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Cavity filters, What is it ?

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StoppTidigare

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Hi all I have attached a picture of some cavity filters 140-200MHz.
I've seen bigger units. Approx. 2 meter high, and about 30cm in diameter.
I beliveve they were 30-60MHz somewhere...

How do these monsters look inside anyway ?

regards,
StoppTidigare
 

They are coaxial transmission lines with the end toward the table top shorted outer to inner conductor. The knobs in the center top are on threaded bolts with capacitive plates near the center conductor end at the top. This tunes the resonant frequency. The input and output are either capacitive coupled to the center conductor or magnetically coupled with loops to the outer conductor of the coax circuit.
 

Hi again !
No djalli, the paper you uploaded are describes waveguide filters and microwave cavity filters...

The cavity filters talked about above work in much lower frequency.
The design principle can't be the same because of the much longer wavelength.....

regards,
StoppTidigare
 

Each cavity contains a quarter wavelength resonator. A coupling loop is connected to each coaxial connector.

As high Q is obtained, this is a common filter used for narrow band duplexers. Also used by hams for providing sufficient isolation between transmit and receive frequencies in a ham radio repeater, where the same antenna is used for simultaneous transmit and receive. See the link below for a 2 m repeater cavity filter diplexer application:

**broken link removed**

What's inside is shown here:

**broken link removed**
 

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