If I may, I would like to bullet-point what I consider to be important
factors that oftentimes lead to confusion. Walt, if what I am about to
present is incorrect or misleading, please step in.
1) For H.F. communications purposes, moderate mismatches between the
transmission line and the antenna can be compensated through a conjugate
match (where the entire system reactance is zero), performed anywhere in the
transmission system between the transmitter and the antenna, inclusive of
the transmitter's pi or link output network (typical of vacuum tube-based
transmitters), resulting in an efficient transfer of power to the
antenna/load, provided that transmission line losses are minimal.
2) In the case of fixed-output-impedance, solid-state transmitters, this
conjugate match can be achieved through the use of an external network (e.g.
antenna tuner or transmatch).
3) Under small to moderate antenna-to-transmission line mismatches, varying
the length of the transmission line will not change the VSWR, but will
result in varying the impedance the transmitter sees looking into the
transmission line. By varying the length of the transmission line, the
transmitter's output network may be able to achieve a conjugate match more
easily.
4) Larger degrees of antenna to transmission line mismatch (higher VSWR)will
oftentimes require a network (external to the pi or link network) capable of
handling a greater mismatch. An "antenna tuner" or "Transmatch" will
oftentimes achieve this. Paramount to keeping an efficient transmitter-to-
load transfer under this condition, it is important to keep transmission
line losses low, as the antenna-to-transmission line mismatch increases.
5) The purpose of the network (e.g., antenna tuner or transmatch) is to
conjugately *tune* the entire system through reactance cancellation, while
achieving a Zo match ( i.e., the transmitter's output impedance) to the
input of the network.
6) In the case of a vacuum tube output, #5 above is achieved by matching
(Zo)the PA plate circuit's output impedance to the input of the pi or link
network, while the network also conjugately tunes (reactively cancels) the
entire antenna system including the transmission line.
7) The above arguments do not necessarily apply to critical transmission
line and/or communication applications including FM multiplex (Stereo +
SCA), and fast-scan, broadcast quality television transmissions. Under
these two examples, a transmission line-to-antenna *match* is absolutely
necessary in order to eliminate the effects of self-induced multipath that
will occur with only a conjugate match. This manifests as "ghosting" on
television reception and poor stereophonic audio separation on FM multiplex
broadcasts
8) Regarding the term "Reflection Gain:" True, while the name implies
"amplification," the use of this terminology is a "well-settled" description
among RF engineers to denote a re-reflection.
-Paul, W9AC
-----Original Message-----
From: sbest@cushcraft.com <sbest@cushcraft.com>
To: towertalk@contesting.com <towertalk@contesting.com>; w2du@journey.com
<w2du@journey.com>
To: <towertalk@contesting.com>
Date: Monday, August 24, 1998 1:10 PM
Subject: [TowerTalk] Mismatch Loss and Tuners
>
>
> 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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>
>
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