At 13:32 6/27/98 -0400, Robert Hummel wrote:
>
>>The mismatch of VF has no more effect than the SWR - it all comes out the
>>other end, even though some of it takes a lot of trips back and forth to
>>get there, losing only what is often called the "copper loss," but is
>>really the loss characteristic of the design of each individual cable.
The above statement is true.
The following is not true:
>With the utmost respect to Press (who knows his cable), the idea that "all
>the power goes to the antenna eventually" is one of the most persistent and
>pernicious myths in ham radio. In my mind, it's right up there with "RG-8
>is essentially lossless at HF."
>
>The idea that an SWR mismatch at an interface doesn't directly cause a loss
>in thru power is valid only in the very special case of steady-state power
>transmission. Amateur radio transmission, however, is not steady-state. It
>is more like impulse transmission, and that makes all the difference.
>
>Consider a single sharp dit going into a mismatched junction. Part of your
>dit gets transmitted, part gets reflected. At some junction back toward the
>transmitter, lets assume that all the reflected power gets reflected back
>toward the antenna. At the junction, some more of that original dit power
>(now delayed by a coax round-trip) gets radiated. Was that your intent? Did
>you mean to transmit DIT Dit dit .... (echoes continue)?
The above statements are plain SILLY, see on below!
>Now, if you really transmitted a series of dits and dahs or a complex audio
>waveform, all reflected/radiated signals that are not time coincident with
>the original signal contribute to your radiated power, but only as noise.
>The effect is that you are transmitting a noisy signal that's weaker than
>you think.
NO, you are not transmitting ANY noise, nor a noisy signal!
>
>Consider the analogy of a 4" diameter pipe that goes through an abrupt
>transition to a 2" pipe. At some certain steady, constant pressure on the
>4" inlet, a measurable amount of water will flow into the pipe and --
>behold! -- the same amount comes out at the 2" end. This is the steady
>state situation.
>
>But, now consider that you turn on the water briefly, enough to fill the
>entire 4" diameter of the pipe for let's say 1 foot in length. This packet
>of water (neglecting friction the way we neglected loss in the coax
>example) will fly down the 4" pipe like a bullet in a gun barrel. But when
>it hits the 2" abrupt transition, part of the water will be reflected back.
>At the 2" outlet, you will see far less water come out than you put in.
>
>Of course, eventually, all the water may drain out of the pipe. But that
>isn't what you really wanted.
After reading what follows you can the do the following:
Repeat this phrase until you believe it: Mismatch loss is REAL loss,
but this loss has NO meaningful significance, nor can it be
detected at amateur HF or UHF frequencies and the usual
modulations, analog or digital!
Why not?
The velocity of electromagnetic waves in a vacuum has long been
known to be 3 times 10 to the eighth power meters per second,
300 million meters per second (well, actually now the figure is
taken to be 299,792,458 m/s give or take a bit of measurement
uncertainty or two in the last couple of characters). Or about
186,000 miles per second.
Now let's stay in metric units for this discussion. Consider a 30
meter length of coax, about 98+ feet of it. Also, let us use
ordinary RG8 coax, so it will have a typical velocity factor of
about 66, or an EM signal will be travelling at only 66%
of the velocity in a vacuum. So, in the coax it slows down
to 0.66 times 300 million meters per second, or our CW
dit signal pulse (to use Robert's example above) is now
slowed down to only about 198 million meters per second!
In LMR coax, however, it would be going a bit faster, at
around 255 million meters per second and on twin line,
ladder line nearly the full 300 million meters per second.
So the signal will travel the 30 meter length (98+ feet)
of the RG8 in the same fraction of time that 30 meters
is a fraction of 198 million meters, or the time to traverse
will be 30/198 millions of one second which is to say
about 0.15 microseconds (0.15 millionths of a second).
Or, the round trip time in the coax for signals reflecting
from the output end back to the beginning, then back
to the output end would be about 0.3 microseconds.
Robert has proposed a series of reflections, lets say
6 for all of the signal to finally come out the end of
the coax "pipe" of his water example. So now, before all
of the initial reflected "dit" power is out of the line
and radiated by the antenna an entire 1.8 millionths
of one second goes by (1.8 microseconds)
Well, so how long is a "dit" at let's say 30 wpm?
The word PARIS is defined as taking 48 "dit" spaces to
transmit, using one space for a dit, another "dit" space
to separate dits and dahs, and a "dah" is three dit spaces,
all as defined by "others" somewhere in the past. So at
30 wpm, we are sending 1440 "dit" spaces per minute,
or, one "dit" uses up 1/1440 the of one minute; or,
60/1440 of one second. which is about 42 milliseconds,
(42 thousandths second).
Now, according to Robert, we should be concerned about
sending out repetitive "dits" or dit echoes because of our
mismatched transmission lines. Well, lets see, it takes all
of 42 msec to send one dit at 30 wpm, and more time than
that at slower CW speeds. And, even if our "dit" reflected around
10 times in our coax line, that would mean all of the initial
"impulse" power from the start of our "dit" would be radiated
from the antenna within 3 microseconds. Let's compare that
to the 42 msec total "dit" time.
Well, 42 msec is 42,000 microseconds!!
Obviously, all of the reflected "initial" and all following
impulse power is going to escape our transmission line
pipe long before we can get off the key to end our "dit".
Furthermore, reflections from the last fraction of a millisecond
of our "dit" will be radiated within 3 microseconds of our key-up
state.
I don't care what amateur radio type of signal you are sending
at HF. Absolutely NO signal distortion, echo, etc. problem can
be detected/heard. And equally obviously, if you are at amateur
microwave band above 1 or 2 gHz somewhere, and are using
some type of pulse modulation, you just might find that the
3 microsecond time (phase error distortion) could become a
problem, depending upon signal and modulation method at
microwave frequencies.
So much for Robert's proposed loss caused by transmission line
impedance mismatches.
73, Jim, KH7M
On the Garden Island of Kauai
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