John, KN5L, has published measurements made of two types of nominally 300Ω windowed ladder line. The measurements of such similar lines gives opportunity to explore the effect of conductor differences on matched line loss (MLL).
Above is an analysis of KN5L’s published measurements of a 19.93m test section of Seminole 1320 (nominal 300Ω windowed ladder line, 0.812mm (#20) 7 strand copper). The line was purchased around 2015. The plot has:
- 101 measurement points (blue);
- a curve fit of the measurements to the model \(MLL=(k_1 \sqrt f + k_2 f)l\) (green);
- a theoretical calculation of MLL of an equivalent line of 0.812mm (#20) round copper conductor and dielectric loss of polythene (adjusted for measured VF); and
- a curve fit to AC6LA’s (Johnson) model using coefficients from ZPLOTS.
It can be seen that:
- the measurements are a very good fit to the model \(MLL=(k_1 \sqrt f + k_2 f)l\),
- measured loss is reasonably close to prediction; and
- measured MLL at 1MHz is 0.0035dB/m.
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Above is an analysis of KN5L’s published measurements of a 14.54m test section of Seminole 1321 (nominal 300Ω windowed ladder line, 1.024mm (#18) 19 strand copper clad steel). The line was purchased around 2015. The plot has:
- 101 measurement points (blue);
- a curve fit of the measurements above 20MHz to the model \(MLL=(k_1 \sqrt f + k_2 f)l\) (green);
- a theoretical calculation of MLL of an equivalent line of 1.024mm (#18) round copper conductor; and
- a curve fit to AC6LA’s (Johnson) model using coefficients from ZPLOTS.
It can be seen that:
- the measurements (over the whole range) are not a good fit to the model \(MLL=(k_1 \sqrt f + k_2 f)l\) below about 20MHz;
- measured loss is reasonably close to prediction above about 20MHz, but way more at lower frequencies; and
- measured MLL at 1MHz is 0.010dB/m (three times the copper 1320).
Above is a model of current distribution in a 1.024mm (#18) round CCS conductor with cladding equal to that of the component 0.255mm (#30) strands 30% IACS (17.8µm) as an approximation of the #30×19 conductor. That model suggests that the MLL is 0.011dB/m. We can fully expect that the loss of a stranded conductor will be a little higher, so measurement and prediction reconcile reasonably well.
Conclusions
- KN5L’s measurement data looks credible;
- AC6LA’s (Johnson) model is a good fit to the measurements;
- the measurements and conductor model are broadly consistent; and
- both show a significant degradation from copper like performance in the CCS line below 20MHz; and
- copper (1320) has half the CCS (1321) MLL @ 1.8MHz..