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Microwave L.O. Filters

Here's another, more compact filter that's fairly easy to make. I designed this one to filter the output of my microwave local oscillators, but one could also use this type of filter on the output of preamplifiers to limit out-of-band responses in the receiver.

It's tunable from about 1000 to 1200 MHz, and by changing the length of the resonators or tuning screws, can cover other frequency ranges as well. Most of the L.O. frequencies required by the DEMI and DB6NT transverters are in this range.

No connectors are needed, but one could use them if desired; I just soldered RG-316 Teflon coax directly into the sides of the enclosure.

This 2-pole filter is a mere 1.25 x 2 x .625" high; the box is made from inexpensive .008 tin sheet, available from most hobby stores. You'll also need about 4 inches of .141 micro-coax, and a couple of 2-56 brass nuts and machine screws.

Here's the response with the filter tuned to 1104 MHz, the L.O. frequency required by my 3456 transverter.

The local oscillator used with this filter begins with a 184 MHz crystal oscillator.

This drives a harmonic comb generator, and the filter picks off the 6th harmonic at 1104.

All of the other harmonics are down at least 45 db after passing through.

This is more than enough, considering the additional filtering that is normally part of a transverter's multiplier chain.


Here's a closer look at the close-in response. This is the output of a microwave crystal detector, as viewed on an oscilloscope; the horizontal resolution is 40 MHz/div, centered on 1100 MHz.  Vertical resolution is unknown, and unimportant for this measurement.

I use this test setup to adjust the coupling between the resonators; too much coupling, and you get a double-peak; too little, and the insertion loss is too high. The idea is to set the coupling just beyond the point where the double-peak disappears.

The size of the shield between the resonators (shown in later photos) is used to adjust the coupling. If you stick with the dimensions shown below, you won't have to worry about adjusting this parameter.

Looking inside (with the lid removed), you can see how simple the construction really is.
  • The input coupling loops are formed using the center conductor of the RG-316 coax.
  • The coupling shield is soldered to the floor and wall of the enclosure at the resonator base wall.
  • The 2-56 nuts are soldered over holes drilled in the opposite wall.
  • The 2-56 tuning screws pass through the nuts, and thread themselves into the Teflon insulation inside the resonators.
  • The resonators are soldered into the base wall, and supported on the opposite side by the tuning screws.
For the resonators, I used two 1.875" long pieces of UT-141 micro-coax.

Start with a 2.25" piece, and trim .375 off of the outer shield and insulation on one end, leaving the center conductor exposed.

Next, secure the coax in a vice (do not crush it), and using a pair of pliers, pull out the center conductor.

Finally, on one end, widen the hole in the Teflon insulation using a 2-56 tap drill (#51). Drill into the Teflon about 1/2 inch. This end will receive the adjustment screw, which will self-tap into the Teflon.

Next, form the enclosure from a piece of tin sheet according to the drawing and picture shown on the right.

I form the box by hand, using nothing more than a small vice and a couple of aluminum scraps cut to the correct length to form a bending guide.
Just use your fingers, and fold the sides over the guide.
When you have all four sides bent, it should look like this.
Using your fingers, bend the corner ears into place.

Solder them to the side walls. I find it helpful to use a small pair of forceps to hold the joint closed while soldering.

This picture shows them bent into place prior to soldering.

Next, cut a .5 x 1" piece of tin sheet for the coupling shield, and solder it in place as shown.


Using a #27 drill, drill holes in the base wall for the resonators.

Insert the base end of the resonators, and solder in place.

On the opposite wall, drill clearance holes for the 2-56 tuning screws. Solder the 2-56 nuts over these holes.

Screw the machine screws through the nuts and thread them into the Teflon insulation of the resonators.

Using a #44 drill, drill the holes for the RG-316 coax 12mm up from the base wall of the box.

Strip the RG-316 as shown, pass the shield through the hole and solder in place. Solder the center conductors to the base wall.

The center conductor (coupling loop) should be spaced 2mm away from the resonator.

Make the top cover using the dimensions shown here.


Form the cover using the same techniques described for the box.

The cover is designed to fit into (not over) the top of the filter box.

With the cover in place, test your filter.

If all is OK, solder the cover permanently into place.


Here's some info on the tin sheet I used; I bought this at a local ACE hardware store, and I've seen it in well-stocked hobby stores as well.

Another material that should work well is plain old PC board, just cut to proper dimension and soldered together on the inside. The top cover should probably be brass, copper or tin sheet, though.