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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
Unread postPosted: Fri Jul 16, 2004 4:56 pm

Joined: Fri Jul 16, 2004 4:51 pm
Posts: 1

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.


Post subject:
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 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.


Post subject:
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.


Post subject:
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 device.


Kirt Blattenberger
Post subject: Coaxial Filters
Unread postPosted: Fri Jul 23, 2004 8:36 am
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Joined: Sun Aug 03, 2003 2:02 pm
Posts: 308
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


Post subject:
Unread 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 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 of coax.
Do you hear the BPL QRM? It jamms all bands, on 80m its peaking 59+

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