Tests of variable capacitors using Q meter including latest "0219" version Oren Elliott cap

Steve Ratzlaff

I received my Mike's Electronics latest version of the Oren Elliot-manufactured latest version "0219" N50-384P variable cap with vernier today.

There is a way to test the relative Q of a capacitor on the Q meter using a reference inductor. You use your best cap as the reference cap noting its Q at the various test frequencies. Then you test other caps and see how their Q's compare to the best cap. My best variable cap is the so-called "holy grail" TRW variable capacitor from the URM-25D signal generator--it has ceramic insulation, silver-plated plates, etc.  Xtal Set DXers prize this cap as the best. It has a 14-495 pF range, quite a bit higher max capacitance than the nominal 385 pF of the N50-384P. Of course one just sets the cap to the same 385 pF when testing. I have two older Oren Elliott caps, both unused, so with the latest one I have 3 of them to compare. I have a number of Russian caps with vernier tuning and ceramic insulation that used to be inexpensive and plentiful on eBay 5-10 years ago (in the $5-8 range)--now only one or two sellers offer them at much higher prices in the $25-30 range; and several unknown brand caps with ceramic insulation (no vernier). Mike's Electronics also offers a custom-made dual gang polyvaricon with about 320 pF per section; it does not have vernier tuning. (The URM-25D cap doesn't have built in vernier tuning either.)

I will say right now that the latest "0219" version cap does not test any better than the previous two Oren Elliott caps and actually tests somewhat poorer/lower Q. This is beginning to lend credence to Dave Aichelman's comments about these caps seem to deteriorate in selectivity/Q when used in FSLs that are exposed to cruddy conditions such as coastal salt air, and that cleaning the cap can restore it to "as-new" condition.

My tests are at three frequencies, near 540 kHz for the N50-384P max capacitance; 1000 kHz; 1700 kHz.


Steve AA7U

URM-25D variable cap ("holy grail" cap)

536 kHz 816Q     996 kHz 716 Q     1698 kHz 505Q

Small Russian dual gang cap with vernier, single gang tested

537 kHz 791Q     1000 kHz 695Q     1699 kHz 495Q

Larger Russian dual gang cap with vernier, single gang tested

538 kHz 780Q     1000 kHz 678Q     1701 kHz 464Q

Unknown brand dual gang cap with ceramic, no vernier, single gang tested

539 kHz 757Q     1000 kHz 660Q     1701 kHz 455Q

Mike's Electronics custom dual gang polyvaricon (about 320 pF max per gang),1 gang

(575 kHz 561Q)     1004 kHz 536Q     1702 kHz 409Q

N50-365P Oren Elliot cap with vernier (385pF)

538 kHz 604Q     999 kHz 503Q     1701 kHz 274Q

N50-384P version 0217 cap with vernier

538 kHz 614Q     1004 kHz 499Q     1700 kHz 276Q

N50-384P version 0219 (latest) cap with vernier (395pF measured)

532 kHz 555Q     999 kHz 386Q      1698 kHz 195Q

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