"Richard Harrison" wrote
Cecil, W5DXP wrote:
"---a two foot long section of 50 ohm coax is all the length needed to
force the V/I ratio to be 50 ohms at HF---"

At 3 MHz?

When power is applied to a transmission line, energy from the power
source doesn`t appear everywhere along the line at once.
( much clippage)
Just how short can a transmission line be and still enforce its Zo? A
1/4-wave matching section inverts impedance between its ends by
enforcing its Zo.
For Zo to equal the square root of L/C, (a resistance), XL must be much
greater than R, and XC must be much greater than G. These restrictions
impose frequency limits on Zo. And, these restrictions may place a low
frequency limit on how short a line can be and still enforce Zo.
______________

For a concept of what that length actually is in the real world, recall that
Bird Corp and others supply directional wattmeters giving reasonably
accurate measurement of forward and reflected power -- leading to an SWR
value. The coax sampling sections for RF frequencies at least as low as 540
kHz. is around 9" in length.

RF

Richard Fry wrote:
"For a concept of what that length actually is in the real world, recall
that Bird Corp. and others supply directional wattmeters giving
reasonably accurate measurement of forward and reflected power --
leading to SWR value."

True, and these work with mismatched loads if you have enough 50-ohm
cable connecting the wattmeter.

The Bird Model 43 wattmeter is 5.125 inches (13 cm) wide. This is the
distance between its input and output connectors. This length of "high
precision 50 ohm coaxial air line designed for insertion between the
transmitter or load" requires either some more 50-ohm line or a matched
load to enforce Zo.

IF you were to insert the Model 43 into most 75-ohm transmission
systems, the precision 50-ohm meter line of 5.125 inches would not
likely enforce the 50-ohm V/I ratio and the meter reading would be in
error. At VHF, 1/2-wave of connecting line including the Model 43
wattmeter is ideal, allowing you to insert and withdraw the meter
without affecting the match.

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote
IF you were to insert the Model 43 into most 75-ohm transmission
systems, the precision 50-ohm meter line of 5.125 inches would not
likely enforce the 50-ohm V/I ratio and the meter reading would be in
error.
________________

Yet a 50 ohm RF bridge or network analyzer with a 75 ohm termination applied
directly at its output port has no trouble showing the true SWR. These
measuring devices are looking at the same transition plane from 50 to 75
ohms as the Bird 43 would see with a 75 ohm load at its output port.

If the Model 43 is unable to make an accurate measurement of this, is that
not due to reasons other than not having the right 50-ohm V/I ratio in its
line section?

RF

Richard Fry wrote:
"If the Model 43 is unable to make an accurate masuremnt of this, is
that not due to reasons other than not having the right V/I ratio in its
line section?"

Many details of desisn, construction, and application must be chosen and
executed right to get accuracy, but the line impedance is essential.

Bird can adjust the current sample to exactly equal the voltage sample,
both taken from the transmission line at any point. But it must work
with a fixed voltage to current ratio. Bird chose 50 onms.

For a directional meter, it`s necessary to respond to one direction
while rejecting the other. When power is applied to a line, the
resulting current is is in phase with the volts. On reflection, the
volts and amps in the reflected wave are 180 degrees out of phase. The
phase difference of the reflected wave is used by Bird to distinguish it
from the incident wave.

By selecting and adjusting for equal samples of volts and amps in the
forward wave, their total is 2X that of either sample. But, the samples
from the reflected wave, being equal but out of phase, cancel.

To get the value of the reflected power samples, it is only necessary to
reverse the polarity of one of the samples. They are now in phase and
the forward power samples are now out of phase and cancel.

If some other voltage to current ratio is used for the power samples
than that of the design, the samples won`t be exactly equal and
cancellation of the undesired direction does not work.

Best regards, Richard Harrison, KB5WZI

Richard Fry wrote:
The coax sampling sections for RF frequencies at least as low as 540
kHz. is around 9" in length.

The guys over on s.p.e said it has something to do with conductor
spacing Vs conductor length. They said a 100/1 ratio is plenty
long enough for Z0 to assert itself.
--
73, Cecil http://www.qsl.net/w5dxp


-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----