N0TZU recently report his perception that a length of Logico COX3520 RG6 Quad cable he purchased exhibited higher than expected Matched Line Loss (MLL) at 10MHz.
Most RG6 type cable sold these days at low cost uses a copper clad steel centre conductor, and much of it has insufficient copper cladding thickness for copper like performance at HF.
Above is a pic N0TZU gave of the centre conductor cross section. It is possible to measure the cladding thickness from the pic knowing that the overall diameter is 1.024mm. The copper thickness measured 13.7µm, lets round it to 14µm.
There is a rule of thumb that where a copper tube (the cladding) thickness exceeds three skin depths (δ), effective RF resistance will be only very slightly higher than solid copper, and can be considered as good as solid copper.
Skin depth in copper at 10MHz is 20.6µm, 3δ is 61.8µm, and this conductor comes nowhere near it… so we should not expect copper like performance.
On the 3δ criteria, a 14µm cladding delivers copper like performance above 200MHz. That is not to say it isn’t usable below 200MHz, just that its MLL will be higher than an equivalent cable with solid copper centre conductor. Indeed, you might be hard put to measure the difference above 85MHz where the cladding is 2δ.
This analysis has focussed on the centre conductor cladding thickness, there may be other issues with the actual cable.
Of course in some applications, the higher MLL might be of little consequence and the cable may represent good value for money.
Beyond rules of thumb, here is a plot of the current distribution in the conductor at 10MHz. The current density magnitude falls exponentially as explained by skin effect, but the cladding is too thin to allow the full exponential decay and the current distribution is essentially truncated at 14µm depth… leading to the higher effective 10MHz RF resistance of about 1.6 times that of a solid copper conductor.
RG6 CCS cables are commonly sold without meaningful specifications, but occasionally they have useful specs. Some cables site ASTM B-869 for the centre conductor, basically 21% IACS conductivity, and for a #18 centre conductor, DC resistance will be around 100Ω/km. The cladding thickness in this case is 30.7µm which is 3δ at 40MHz, 2δ at 18MHz.