I will use this one message to respond to many.
*****
Ward's (N0AX) statement:
"I think there is some confusion on nomenclature. I disagree that
VSWR affects antenna gain, but agree that VSWR affects *system
efficiency*. Antenna gain is only a function of antenna geometry.
System efficiency includes antenna gain and line loss, but the two are
independent."
I will agree with this statement. The IEEE standard definition for
antenna gain does not include the effect of VSWR. VSWR does impact
system gain. However, as an antenna manufacturer, we directly compare
antenna performance to a reference antenna with a good match. Since
the difference in the two antenna's VSWR's results in a "gain"
difference between the two, we include it in our antenna gain figures.
The real issue is working with system engineers - they always need to
know how we define "antenna gain". Most end users (maybe commercial
NOT amateur) are not aware of these subtle definitions.
*****
Ron's (KK1L) statement:
"Now wouldn't this also mean that you are creating a nice reflection
back up the line to the antenna? This should provide more power at the
antenna for it to radiate."
Most tuners will totally dissipate the reflection such that it will
not be delivered to either the transmitter or the antenna.
*****
Guy's (K2AV?) statement: My understanding of "loss" is power
dissipation. If the 7.41 db is dissipated, and not in the feedline,
where is it dissipated? Or do you intend "loss" as some ratio concept
that does not actually dissipate power?
Absent the presence of a tuner, the transmitter will internally
dissipate the power as heat. If the transmitter source impedance is
not the same as the characteristic impedance of the cable then energy
will be reflected back towards the antenna.
****
Gerald's (K5GW) statements:
"You may or may not be aware that virtually all amateur transmitters
and amplifiers contain tuning networks that are either manually or
automatically adjusted to minimize the vswr seen by the internal
amplifying device. This same proccess also provides (through conjugal
matching theory) the needed shift or distortion of the output z of the
matching device to re-reflect the wave back toward the antenna.
In other words, after matching adjustments are completed, the
transmitter or amplifier no longer "looks" like 50+j0 ohms.
For this reason, the vswr of the line does cause extra losses, but
only the by the amount caused by the multiple trips up and down the
line.
This proccess is of course not valid for broadband systems with fixed
50 ohm output impedance as there is no mechanism to get the reflected
wave turned around at the output of the device."
Gerald, thank you. My background has virtually all been with 50 ohm
broadband systems, although, I do understand the concept and theory of
conjugal matching. I am going to think about this one for a bit.
Might have to reconsider a few statements I made. I am confident that
I am correct, given my assumptions. Now I need to rethink my
assumptions.
*****
More from Gerald:
"A more accurate statement about loss of antenna gain by vswr would be
"loss of antenna SYSTEM gain. A 20 to vswr created by a 1000 ohm feed
impedance can easily be matched to 50 ohms via an LC network with no
apparent loss of gain. With no matching network, there would be
considerable power loss in most 50 ohm cables used by amateurs. With a
matching network, the the antenna gain is still the same, but the
SYSTEM gain will be much greater."
This is correct. From an antenna designer's perspective, the LC or
other type of matching network becomes part of the antenna, reducing
"antenna" VSWR such that the antenna is now matched. However, not all
of the power will be radiated since some will be lost into the
matching components.
73
Steve Best
--
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