Transmission line filter for a field day station – designs

Bruce, VK4MQ, was canvassing ideas of a simple way to reduce second harmonics from a 40m field station interfering with operations on 20m at the same site.

A shunt OC stub of 90° electrical length was proposed to start thinking. My thoughts were that online experts often propose such as a cheap and effective solution… but I suspect they had read about it rather than speaking from actual experience.

The models and calculations assume that linear circuit theory applies, that the source is well represented by a Thevenin equivalent circuit with Zth=50+j0Ω. Most ham transmitters are not well represented by such a circuit, and the calculated results may not apply exactly. The calculated results should be observed when measuring with a good VNA.

Here is the problem

Above is a Simsmith model of a shunt stub in a linear matched 50Ω system. The stub achieves a reduction of more than 20dB over about 900kHz, and a maximum reduction of around 35dB at 14.2MHz.

But, it ruins the VSWR seen at G at 7.1MHz, VSWR is 2.6.

Expected InsertionLoss is 1.0dB.

This is why this is a pretty naive approach in a system that requires low InsertionVSWR for a transmitter.

This VSWR could be fixed with an ATU at the transmitter. So in this case, the length of the stub determines the notch frequency, and an ATU restores the load to suit the transmitter (as needed).

Two stubs are better than one

An option that has low insertion VSWR and better harmonic reduction uses two stubs.

Above is a Simsmith model of the two stub harmonic filter. It has better harmonic reduction at 14.2MHz, and input VSWR50 is low.

The length of T1 and T3 set the notch frequency, and the length of T2 is chosen of best input VSWR.

Expected InsertionLoss is 0.2dB.

7MHz pass 21MHz reject

Third harmonic more commonly a greater problem that second harmonic.

Above is a design for 7MHz pass and 21MHz reject.

Expected InsertionLoss is 0.1dB.

7MHz pass 14+21MHz reject

Above is a design for 7MHz pass and 21MHz reject.

Above is the full schematic.

Expected InsertionLoss is 0.4dB.


A follow up article will describe the implementation.