What is a 50-ohm environment. ???
 

On Sat, 15 Oct 2005 21:20:16 GMT, Owen Duffy wrote:

Look inside a HF transceiver, it is full of
discontinuities between the PA collector and the coax socket. Design
practice for HF transceivers does not consider potential losses from
evanescent modes.

Hi Owen,

Some of the minor points here would be, to use the term you use
yourself later in your post, swamped. The swamping contributors are
generally attended to in design, and those corrections draw in and
encompass these smaller points.

A lot of this is handled by the low pass filtering following the final
stage's Z transformer in a transistor set.

If there were significant dissapative losses from such
discontinuities, don't all the texts on stub tuner design need to be
rewritten.

These would be called mismatch loss, which is not dissipative, but
rather what power was not delivered, that could have been if not in
the face of the discontinuity. Think of it as a loss of opportunity,
not a loss down the drain. Consider that same low pass filter
following the transformer. It withholds the power found in the
harmonics, and yet few filters need as much cooling as do the finals
transistors that are arcin' and sparkin' with those transients.

The low pass filter is a cheap alternative to linearizing the finals
stage and thus reducing those spurs. This would run the finals at a
lower gain however (to be put into what Bode calls the noise gain, or
negative feedback), and you would need more dollar investment for the
same power out.

Where is the experimental evidence that significant power is diverted
in practical circuits and transmission line discontinuities?

A practical example can be found in any RADAR system. You have ATR
tubes that perform power steering in waveguides. The list goes on
with lots of goodies (can anyone explain the Magic-T?) as RADAR is
particularly theatrical in this arena.

Every connection to a waveguide uses the physics of discontinuity to
suppress leakage. Examine the choke fittings, they are series shorted
tuned cavities used to bridge joints that necessarily have some
prospect of not maintaining entirely mating interfaces.

Perhaps it does demonstrate that the loss caused by the discontinuity
is not significant in terms of the specified accuracy of the Bird 43.

And then there is that distinct possibility. In such cases you design
the test to discard that perturbation by increasing the effect being
investigated. This is why I suggested a large mismatch would render
this chatter about 4W as being inconsequential.

73's
Richard Clark, KB7QHC

What is a 50-ohm environment. ???
 

On Sat, 15 Oct 2005 15:01:45 -0700, Richard Clark
wrote:

On Sat, 15 Oct 2005 21:20:16 GMT, Owen Duffy wrote:


Where is the experimental evidence that significant power is diverted
in practical circuits and transmission line discontinuities?

A practical example can be found in any RADAR system. You have ATR
tubes that perform power steering in waveguides. The list goes on
with lots of goodies (can anyone explain the Magic-T?) as RADAR is
particularly theatrical in this arena.

Every connection to a waveguide uses the physics of discontinuity to
suppress leakage. Examine the choke fittings, they are series shorted
tuned cavities used to bridge joints that necessarily have some
prospect of not maintaining entirely mating interfaces.


Yes, I should have qualified the statement to scope it at HF. I am
aware of the risk of excitation of undesirable modes in waveguide and
the need for mode traps to deal with them where their propagation is
undesirable.

I am not questioning whether physical discontinuities give rise to
electrical changes that can be explained or modelled with by lumped
constants or excitation of other propagation modes, I am questioning
whether the effects are significant in practical applications below
microwave frequencies, and especially at HF.

Owen
--

V/I ratio is forced to Z0:was Mythbusters
 

Richard Clark wrote:

On Sat, 15 Oct 2005 20:37:11 GMT, Cecil Moore wrote:
http://www.mfjenterprises.com/man/pdf/MFJ-816.pdf
What do you think is the purpose of the 10pf variable cap
if not to vary the voltage in the voltage divider?

You don't know, do you? :-)
It adjusts the frequency correction at the high end of the meter's
frequency range.

From page 27-8 ARRL Antenna Book 15th edition. "Capacitive
voltage dividers, C1-C3 and C2-C4, are connected across the
line to obtain equal-amplitude voltages in phase with the
line voltage, the division ratio being adjusted so that
these voltages match the voltage drops across R1 and R2
in amplitude."

In other words, it is the main Z0 calibration setting that
sets the sampled voltage equal to the sampled current while
driving a dummy load equal to Z0. Frequency response is
a completely secondary consideration.

If the range of C1 and C2 are great enough, the wattmeter
could be calibrated for 75 ohms rather than 50 ohms.

The question is: Between the "transmitter" terminal and
the "antenna" terminal, what determines the physical
characteristic impedance of the sampling circuit?

It is very lightly loading as a series load by
design and as evidenced by Dave's measurements.

Exactly how much effect does that light loading have on
the primary voltage/current amplitude and phase? Enough
to be detectable if the V/I ratio is not 50 ohms?
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

Richard Clark wrote:
so much for relying on an obscure poster quoted indirectly by
paraphrase to a new context. Such is third hand information.

Which is better? Third hand or under hand? :-)
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

On Sat, 15 Oct 2005 23:01:02 GMT, Cecil Moore wrote:

Owen Duffy wrote:
How does rearranging the terms here increase 0.203" to 2.03"?

Already caught that boo-boo. Correcting it brings Kevin's math
in line with Dave's.

So you were aware of the apparent defect in the work you were citing
as recently as less than two hours ago, and it took someone else to
notify the defect here?

Owen
--

What is a 50-ohm environment. ???
 

Owen Duffy wrote:
If there were significant dissapative losses from such
discontinuities, don't all the texts on stub tuner design need to be
rewritten.

We're not talking about losses, Owen. We are talking about the
changing relationships between V and I at an impedance discontinuity.
Why do you think they call them discontinuities?
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

Dave wrote:
who is kevin rhodes?

The guy from sci.physics.electromag whom I quoted previously
in the "V/I forced to Z0" thread. It appears that he
accidentally replaced the conductor spacing of 0.203" with
2.03" in his calculations. If that is corrected, his values
tend to agree with yours given the different conductor
spacing between RG8X and RG213.

So can we say, the lowest order undesired mode should reduce
intensity by a factor of 1/e in about one conductor spacing,
1/e^2 in two conductor spacings, etc.?
--
73, Cecil http://www.qsl.net/w5dxp

V/I ratio is forced to Z0:was Mythbusters
 

Dave wrote:
obviously between the tx and ant terminals it looks like a 50 ohm
transmission line.

So I repeat, what causes that characteristic? Is there
some coax inside the MFJ box?
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

Owen Duffy wrote:
How does rearranging the terms here increase 0.203" to 2.03"?

Already caught that boo-boo. Correcting it brings Kevin's math
in line with Dave's.
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

"Owen Duffy" wrote in message
...
On Sat, 15 Oct 2005 23:01:02 GMT, Cecil Moore wrote:

Owen Duffy wrote:
How does rearranging the terms here increase 0.203" to 2.03"?

Already caught that boo-boo. Correcting it brings Kevin's math
in line with Dave's.

So you were aware of the apparent defect in the work you were citing
as recently as less than two hours ago, and it took someone else to
notify the defect here?

Owen
--

ugh! all of this was over a slipped decimal point??? so we are down to .2"
transition, which pretty much agrees with the one i came up with, and which
basically means that by the time you are out of the connector shell you are
back at Z0. and since the meter takes its own 50 ohm 'environment' with it
for sampling it is reading every thing exactly as it should... and exactly
as has been measured... and there is no requirement for some particular
length of 50 ohm coax on either side of a meter... what a waste of a
perfectly good argument, you better apologize big time for this one cecil!