RG174 for portable HF operation

There are frequent recommendations of RG174 for portable stations (eg SOTA), usually running QRP, principally because it is light and easy to wind up into a small package to fit into a pack. RG174 commonly uses silver coated steel (SCS) centre conductor, sometimes copper clad steel (CCS), sometimes copper.

Recent posts on eHam recommended RG174 citing a coax loss calculator at http://www.qsl.net/co8tw/Coax_Calculator.htm which gives the matched loss at 1MHz of Belden 8216 (RG174) at 2.5dB/100m.

But, wait a minute, the Belden 8216 datasheet gives matched line loss at 6.2dB/100m @ 1MHz, that is 150% higher!

There will be plenty of calculators that extrapolate high frequency performance (where the silver plating on the steel inner conductor strands is larger wrt skin depth) down to lower frequencies where the simple model used is not adequate.

What about TLDetails, does its algorithm work better?

Screenshot - 25_01_2014 , 08_46_33

Table 1 compares Belden’s 8216 datasheet MLL with those of TLDetails and TLLC. TLLC uses a regression based on Belden’s data with the exclusion of the datapoints at 1 and 10MHz because they are significant departures from the model. The consequence is that TLLC predictions below 50MHz would be extrapolations and less accurate so they are not shown. (TLLC shows the frequency range on which its model is based, and that frequency range is shown in red when the user has requested a frequency outside that range, users are cautioned in the text against extrapolated results.)

TLDetails error is 4-9% over 1-100MHz, which questions Johnson’s and Maguire’s method, at least for this cable type.

A closed form solution that properly deals with the range of centre conductor constructions and materials and shield constructions and materials used in practical cables would be very challenging.

Without some assured method for predicting the change in R and L, a method that works in this case of stranded plated steel centre conductor, and in the more general case for the range of centre conductor constructions and materials and shield constructions and materials, one must resort to measurement.

But, the upshot of all this technical mumbo jumbo for the average portable operator is that transmission lines that use copper clad or silver plated inner conductors are intended for higher frequency operation (where the cladding is a few skin depths in thickness), and below that, their loss might be more than predicted by extrapolation of the higher frequency characteristic.

When in doubt, measure it or use something that you do understand. RG174 that uses copper centre conductor may perform significantly better than that with silver coated steel, depending on frequency, length, application etc.

PS: The matched line loss of a half wavelength of Belden 8216 at 7MHz is more than three times that of Belden 1694A, a RG6 with copper centre conductor. The cost of the convenience of RG174 is around 15% of EIRP which is small, but handing up a dB here, a dB there soon means you are QRP^2.