There are often times when it would be useful to transform measurements made looking into a feed line to the other end of the feed line.
Common advice given by online ham experts include:
- it just cannot be done, the best (only) point to measure an antenna is at the feed point;
- it can be done, but only with an integral number of half waves of feed line;
- use the port extension facility in your software;
- use software package x;
- do an OSL cal with the feed line being part of the fixture.
It just cannot be done
That is simply wrong.
It can be done, but only with an integral number of half waves of feed line
This usually assumes that the impedance at the load end repeats every electrical half wave. This is true for lossless lines, but there is error in applying that assumption to real world lines and the error may be significant. If a range of frequencies is measured with a fixed length line, then further error is introduced as the line is not exactly a half wave (or multiple) at all frequencies.
It is classic ham pseudo science.
Use the port extension facility in your software
Some software includes a facility to apply a constant time offset to measurements (at the calibration Zo). This addresses the multiple frequency problem described for the last method, but it does not address the loss or more grossly, a different Zo.
Use software package x
Some software packages do de-embed transmission line using a loss model for the specified line. Trust these only to the extent that you understand the accept that the loss model is appropriate to your application.
Most packages I have evaluated use a transmission line model that I would prefer to not use for many purposes, so I tend to not turn to those packages.
I do use Rigexpert Antscope 4.2.57 which has a simple add/subtract line facility which I used with line data derived from TLLC, it is quick, interactive, convenient and effective. It is the only off the shelf software package that I use for this purpose.
I do use custom spreadsheets, and PERL, Python, and iPython scripts for de-embedding.
Do an OSL cal with the feed line being part of the fixture
This does work, and it does properly place the reference plane at the desired measurement terminals.
It may not be convenient or even possible in some scenarios, but can be an excellent solution.
If you are able to save the calibration files and restore them at a later time, new measurements can be compared with historical archives and it becomes more practical as the calibration is a once only operation.
Unfortunately, some VNAs and analysers invalidate old version calibration files apparently with disregard for their possible value… so don’t count on this method beyond the current measurement set.
The following examples show different techniques and an explanation of why the transformation was useful.
UHF mobile vertical
In these examples a scan of a UHF mobile vertical was made using a nanoVNA looking into 4.025m of Belden 8259 coax and saved as a s1p file.
This is a case where it is not practical to measure directly at the feed point as the feed point is hidden between the vehicle roof and roof lining. Nevertheless and understanding of the feed point characteristics is useful, especially more complex antennas as such as this ‘gain antenna’.
Above is a Smith chart of the actual measurement data looking into the feed line.
Above is the measurement data with the transmission line de-embedded using custom Python scripting and graphics to transformed the saved raw .sp1 data.
Antscope provides a similar facility with its add/subtract cable feature.
Above is the actual measurement data rendered in Antscope v4.2.57.
Above is the Smith chart plot with the known transmission line ‘subtracted’ using its interactive facility. This used the same transmission line data as in the Python scripts, but the internal algorithms are unknown.
Above is the transformed s1p file from the Python scripts rendered in VNWA, it does not have a facility to de-embed transmission line of this type (its port extension feature does not model line loss effects).
These graphs are based on measurements made with a Rigexpert AA-600, and de-embedded in spreadsheet calculations. In this case, it was simply convenient physically to measure looking into a short length of feed line.
Above is a set of measurements looking into 3.2m of Belden 8262 coax.
Above, the coax de-embedded to give the feed point Z which give a clearer understanding of the antenna behavior.
Another 7MHz vertical
These are measurements of a 7MHz vertical using a MFJ202B noise bridge through 25m of LMR400 coax. In this case, the feed point was covered in snow and direct measurement was not convenient. Note the instrument (which needs a receiver), it takes quite a long time to make a measurement set like this ‘by hand’ and it was much more convenient to do this from indoors and de-embed the feed line effects.
Above, the actual measurement data. Minimum VSWR is just below the band, but why?
Above is the measurement with the coax de-embedded to give a good estimate of the feed point impedance. This perspective shows that R is changing relatively slowly with frequency, and X is just too negative, it is classic behavior of a near quarter wave vertical and in this case, the vertical needs to be a little longer or include some series inductive loading.
There are methods to de-embed feed line, they each bring their own uncertainty and have their own practicality, repeatability, etc.