Category: Transmission lines

Shawn, KB1CKT, set about measuring the matched line (MLL) loss of a roll (~152.4m) RG6/U on the roll and published his results in an online discussion.

A word of warning: measuring on the roll can give unusual results if the cable is deformed significantly on the roll. A higher risk for foamed dielectric cables, and this is one of that type.

The technique he followed was to make a series of measurements of Rin at low Z resonances of the length of line with open circuit at the far end, and to calculate the MLL using Calculate transmission line Matched Line Loss from Rin of o/c or s/c resonant section.

Impedance measurements were made using a nanoVNA which was OSL calibrated in several ranges through the series of measurements. Also notable was that there were several coax adapters used to connect the RG6 to VNA.

Though the calculator can use high Z resonances, the high Z resonances are very narrow and it is very difficult to measure Rin at the true resonant frequency. So, only his reported low Z measurements will be used.

So. let’s graph the measured Rin at zero crossing of Xin.

The circled measurements appear out of line with the others. They were made with a different OSL calibration that the ones immediately below and above, so that hints something may have gone wrong in the calibration. Two of the possible explanations are: Continue reading KB1CKT measurement of a roll of RG6/U

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: Continue reading Loss of ladder line: copper vs CCS (Seminole 1320 vs 1321) – KN5L’s measurements

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).

Continue reading Loss of ladder line: copper vs CCS (DXE-LL300-1C) – KN5L’s measurement 11/2020

RG6/U with CCS centre conductor – shielded twin study discussed a synthesised synthesised shielded twin instead of ordinary two wire line for an example HF multiband antenna.

The original 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 ~2000+j0Ω (a lowish estimate, it could be double that depending on height).

We will consider the system balanced and only deal with differential currents, and matched line loss is based on measurement of a specific sample of line (RG6/U with CCS centre conductor at HF).

This article will calculate the same scenario with three feed line variants:

• 150Ω twin line with the same CCS conductors as the RG6;
• 600Ω twin line withthe  same CCS conductors as the RG6 (ie the spacing is increased to increase Zo); and
• 600Ω twin line using 2mm HDC.

The loss under mismatch depends not only on the transmission line characteristics and length, but also on the load and the current and voltage distribution.

Above the 150Ω twin line with same CCS conductors as the RG6 has loss almost identical to the synthesised twin shielded in the original article. Almost all of the resistance in the coax is in the CCS centre conductor, so I assume that the loss in the twin CCS is approximately equal to that of the synthesised twin. Dielectric loss is less than 1% and can be ignored. Continue reading RG6/U with CCS centre conductor – shielded twin study – why is it so lossy?

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 Continue reading Loss of ladder line: copper vs CCS (DXE-LL300-1C) – revised for 25/07/2018 datasheet