> Some time ago, I gave a seminar to the local radio club
about SWR. To provide an illustration, I used a visual
explanation including a garden hose--actually a series of
various diameter garden hoses tied end-to-end. I explained
that any transition from one diameter to another caused an
impedance 'bump' that had a negative impact on flow. >
Well, I can't think of a way to make that work very well
Ford. Any picture it paints is not an accurate
representation of transmission line.
In a system of transmission lines, a bump does not
necessarily have a negative impact on energy transmission.
In a hose, it represents a change in backpressure and flow
rate vs time. There are no standing waves. Only pressure and
volume. In a transmission line, a bump changes the
relationship between voltage and current for every point
upstream towards the transmitter. That might decrease loss,
increase loss, or not change loss at all.
<< In my analogy, going from 5/8" hose to 1/2" hose induced
a higher pressure and reduced flow due to back pressure.
Going from the 5/8" to 1/2" to 3/4" caused a loss of
pressure after the 1/2" hose and likewise an additional
reduction in flow. The analogy went over pretty good with
those members that, until that point, had no explanation for
using 50 ohm coax vs 75 ohm coax or why there was a need for
antenna matching network--basic stuff.>>
There is no reduction of flow based only on impedances. The
only thing impedance changes mean is the ratio of across
vector to through vector changes. There are actually cases
where an increases SWR reduces loss. For example, in a
fractional wavelength transmission line on HF lowest loss
occurs when the line is operated with a mismatch in the
direction of reduced current on the line. This is because
loss is primarily resistive, and dielectric losses have
minimal effect.
As a matter of fact this is how we can sort out a good
transmission line model from a bad one. If you have a very
short transmission line and mismatch it some reasonable
amount in a direction that reduces current, loss should
decrease. If you mismatch the same line in a direction (even
a small amount) that increases current, loss should
increase. Once the line is long in terms of the wavelength,
this variation goes away and any mismatch direction
increases loss a similar amount.
I can have a transmission line of 600 ohms feeding an
antenna of 60 ohms, and have negligable loss caused by SWR.
Energy transfer can be nearly 100%.
> Lately I've been pondering the notion of the impedance of
free space. I'm told that the characteristic impedance of
free space is 377 ohms. I have no idea where that comes
from, or how it is measured. But I do know that the
characteristic impedance of a 1/4wL monopole, properly
constructed, is about 38 ohms. That's a ~10:1 impedance
ratio. What am I missing here? Is there any intrinsic loss
attributable to coupling a relatively low impedance system
to the high impedance of free space? E.g. is there any
benefit to using a higher impedance system?
We'd have to firmly grasp what they mean by "impedance" of
freespace Ford.
Humans have a very limited perception of what the forces
caused by charges are. We can only use a limited number of
terms to describe the effects of one charge on another. We
can measure a force we know is caused by charge distribution
differences between two points. That's the electric field.
We can measure a force caused by the charges moving, that's
the magnetic field. We can't directly measure radiation, so
we measure one of those two side effects. We call the ratio
of those two effects "impedance".
Radiation comes from NEITHER of those two static or energy
storage effects. It is a different force that is caused by
acceleration of charges. This is where the guys who invent
goofy antennas like the Super C, the EH, and the CFA all go
wrong. They think the fields, which are mathematical
"things" we use to describe the effects we can observe, are
what really happens. That we can mix two effects created by
different causes and make a third thing! It's like saying a
brick is 4" and weighs three pounds, so if we mix 4" with
three pounds we must have a brick.
When a radiation effect is far from the source and not
around anything that is re-radiating it, it settles on a
certain ratio of the effects we can observe. We call that
ratio the "field impedance" or "impedance of freespace".
That does not mean in any way, shape, or form your antenna
matches to the "impedance" of free space. Thinking it does
will get you in deep trouble quickly. The impedance we
measure at the antenna terminals has little to do with the
impedance of a wave traveling in space. There isn't any set
mismatch loss involved, it is a transition from one ratio
just like in transmission lines.
I have something about this at:
http://www.w8ji.com/radiation_and_fields.htm
73 Tom
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