Ian Jackson wrote:
In message ,
writes
Ian Jackson wrote:
In message ,
writes
Ian Jackson wrote:
In message ,
writes
Ian Jackson wrote:


Even when the only transmission line consists the output connector of
the SWR meter, and maybe an inch of internal coax, there will still BE a
standing wave - but it will only be a tiny portion of longer one.

There will NOT be standing waves and there will not be a voltage
maximum and a voltage minimum unless there is a transmission line.

Are you saying that for a standing wave to qualify as a standing wave,
the transmission line needs to be long enough for there to be a voltage
maximum a voltage minimum?

I am saying that if the connection between the two things of interest
is short enough in terms of wavelengths at the frequency of interest,
the connection no longer functions as a transmission line, there are
no standing waves, but the measurment we call SWR still exists.

Pray tell me exactly (in wavelengths) when something which is too short
to be a transmission line suddenly changes into something which IS long
enough to be a transmission line.

Sure.

A transmission line is distinguished from a wire by the fact that a
transmission line carries the energy in the form of an electromagnetic
field contained by the structure of the transmission line while a
wire carries the energy in the form of conduction in the wire.

This is true for all transmission lines, be they parallel, coaxial,
wave guide, microstrip, stripline, or any other type of transmission line.

A conducting structure becomes a transmission line when it's length in
wavelengths becomes long enough to allow the establishment of an
electromagnetic field within it's structure.

The general rule of thumb is that the dividing point is about 1/10 of
a wavelength.

For the pendatic, this does NOT mean that at exactly 1/10 of a wave
length things suddenly change, it means that in general transmission
line effects become negligable below 1/10 of a wave length.

A piece of coax will not function as a transmission line at 1/100
of a wavelength even though it is constructed to be a transmission line
because it is too small to establish an electromagnetic field between
the center conductor and the shield.

Note: This is a slightly simplified explaination, for details and
mathematical derivations:

https://en.wikipedia.org/wiki/Transmission_line
http://www.antenna-theory.com/tutori...ine.php#txline
http://www.ece.uci.edu/docs/hspice/h...001_2-269.html
http://www.allaboutcircuits.com/text...nt/chpt-14/lon
g-and-short-transmission-lines/

Standing waves only occur on a transmission line and are due to reflections
on the line.

If the line length is too short to act as a transmission line, there
can be no reflections and no standing waves.

I haven't checked those references yet, but regardless of what they may
say, if that 10' of coax between my TX and my 160m ATU is NOT a
transmission line - just what IS it? Do different laws of physics
apply?

No, it means that you can view the coax as just a wire and that the
transmission line effects are negligable.

About the only practical consequence of this that I can think of off
the top of my head at the moment, is that a very short, in wavelengths,
piece of coax does not have a characteristic impedance so it would
not matter what kind of coax you use.

To put it another way, if your system is the common 50 Ohms and you
had the very best of lab grade test equipment, for very short lengths
you would see no difference between using 50 Ohm coax and 100 Ohm coax.

As you increase the length, you begin to see differences, and at some
length around 1/10 of a wavelength the differences become big enough
to be significant.


--
Jim Pennino