where does that come from?

"Reg Edwards" wrote in message
...
For anyone who may be interested.

Typical of RG-58 and RG-11 type cables.
Zo = Ro - jXo
Xo is always negative.
Angle of Zo in degrees. Always negative.
VF = relative velocity.

Freq Ro jXo Angle VF
------ ------ ------ ------ ------
50 Hz 967 -965 -44.95 0.034
1 kHz 220 -213 -44 0.151
10 kHz 80 -58 -36 0.41
100 kHz 56 -9.3 -9.5 0.59
1 MHz 52.4 -2.4 -2.7 0.63
10 MHz 50.7 -0.76 -0.86 0.65
100 MHz 50.2 -0.23 -0.27 0.66

Smith Chart calculations begin to be inaccurate around 2 MHz and below.
So
do SWR meters.

I never thought about coax having a low frequency effect, but makes sense.
Twisted pair telephone wire is considered to be 600 - 900 Ohms with a large
capacitive component at voice frequencies, by 25KHz it is about 135, and at
T1/E1 frequencies, which are over 1 MHz, about 100 Ohms.

Tam/WB2TT
"Reg Edwards" wrote in message
...
For anyone who may be interested.

Typical of RG-58 and RG-11 type cables.
Zo = Ro - jXo
Xo is always negative.
Angle of Zo in degrees. Always negative.
VF = relative velocity.

Freq Ro jXo Angle VF
------ ------ ------ ------ ------
50 Hz 967 -965 -44.95 0.034
1 kHz 220 -213 -44 0.151
10 kHz 80 -58 -36 0.41
100 kHz 56 -9.3 -9.5 0.59
1 MHz 52.4 -2.4 -2.7 0.63
10 MHz 50.7 -0.76 -0.86 0.65
100 MHz 50.2 -0.23 -0.27 0.66

Smith Chart calculations begin to be inaccurate around 2 MHz and below.
So
do SWR meters.

"David Robbins" wrote
where does that come from?
=========================

Out of my head. I've just said so, havn't I ?

"Dave Shrader" wrote
What's the source for this data?
=========================

In my head. I just calculated it from first principles.

I did copy a teeny bit from Volta, Ampere and Ohm.

Isn't it in the ARRL Handbook?

Tarmo:

[snip]
"Tarmo Tammaru" wrote in message
...
I never thought about coax having a low frequency effect, but makes sense.
Twisted pair telephone wire is considered to be 600 - 900 Ohms with a
large
capacitive component at voice frequencies, by 25KHz it is about 135, and
at
T1/E1 frequencies, which are over 1 MHz, about 100 Ohms.
[snip]

Depending upon it's length and how it is terminated, telephone twisted pair
is
approximately 5 - 10K Ohms and highly capacitive at the very low end near
100 - 200 Hz and only reaches 600 Ohms somewhere in the range of
1 - 2 KHz and then as you noted it continues downward to around 100 Ohms
at over 1MHz. In the usual lengths in which it is deployed by the telephone
companies,
i.e. [1500 Ohm design rules which "reach" to a maximum of 18,000 feet] it is
quite lossy usually having a DC resistance in the neighbourhood of 1500
Ohms.

For modern xDSL systems which span from a few hundred Hz up into the MHz,
the telephone twisted pair can fairly be considered a very lossy broad band
channel with
essentially no constant Zo. Zo varies all over the map! Cecil would have a
problem
calculating his "reflections" without a constant Zo, heh, heh... To
complicate things, such lines
often contains bridged taps, which are open circuit stubs hanging off the
line for "sparing"
purposes, and these stubs causes 1/4 wave "suck out" notches in the broad
band
transmission bandwidth.

Full duplex echo cancelled data transmission by xDSL techniques over such
channels
is problematic to say the least, but occurs in millions of instances daily.
In some xDSLs
the transmitters are operating simultaneously in the same bandwidth from
both ends while
the receivers on both ends have to operate in the presence of those local
transmitters over
18,000 foot distances with high attenuation and lots of cross-talk without
errors.

All of this is accomplished with dedicated DSP processors.

--
Peter K1PO
Indialantic By-the-Sea, FL

"Reg Edwards" wrote in message
...
"David Robbins" wrote
where does that come from?
=========================

Out of my head. I've just said so, havn't I ?


ok, just wanted to make sure you weren't pulling it out of somewhere else.

Peter,

I once worked on stuff that operated on leased 4 wire lines in the frequency
range of a few KHz to about 200 Khz. We were told by Telco systems engineers
to design for 135 Ohms. There were restrictions on what cable was usable. 19
ga was OK, 26 was definately not. Loading coils had to be removed. I think
it was called a "G" something conditioned line.

There is an FCC approved model for a telephone line input impedance at voice
frequencies. I can't remember the details, but it was something like two
resistors in series with a sum of around 1000 Ohms; the larger resistor
being shunted with a fair sized capacitor. You need this model to design,
for instance, a hybrid for a modem.

When the line is very short relative to a wavelength you could treat it as
something other than a transmission line. For instance, I once used a 1 inch
length of RG 58 as a neutralizing capacitor for an HF traansmitter.

Tam/WB2TT

On Thu, 14 Aug 2003 10:52:49 -0400, "Tarmo Tammaru"
wrote:

There is an FCC approved model for a telephone line input impedance at voice
frequencies. I can't remember the details, but it was something like two
resistors in series with a sum of around 1000 Ohms; the larger resistor
being shunted with a fair sized capacitor. You need this model to design,
for instance, a hybrid for a modem.


Hi Tam,

Sounds more like the model for the K-1 relay, or a hybrid decoupling
network or balancing network description. Telephone is the land of
600 Ohm specifications (min. 550, max. 850) for many good reasons
related to their networks.

The passband for voice frequencies does not demonstrate any simple one
time constant roll-off characteristic. Ma Bell couldn't afford that
kind of waste.

73's
Richard Clark, KB7QHC

An interesting number of significant figures, if it's to cover both
RG-58 and RG-11 types. I'd expect the RG-11 to converge a bit better
to 72-75 ohms, actually. I have a note I made some time ago in a
reference book that says, "Neglecting dielectric loss, if Zo = Ro+jXo,
Xo is approximately -0.180*Ro*A100*VF/f" where A100 is the attenuation
in dB/100 feet and f is the frequency in MHz. There. Now, at least
for RF, people should be able to generate their own tables.

Cheers,
Tom

"Reg Edwards" wrote in message ...
For anyone who may be interested.

Typical of RG-58 and RG-11 type cables.
Zo = Ro - jXo
Xo is always negative.
Angle of Zo in degrees. Always negative.
VF = relative velocity.

Freq Ro jXo Angle VF
------ ------ ------ ------ ------
50 Hz 967 -965 -44.95 0.034
1 kHz 220 -213 -44 0.151
10 kHz 80 -58 -36 0.41
100 kHz 56 -9.3 -9.5 0.59
1 MHz 52.4 -2.4 -2.7 0.63
10 MHz 50.7 -0.76 -0.86 0.65
100 MHz 50.2 -0.23 -0.27 0.66

Smith Chart calculations begin to be inaccurate around 2 MHz and below. So
do SWR meters.

Tom sez,
I have a note I made some time ago in a
reference book that says, "Neglecting dielectric loss, if Zo = Ro+jXo,
Xo is approximately -0.180*Ro*A100*VF/f" where A100 is the attenuation
in dB/100 feet and f is the frequency in MHz. There. Now, at least
for RF, people should be able to generate their own tables.
=============================

Your formula is correct and indeed approximate. But it's not of much use
unless you already know Ro, A100 and VF, all of which vary with frequency.
It is approximate because it ignores a correction factor related to
Sqr{(Wire Resistance) / (Wire inductive reactance)} which varies fast with
frequency.

At sufficiently high frequencies, where Ro (whatever that value may be),
A100 and VF have settled down, all that is necessary is to set Xo equal to
zero and be done with it, just like book-cooking Mr Smith of chart fame did
in 1938.

I produced the table to illustrate the peculiar unexpected effects which
occur with the smaller, popular diameter coax cables as used by amateurs. I
used the words "typical" and "type" in anticipation of remarks such as yours
from already educated people. The table applies closely to RG-58. RG-11, as
you pointed out, is 75-ohm nominal but its characteristics behave in a
similar fashion.

It should be stated that twin, balanced lines of all impedances behave in
the same way except that their HF characteristics extend down to
considerably lower frequencies.

I had in mind there are now LF amateur bands and anyone who may wish to do
transmission line calculations or use his SWR meter at low frequencies
should be warned against measuring errors. It's obvious he wouldn't get much
help from the present un-ending thread on that subject. ;o)

Use programs COAXPAIR and RJELINE3 for design and performance of
transmission lines from power frequencies, audio frquencies, up to UHF using
exact classical methods of analysis. Enter data in practical units and
obtain results in the form you are accustomed to using. Everybody is
familiar with the 1.5-inch diameter wires spaced 12 feet apart striding
across the countryside. Have you ever wondered what a 500-mile length would
sound like with a telephone at each end?
----
=======================
Regards from Reg, G4FGQ
For Free Radio Design Software
go to http://www.g4fgq.com
=======================





Cheers,
Tom

"Reg Edwards" wrote in message
...
For anyone who may be interested.

Typical of RG-58 and RG-11 type cables.
Zo = Ro - jXo
Xo is always negative.
Angle of Zo in degrees. Always negative.
VF = relative velocity.

Freq Ro jXo Angle VF
------ ------ ------ ------ ------
50 Hz 967 -965 -44.95 0.034
1 kHz 220 -213 -44 0.151
10 kHz 80 -58 -36 0.41
100 kHz 56 -9.3 -9.5 0.59
1 MHz 52.4 -2.4 -2.7 0.63
10 MHz 50.7 -0.76 -0.86 0.65
100 MHz 50.2 -0.23 -0.27 0.66

Smith Chart calculations begin to be inaccurate around 2 MHz and below.
So
do SWR meters.