Jeff's Hardline 6m Duplexer...

This page documents my construction of a set of WB5WPA-style 6-meter notch duplexer from sections of Andrew LDF semi-rigid line.

Please consult Jim's site for more information about the design. I'm really only trying to show what I did differently...

With that said, I used a bunch of Andrew LDF7-50 1 5/8" semi-rigid transmission line. I cut it to length based on the calculator on Jim's site. Instead of hollowing out the top end of the stubs, or making a fancy box for the tops of the stubs, I wound some copper strap around the tops of the stubs and soldered it to the shield with a small torch... I mounted the BNC connectors through this cylindar. When tuning was complete, I soldered a copper disc over the top to keep the bugs out and the RF in...

I was pretty sure that having a very low impedance connection to the lumped inductance of the stub was very important... So I tried to make sure that the short at the end of the stub had a very low impedance. I cut a disc of copper that fit inside the shield, with a hole for the center conductor. Then I notched and folded the shield over the disc, and soldered the whole thing with a mini-torch. I used a hole plug from The Home Depot to close up the end of the center conductor, to keep the bugs out.

View of bottom end of hardline filter.
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This is a close-up if the network inside of the top of the filter. Note the piston trimmer. This would probably not be a good thing (tm) for a high-power application, as the voltage could exceed the rating of the cap.
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This is a view of a high-side and a low-side filter. You can see that one filter uses a silver-mica capacitor to ground, and the other uses a small inductor.
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This is the response of a filter before adding the capacitor or inductor to adjust it for high- or low-side use.
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Frequency response of two high-side notch filters.
(This was at 1 MHz/div horizontal, and 10 dB/division vertical.)
This shows a notch depth of nearly 70dB!
With three filters, the notch was deeper than my instruments could detect.
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Here is the response curves of all 6 stubs.
Here is some DB-212 Info.

There is more to come.

N1KDO, 20061103
Made the old-fashioned way, with emacs.