I guess I didn't do too well in trying to stay within the two ends of the
coax, and I apologize. I hope I caused no harm.
Robert, I'm not qualified to argue very deeply at the theoretical level
here. Attempting to make comments useful at a practical level, I roared
past the point that Dan cited a working, matched system which assumes a
50 ohm load at the feedpoint, with 50 ohm coax and a transmitter to
match. This being the case, the interface of the cables of different
capacitance would, I guessed, cause a slight reflection, insufficient to
consider replacement of the coax.
I do understand that the system, if seriously mismatched due to the
antenna's presentation of an improper load, will show significant loss
due to the resultant high SWR, but that loss should not be exacerbated by
the characteristics of the coax.
Moving on,
Robert, it is hard for me to handle the analogies of comparing SSB, etc
to impulse and to visualize echoes or even much in the way of discernible
noise from reflections of signals moving in their respective directions
in the relatively short length of coax at some 650,000,000 to 850,000,000
feet per second. I guess I feel better in terms of current and the
mismatch capability of cooking the finals! I liked the water better!
Even at my very best at CW I had trouble with dits in succession - I
gave up on a call from N5EE in complete desperation years ago.
Thanks again, Robert, for the wake up call!
73
Press Jones, N8UG, The Wireman, Inc., Landrum, SC, 29356
Sales(800)727-WIRE(9473) or cqwire@juno.com.
Tech help (864)895-4195 or n8ug@juno.com
www.thewireman.com
Our 21st year!
On Sat, 27 Jun 1998 13:32:32 -0400 Robert Hummel <rhummel@cheshire.net>
writes:
>
>>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.
>
>Can't help but throw in my two cents here...
>
>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)?
>
>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.
>
>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.
>
>Repeat this phrase until you believe it: Mismatch loss is REAL loss.
>
>
> *=======================*
>* Robert L. Hummel *
>* phone: 603-847-9444 *
>* email: rhummel@cheshire.net *
>* US mail: P.O. Box 214 *
>* Ground: 276 Gilsum Road *
>* Sullivan, NH 03445 *
> *=======================*
>
>
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>
>
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