Amen Steve! It is also interesting to contemplate what happens on the
receive side. The 20:1 SWR doesn't do any good for the receive signal
either.
73, Steve Thomas, N6ST
> -----Original Message-----
> From: owner-towertalk@contesting.com
> [mailto:owner-towertalk@contesting.com]On Behalf Of sbest@cushcraft.com
> Sent: Monday, August 24, 1998 5:16 AM
> To: towertalk@contesting.com; w2du@journey.com
> 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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