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Anyone know how a coax filter works - RF Cafe Forums
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Post subject: Anyone know how a coax filter works
postPosted: Fri Jul 16, 2004 4:56 pm
Joined: Fri Jul 16, 2004 4:51 pm
a device that is a microwave filter, the manufaturer says it is a coax
device. How does a coax cable get a sharp enough cutoff to be used as
a filter? Does it have to do with TEM wave modes?
Thanks in advance.
Tue Jul 20, 2004 1:38 pm
Coaxial filters are basically cascades
of differing lengths of differing characteristic impedances. They're
usually made up of a constant diameter outside shield and lengths of
differing diameter center conductor to get the different characteristic
The (differing) lengths allow a segment of the line
to appear capacitive or inductive, depending on its length and its termination.
(Think Smith Chart!) The differing characteristic impedances cause the
mismatch reflections which the line segment lengths then transform into
capacitive or inductive reactance, out of which the filter is built.
That's a bit hand-wavy, but one of Kirt's "books of the day" a week
or so ago goes into some depth if you really need to know.
Unread postPosted: Tue
Jul 20, 2004 1:51 pm
Thanks!! Where can I find Kirt's book of
the day? How does coaxial compare to waveguide (interdigital) filter?
I would think that a waveguide filter would have a better VSWR, loss,
and steeper cutoff than a coax device.
Unread postPosted: Tue Jul 20, 2004 4:00 pm
Thanks!! Where can I find Kirt's book of the day? How does coaxial
compare to waveguide (interdigital) filter? I would think that a waveguide
filter would have a better VSWR, loss, and steeper cutoff than a coax
Unread postPosted: Fri Jul 23, 2004 8:36 am
Joined: Sun Aug 03, 2003 2:02 pm
Location: Erie, PA
I believe the
book being referred to is "Theory and Design of Microwave Filters,"
by Ian Hunter.
When the vendor said the filter is a coaxial device, did he/she/it
mean built of multiple lengths of coaxial cable, or is it a tubular
filter like what K&L, Lorch, Salisbury Engineering, etc., sell?
- Kirt Blattenberger :smt024
Unread postPosted: Fri Jul 23, 2004 9:41
A coax filter is nothing more than a filter made with series
L and shunt C elements, except the elements are made with coax. Remember
a low impedance coax (think fat center conductor) that is short vs a
wavelength looks like a capacitor to ground and a high impedance (think
skinny center conductor) piece looks like an inductor. So, in a coax
low pass filter, you'd find alternating fat and skinny center conductor
pieces that make up the elements. They work quite well and are super
inexpensive once you get the initial design figured out. Most always
they are setup to be low pass filters because of the topology C,L,C.
One drawback is a coax filter can have a pretty severe flyback area
(usually about 3x the cutoff frequency) where the rejection isnt so
great. Waveguide and interdigital filters dont usually have this problem,
but are more expensive and harder to get built right and tuned.
Post subject: W6
Unread postPosted: Fri Jul 30, 2004 5:58 am
it could be whatever.
Take it apart. Some filters use shiny expensive box and you will find
cheap dicrete L C inside.
There are very good narrow filters made
Do you hear the BPL QRM? It jamms all bands, on 80m its