To All:
I enjoyed all of the comments, although I will say some more than
others.
To Jim Reid:
The history of mankind is quite interesting. Have you ever noticed
that anytime someone challenges another person's (or group of
people's) beliefs that they respond immediately with persecution. Is
the world still flat and does it still revolve around the sun?
I was very disappointed to see your personal attacks towards me
regarding my discussion, my background and the DOD $$. Given my
educational and professional background, I could respond much more
severely to you, but I won't.
First, I could not respond to questions last night because my e-mail
at home does not access the reflector. I enjoy the technical exchange
of opinions on the reflector. When I prepare my submittals, I realize
that I am challenging many opinions and that I will be subject to
great disagreement. I do not, however, expect personal ridicule.
Given my extensive technical background on antenna and transmission
theory and design, I attempt not to state technical opinions that are
invalid. Although, I will admit that I am not familiar with the
operation of ALL amateur tuners. Maybe I should have said some
dissipate the power rather than most.
My only reason for posting the discussions about transmission line
theory (reflections, VSWR, etc.) was to educate those people that
might be interested in broadening their base of knowledge. I am not
an expert class ham but I am somewhat of an expert at the topics I
address. (On the 200 Watts on the bird wattmeter - you don't provide
enough information for me to explain it properly too you. In the
meantime, you might want to read up on the theory of "conservation of
energy").
As far as reading materials - I have not read Maxwell's reflections
and cannot comment on it until I have done so. K4SB has been kind
enough to send me a copy to review. The only Maxwell I have read is
James Clerk Maxwell - you might find this interesting Jim. Anyway, I
have numerous electrical engineering books completely devoted to
transmission line theory and practice. I would highly recommend you
read "Schaum's Outline Series - Transmission Lines" by Robert Chipman.
A quote from page 203 "At the signal source end of the line the
reflection coefficient, ps = 0 because Zs = Zo, so that none of the
power reflected by the terminal load impedance is re-reflected on
returning to the input end of the line." Another one is "Transmission
Lines and Networks by Walter Johnson". A quote from page 16 "Thus, a
terminating impedance different from Zo will give rise to a reflected
wave which travels away from the termination. The reflection, upon
reaching the other end, will itself be reflected IF the impedance at
that end is DIFFERENT from Zo".
I don't believe I have said anything technically incorrect. My
assumptions regarding source impedance being equal to Zo, etc. have
always been clearly stated. A great many physical systems operate
this way. I challenge you to open your mind to new concepts. Read
the short article I wrote.
Oh, one last thing, I too have designed many, many antennas that are
ALL still working fine. Some even for satellites.
To Anyone Else Interested:
I guess that we are beyond the semantics of "antenna gain" versus
"system gain".
I also think that all of us understand that an antenna has a complex
impedance and that VSWR cannot be defined until the antenna is
connected to a transmission with a characteristic impedance Zo.
In any event, the real issue is not VSWR but rather what happens to
the reflected energy? Most on the reflector appear to believe that it
is never really lost and that it does not impact system performance -
like operating into an antenna with a 20:1 VSWR and not experiencing
even a fraction of dB loss. Why do ALL the antenna engineers in the
world waste their time minimizing VSWR?
I made several controversial statements yesterday. One was that the
reflected energy is dissipated in the transmitter if the source
impedance is 50 ohms. If Zo is 50 ohms and Zs is 50 ohms, how can
energy be reflected from the source? I also stated that if the source
impedance is not equal to Zo, then SOME, BUT NOT ALL, energy would be
reflected back towards the antenna. The only way ALL of the energy
could be reflected back towards the antenna is if the source impedance
had an infinite VSWR, i.e. it was either an open circuit or a short
circuit. This also applies to tuners and amplifiers as well. A tuner
(even Jim's) will never reflect all energy unless the VSWR seen by the
antenna reflected wave is infinite. Again, some energy will be
reflected if the VSWR seen by the antenna reflected wave is not 1:1
and some will be absorbed and dissipated as heat OR some will be
canceled (maybe a new concept for some).
My discussions and examples assumed a source with an impedance of 50
ohms. I clearly stated this and many real transmitters and amplifiers
operate in this manner. The concepts I discussed in the article I
wrote (which some of you asked for) are valid for all situations (real
and theory) and should never "be left in the lab". The formulas
presented in the article are the basis for all transmission line work.
Ask yourself why transceiver manufacturers, even ones with built in
tuners, specify a maximum VSWR load (like 3.0:1) and why the output
power is lowered when operating into a high VSWR? If all of the
antenna reflected power was reflected back to the antenna, why would
it make a difference? And yes, some transceivers and tuners do burn
up because of reflections.
One other concept that has to be considered relates to "lost energy".
This is important. Electromagnetic energy in a transmission line
system can be dissipated as heat, it can be reflected by an impedance
mismatch, or it can be canceled. Cancellation is the issue that is
sometimes forgotten and hard to understand in practice. On a
transmission line, two voltages that are of equal amplitude and 180
degrees out of phase will cancel completely. Many matching devices
use this method to eliminate or minimize reflections on the line.
Conjugate Matching: Actually, I think Dick Green gave a pretty good
summary but was challenged by someone (was it Jim?). Conjugate match
theory was derived for a transmission line with a complex
characteristic impedance, complex source impedance and complex load.
Anyway, conjugate theory states that more power will be delivered to a
terminal load whose impedance is the conjugate of characteristic
impedance. There are still reflections on the line but what happens
is the forward and reflected waves combine in such a manner that the
total line loss is not increased as with a transmission line whose
characteristic impedance is real. With tuners, the conjugate match
theory is a bit different. Once the complex input impedance to the
line is determined (after reflections reach the tuner) the tuner
impedance is adjusted to be the conjugate of the input impedance.
(Assumption on my part as to how they work) In this case, the
reflections will still exist on the line between the tuner and
antenna. When the reflections arrive back at the tuner, they will be
prevented from going to the transmitter (so it sees a 1:1 match and
thinks there are no reflections). Three things can happen to the
energy at the tuner - some will be reflected back to the antenna
because the reflected wave no longer see a matched condition, some
will be dissipated as heat in the tuner and some can be canceled as
described in the previous paragraph.
Enough said.
73
Steve Best
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