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Author Topic: Capacitor Ratings for Tuners  (Read 702 times)

Posts: 49

« on: November 02, 2009, 07:32:50 PM »

I have a LDG Z100 Autotuner that I have difficulty loading on 80m so I am looking into the possibly getting a traditional T network tuner (MFJ style) with the big air capacitors and inductor.  I have read several reviews that talked about arcing in MFJ style tuners, so I am trying to understand capacitors better.  The LDG has silver mica capacitors that are rated as 500V.  The MFJ variable capacitors are rated at least 1500v.  With such a higher rating, how is it possible that an air variable cap will arc while the silver mica cap won't (that I know of)?


Posts: 21757

« Reply #1 on: November 02, 2009, 08:14:53 PM »

The MFJ tuners with 1500V air variables arc when used at 100W or less?

Posts: 1

« Reply #2 on: November 02, 2009, 09:11:28 PM »

Pretty soon now you will hear the same old song about how T tuners will tune the widest range of Load Z with the most limited value of caps and inductors.

The reality is that tuners are likely to arc when the total circuit (network) Q rises.  This happens in T tuners quicker than L tuners.

Buy a wide range auto tuner that usually use an L configuration with high values of capacitance.  Auto tuners use separate uncoupled inductors that get rid of the effects of shorted turns in T tuners.

Auto tuners will have lower losses and will crush a comparable T type tuner in performance.

Posts: 833

« Reply #3 on: November 03, 2009, 01:25:53 AM »

The problem of the capacitors in T-network matching units arcing on 80 and 160, with attendant high losses, is well documented. See the Cebik site.

The problem of T-networks on those bands is basically the fact that C1 and C2, which are in series with each other and the load, are too low in capacitance.  Typically, around 240pF max, when they should be around 600pF or more to achieve better matching performance on 80/160.

With typical (ie 240pF C1/C2) T-networks, even with loading a resistive 50 ohm load on the antenna output, the peak voltages across C1 and C2 are huge - over 2.5KV at 1500 watts, and even around 600V at 100 watts - when 'matching' a 50 ohm purely resistive load. If the load value of the antenna is reduced below 50 ohm, and particularly if it is a low resistance capacitative load (eg short vertical or whip), the peak voltage across C1/C2 increases again dramatically.

L-networks (both shunt and series type) generally carry considerably lower voltages across the capacitor and inductor, when matching such loads, than if a T-network was used but are less versatile in their matching ability range than T-networks.

The improved (and expensive) commercial T-networks, such as the X-match, use C1 and C2 of at least 600pF, either both variable or one variable and one fixed (such as a bank of six 100pF 'doorknobs in parallel). This keeps the voltages down and reduces the tendency to arc on low bands.

Posts: 6582

« Reply #4 on: November 03, 2009, 04:41:06 AM »

Welcome to the wonderful world of "Q"!
Most of the autotuners are an L design, switching fixed caps and/or coils into the circuit, hence the lower voltages.
Manual tuners use variable caps and either variable or tapped coils.  The early ones used a low-pass design, but the T network has become popular because of it's large matching range.  Thre are trade-offs, however.  Using LARGE value caps works best for the lower bands, but the higher bands get touchy. Smaller caps make it easier to adjust on the higher bands but limits the range on low bands.  For HF, as I recall, about 200-250 uf was the usual compromise.  The coil value usually was in the 27-32 uh range.
Capacitor plate spacings controlled the voltager, while the coil controlled the current it would handle.
And remember the values (voltage and current) in a tuned circuit can be several times what is outside the circuit!
Hope this helps, 73s.


Posts: 9749


« Reply #5 on: November 03, 2009, 06:36:32 AM »


You are getting a lot of noise and rubbish for answers. Here is how it really works:

You control the Q in a T network. Let's say you want to match two impedances with a T network using two 250 pF 1500 volt capacitors and 30uH inductance available. The tuner is good for 100 watts into a load from 30-2000 ohms on 160, and considerably more power on higher bands. For example with the same values it would handle 300 watts on 80 meters, and 800 watts on 40 meters into 30 ohms.

160 meters is a performance limit in a C-L-C network. That's where it is most limited.

The problem is NOT the voltage rating of the caps, the real problem is the capacitance just isn't enough. If it had more capacitance the power rating and lowest matched impedance would go way up on 160. But keep in mind you can aggrivate that by not using the maximum capacitance when you tune.

There are a limitless number of settings that allow a match. The CORRECT setting is the one that uses the most capacitance possible to obtain the match. Other settings will increase capacitor voltage and network loss.

The most likely problem you have is either something wrong with the tuner, the load is out of range for the T network, or you are not tuning the tuner correctly. Most commonly it is the latter.

Try tuning it this way. Fully mesh both air variables and adjust the inductance for lowest SWR. Then open one or the other variable to further reduce SWR and readjust L. Try to use the very maximum most fully meshed C that allows a match and see what happens.

The T network will outdo the L network overall for matching range and power if you tune it correctly. If you misadjust it,the same tuner that could handle 500W can arc with 50W.


Posts: 9749


« Reply #6 on: November 03, 2009, 03:06:45 PM »

I really didn't mean for that answer to sound like it did.

Most of the problems with tuners arcing come from two things:

1.) Manufacturers rate the tuners way too high, and do not give load impedance restrictions or warnings.

2.) Users do not tune the tuner correctly. They don't use the maximum possible C that allows a match.

What people always overlook is the compromises that have to be made.

For example an L network has the most limited range of load impedances it can match. The capacitor has to handle the full peak voltage of the very highest voltage load it will see, and yet has to have the maximum capacitance value that will allow it to match the lowest impedance load. This means the capacitor has to have big voltage and big capacitance range.

That makes it very expensive to do the same job as a T network in power, impedance, and frequency range. As a matter of fact it will never handle the impedance range without really getting complicated and expensive.

The T network has a compromise at low load impedances on low frequencies, and with capacitive loads on those low frequencies. It's limitation is not matching range, but power rating on the lowest band especially into a low impedance. That's greatly aggravated when someone tunes it wrong.

For an equal amount of money and size, the T is way beyond what you can do with anything else but nothing is magical.

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