Electronics > RF, Microwave, Ham Radio

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

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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?


Depends on your intentions and requirements.

One obvious possibility is to divide the capacitance into a fixed part (e.g. 25pF) and in your case a somewhat smaller variable part (15pF) which is e.g. in parallel with the fixed part.  Whether that helps you achieve higher Q and stability and range constraint / mechanical improvement depends on your application.

You're not going to get much higher Q than just having two high quality areas of copper which have a mechanically variable degree of overlap.  The butterfly type variable capacitors do that with paralleled stacked plates between a rotor and a stator with air in between the rotor plates and stator plates. 

For "only" 15-40pF range capacitance the necessary plate area for a DIY variable capacitor is not huge if you keep the insulating gap between them well under 1mm for instance (e.g. 100um, 10um, whatever).  Whether that's enough clearance or too large overall depends on the frequency, voltage, size constraints, whether this is a production circuit or an bench experiment etc.

But for a DIY variable capacitor just two PCBs laid on top of each other with variable overlap and a thin plastic film separating the plates would certainly work and have relatively high Q at some frequencies / power levels if you keep the terminals wide and the sheet resistance of the copper foil is relatively low compared to your losses to achieve the desired Q.  For low frequencies you can use thicker copper, of course to achieve sufficiently low plate / terminal resistance.


--- 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.

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.


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