With the popularity of the nanoVNA, one of the applications that is coming up regularly in online discussion is the use to measure velocity factor of cable and / or tuning of phasing sections in antenna feeds.
‘Tuning’ electrical lengths of transmission line sections
Online experts offer a range of advice including:
- use the datasheet velocity factor;
- measure velocity factor with your nanoVNA then cut the cable;
- measure the ‘tuned’ length observing input impedance of the section with the nanoVNA; and
- measure the ‘tuned’ length using the nanoVNA TDR facility.
All of these have advantages and pitfalls in some ways, some are better suited to some applications, others may be quite unsuitable.
Let’s make the point that these sections are often not highly critical in length, especially considering that in actual use, the loads are not perfect. One application where they are quite critical is the tuned interconnections in a typical repeater duplexer where the best response depends on quite exact tuning of lengths.
1. Use the datasheet velocity factor
Datasheet velocity factor will be fairly good at VHF for good quality solid PE or PTFE dielectric and sufficiently good for most purposes, but does not take into account connectors.
Foam dielectrics are less well controlled for velocity factor, and warrant measurement of the actual cable to be used.
I make the observation that most attempts to actually measure such cables demonstrate a failure of measurement technique, the datasheet is better than many measurers.
2. Measure velocity factor with your nanoVNA then cut the cable to formula
It is difficult to connect nothing but the test cable to the reference plane of the instrument, there is likely to be some transmission line section involved that is not purely test cable, and if the length and Zo are significant then results are degraded. This is more so if there are connectors on one or both ends.
The article Finding velocity factor of coaxial transmission line using the velocity factor solver offers a tool and explains how to use it to overcome some of these problems.
Velocity factor of good cables does not change much above 30MHz, so for VHF/UHF application measure velocity factor at a frequency where measurement is not so sensitive to small errors in length, effects of fringing capacitance etc.
3. Measure the ‘tuned’ length observing input impedance of the section with the nanoVNA
This approach sounds an obvious solution, but by itself it does not properly account for connectors and effects.
4. Measure the ‘tuned’ length using the nanoVNA TDR facility
This approach also sounds an obviously good solution as you leverage the smarts of the TDR… but it is subject to effects of connectors and the parameters of the TDR sweep. Better resolution and accuracy can usually be obtained by a conventional s11 sweep around section resonance… but connector effects have to be deducted… see 2.
if you come to an accurate figure for velocity factor of cable and measure the resonance of a known length of cable with connectors and terminated in a calibration grade open or short, you can calculate the equivalent electrical length of the connectors.
Finding other important reference points
Sometimes you may wish to find the electrical length to a key circuit node, like the node within a coaxial resonator. Where the node is in a T coax adapter, the distance is not hard to find, but for a resonator with separate input and output connectors, it takes more effort with calibrated cables and terminations to find the point of action of the internal circuits of the resonator. Establishing these reference points is important in finding the physical size of interconnecting cables in a cascade of resonators so that the points at which each resonator section acts are properly spaced electrically for maximum reinforcement of response of the whole duplexer.
- Beware of too much science applied badly.
- If measured velocity factor departs greatly from datasheet values, investigate.