[Antennas] Complex Transmission Line Impedance

[George, W5YR]

Thanks, Barry - your comments and opinion are much appreciated.

Your strain-gage experience is interesting. I had a similar experience
before retirement at Texas Instruments where I was responsible for all of
our worldwide network (our private Internet!) performance and availability
of service.

One problem involved our IMS terminals which were connected to central
controllers via shielded twisted pair. The original system engineers - I use
the term loosely - failed to consider that not only did the line function in
the manner of a transmission line but that at the frequencies involved, its
Zo was considerably reactive, thereby doing a wondrous dispersion job on the
packet pulses.

With thousands of such terminals in the system, replacing the distribution
system and/or modifying the terminal circuitry was out of the question. So,
I took the easy way out and just swamped the terminal input connection with
an appropriate resistor to more or less terminate the line. This reduced the
errors due to multiple reflected pulses. We had to live with the dispersion
but the data rates were sufficiently low that this could be tolerated.

I hope that we can terminate this discussion soon before it hatches the
other seasonal jousts such as the *real* direction of "current" flow and the
*real* difference between "voltage decibels" and "current decibels."

My apologies to the list if my last response to Jim seemed to lack my usual
positive outlook and overall good humor approach.

And for the record, there are many references in the ARRL Antenna Book, for
example, to complex characteristic impedance, including one HF amateur
example where it is involved in the solution of the problem.

73/72, George
Amateur Radio W5YR -  the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13QE
"In the 57th year and it just keeps getting better!"
<mailto:w5yr@att.net>





----- Original Message -----
From: "Barry L. Ornitz" <ornitz@tricon.net>
To: <antennas@mailman.qth.net>
Cc: <n7ws@yahoo.com>; <dufferjames@hotmail.com>; <w5yr@att.net>
Sent: Saturday, May 10, 2003 12:46 AM
Subject: [Antennas] Complex Transmission Line Impedance


> I agree fully with George's comments:
>
> > Jim, you seem to prefer keeping this discussion on the ]
> > reflector when your objections properly belong in a private
> > forum. However, that is your choice. . .
> >
> > Let me be as brief as I can.
>
> > You have chosen to view a very complex subject through the
> > convenient knothole of "typical amateur radio HF" practices.
> > Consequently, you have reached conclusions which while they
> > work well enough in that area, are NOT generally true, and
> > neither Wes nor I are in error for pointing this out to
> > you.
>
> Earlier George had written:
>
> >> 2. depending upon the frequency range involved, the
> >> reactive component Xo of Zo may or may not be of
> >> importance.
>
> And Jim replied:
>
> > *****DEFINITELY********  Especially when the frequency range
> > is within the normal range for the particular transmission
> > lines application (where line losses are tolerable)
>
> This clearly shows he still does not understand and is trying
> to pick bits and pieces of information out of context from
> accepted texts.
>
> Actually transmission lines show the most complex
> characteristic impedance effects at low frequencies - where
> their losses are the lowest.  At such frequencies, the
> dielectrics are nearly perfect (polyethylene and
> polytetrafluoroethylene have negligible losses at such
> frequencies).  Likewise below the frequency where the inner
> conductor diameter is the same as the skin depth, the series
> resistance is the least.
>
> I discovered the reality of a complex characteristic impedance
> several years ago in a particular instrumentation problem I
> was working on.  I was trying to measure extreme high
> frequency mechanical vibration on a piece of equipment located
> in a hazardous area (explosive vapors).  I was using a 100 ohm
> half-bridge strain gage to measure the mechanical deflection
> of the equipment.  Thus my output signal (millivolts) came
> from an essentially 50 ohm resistive source.
>
> In the lab, I was easily able to measure signals on a FFT
> spectrum analyzer up to several hundred kilohertz with a mock-
> up of the mechanical system.  Because of the explosive
> environment, my strain gage was connected by several hundred
> feet of 50 ohm, double shielded coaxial cable to the analyzer
> in a safe location.  Power to the bridge came from an
> intrinsically safe supply.  Double shielding was used to
> minimize noise pickup on the low level signals.  In
> retrospect, shielded twisted pair with a full bridge gage
> might have been a better choice.
>
> Cable losses at these frequencies should have been a small
> fraction of a decibel, yet I saw great differences in the
> spectrum between the lab hookup and the process hookup.
>
> It turned out that I had not considered the fact that the
> cable no longer had a resistive 50 ohm characteristic
> impedance.  Even though the cable length of a few hundred feet
> was a small fraction of a wavelength, it was enough to cause
> considerable dispersion in the waveforms and very significant
> apparent attenuation versus frequency effects.
>
> Once I realized what was happening, I calculated the real
> cable characteristic impedance as a function of frequency and
> used this to mathematically "deconvolve" the data to get the
> correct spectra.  It matched the lab data perfectly after
> this.
>
> This example shows the importance of a complex characteristic
> impedance.  In fact, any shielded wire at low frequencies
> shows considerable capacitive effects.  Knowing when and where
> to use the resistive approximation to characteristic impedance
> is important and well worth discussing here.
>
>         73,  Barry L. Ornitz     WA4VZQ     ornitz@tricon.net
>
> - - -
>
> Your moderator for this list is:
> Larry Wilson KE1HZ antennas-owner@mailman.qth.net
> _______________________________________________
> Antennas mailing list
> Antennas@mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/antennas