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.
This article revises Loss of ladder line: copper vs CCS (DXE-LL300-1C) for revised published datasheet MLL figures with internal PDF date of 25/07/2018.
Let’s start by assuming that the new offered data is credible, let’s take it at face value.
The line is described as 19 strand #18 (1mm) CCS and the line has velocity factor (vf) 0.88 and Zo of 272Ω.
Let us calculate using TWLLC the loss at 2MHz of a similar line but using pure solid copper conductor with same conductor diameter, vf and Zo. We will assume dielectric loss is negligible at 2MHz
|Twist rate||0 t/m|
|Length||83.18 °, 0.231 λ, 30.4800 m, 1.155e+5 ps|
|Line Loss (matched)||0.121 dB|
Spacing has been adjusted to obtain Zo.
At 2MHz MLL of a copper line is 0.121dB for 30.48m (100′) as against 0.32dB measured for the stranded CCS line.
At 50MHz MLL of a copper line is 0.641dB for 30.48m (100′) as against 0.89dB measured for the stranded CCS line.
If the measurement data was valid and correct, the difference would almost certainly attributable to CCS and stranding. The copper cladding on the very thin strands is way less than skin depth at lower frequencies, effective RF resistance is higher than that of a solid copper conductor.
You might regard that the difference is tenths of a dB and insignificant, but this line is almost always used at high VSWR and the difference between the two lines is likely to be significant.
But is the measured data credible?
If we take the measured data and fit a model that matched line loss is per unit length of line (m) is:\(MLL=(k_1 \sqrt f + k_2 f)l\)
|Where||Loss =||loss per unit length|
Such a model is usually a good fit for practical transmission lines where skin effect is well developed, and dielectric loss is proportional to frequency. A solution for k1 and k2 for least squares error has been found for the DXE published data.
Above is a plot of the measured data and the model.
The measured data curve exhibits some form of oscillation about some possibly smoother curve. The oscillation is unexpected and ought prompt review of the measurement setup to see that there is not some other effect being captured, eg unbalanced drive exciting common mode resonances.
Nevertheless, it we treat the data as correct, the issue that arises is that the value for k2 is significantly negative, and we ought to expect it is positive and smaller than k2 at these frequencies.
We might expect and excuse some obvious departure from the model at frequencies below 5MHz due to the copper clad steel conductors.
So, the extent of oscillation and higher frequencies and poor fit to the model raises some questions about the validity of the measurement data.