# Choosing antenna tuner capacitor

Discussion in 'Radio Circuits, Repair & Performance' started by 9A5DLZ, Feb 16, 2016.

1. ### 9A5DLZHam MemberQRZ Page

I am looking for a capacitor for building T match antenna tuner. I found one that is within my budget https://www.rfparts.com/capacitors/capacitors-antennaload/56-36.html

It says maximum voltage is 250V across the capacitor. I know how to calculate what voltage and current will be when the load is 50 ohms using 100W of power. But this circuit complicates things. It uses inductor and as far as I know inductor will actually increase voltage across it's leads when AC is applied to it. I guess I could research into it.

I don't know if the voltage will cross capacitor's max rated voltage. Can anybody help here?

The short answer: Look for a bigger capacitor with wider plate spacing for your antenna tuner unless it's strictly a QRP tuner.

Longer answer: Actually determining necessary capacitor values and breakdown voltages for tuner capacitors takes several steps:

- Figure out what the extremes of tuning impedance you want to support with your tuner. For instance you might want to be able to tune a load from say 20 +/- j100 to 1000 +/- j200 ohms. (these aren't typical, necessary or even desired values, just examples of putting a stake in the sand for the matching range of your tuner).

- Then work with either a Smith chart like: http://cgi.www.telestrian.co.uk/cgi-bin/www.telestrian.co.uk/smiths.pl or a matching network calculator like: http://home.sandiego.edu/~ekim/e194rfs01/jwmatcher/matcher2.html to figure out what values you need to match your desired loads to a 50 ohm source with your desired matching network configuration (e.g. T network). In the case of a T network strive for a minimum Q solution, IOW a solution with minimal shunt inductance and maximum series capacitance that yields an acceptable match across your desired matching range.

- Once you have picked inductor values for the bands of interest and cap max and min values to satisfy your tuning range needs you model those extreme values in terms of reactive values at frequencies of interest and then perform VAR analysis to see what that translates to in terms of voltage across and current through components of interest: https://en.wikipedia.org/wiki/Volt-ampere_reactive

The whole process is typically iterative for a multiband tuner as you refine what you can actually achieve for impedance matching range and breakdown voltages based on components that you can actually source or are within the design budget. Of course you can go the other way, pin down the components you wish to use and then see what they yield in terms of impedance matching range and breakdown voltage. Either way you'll likely find you're going to need transmit specific variable caps with much wider plate spacing than what's shown in that link unless you keep your RF power very low.

One thing that becomes clear if you do this sort of analysis is that supporting both a wide matching range AND high power support in a T-network tuner requires wide value range variable capacitors with high breakdown voltage ratings, especially for the lower bands like 160 and 80 meters.

-Dave

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3. ### 9A5DLZHam MemberQRZ Page

Dave, thanks for the long answer. Do you know if anybody tried immersing the capacitor in mineral oil to get bigger breakdown voltage? What about dielectric loss in that case?

And how about using smaller variable capacitor with high breakdown voltage and using switches to switch in additional capacitors for wider range of capacitance?

Last edited: Feb 17, 2016
4. ### G4LNAHam MemberQRZ Page

As a matter of interest, I've built a remote double L match unit to feed my open wire line using relays and fixed capacitors and inductors in decades so I could switch them from the shack.

The capacitors are made from 0.25 mm PTFE (Teflon) Sheet and brass shim about 5 thou and interleaving the shim with the PTFE sheet. I was able to get the exact values I wanted very cheaply and with a breakdown voltage in the region of several Kv.

If you don't mind spending a little time it is very easy to do, it took my about a week to build the unit over Christmas and it works perfectly.

One of these days I must publish it.

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5. ### G4LNAHam MemberQRZ Page

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6. ### PA1ZPHam MemberQRZ Page

Hi

For capacitors at T-tuners, for 100 watts 20-500 pF and about 1KV.
For QRO 20-500 pF about 3KV or more.

73 Jos

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7. ### 9A5DLZHam MemberQRZ Page

Very cool. And I can make that with a microcontroller so it automatically chooses correct LC combination!

8. ### G4LNAHam MemberQRZ Page

You could use a micro, but I choose to use decade switches.

W7DEX and 9A5DLZ like this.

Can't speak for immersing the capacitors in oil but I suspect you'd create more troubles than you'd solve with that approach. Vacuum variable caps are a great solution but they can be hard to find at a good price.

But yes, as G4LNA suggests, switched capacitor banks work very well and are used in several high end commercial tuner designs at least for lower band support. You still want to pay attention to things like breakdown voltage for both the added caps and any switches or relays you use to switch them in and out of circuit. Doorknob caps are a good choice and are easy to find on the surplus market.

There's also a ham that's been offering homebrew tuner cap kits on these forums. They look pretty nice: https://forums.qrz.com/index.php?threads/butterfly-capacitor-kits-va6pop.478016/ that link is for butterfly caps which may or may not fit your design but I'm pretty sure he offers traditional tuner cap kits as well.

PAZP's post above is really the bottom line. If you want wide matching range and high power operation you're looking at something like 20-500pf (though many very good tuners get by with 10-360pf or so but may struggle for matching on 160m or add one or more external doorknob caps that can be switched in for lower bands) and 1-3kV breakdown voltage.

-Dave

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10. ### REASTONQRZ Member

It sounds to me you are on a road to waste money, time, and effort building a scaled down tuner when you can get something like a used Murch on ebay for just over \$100 if you are patient looking for one to appear for sale at a good price. There are other used tuners that would also work well that have the capability to match wide range loads at high power which is to advantage even if you run low power.

Save your money and buy something that works for a reasonable amount and you don't have to build something that turns out to be very limited.

I'm all in to see hams build their own equipment but when hams don't have much building experience, tools, and materials, building can turn into a big waste of time and money. Building stuff always turns into costing more money than planned, grief trying to find metal and materials, and ending up maybe getting something too big, too ugly, and too heavy besides not working.

Last edited: Feb 17, 2016