2kW Low Pass Filter for SW Amps

Building the filter paths

This is an update for building the filters. The previous version works well, so if you have built the filter already there is absolutely no need to make any changes. This description is for building a new filter because it makes assembly easier.

The goal of this update is:

* avoid silver plated wire where possible, use enamelled copper wire.
* avoid the need for fine tuning the inductors. All filters are calculated with the un-tuned inductances.

Building low pass power filter banks:

1. measure each inductor with a reliable L measurement instrument after you wound the coils
2. measure the filter path with a spectrum analyser after assembly

The minimum requirement is to use an LC measurement instrument. Reliable and cheap instruments are available as kits from various sources.

This article describe how I built the filter and shows which results I got from the air coils and toroidal cores.

How to adjust the inductance of a core:

The inductors desribed here do not need any tuning (maybe except 15 or 10m band).

If tuning is needed it works like that:

The inductance of an air coil can be tunded by streching or compressing the length of the coil.

The inductance of a toroidal core can be tuned by compressing a couple of windungs in the middle of the core and streching the windings near the connections. This works very effective and gives a tuning range of > 10%.

Building tips for the toroidal cores:

* The wire should be tight on the core and the turns should be spaced evenly.
* Adjust the connections so they fit into the holes of the board (Distance: 20mm).
* count to number of windings going "through" the core. This is the correct number. It does not matter what the wire is doing outside of the core.
* Keep the connections short to avoid additional inductance caused by the connection wires.

Building the filter:

All the relais and the driver transistors and Rs for the relais are already mounted.

Click these links for detailed information about the various filter paths:

160m low pass filter

80m low pass filter

40m + 60m low pass filter

30m + 20m low pass filter

17m + 15m low pass filter

12m + 10m low pass filter

6m low pass filter