I have some data to make one to try out.... 6mm dia, 28 mm long but.... Does it matter what the spacing between the coils is, i.e. no of turns ? Is the effective 'length' (pi x Dia x no of turns) = 1/4 wave ? If I 'pot it' with hot melt glue, to make it 'rigid' will that screw it ? Can I use any solid wire? 18swg TCW ? Or does it have to be enammel covered ? TIA
A helical resonator is essentially a length of slow-wave transmission line, much like a coaxial stub. The helix is what makes it a slow-wave structure, i.e. the physical length is short compared with the electrical length. (Resonators also have much weaker coupling to the external circuit than stubs.)
Filling a helical resonator with *lossless* dielectric would reduce its characteristic impedance and make the slow wave even slower--so the resonant frequency would go down. It would also affect the Q by changing the coefficient of coupling into the resonator.
Part of the point of helical resonators is to get high Q, which makes them very sensitive to dielectric loss. Air is a good candidate for the job, but most kinds of wire insulation are poor--using enamel makes good sense because it protects the wire from oxidizing (which increases the loss) with a minimum of dielectric.
I don't know the exact dielectric properties of hot melt, but I doubt they're all that good. Another problem will be the temperature coefficient, which will get much, much worse if you pot it.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
out.... 6mm dia, 28 mm long but....
For sticking things together use "Q-Dope". It is made for RF.
y out.... 6mm dia, 28 mm long but....
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dto the op... check out this book
Mark
Yes. Like Mr. Hobbs said, it's a slow wave structure, and the coil geometry (including turn spacing and wire diameter) affect the speed of the wave along the coil.
Probably close, but it could be 1/2.
Almost certainly. I doubt that hot melt glue is a really high Q dielectric, and it'll change your resonant frequency.
Uninsulated is probably best. Your wire insulation is a dielectric, see my answer to your hot glue question.
-- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
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