I'ts been interesting reading the various information and mis-information
about tuners on here lately.
The great rise in the popularity of tuners began with the solid state
rigs which were designed to match the very low impedance of their
transistor finals to a fixed 50 ohm impedance. If the impedance of the
antenna/feedline combination you were using was too far away from this 50
ohm design impedance, the impedance reflected by the fixed matching
circuit to the final transistors could cause them to draw excessive
current and self destruct. Read your Ten Tec owners manual section about
solid state finals for more information on why this happens. Most
Japanese equipment has built-in protection circuitry which cuts the power
back if this mismatch becomes too great. Ten Tec does not use circuitry
which limits power output but instead monitors the current drawn from the
power supply and when it starts to exceed a preset level, the power
supply shuts down.
When this problem occurs, your solution is to either change the
antenna/feedline arrangement so it presents an impedance closer to 50
ohms to the rig or to insert an impedance matching device between the rig
and the feeders to match the different impedances and thus get the
optimum power transfer. This impedance matcher is commonly called an
Antenna Tuner, Transmatch or Antenna Coupler. It does not really "tune"
the antenna but instead matches the impedance presented by the
antenna/feedline combo to the 50 ohms the rig wants to "see".
It the days of vacuum tube finals, it was seldom necessary to use a tuner
as the high impedance of the final tubes was matched to the low impedance
of the antenna through an adjustable PI network. A PI network consists
of two shunt capacitors (one on the input and one on the output) and a
series inductor. In most cases you could match antenna impedances
ranging from 25 to 150 ohms with these adjustable PI network output
tanks.
The most popular type tuner available today is the T network type. A T
network consists of two series capacitors and a shunt inductor - notice
the difference between this arrangement and the PI. By making all
elements of the circuit adjustable (variable capacitors and rotary or
tapped inductor) a wide range of impedances can be transformed to 50
ohms. The drawback to the T network is that more than one setting of the
components will allow a match and it is inherently a high pass filter
which provides little if any harmonic attenuation. The plus is that it
will match just about anything to your 50 ohm rig!
The forerunner of the T network tuner was the Ultimate Transmatch and the
SPC tuner. These had one dual section capacitor and one single capacitor
for a total of three instead of two capacitors. It was really a T
network with an added capacitor which, in the case of the SPC, offered
improved harmonic attenuation. It was found that the basic T network did
much the same thing without the need for the more costly dual section
capacitor so the T's pretty much replaced the Ultimate and SPC.
Another matching device is the L network which consists of just two
components, one capacitor and one inductor in various configurations.
The circuit is simplified by eliminating one variable capacitor. Ten Tec
uses what they call a reversable L network in their high power tuners.
By switching a single shunt capacitor to either the input or the output
of the series inductor a wide range of impedances can be matched. The
advantage of the L network is that only one setting of the capacitor and
inductor will yield a match (and it is the optimum setting). Also, in the
series inductor, shunt capacitor configuration, an L network has low pass
pass filter properties and so may provide some harmonic attenuation.
All of the above devices are unbalanced types, i.e. they are designed to
match coax or single wire feed antennas to your unbalanced output rig.
If you want to use an antenna with balanced feeders (ladder line or open
wire) you will need a balanced tuner or a device called a balun
(BALance/UNbalanced) with an unbalanced tuner. This balun can be be put
in various places in the circuit (in the antenna feed line in the case of
the G5RV, at the output of the tuner, or at the input of the tuner). I
don't know of a real balanced tuner (one with no balun) in production
today. You either have to build one or look for an old Johnson Matchbox.
The trouble with most baluns is that, while they do the unbalanced to
balanced trick, they are also designed for specific impedances. A 4:1
balun is commonly used at the output of tuners and is either 50:200 or
75:300 ohms. They work best at those design impedances. In a tuner they
could see impedances ranging from 10 to 3,000 ohms or more! If the load
you are trying to match is too far from the design impedance of the
balun, you could have problems with losses and heating of the balun core.
All tuners dissapate some power. Since a loss of as much as half of the
power is only 3 db which is one half an S-unit, I'm not overly concerned
by these losses. On the other hand, why pay big bucks for the power and
then waste part of it warming up the components in your tuner. If I had
my druthers all my antennas would be resonate and present a 50 ohm
impedance to my rig on all frequencies! The less "stuff" you have
between the RF generator (your rig) and the RF radiator (your antenna)
the less losses you have and the more power you will radiate! But it's
not a perfect world, and some of us have to make do with one antenna for
all bands. The transmatch allows us to fairly efficiently put our RF up
into whatever type of sky wire we can manage.
All of the above is offered for whatever it's worth! I'm neither an
engineer nor a technician but I have used various transmatches over the
years and have gathered some knowledge on the subject through reading the
more scholarly work of others.
73,
Puck, W4PM
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