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Author Topic: LCL tuner  (Read 1664 times)
K1BXI
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« on: October 20, 2008, 06:35:33 PM »

All the T network antenna tuners seem to be a CLC configuration. Any advantage or disadvantage to a LCL configuration?

John
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NA0AA
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« Reply #1 on: October 20, 2008, 07:19:04 PM »

Cost?  Speed of tuning [in the sense that it's slower to move taps if you don't have a variable inductor set]?  I don't really know myself.
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W5FYI
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« Reply #2 on: October 20, 2008, 07:37:13 PM »

A big advantage is the reduction in harmonics.
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K1BXI
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« Reply #3 on: October 20, 2008, 07:37:49 PM »

Maybe I should have asked, given a certain load to match, will one be more efficient than the other?
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N3OX
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« Reply #4 on: October 20, 2008, 08:01:42 PM »

Some loads would be better matched by an LCL tee, I expect, very capacitive loads certainly would.

But I think you give up fast continuous adjustment (either you need to switch 'em or crank crank crank).

I also think you sometimes need a pretty small shunt reactance.  A small shunt reactance requires a HUGE capacitor, while it just requires a few turns of inductor.

I'm sure there are all sorts of reasons.  I think the CLC tee sits at the intersection of cost, complexity, loss performance and matching range.


73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB6BYU
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« Reply #5 on: October 20, 2008, 08:56:18 PM »

As N3OX said, the problem is when you need a small shunt reactance,
which primarily will be to match low impedances.  A small inductor is
much cheaper than a large capacitor.
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W8JI
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« Reply #6 on: October 20, 2008, 11:10:27 PM »

John,

It is all very logical.



It is very difficult to obtain low values of reactance in a capacitor, or high values of reactance in an inductor. Keep this in mind.

In a T circuit, it takes a LOW reactance shunt to have a reasonable Q and high series reactance to have a wide impedance matching range, including matching very low or very high impedances.  This is why a C L C "T" network works so well over very wide impedance ranges. An L C L network would have terrible matching range and unless the capacitor was huge have a problem with lower bands.

In an L network, both components have to have a very wide reactance range and the shunt element has to be very large for low impedances. They can also only, unless they are reversed, match with a high impedance on one end and a low impedance on the other. This is why L networks have a very limited matching range, especially over wide frequency ranges, and they have to be able to be reversed when used in a tuner.

In a pi network, the shunt elements have to have a very wide reactance range and have very low reactance for low impedances and fairly high reactances for high impedances. This seriously limits matching range especially over wide frequency ranges.

There are very good electrical reasons why almost every tuner you see winds up being a C-L-C "T" network. The reasons are better:

Power handling
Matching range
Cost
Simplicity
Ease of adjustment

 If you look at any LCL T, or LC network, or CLC Pi, you will see they are much more complex in switching because they require much wide component adjustment ranges, or they just won't match anything.

Pi's work well in amplifiers because the load impedance does not change much and the source impedance is pretty much fixed.

L's work well with single impedance loads and sources that are always high on one end and low on the other.

T's with a CLC work best by far when you have no idea what the load impedance will be.

73 Tom
 
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K1BXI
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« Reply #7 on: October 21, 2008, 05:42:11 AM »

Thanks everybody........I thought there was a reason mechanically and cost wise in manufacturing an LCL type but wasn't sure of the electrical reasons.  

But now I know.........one more question. When Lou McCoy published his design of the T tuner back in the 60's he used a split stator input capacitor with the bottom end to ground. I seem to remember it may have had better harmonic rejection. Was there any other reason electrically?

I asked all this because I found another roller inductor of the same type as I have in my tuner that I built and had the idea of changing it to an LCL configuration just for kicks.

I'll find another use for it.

John
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WA3SKN
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« Reply #8 on: October 21, 2008, 07:42:09 AM »

Just about all the loss in a tuner is in the inductor.
As for the inductor, make another tuner! You can always find a use for one around the shack.
73s.


-Mike.
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WB6BYU
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Posts: 12973




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« Reply #9 on: October 21, 2008, 12:42:52 PM »

I seem to recall that one advantage of the McCoy "Ultimate
Transmatch" was that it used a slightly smaller coil on
160m, and the coil he had was marginal otherwise.

Given that the losses are slightly higher in that circuit,
you probably would be better off using the two sections
of the split stator capacitor in parallel to get more
capacitance.

There are differences in harmonic attenuation, depending
on what sort of impedance you are matching.  That might
have been important with some of the transmitters from the
'50s and '60s, but if your rig is clean enough that you
would feed it into a multiband antenna without additional
filtering, the harmonic attenuation of a tuner shouldn't
matter.
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