There is a frequency region between around 600 MHz and 1500 MHz where the selfresonant frequencies of discrete
components make filter design very difficult, and where the physical dimensions of transmission lines and cavity
filters are too large for practical implementation.
One of the most prominent ISM bands (900 MHz) falls
squarely in the middle of the region. Thanks to the wireless revolution, there are a plethora of SAW and
dielectric filters available for the 840 – 980 MHz band, but that is about it, and they are only rated for
relatively low powers (maybe 20 dBm). Helical filters fill that gap nicely, and are a combination of all three
formats.
Like cavity filters, the Q of the helical resonators is very high if constructed properly. That is
because at the frequencies of operation, the skin thickness is getting very small and most of the current is
flowing on the surfaces. Plating the cavity walls and helix with a high conductivity material increases the Q even
more that bare of tinned copper.
Zverev was the first to publicly publish the equations for helical filter design (link to book on Amazon).
While the book goes into more detail, the basics of those equations are replicated here, and should serve as a
good enough springboard to get you going on a design. I actually worked with, but did not design, a multicavity
helical filter as part of a transmitter for a remote utility meter reading transponder back in the early 1990s,
produced by Itron. Choosing shield and helix sizes is largely an iterative task once you know what the resonant
frequency for each cavity will be.
Sorry,
but I do not have equations for the tuning slug’s diameter or how it affects the resonant frequency as it is
advanced into the helix. A wag would be that if the outside diameter of the tuning slug is very close to the
inside diameter of the helix, then the resonant frequency is modified as if the total height of the helix and
shield are shorter than when the tuning slug is fully retracted.
The graphs illustrate how the equations
are affected when the diameter of the helix, d, is varied. Notice that the wire diameter, g, increases with
frequency so that an adjustment of the helix diameter, d, is needed to keep the wire size and space between the
turns reasonable.
Check this out  someone reference this page on
Wikipedia!
