To: <towertalk@contesting.com>
> 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
Q.
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
component.
IMPORTANT TIP!
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
w8ji.tom@MCIONE.com
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search
|