I apologize that this is a bit long, but I consider this to be
important.
I would like to comment on the remarks made by Walter Maxwell (W2DU)
regarding statements I made about mismatch loss and tuners. I would
also like to comment on Mr. Maxwell's statement regarding the
operation of tuners.
Mr. Maxwell stated that several of my previous statements are
incorrect and "shows a misunderstanding of transmission line operation
and technique."
I made a previous statement that an antenna with a 20:1 VSWR results
in a mismatch loss of 7.41 dB. This statement was made and is valid
based on my assumption that a transmitter would not re-reflect any of
the initial power reflected by the antenna. I recognize that this
assumption is incorrect for amateur transceivers and that I should
have been more specific in my statements. My assumption was made
based upon my experience with some military (US Navy) HF transmitters
which are inherently designed to dissipate power reflected by the
antenna. These transmitters have a VSWR (reflected power) detector
which reduces output power as VSWR increases. At a 4:1 VSWR they shut
down completely because the reflected power cannot be dissipated
without damaging the transmitter front end. I believe that a passive
network is used to dissipate the reflected power.
In any event, in the design of antennas for use in a transmit circuit,
it has always been good engineering practice to design the antenna to
have a minimum VSWR relative to the transmission line. Aside from
reflecting power at the connection point, the two most critical issues
that result due to a high antenna VSWR are the following:
1. Increased voltages and currents at some points along the length of
the transmission line. At high current points, the increased power
dissipated can cause the line to become hot (temperature), sometimes
to a point where the line will "burn" through. If higher voltages
exist at the transmitter front end or the antenna (relative to a
matched condition), internal components can be damaged. It is
important to note that most manufactures specify power ratings in
components and cables under a matched condition.
2. Increased attenuation in the transmission line due to the increased
power dissipated at high current points along the length of the line.
Tuners:
In his discussions regarding tuners, Walter Maxwell states that a
tuner "compensates for the reflection loss by introducing an equal
amount of reflection gain." Tuners do not "introduce" any "gain" and
in fact, the term "reflection gain" is somewhat misleading. The term
"gain" would imply that the signal intended to be radiated is
undergoing some form of amplification process. This is not the case.
At any single point in time, the total steady state power delivered to
the antenna is sum of the signal intended to be radiated plus the
reflections that resulted from signals arriving at the antenna at
previous points in time. Basically, the total steady state power
delivered to the antenna is the intended signal plus the reflected
echoes from previous signals. These echo signals are similar to
multipath signals and in some communication systems they are disastrous
to signal quality. In practice however, these echo signals may not be
detrimental to amateur communications. This would be the topic of a
different discussion.
If we assume a perfect lossless fixed component value conjugate match
"tuner" and a lossless transmission line between the tuner and the antenna
the following will occur. (Let's also assume a 20:1 antenna and 100 Watts
of power delivered into the tuner.)
In the steady state condition, the VSWR between the tuner and the
transmitter will be 1.0:1. The VSWR between the tuner and the antenna
will be 20:1. The steady state power delivered to the antenna will be
100 Watts. The forward power measured by a wattmeter located at the
tuner output will be 551.25 Watts. The reflected power measured by a
wattmeter located at the tuner output will be 451.25 Watts. It is now
important is discuss how this occurs.
In order to arrive at the steady state condition, where the VSWR between
the tuner and the transmitter is 1.0:1, a lot must happen. First, in the
initial state, where no signal has yet arrived at the antenna, there will
be an initial voltage reflected at the tuner input (the tuner input will
have an initial VSWR of 20:1). The initial power delivered to the tuner
and to the transmission line connecting the tuner to the antenna will be
18.14 Watts. Since the transmission line is lossless, this 18.14 Watts of
power will arrive at the antenna where only 3.29 Watts will be initially
delivered to the antenna for radiation (20:1 VSWR). The initial 14.85
Watts
of power reflected at the antenna will then be re-reflected back and forth
between the tuner and the antenna until the steady state condition is
reached. These multiple reflections between the tuner and the antenna
will
result in 100 Watts of steady state power being delivered to the antenna.
It is important to note that of the 100 Watts of steady state power
delivered to the antenna 3.29 Watts is the intended signal and 96.71 Watts
is echo signal. It is also important to note that at the tuner output, a
20:1 VSWR exists for the voltage reflected from the antenna. If we do a
further analysis of the steady state voltage at the tuner output, we will
find that sufficient voltage is delivered back towards the transmitter (at
the tuner input) to cancel the initial voltage reflection mentioned above.
In the steady state condition, the tuner achieves a 1.0:1 match at its
input by canceling the initial reflected voltage.
If the transmission line between the tuner and the antenna has loss, let's
say a TOTAL one way loss of 1 dB, then the steady state power delivered to
the antenna will be 24.19 Watts. 12.24 Watts of this will be echo power.
The forward power measured by a wattmeter located at the tuner output will
be 167.84 Watts. The reflected power measured by a wattmeter located at
the tuner output will be 86.69 Watts. Note that the difference in the
wattmeter readings is not equal to the steady state power delivered to the
antenna because of the line attenuation. It must be recognized that
reflections from the antenna must travel the transmission line twice prior
to arriving back at the antenna.
If we were to connect an antenna with a VSWR of 1.0:1 to the same
transmitter with a TOTAL one way transmission line loss of 1.0 dB then
79.43 Watts of power would be delivered to the antenna. This is far
greater than with the 20:1 antenna and tuner.
If anyone would like a more detailed proof of what I have outlined above,
I
have an HTML document that I would be happy to forward it along.
SUMMARY - A matched antenna will significantly outperform a
mismatched antenna with a tuner. A tuner cannot introduce "reflection
gain" to change this fact.
73 Steve Best
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