[TowerTalk] Mismatch loss is REAL loss

[Robert Hummel]

>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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