RG-6/U for lower HF

RG-6 has become a popular 75Ω transmission line for ham stations, and I have used it to good effect in many applications.

(Duffy 2007) extolled the virtues and gave implementation information, but cautioned:

Some types of RG−6/U use a CCS centre conductor and will have higher loss at low frequencies that shown in Fig 1, depending on the thickness of the copper cladding which may vary from cable to cable.

I have used RG-6/U with solid copper centre conductor widely on HF, and measured performance has always been consistent with expectation.

However, RG-6/U with solid copper centre conductor has become very hard to obtain, and products that remain available such as Belden 1694A are quite expensive.

This article documents measurements at low HF on a 100m roll of Quad shield RG-6/U purchased for UHF TV cabling.

The method used was to measure input impedance of the open circuit terminated 100m line section at a range of resonant and antiresonant frequencies, and from those to calculate Matched Line Loss (MLL) in dB/m.

Screenshot - 25_02_16 , 10_17_41

Above is an example measurement around 3.74MHz. Zin is 213.4Ω at 3.74MHz. In this case I have used an AIMuhf one port analyser, but any instrument that can measure impedance in the range 10-500Ω would suit this particular scenario. Measurement of short low loss cables will yield more extreme impedances and may not be within range of some instruments.

Screenshot - 26_02_16 , 09_36_21

Above is calculation using Calculate transmission line Matched Line Loss from Rin of o/c or s/c resonant section of MLL of 0.031886dB/m from the measured Zin, 3.2dB/100m.

Clip 130

The chart above compares MLL for the CCS cable with interpolated values from the Belden 1694A (copper centre conductor) cable. The copper centre conductor cable is significantly better at all frequencies shown, and the gap is greatest at the lower frequencies.

Note that there is no specification for the copper depth on CCS cables, so actual losses for CCS at lower frequencies will vary and warrant measurement of cable before application below say 20MHz.

To illustrate that point, at 5MHz Belden 1189A (CCS) is specified at 0.0220dB/m, and the cable tested above looks to be around 0.033dB/m.

All transmission lines using non-homgeneous or coated conductors (CCS, SCS in coax and open wire lines) are potentially subject to this issue, they should be evaluated for the intended application. See Loss of Wireman 553 windowed ladder line at MF/HF for another example.

Conclusions

  • RG-6/U that uses a CCS centre conductor may have insufficient copper cladding to deliver copper like performance at HF, and the deficiency is worst at lower frequencies.
  • Enthusiasm for RG-6/U on HF without regard to centre conductor type is naive.
  • All non-homgenous conductors are a risk and should be evaluated for the intended application.
  • MLL can be calculated from a simple measurement.

References