DXE sell a nominal 300Ω ladder line, DX Engineering 300-ohm Ladder Line DXE-LL300-1C, and to their credit they give measured matched line loss (MLL) figures.
Loss of ladder line: copper vs CCS (DXE-LL300-1C) – revised for 25/07/2018 datasheet was a revision of an earlier article based on an updated datasheet from DXE. I noted that the specification data had artifacts that one would not expect of such a line, and I questioned whether the datasheet was credible.
John, KN5L, recently purchased, measured and published measurements of a 10.06m (33′) section of new DXE-LL300-1C which provide an independent dataset that might cast some light on the matter.
The chart above plots:
- DXE's datasheet MLL figures (converted to dB/m);
- MLL calculated from KN5L's S11 open and shorted measurements; and
- theoretical MLL for round copper conductors of the same gauge as specified for the LL300 (dielectric loss is assumed insignificant).
The theoretical line is based on well developed skin effect and \(MLL \propto \sqrt f\), resulting in a straight line on the log-log graph.
KN5L's 100 point measurement dataset is for the most part smooth and quite credible, though it shows departure from ideal homogenous conductors with well developed skin effect… and for good reason, these are not homogenous conductors and skin effect is only developed at higher frequencies.
At 60MHz, KN5L's measured MLL is a little worse than theoretical, quite probably due to the fact that these are 19 strand conductors, and the cladding thickness may be just too little to deliver copper like performance even at 60MHz. At lower frequencies, MLL is better, but a good deal worse than the theoretical MLL for copper conductors.
Whilst the MLL might seem small, these types of line are commonly used in scenarios with high VSWR. Let's calculate the loss under mismatch of a scenario used for some recent articles.
The scenario then is the very popular 132′ multi band dipole:
- the famous 40m (132′) centre fed dipole;
- 20m of feed line being parallel RG6/U CCS quad shield with shields bonded at both ends;
- 7MHz where we will assume dipole feed point impedance is ~4000+j0Ω.
We will consider the system balanced and only deal with differential currents.
Taking the MLL of the LL300 as 0.018dB/m, the calculated loss under mismatch is 2.3dB. It is not huge, but any assumption that the loss in open wire line is insignificant is wrong.
Lets evaluate the loss using a home made open wire line of 2mm copper conductors spaced 150mm. The calculated loss under mismatch is 0.114dB, a lot better than the previous case.