Re[2]: [TowerTalk] Mismatch Loss and Tuners
sbest@cushcraft.com
sbest@cushcraft.com
Tue, 25 Aug 98 08:52:22
I think Hank (KN6DI) has made a few good points.
With the exception of responding to the question posted by Pete
(N4ZR), this will be my last post on this subject for awhile. After
this post, I think it only fair that Walt Maxwell be given a chance to
provide his rebuttal to my comments. In the meantime, I have sent
Walt a copy of my "proof" of the concepts I have outlined. I will
value his review, comments and opinions.
In plain English:
One of the objectives in the transmit system is to eliminate
reflections arriving at the front end of the transmit circuit. This
can be accomplished in many ways. The most common are to use an
antenna tuner or to use an antenna with a feedpoint impedance equal to
the characteristic impedance of the transmission line.
With a matched antenna, all power arriving at the antenna will be
delivered to the antenna with no reflections. The significant source
of loss in the system will be that which occurs as a result of cable
attenuation between the transmitter and the antenna.
To properly analyze a tuner we must first describe the initial state
of the tuner and then develop into the steady state of the tuner. To
do a full analysis, voltages must be used in all calculations. Steady
state power levels can be determined once the steady state voltages
are known.
In the initial state (the state prior to reflections arriving at the
tuner output from the antenna), the tuner will not present a 1.0:1
VSWR at its input. Because of this, there will be an initial
reflected voltage present at the tuner input. Therefore, the initial
power delivered through the tuner to the antenna will be less than the
total power delivered to the tuner from the transmitter. The voltage
(power) delivered through the tuner will then arrive at the antenna
where some will be reflected (based upon antenna VSWR). This
reflected voltage will arrive back at the tuner output. At the tuner
output, another reflected voltage will be created based upon the tuner
VSWR. Note that the tuner output VSWR will not be infinite therefore,
ALL of the voltage will not be re-reflected. This process will
continue until the steady state condition is reached. In the steady
state condition, the multiple reflections will create a steady state
voltage at the tuner output. This voltage will be delivered back to
the tuner input through the tuner components. The voltage delivered
back to the tuner input will be the negative of the initial reflected
voltage at the tuner input causing a complete cancellation of the
initial reflection. Hence, the steady state 1.0:1 VSWR.
The difference between this discussion and that of Walt Maxwell's is
that Walt does not start by considering the initial state of the
tuner. He immediately "assumes" that all of the transmitter power is
delivered through the tuner, some is reflected by the antenna and then
ALL is re-reflected at the tuner output. His initial assumption may
support his conclusion but it does not accurately describe the
operation of the tuner, nor all of the voltage reflections and
cancellations that occur. With a lossless transmission line between
the tuner and antenna his results are numerically correct, however, I
am uncertain if this would be true when considering a case with
transmission line attenuation.
In any event, even considering the transmission line losses, the
matched antenna will radiate more steady state power than the
mismatched antenna and tuner. Remember that with the tuner, the
reflections from the antenna must travel the transmission line twice
before arriving back at the antenna.
Which "works" better - matched antenna or mismatched antenna/tuner?
The answer to this question requires a much more involved analysis.
In order to "work", the transmit system only needs to provide a
useable signal level at the receive site. In many cases, both will
"work" quite well.
73, Steve, VE9SRB
______________________________ Reply Separator _________________________________
Subject: Re: [TowerTalk] Mismatch Loss and Tuners
Author: kn6di@groupone.net
Organization: Global Pack & Mail at InterNet
Date: 8/24/98 10:22 PM
Wouldn't be great if people could talk in plain english and discuss this thread
in a straight forward manner instead of every one trying to get the last piece
of fly S--- out of the black pepper.
It would be nice for a plain statement of facts of what works and does not work.
As we know no 2 antennas work the same because the conditions are not/never the
same. This could go on until cycle 30 at least.
Hank KN6DI
J
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