868Mhz helical coil questions....

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

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....

s ?

it ?

overed

ve

to 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.