I see online discussion of specification bending radius for coax cables, and their application to ferrite cored common mode chokes.
A low Insertion VSWR high Zcm Guanella 1:1 balun for HF and follow on articles described a balun with focus on InsertionLoss.
Let’s remind ourselves of the internal layout of the uncompensated balun.
The coax is quality RG58A/U with solid polythene dielectric. The coax is wound with a bending radius of about 10mm, way less than Belden’s specified minimum bending radius of 50mm.
So, the question is does this cause significant centre conductor migration that will ruin the characteristic impedance:
- when it was first constructed; and
- through life.
Note the pigtails at each coax connector, they are a departure from Zo of the coax and the N type connectors. They can be seen as short sections of transmission line with Zo perhaps 200Ω or more. The effect of these is to transform impedance and so cause the input VSWR to depart from ideal.
When first constructed
Above is a chart from the original construction articles. InsertionVSWR @ 30MHz is about 1.15.
Note the pigtails at each coax connector, they are a departure from Zo of the coax and connectors. They can be seen as short sections of transmission line with Zo perhaps 200Ω or more. The effect of these is to transform impedance and so cause the input VSWR to depart from ideal.
After 5 years of service
This article presents measurement of the balun 5 years after it was made, 5 years in use, but not operated at temperatures above datasheet maximum.
As mentioned, the pigtails are the main contribution to InsertionVSWR. The balun was compensated using 10pF shunt capacitors at both coax connectors. (Possible compensation solutions were discussed at A low Insertion VSWR high Zcm Guanella 1:1 balun for HF – more detail #3).
Above is a NanoVNA screenshot of measurement of the compensated balun using the calibration LOAD, a couple of SMA(F)-N(M) adapters and a short SMA(M)-SMA(M) cable.
InsertionVSWR is highest at 1.01 @ 16MHz, at the limits of accuracy with this equipment. These 3kV 10pF ceramic capacitors have measured Q around 500 at 30MHz, expected loss is less than 200mW @ 1kW through.
The InsertionVSWR is not significantly affected by the quite small bending radius.
Voltage withstand
Does tight bending of this cable degrade voltage withstand of the balun?
Experience Hipot testing lots of baluns with this type of coax winding shows that the voltage withstand weakness is over the surface of the dielectric from braid to centre conductor, and pigtails of less than 15mm will flash over before internal flashover.
Alternative coax types
Coax with a solid PTFE dielectric is more suitable as the dielectric is harder and withstands higher temperatures before deformation.
Foamed dielectric cables are much more prone to migration of the centre conductor on tight bends, even at room temperature and are probably unsuitable for tight wraps.
Small diameter cables might seem the obvious answer, but they are higher in loss and will run at higher temperature.
Conclusions
Though the coax bend radius is substantially smaller than specification minimum bend radius:
- when first constructed, there was little evidence that the coax characteristic impedance was altered by the winding radius, and that the pigtails were the main contribution to InsertionVSWR; and
- after five years of service, the InsertionVSWR of the compensated balun is excellent, at the limits of accuracy of the test equipment and again, little evidence that the coax characteristic impedance was altered by the winding radius.
Solid dielectric coax may be quite satisfactory at static tight bend radius, subject to the temperature of operation and applied forces.