Brainteaser
 

Last week I posted a brainteaser over on qrz.com. Nobody figured
it out. I wonder how many on r.r.a.a can figure it out.

Given: A one second long lossless transmission line with a
steady-state forward power of 200 watts and a reflected power
of 100 watts. How many joules are contained in that feedline?
--
73, Cecil, W5DXP

Brainteaser
 

I had Beaker set up an experiment to test this, and after he put out
the fire he started in his hair when the first load he tried melted a
hole in the lab bench, he finally got it set up right. He determined
that the mass of the line increased by about 3.338 femtograms when
powered as compared with it unpowered. We're now looking into what to
do with the line, as it's cluttering up the lab, and we're open to
suggestions.

Regards,
Bunsen

Cecil Moore wrote:
Last week I posted a brainteaser over on qrz.com. Nobody figured
it out. I wonder how many on r.r.a.a can figure it out.

Given: A one second long lossless transmission line with a
steady-state forward power of 200 watts and a reflected power
of 100 watts. How many joules are contained in that feedline?
--
73, Cecil, W5DXP

Brainteaser
 

Cecil, W5DXP wrote:
"How many joules are contained in that feedline?"

I`ll speculate that after one second, 200 joules are contained in the
forward wave on that line. Then, after two seconds, another 100 joules
has been reflected back toward the line feedpoint where it opposes
growth of power input to the line. Total joules on the line is 300.

Forward power minus the reflected power equals 100 watts being supplied
by the generator to the load with 200 watts forward power and 100 watts
reflrcted power in the line.

Best regards, Richard Harrison, KB5WZI

Brainteaser
 

Richard Harrison wrote:
Cecil, W5DXP wrote:
"How many joules are contained in that feedline?"

Total joules on the line is 300.

That can be shown to be true by noting that during the
transient buildup to steady-state, 300 joules sourced
by the generator have not yet reached the load. That
remains true until the generator is powered down, i.e.
all during steady-state.

Forward power minus the reflected power equals 100 watts being supplied
by the generator to the load with 200 watts forward power and 100 watts
reflrcted power in the line.

But what about the people who say there's no energy in the
reflected wave? Reckon how they sweep all those joules,
whose energy must be conserved, under the rug? :-)
--
73, Cecil http://www.qsl.net/w5dxp

Brainteaser
 

One should also carefully consider the
more interesting variation of the
problem: an open transmission line. In
the steady state we have 100 watts
forward, 100 watts reflected, 200 Joules
in the line, and 0 watts being sourced
by the generator. :-)

ac6xg

Cecil Moore wrote:
Richard Harrison wrote:

Cecil, W5DXP wrote:
"How many joules are contained in that feedline?"

Total joules on the line is 300.


That can be shown to be true by noting that during the
transient buildup to steady-state, 300 joules sourced
by the generator have not yet reached the load. That
remains true until the generator is powered down, i.e.
all during steady-state.

Forward power minus the reflected power equals 100 watts being supplied
by the generator to the load with 200 watts forward power and 100 watts
reflrcted power in the line.


But what about the people who say there's no energy in the
reflected wave? Reckon how they sweep all those joules,
whose energy must be conserved, under the rug? :-)

Brainteaser
 

Jim Kelley wrote:
One should also carefully consider the more interesting variation of the
problem: an open transmission line. In the steady state we have 100
watts forward, 100 watts reflected, 200 Joules in the line, and 0 watts
being sourced by the generator. :-)

Yes, but the 200 joules in the line was previously sourced
by the generator during the transient state. It's hard to
sweep 200 joules under the reflected power rug.
--
73, Cecil http://www.qsl.net/w5dxp

Brainteaser
 

Jim Kelley wrote:
One should also carefully consider the more interesting variation of the
problem: an open transmission line. In the steady state we have 100
watts forward, 100 watts reflected, 200 Joules in the line, and 0 watts
being sourced by the generator. :-)

Expanding on my earlier response - For the first two seconds,
the source doesn't know it is looking into an open transmission
line so a 100 watt source would faithfully output 200 joules
into a one second long open circuit transmission line. That
200 joules cannot be destroyed. Is it mere coincidence that
the forward and reflected waves are 100 joules/sec*(one second),
exactly equal to the 200 joules supplied by the source?
--
73, Cecil http://www.qsl.net/w5dxp

Brainteaser
 

Dr. Honeydew wrote:

I had Beaker set up an experiment to test this, and after he put out
the fire he started in his hair when the first load he tried melted a
hole in the lab bench, he finally got it set up right. He determined
that the mass of the line increased by about 3.338 femtograms when
powered as compared with it unpowered. We're now looking into what to
do with the line, as it's cluttering up the lab, and we're open to
suggestions.

Regards,
Bunsen


That is the best response we will get.

Even if there are correct ones!

tom
K0TAR

Brainteaser
 

On Thu, 25 May 2006 22:04:12 GMT, Cecil Moore
wrote:

But what about the people who say there's no energy in the
reflected wave? Reckon how they sweep all those joules,
whose energy must be conserved, under the rug? :-)

with a jouler's brush?

--
73 for now
Buck
N4PGW

Brainteaser
 

Buck wrote:
Cecil Moore wrote:

But what about the people who say there's no energy in the
reflected wave? Reckon how they sweep all those joules,
whose energy must be conserved, under the rug? :-)

with a jouler's brush?

All kidding aside, whatever number of joules of energy are
required to support the forward power and reflected power
is provided by the source during the transient condition
following power up.

If the source power is 100 watts, the forward power is 200
watts, the reflected power is 100 watts, and the load power
is 100 watts, all the joules per second needed to support
that forward and reflected power was supplied by the source
before steady-state was reached. Anything else would violate
the conservation of energy principle.

This is easily proven using a one second long lossless
transmission line as a conceptual training aid for the
uninitiated.
--
73, Cecil http://www.qsl.net/w5dxp