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[TowerTalk] Antenna Tuner

To: <>
Subject: [TowerTalk] Antenna Tuner
From: (Tom Rauch)
Date: Wed, 08 Apr 1998 12:06:01 +0000
To: <>
> Date:          Tue, 07 Apr 1998 22:07:47 -0400
> > XMatch is best tuner currently sold assembled.
> > Nye is next and a good buy used.
> > All the rest are pretenders, sorry to say.
> > de K4VUD

The X-match, despite the marketing hyperbole all modern-age 
manufacturers engage in, is really just a conventional T network 
tuner. The unique feature is the network employs a fixed capacitor at 
one end of the T, and so must be "reversible" in order to match 
impedances higher and lower than 50 ohms. The cover is poorly 
grounded (allowing considerable case radiation), but that is easily 
cured with a drill, tap, sandpaper, and some machine screws.

The real technical "secret" to its better low band performance is in 
the amount of capacitance used. The amount of capacitance is very 
high, and the increased capacitance lowers the loaded or operating 

The efficiency of this type of  matching network is governed by the 
ratio of unloaded Q to loaded Q. Having a large amount of 
capacitance decreases the loaded Q **IF** the load impedance is low 
or has capacitive reactance.  The X-match capitalizes on this 
very simple basic network principle.

This advantage totally disappears (and can actually be worse) if the 
load has high values of inductive reactance, is a high impedance, 
or the tuner is operated on higher frequency bands.

For example, a tuner like your Vectronics would be optimized into a 
few thousand ohm load on 160 and 80 meters, because the series C is 
so small (only a few hundred pF). In this case efficiency would be 
very high, and power handling would be very good (well over 2000 
watts RF output). 

Put the very same Vectronics tuner into a 10 ohm load, and 
performance would be poor, and the tuner would dissipate 
approximately 20% of the applied power. While the actual power loss 
is very small and not noticeable over the air, the heat (mainly 
concentrated in the inductor) would be beyond the rating of the 


If you want to maximize the power handling of any CLC T network 
tuner, ALWAYS operate it with the capacitors as fully meshed and as 
equal in value as possible. As a general rule, the more C used, the 
lower the Q and the lower power loss will be.

By the way, lower power loss does not necessarily mean less 
heat damage to the inductor. As you increase capacitance it requires 
reducing inductance, and that concentrates whatever heating there 
is in a smaller physical area (fewer turns) of the coil.

That's a major advantage of an edge wound inductor, the surface area 
is larger and it can dissipate more heat without undue component 
temperature rise. You would "perceive" the tuner as much better, 
because the coil would appear to be much cooler.

In almost all cases, the losses would be so low the over the air 
results would be no different. It's just a matter of when the tuner 
breaks, or what it won't match.

Keep in mind that what works very well in one particular situation 
might be a disaster in another, there is no magic elixir. The optimum 
design is controlled by the frequency range and the load impedance 
YOU have, and nearly every case is different.
73, Tom W8JI

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