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!

Scott
N0EDV

K7ITM wrote:


Also, if the goal is to kill harmonics of a
single narrow band, you may do better with notches at the harmonic
frequencies.

Cheers,
Tom

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

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

Offhand, I'd be tempted to use an interdigital filter. There was a nice
article about this, including a computer program in basic for the
caculations in Ham Radio magizine, Ummm... November 1985 (?).

I used this in a 2400 MHz filter I needed one time and I found the skirt
selectivity to be pretty good. Of course, like a lot of wavelength
related filters, it had harmonic responses, but a low pass filter fixed that
quite nicely.



Jim Pennell
N6BIU

--

21:05 Pacific Time Zone
Oct 20 2006

International Time
04:05 UTC
21.10.2006