so then, does RF Tx inside a Non-grounded, 100% sealed, metal enclosure radiate out? like with Van De graaff

nsaspook

Joined Aug 27, 2009
13,087
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.
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.

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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nsaspook

Joined Aug 27, 2009
13,087
This is why an SWR of 1.5 or less is considered "good" acceptable resonance.
SWR is not really a condition of antenna resonance, it's a condition of energy movement and energy reflection in a transmission line. A properly matched traveling-wave antenna will have a low SWR on the transmission line just like a resonant antenna.

https://en.wikipedia.org/wiki/Standing_wave_ratio
https://en.wikipedia.org/wiki/Standing_wave_ratio#Practical_implications_of_SWR

https://en.wikipedia.org/wiki/Traveling-wave_antenna
 
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Thread Starter

genekuli

Joined Oct 21, 2018
113
Actual physical antenna resonance (a special condition but not a necessary condition for EM radiation) is not a much of a factor in the totally enclosed shielding effect just like it's not in the small gap leakage case. The leakage dimensions needs to be on the order of 1/50 of a wavelength for significant direct radiation escape from the crack but the skin effect of the conductors on the crack surfaces provides a transmission line short to the exposed outer surface for energy transfers across the RF nonconducting middle of the shielding.

https://palomar-engineers.com/non-resonant-vs-resonant-antennas
I notice that you are particularly knowledgeable in this field and who ever wrote the content of the article you linked sure understood this field of science/engineering. thanks for the info
 

Thread Starter

genekuli

Joined Oct 21, 2018
113
SWR is not really a condition of antenna resonance, it's a condition of energy movement and energy reflection in a transmission line. A properly matched traveling-wave antenna will have a low SWR on the transmission line just like a resonant antenna.

https://en.wikipedia.org/wiki/Standing_wave_ratio
https://en.wikipedia.org/wiki/Standing_wave_ratio#Practical_implications_of_SWR

https://en.wikipedia.org/wiki/Traveling-wave_antenna
so would that mean that with a high RF voltage that a normally nonmatching antenna can be made to match?
 

nsaspook

Joined Aug 27, 2009
13,087
so would that mean that with a high RF voltage that a normally nonmatching antenna can be made to match?
It depends on the total system, as what's possible and what's practical for Joe-six pack are two separate things. We had 2-30MHz antenna tuning systems that could combine (provide the required electrical match) the output of four separate several thousand watt transmitters (each in it's couplers frequency range) into one high voltage and low loss Heliax transmission line to a broadband fan antenna system.
https://www.rfparts.com/coax/heliaxcoax.html

sra56-31.JPG
AN/SRA-56
AN/SRA-57
AN/SRA-58
Multicoupler

https://www.globalsecurity.org/military/library/policy/navy/nrtc/14092_ch2.pdf
 
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SamR

Joined Mar 19, 2019
5,031
Properly matching the impedance of the connection between the transmitter and antenna allows maximum power transfer to the antenna. Having the antenna tuned to the Tx frequency further maximizes the transmitted power by reducing the SWR IF the impedance of the connection between the transmitter and antenna is matched giving 90+% antenna efficiency. It is a two-pronged effort. You need both impedance matching and tuning. When tuners started being used by Hams a gimmick was using one to "tune" a folding aluminum lawn chair to the Tx frequency. It was NOT efficient but you could transmit and receive doing so. Impedance matching is typically done with a Balun. A balanced to unbalanced converter composed of a fixed coil and capacitors to match impedance between feedline and antenna. In most cases for Hams it is for 50Ω impedance of the transmitter and 50Ω RG-8 coax connected by balun to an unbalanced impedance antenna.

See: antenna design (pdfdrive.com) for more in depth information
 
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nsaspook

Joined Aug 27, 2009
13,087
We used both type of antenna connection systems. Some used a proper transmission line match using a remote tuner (for proper electrical resonance using tunable LC reactance and transmission line impedance matching) at the base of the antenna but others used unmatched transmission lines often of several hundred feet designed to handle the high SWR generated voltages and currents as the energy moved down the line into the antenna. Matches like the SRA-5X series only provided the necessary electrical match between the short transmitter and match coax line, usually in the same room.

x1a.jpg
A typical transmission line cable.
http://www.kh6bb.org/photos1.html
The transmitter compartment termination of the 1 5/8" Heliax transmission line to the wire fan transmitting antenna, with a temporary connection to a patch cable back to FACCON 1. Large diameter Heliax cable was used for most transmitting antennas, not so much because of its low loss but because the SWR was generally high and so high voltage peaks were likely.
DSC02094.jpeg
 
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