6" 2 metre Cavity Filter Conversion 


There are many 6" (19cm) diameter cavity filters available on the surplus market and they are of high quality.  Suitable for use on 2 metres, they provide at 600KHz separation in standard bandpass mode, about 16dB attenuation. The normal 4" cavity filter provides about 10dB attenuation 600KHz away.

However when used in notch mode, which requires the addition of a capacitor for high pass or an inductor for low pass, the difficulty is how to connect these components into the cavity filter.

The method described below worked well and provided notch depths in excess of 40dB.

6" cavity filter

Standard 6" VHF cavity filter

The "N" type connectors are mounted on the top of the cavity filter. These are not removed and a copper circuit box is constructed and mounted on the top of box with holes drilled in the bottom of the box to fit over the centre tuning assembly and the two "N" type connectors as shown below.

Box hole sizes

Hole sizes for bottom of copper box

The connection to the on top "N" type female connectors is done by using an "N"male centre pin or other suitable piece of wire.

Coax connection

Connection to "N" type female connector

There are two "N" type female connectors mounted on the copper box, one of which can be seen above. This is the connection to the outside World. These outside connectors are 2.25" (95mm) apart. For a 600 KHZ notch on the high side, an inductor length of  3.5" (13cm) is required but the connectors are wider than this. This was solved if you read below.

Box dimensions
Copper box dimensions

You will have noticed the copper circuit board soldered between the outside "N" type connectors. This 1 cm wide copper board has a break in the copper foil mid way between the two "N" type connectors.

break in copper foil
Break in copper straps

The high pass (notch on low side) use the standard shunt capacitor as shown above and the low pass (notch on high side) use the method below.

Increasing the Inductance

The length of the copper strip between the external "N" types matches the distance between the existing "N" types on the top of the cavity filter. My thoughts were that to have the same inductor length as the 4" cavity filter but as the top "N" types were closer together I decided to go with the shorter length and just add a couple of turns of wire to increase the Inductance to match the 3.5" (13cm) required for the low pass notch filter. However.........

In the process of adding the extra inductance in the middle between the connectors at one point I left in the shunt 10pF capacitor and I discovered that with the added inductance I could, by varying the shunt capacitor, adjust the notch point of the low pass cavity filter (the one that normally has the added inductor).

So the point is make the inductance longer and then use the capacitor to tune out some of this extra inductance and you have a means to fine tune the notch position on the low pass cavity.


Inductor and capacitor together in low pass (notch on high side)

The above shows how the introduction of a capacitor overcomes the extra length of inductance. Because the distance between the two connectors means the inductance is too high to place the notch the correct 600 KHz away (higher), I found the addition of the red wire and the capacitor allowed for the notch to be moved to the correct 600 KHz away.

The extra inductance (the red wire) is about 13cm long.

The top of the copper boxes have copper tops screwed on.

6" duplexer

The finished 6" duplexer.


The copper boxes are attached to the top of the cavity filters with small (5mm long) self tappers. I drilled carefully into the top of the cavity filter, but not all the way through, for the self tappers. The earth connection that the self tappers provide are most import as the earth connection between the copper box and the cavity filter can not be over stated. I used 4 self tappers spaced around the bottom of the box, but make sure two are as close to the out side World "N" types as possible.

self tappers

Self tappers location