the feedpoint impedance, but until I know what my actual feedpoint impedance even is, I'm flying blind.
So... how, exactly, does one go about accurately MEASURING feedpoint impedance for a given frequency? It seems like a chicken-egg problem... the only tool I presently own (NanoVNAv2) needs 50 ohms, but (AFAIK) can't itself determine whether the feedpoint it's looking at actually PRESENTS a 50-ohm impedance, or directly determine it.
I assume I'd also need a frequency source, since (AFAIK) impedance isn't constant, and depends upon the frequency you're measuring it at. I could swear I remember reading somewhere that NanoVNAv2 is itself capable of directly outputting a square wave at a frequency within range of its frequency generator. If not, I have a Si5351a on a breakout board, too.
The big million dollar question is... could a cheap (sub-$40) LCR-capable multimeter from Amazon be used in conjunction with a NanoVNAv2 or Si5351a+Arduino to reasonably measure the ACTUAL feedpoint impedance? Or would I still need other tools (possibly, tools that are cost-prohibitive) to do it?
(**) Mechanical details of connection to ground:
I drilled 4 holes in the screen enclosure's aluminum frame to match the hole pattern on the 3/8-24 mirror mount
I screwed 4 rare-earth donut-shaped magnets into the holes (one per hole) using flathead stainless-steel screws, and verified 0 ohms resistance between the screw heads and enclosure frame.
At the other end, I used stainless steel bolts with flathead machine screws and 4 more magnets facing in the opposite direction to attach the 3/8-24 mirror mount to a PVC pole. When brought together, the 4 magnets on the mount grab the 4 magnets on the enclosure frame, bringing the screw heads together and making what appears to be good electrical contact.
the 10 foot PVC pole serves two purposes... it enables me to take the antenna down and put it up without needing a ladder, and provides vertical support to the antenna (the magnets secure it horizontally)