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[K8AC]

Not an amplifier question, but considering the component in question and the power level, this is probably the best place to ask the question.
A while back I built an L-network tuner for use with my 1.5KW amplifier and 160M dipole, fed with ladder line with an 8' coax to the outdoor balun where the ladder line terminates.  The tuning cap is a 3.5 KV Cardwell with around 490 pF maximum capacitance which isn't quite enough on 160M so I switch in a 470 pF doorknob cap and that gives me a perfect match.  The doorknobs I used in the tuner are some of the cheaper Russian surplus units sourced from eBay.  The doorknob cap tends to drift in value when heated up and the question is how to best deal with the situation.  I ran across a couple of posts here where it was suggested to use multiple lower value doorknobs (say, 100 pF) instead of one larger unit because the smaller values have a more favorable temperature coefficient.  At least, the total current would be divided between several units resulting in less heating of the present one unit.  Anyone have advice on the best approach?

[WB8PFZ]

Can you purchase a vacuum cap with 500 pf?

[W9IQ]

Have you measured the input impedance of the transmission line so that you know what voltages and currents are involved in the L match circuit? Or alternatively, based on the current matching conditions, what are the values of L and C?

- Glenn W9IQ

[K8AC]

Can you purchase a vacuum cap with 500 pf?

Yes - used prices ranging from $200 to $895!  Not an acceptable solution for me. 

[WB8PFZ]

Is that for a fixed capacitance or a variable?

[K8AC]

Have you measured the input impedance of the transmission line so that you know what voltages and currents are involved in the L match circuit? Or alternatively, based on the current matching conditions, what are the values of L and C?

- Glenn W9IQ

No.  Maybe you can help me understand why I'd need to do that to answer my question.  The L network has variable L and C to match a wide range of impedances.  If the goal were to design a fixed matching network then I understand the value of having that information.  The only tool I have for measuring the input impedance of the line is an old MFJ 259B and I connected that to see what the value are.  At 1.825 MHz (this is approximately a 1/2 wave dipole on 160) the R value is 321 ohms, the reactance is 255 ohms.  Capacitance shows 315 pF and inductance is 22.22 uH. At the match point, I measure 738 pF  of capacitance in the L network and the inductance measures 13.5 uH.  The L network capacitor is on the input side of the inductor.   The ladder line terminates in a 4:1 balun and then there's a run of about 10 feet of 50 ohm coax to the indoor tuner. 

[K8AC]

Is that for a fixed capacitance or a variable?

Fixed.  I remember when you could buy them surplus for $5 or less.

[WB8PFZ]

Yes, I checked Ebay and the high value capacitance caps are expensive.

[G3RZP]

Use several parallel caps of under 150 to 200 pF. NP0 caps are not generally available under 2 to 300 pF, mainly because the NP0 dielectric has a lower dielectric constant.

[W9IQ]

Maybe you can help me understand why I'd need to do that to answer my question.

The reason for the question is to determine the efficiency of your current setup and if it is possible to use a more efficient configuration rather than simply beefing up the existing design.

Do you have the L network configured as a low pass (inductor in series) or a high pass (capacitor in series) network?

What are you using for the balun at the coax to ladderline junction?

Could the "drifting" you are observing be due to heat build up in the balun?

- Glenn W9IQ

[K8AC]

Use several parallel caps of under 150 to 200 pF. NP0 caps are not generally available under 2 to 300 pF, mainly because the NP0 dielectric has a lower dielectric constant.

I noticed that the larger caps weren't available in NPO but didn't know why.  Thanks for the explanation. 

[K8AC]

The reason for the question is to determine the efficiency of your current setup and if it is possible to use a more efficient configuration rather than simply beefing up the existing design.

Do you have the L network configured as a low pass (inductor in series) or a high pass (capacitor in series) network?

What are you using for the balun at the coax to ladderline junction?

Could the "drifting" you are observing be due to heat build up in the balun?

- Glenn W9IQ

OK.  The L network is a low pass configuration, with the capacitor switchable to the input or antenna side.  Almost identical to the Tentec 238 series in the switching arrangement. The balun is a homebrew unit from Jerry Sevick's book, a Ruthroff design using a T400A-2 core.  I considered that it may be the balun causing the drift, but when I substituted my Ameritron ATR-30 tuner for the homebrew unit, there was no drift at all when running 1,500 watts through it on 160M. 

[W9IQ]

Is the balun a 1:4 design with the '4' side connected to the ladderline?

- Glenn W9IQ

[K8AC]

Is the balun a 1:4 design with the '4' side connected to the ladderline?

- Glenn W9IQ

Yes.  The conventional "wisdom" seems to be to use a 1:1 with a multiband antenna, but I was never able to achieve a match across all bands with the 1:1 and my ATR-30 tuner.  I used to use this antenna on 160 through 10, but these days just on 160, 80, 60, 40, 30. 

[VK6HP]

I have a collection of Russian eBay doorknob ceramic capacitors and they are quite a variable lot in terms of loss tangent and temperature coefficient.  Most recently, I grabbed a few for inclusion in my dummy LF/MF antenna (not resistive dummy load) and I was unable to find any that were very temperature stable.  Of course, the physically smaller doorknobs are the wrong capacitors to be using in such applications but these were worse than any I'd previously used.  As a quick fix, I'd suggest you see if you can find some of the larger "pancake" ceramics which, in my experience, are more stable in high current applications.

73, Peter.