Low pass filter design - Microstrip

Hello!

I am working on a Third Order equiripple Low pass filter design suitable for Microstrip ( Distributed structure). The cut-off frequency varies from 400-800 MHz and the range of capacitnace values varies from 1-7 pF for the following setup ::

SMA connectors at board edge to filter ports via 50 Ohm Microstrip,
unspecified length.

The Filter comprises of five transmission lines directly coupled
together.

Line 1: shunt, 64.9 Ohm, terminated in VarCap.
Line 2: series, 217.5 Ohm.
Line 3: shunt, 70.3 Ohm, terminated in VarCap.
Line 4: series, 217.5 Ohm.
Line 5: shunt 64.9 Ohm, terminated in VarCap.

All transmission line lengths are 2.5 cm.

A 'diagram' of layout is below:

Cap Cap Cap
* * *
* * *
* 1 *3 *5
* * *
********************************
Feed 2 4 Feed

I have to design this on Microwave Office. The problem is that these measurements were based on assuming the Er of the substrate as 2.2 I would like to know the following things :

1. If the Er of the substrate is changed to 4.3, how will the above values change? On what basis can they be recalculated?

2. How to find the length of the transmission line connecting the feed( with impedance 50 Ohms) to the shunt transmission line (1 & 5)?

Thanks in advance for your feedback.

Hello,

Use the TxLine -Transmission line Calculator to get the exact width & Length of the Microstrip lines for say Er=4.3 (with substrate height =0.8mm) & 64.9 ohm then the approax values are W=1.5 mm & lamda/8 electrical lenngth L=25 mm...
Follow the similar steps to calculate other lines...

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---manju---