High and low Z responses of a resonant shorted / open transmission line section

At KB1CKT measurement of a roll of RG6/U I stated:

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.

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.

This article illustrates the response expected from a 13.7m length of Belden 8262 (RG58C/U).

Above, the response at the first low Z resonance with open end. The rate of change of X with frequency is 2.2Ω/kHz, quite modest and given the low slope of the Rin line, frequency need only be set to within 1kHz for reasonably good accuracy of the Rin value obtained. (Slope will be steeper on a lower loss line section.)

Above, the response at the first high Z resonance with shorted end. The rate of change of X with frequency is -12.7Ω/kHz, quite rapid and smaller frequency steps (~0.1kHz) are needed to resolve an accurate value for Rin. (Slope will be steeper on a lower loss line section.)

The response depends on the line type, length and frequency.

Conclusions

  • It takes more care to make good measurements at a high Z resonance.

References

KB1CKT measurement of a roll of RG6/U

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

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

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

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

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

A low cost two wire transmission line using high strength aluminium MIG wire and agricultural trellis clips

This article documents a weather test on a trial section of two wire line using:

  • 1.6mm 5356 Aluminium MIG wire;
  • agricultural clips (Trellis Clips, Rose Clips) found on eBAY.

Construction

Above is a pic of the Rose Clip as a line insulator / support. The clips are pretty flimsy but pretty cheap. They click onto the 1.6mm diameter wire reasonably firmly. Continue reading A low cost two wire transmission line using high strength aluminium MIG wire and agricultural trellis clips

RG6/U with CCS centre conductor – shielded twin study – why is it so lossy?

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?

nanoVNA – evaluation of a voltage balun – RAK BL-50A

In this article, I will outline an evaluation of a ‘classic’ voltage balun, the 1:1 RAK BL-50A voltage balun, specified for 1.8-30MHz.

These were very popular at one time, but good voltage baluns achieve good current balance ONLY on very symmetric loads and so are not well suited to most wire antennas.

Above is a pic of the balun with load on test. It is not the greatest test fixture, but good enough to evaluate this balun over HF.

Mine has survived, but many users report the moulding cracking and rusted  / loose terminal screws, and signs of internal cracks in the ferrite ring.

Continue reading nanoVNA – evaluation of a voltage balun – RAK BL-50A

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

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

nanoVNA – evaluation of a voltage balun – W2AU 1:1

In this article, I will outline an evaluation of a ‘classic’ voltage balun, the 1:1 W2AU voltage balun, specified for 1.8-30MHz.

These were very popular at one time, but good voltage baluns achieve good current balance ONLY on very symmetric loads and so are not well suited to most wire antennas.

Above is W2AU’s illustration of the internals.

Mine barely saw service before it became obvious that it had an intermittent connection to the inner pin of the coax connector. That turned out to be a poor soldered joint, a problem that is apparently quite common and perhaps the result of not properly removing the wire enamel before soldering.

Having cut the enclosure to get at the innards and fix it (they were not intended to be repaired), I rebuilt it in a similar enclosure made from plumbing PVC pipe and caps, and took the opportunity to fit some different output terminals and an N type coax connector.

W2auBalun01

Above is the rebuilt balun which since that day has been reserved for test kit for evaluating the performance of a voltage balun in some scenario or another. Continue reading nanoVNA – evaluation of a voltage balun – W2AU 1:1

what-exactly-happens-to-the-signals-hitting-a-common-mode-choke?

An image from what-exactly-happens-to-the-signals-hitting-a-common-mode-choke doesn’t quite look right.

In respect of the first part, inductance \(L=\frac{\phi(i)}{i}\) so if the windings are equal, half the total current flows in each winding and each contributes flux due to i/2, total current is i, total flux is twice that due to i/2, so the inductance of the parallel equal windings is the same as if i flowed in a single winding, ie L of the combination is the same as the inductance of each of the equal windings alone. Continue reading what-exactly-happens-to-the-signals-hitting-a-common-mode-choke?