[Antennas] How do you use an antenna tuner?

2002年3月11日 09:27:45 -0600

And while we are at it, none of it is "matched to the transmitter" if you
want to get into the fine print.
A transmitter has a specific design load *resistance* into which it will
deliver rated power at rated efficiency within rated distortion limits. It
is misleading to think that the transmitter "looks" like this resistance
value when one peers back into the box from the antenna terminals. In fact,
what can be seen from that vantage point is a whole universe of different
time-varying values, all depending upon the devices involved, how they are
being operated, the filter circuits being used, the precise time of the
observations, etc. 
A highly sophisticated debate among two of our most knowledgeable engineers
has raged for several years on this very subject. Each has claimed victory
for his viewpoint, based upon analysis and laboratory measurements. As a
practical matter, it really matters little who wins, since amplifiers are
not designed to "have" specific internal or source impedances. Rather they
are designed *for* specific *load* resistances selected for optimum
amplifier performance. So, while lmpwledge of internal or source impedance
is of interest, it is not a required parameter for amplifier design.
Because of this misleading notion, the confusing viewpoint has developed
that the purpose of a tuner is to "match the transmitter to the
transmission line and antenna." While vaguely true in a sense, it is much
more instructive and to the point of what is really happening to say that
the tuner transforms the input impedance of the transmission line to the
antenna - some R + jX value - to typically the value 50+j0, that being the
most common load resistance requirement for modern amateur radio
transmitters. 
In other words, the transmitter is being provided with a proper load which
may or may not actually "match" any internal impedance(s) within the
amplifier. The role of the tuner is to make the driving-point impedance of
the antenna, as transformed by the transmission line running to the tuner,
appear to the transmitter as its required load resistance (not impedance).
Then the transmitter will deliver rated power, etc.
The almost universal use of 50 ohm coax between the tuner and the
transmitter is another source of confusion in this area. The use of 50 ohm
line is a real-world convenience, not an inherent requirement because "the
transmitter output impedance is 50 ohms." Again, we are not matching the
transmitter to anything - we are merely providing it with the load
resistance it requires for rated performance. If we desired to use 83 ohm
coax,for example, we easily could in any of three ways:
1. just use it and recognize that the SWR in the coax will not be 1:1 since
by definition we have adjusted the tuner to present a 50+j0 input
impedance. The effect of this mismatch would be to present something other
than 50+j0 at the transmitter output terminals unless the 83-ohm line were
an electrical half-wavelength at the operating frequency. So the result
would be failure to properly load the transmitter, and its performance
would not be within specs. Plus, depending upon power levels and line
length and frequency, etc. there could appear at the transmitter output
terminals a high voltage or current resulting from the N:1 SWR in the coax
line - damage could result.
2. merely change the tuner adjustments to provide a tuner input impedance
of 83+j0 so that the SWR in the line to the transmitter is 1:1 and then add
a second tuner at the input of the 83-ohm coax to make its input impedance
at that point look like 50+j0. Seems pointless, but feasible if one had a
long coax run from rig to tuner and had a lot of low-loss 83-ohm coax on
hand.
3. redesign the internal impedance matching circuits in the transmitter to
require an 83+j0 load instead of a 50+j0 load. This usually is a matter of
adjusting the number of turns on a transformer in the output filter
circuits of the amplifier, plus redesigning the filters for the different
base impedance level. That same transformer and its filters were originally
designed to raise the required, for example, 3.78 ohm load resistance - at
the device outputs - to the conventional 50 ohm level. Making the
transformation to 83 ohms instead is not a major problem.
The bottom line to all this is that an appropriate tuner, adjusted with
respect to the transmission line and its length, the frequency, and the
antenna driving-point impedance, will present to the transmitter its
required load. The transmitter will then deliver its rated output power to
the entire antenna system: coax to tuner, tuner, t-line to antenna, and the
antenna itself. Everywhere along the line there is opportunity for loss,
but loss can be controlled by design and proper adjustment. When done
properly, the bulk of the loss will be in the line to the antenna and a
properly chosen line will have minimal loss even when operating at a
relatively high SWR. 
So, yes, most of the power from the transmitter goes to the antenna and is
radiated. A small amount of power is lost in the coax to the tuner, a
little more in the tuner, and most in the line itself due to the interplay
between forward and reflected waves. But every net watt (forward minus
reflected) that reaches the antenna will be radiated, minus any power that
is lost in the antenna structure itself.
Interesting stuff, eh?
73/72/oo, George W5YR - the Yellow Rose of Texas 
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe 
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
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Pat W wrote:
>> You wrote:
> "you haveto get power into it. That only happens when
> the transmitter AND the antenna is matched to the
> transmission line."
>> Wrong Wrong wrong!
>> THe antenna does NOT have to be matched to the
> transmission line in order to get power into it.
>> Please get a copy of Maxell's 'Reflections II"
>> Pat W0OPW