Anyone Know How a Coax Filter Works - RF Cafe Forums
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
I have 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.
Unread postPosted: 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 impedances.
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
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
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.
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 device.
Post subject: Coaxial Filters
Unread postPosted: Fri Jul 23, 2004 8:36 am
Joined: Sun Aug 03, 2003 2:02 pm
Location: Erie, PA
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
postPosted: Fri Jul 23, 2004 9:41 am
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
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.
are very good narrow filters made of coax.
Do you hear the BPL
QRM? It jamms all bands, on 80m its peaking 59+