Electronics > RF, Microwave, Ham Radio

Highest-Q 25pF-40pF variable capacitors for VHF?

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szoftveres:
Hi All,

What are the recommendations for high-Q variable capacitors in the 25pF-40pF range?
So far I've found the old style ceramic trimmer to work the best, but I'm curious if there are any other good options. Anything else I should try?

Thanks!

radiolistener:

--- Quote from: szoftveres on October 17, 2021, 07:51:04 am ---high-Q variable capacitors in the 25pF-40pF range?

--- End quote ---

КП1-4, 5-100pF, 25 kV vacuum variable capacitor:


it cost about 75 USD on internet.

Very good for magnetic loop antenna  :-+

Ups... didn't notice you're needs it for VHF, КП1-4 max frequency is 30 MHz...

David Hess:
Piston trimmers achieve Qs in the 1000s.

TimFox:
Piston trimmers are great.
Most have one terminal to the mounting bushing (normally grounded) but some have two leads for non-grounded operation.
More expensive ones have sapphire dielectric between the internal moving piston and the outside cylindrical electrode;  cheaper ones use glass.  Others are PTFE or air dielectric.
If this be a onesie (not manufacturing) project, Surplus Sales of Nebraska has a good selection of pistons.   https://www.surplussales.com/Variables/PistonTrimmers/index.html
For example, on the "high-Q" page, their catalog no. CTP5452 is a Johanson 1.0 to 16 pF panel-mount unit, air dielectric, with Q > 3000 at 100 MHz, for $18.99.
On the "standard" page is CTP MPT62004A from JFD, Q not specified, 0.8 to 18 pF for $6.00
Both of these have enough change in C to reach your 25 to 40 pF range with a suitable fixed capacitor.  Higher capacitance units are available there.

szoftveres:
Thanks all, the piston trimmer seems like a good choice. I'm building a coaxial type resonator filter for low power level on vhf.

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