Resonance of Various Pipe Cap Filters

Pipe cap filters are one of the least expensive microwave filter design for ham applications that can be built at home without special tools.

There is an example of 2.2 GHz filter for a satellite receiver at my Pipe Cap page:

http://www.ko4bb.com/ham_radio/Pipe_Cap_Filters/

Here is some more information:

Knowing that anything inside a cavity will lower it's Q, I wanted to find a pipe cap as close to 2.2 GHz as possible to minimize the amount of screw intrusion needed to bring it down. On the other hand, if the cavity is too big, I won't be able to (easily) bring it's resonance up.

So I made a new PWB filter base and checked the pipe caps I had for resonance without any screw.

Here are the results.

The dimension is the inside diameter of the pipe cap, which is not the same as the standard pipe cap size. The standard pipe cap size (the one you see listed at the hardware store) is the inside dimension of the copper pipe that matches the pipe cap, so the pipe cap inside dimension is equal to the outside dimension of the copper tube plus a small gap for the solder to flow.

The first dimension is the standard dimension, the second dimension is the actual inside diameter.

Standard Size	Actual inside diameter	Diameter and depth in mm	Resonance frequency without screw
1 1/2"	1 5/8	41x31	5920 MHz
1 1/4"	1 3/8	35x27	6290 MHz
1"	1 1/8	28x21	7020 MHz
3/4"	7/8	22x19	10350 MHz
1/2"	5/8	16x13	not checked

I did not check the 1/2" cap because my probes were too far apart. I did not check the 2" cap that was at the hardware store because they wanted $7.50 for one, compared to $2.50 for the 1 1/2" cap.

Interestingly, it looks like the 3/4" cap is almost good for 10.368. I will try to file it and see if I can bring it up some. It would probably work better (higher Q) than the 1/2" at 10.368.

The probe size was the same for all the tests (0.25") and spacing (almost 3/4") which made it hard for the 3/4" cap. So I did not try to measure Q or insertion loss.

In any case, since the pipe caps were held with a C clamp, the Q was probably not very good anyhow.

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