Using Tuner to Determine Line Input Impedance
 

On Jun 7, 6:57*pm, Richard Clark wrote:
On Sun, 7 Jun 2009 10:54:46 -0700 (PDT), dykesc

wrote:
Do you use the MFJ-259B at the load, or through the line?

Through the line Richard.

OK. *Then you need to consider that the line is transforming the load
Z to the Z measured (by both methods) at the line input. *I realize
you already appreciate this. *However, the length of the line in
wavelengths also casts more exaggerated results into your computation
when that length is not a multiple of odd eighth wavelengths. *This
suggestion comes from Walt Maxwell's own work and sidenotes to his
measurements as published in any of his several releases of
"Reflections." *Read his commentary on this for more detail.

73's
Richard Clark, KB7QHC

When you said "At the load" I thought you meant at the antenna. When
you said "through the line" I thought you meant at the line input
(source end). I am measuring at the line input (source end) with both
methods. Are you saying that line length could be a factor in the
quality of the line input impedance measurements?

Thanks. Still learning here.

Dykes AD5VS

Using Tuner to Determine Line Input Impedance
 

On Sun, 7 Jun 2009 19:31:34 -0700 (PDT), dykesc
wrote:

When you said "At the load" I thought you meant at the antenna.

Hi OM,

Quite so, that is the convention.

When
you said "through the line" I thought you meant at the line input
(source end).

Quite so again.

I am measuring at the line input (source end) with both
methods.

I anticipated that.

Are you saying that line length could be a factor in the
quality of the line input impedance measurements?

Very much.

Thanks. Still learning here.

If you were to observe your line distance from the measurement out to
the load, and plot that, you want the line distance to the load to be
some odd-eighth interval of a wavelength long (1/8ths, 3/8ths, 5/8ths,
7/8ths, ... and so on). The reason being that your load Z will be
transformed through that odd eighth to a region on the Smith Chart
that has a milder shift in reactances and resistances for a slight
change in frequency. This means errors of line-length contribution
have a reduced impact on that transform. If you were in quarter
wavelength relationships, those chart lines of reactance and
resistance would change far faster for the same errors in line-length
determination.

This topic is covered at:
http://www.w2du.com/r2ch15.pdf
under the section at:
Sec 15.3 Antenna Impedances From Measured Line-Input Impedances

73's
Richard Clark, KB7QHC

Using Tuner to Determine Line Input Impedance
 

Richard Clark wrote in
:

....
This topic is covered at:
http://www.w2du.com/r2ch15.pdf
under the section at:
Sec 15.3 Antenna Impedances From Measured Line-Input Impedances

In S15.3, Walt lays out a method of characterising a length of line used
for measurement, and then using the characterisation, to refer
measurments of a load at the line input to the load end of the line.

TLLC at http://www.vk1od.net/calc/tl/tllc.php will perform the second
part of that using published line characteristics.

A quick check shows that TLLC produces very similar results when the VSWR
is low, and less so at the end of the data range where the VSWR is 10.
Reasons for that include that Walt used a characterisation of the cable
used for measurement, and those characteristics are not exactly the same
as published spec, and Walt's calculation engine makes some
approximations that are not made in TLLC.

The discussion highlights that the method is only as good as knowledge of
the coax characteristics, and the accuracy of the measurement instrument.
Errors will be worst when the line operates at high VSWR.

I would argue that if you use an MFJ259B, for measured values that are
not near the limits of the instrument, and |arg(Z)|70°, you will get
relatively good results from TLLC. Validating the length and loss of the
measurement coax to ensure it complies with spec is a precursor to using
TLLC with best accuracy.

Owen