Yes, that's all a helical filter appears to be...a coil inside a metal
box. I have no idea how many turns, turn spacing and anything else that
might be important to the design of one. Fortunately, Downeast
Microwave sells a bunch of different helicals for a LOT less than
Digikey, but I need the 1305 MHz set and I don't see them at DEMI, so
I'll have to fork out $26 each from Digikey (and probably will end up
using two) ;(

Scott
N0EDV




Roy Lewallen wrote:
Scott wrote:

That's exactly what I'm trying on my present project at 902. I have a
50 microstrip line on the circuit board where the RF connector hooks
up. I will etch it to be a 1/4 wavelength at 1802 MHz (2nd harmonic)
and see how that works. If it works OK, I may try the same at
1296...I might also go with a helical filter...they work nice but can
be a bit pricey!


Isn't it possible to make a helical resonator with a helix of wire
inside a box made from soldered-together pieces of PC board?(*) A filter
could be made from a series of these. The Q might not be optimum, but
should be good enough for a lot of purposes.

(*) I actually made a two-resonator filter for 450 MHz using this method
long ago, but it didn't work well. I came across it recently and checked
it with a network analyzer -- I had no decent test equipment back when I
made it -- and found that the sections were severely undercoupled. But I
haven't tried modifying it to see if it could have been made to work as
intended.

Roy Lewallen, W7EL