Monday, 3 October 2016

Duplexer DVB-T TX filter:Technical details (Draft)

Duplexer DVB-T TX filter: Technical details

Introduction


Analysis


Bandpass is addition of two notches


 Three cavities RX to ANT

Reverseded notch TX to ANT

Sum of two notches with six cavities RX to TX

Two left hand couplings are conventional, the two right hand couplings, with plastic spacers are unusual, give reverse notch (not sure how).
The image shows internal construction of 4 cavity duplexer. Tuning is achieved by slug into the top of the probes, like a capacitive hat on an antenna. Tubes or probes are slightly less than a quarter wave length. Basically antennas in a box with couplings in and out. Ratio of tube diameter to outside diameter determines impedence. One to three gives about 50 Ohm.

SWR of Duplexer filter

7 MHz on 70 cm, not great but probably affected by tuning, possibly by coupling design.

Conclusion


Coupling for reversed notch needs further investigation


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