I work with RF resonance chambers (vacuum sealed enclosures with a tank coil and wall capacitance tuning) as low as 13.58MHz for linac accelerator systems. They are tuned for precise phase, voltage and waveform placement (to accelerate ion bunches) with very low reflected power >10 from a 3,000W amplifier per section. Standing Waves would ruin the RF phase to moving ion beam alignment so it's usually very low when the enclosure is properly tuned.I really can't even conceive of an enclosure archiving resonance. Since it can't and most probably is nowhere near resonance the SWR would have to be enormous with effective radiation near zero. Resonance is the Golden Ring for antennas and 100% actually near impossible if not truly impossible. As you fall away from that ideal the antenna effectiveness diminishes rapidly. Even with small bands of frequency for an antenna, tuners are used to improve matching and radiation effectiveness.
https://www.kb6nu.com/does-a-resonant-antenna-work-better-than-a-non-resonant-antenna/
Does a resonant antenna work better than a non-resonant antenna?
Like most things, 'it depends'. A physically resonant antenna is not a necessary condition for efficient operation. Resonance implies zero reactive power, but not necessarily a good match to the feedline, so matching of impedance is still needed to have optimal power transfer.
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