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

The devil is in the detail – real world transmission lines and loss under standing waves

We are traditionally taught transmission line theory starting with the concept of complex propagation constant γ, and that loss in a section of line is \(Loss=20log_{10}( l |\gamma|) dB\) where l is length. That is the ‘one way’ loss in a travelling wave, also the the matched line loss (MLL) (as there is no reflected wave).

There are some popular formulas and charts that purport to properly estimate the loss under standing waves or mismatch conditions, usually in the form of a function of VSWR and MLL, more on this later.

Let’s explore theoretical calculations of loss for a very short section of common RG58 at 3.6MHz with two different load scenarios.

The scenarios are:

  • Zload=5+j0Ω (VSWR(50)=10); and
  • Zload=500+j0Ω (VSWR(50)=10).

Above is the RF Transmission Line Loss Calculator (TLLC) input form. A similar case was run for Zload=500Ω. Continue reading The devil is in the detail – real world transmission lines and loss under standing waves

The devil is in the detail – real world transmission lines and ReturnLoss

We are traditionally taught transmission line theory starting with the concept of complex propagation constant γ and then dealing with them as lossless lines (means Zo is purely real) or low loss distortionless lines (means Zo is purely real).

Let’s explore theoretical calculations of ReturnLoss for a very short section of common RG58 at 3.6MHz.

By definition, \(ReturnLoss=\frac{ForwardPower}{ReflectedPower}\) and it may be expressed as \(ReturnLoss=10log_{10}\frac{ForwardPower}{ReflectedPower} dB\).

The scenarios are:

  • open circuit termination; and
  • short circuit termination.

Above is the RF Transmission Line Loss Calculator (TLLC) input form. Note that it will not accept Zload of zero or infinity, instead a very small value (1e-100) or very large value (1e100) is used. Continue reading The devil is in the detail – real world transmission lines and ReturnLoss

Measurement of recent ‘FT240-43’ core parameters

This article reports measurement of two ‘FT240-43’ cores (actually Fair-rite 5943003801 ‘inductive’ toroids, ie not suppression product) purchased together around 2019, so quite likely from the same manufacturing batch. IIRC, the country of origin was given as China, it is so for product ordered today from element14. The measurements are of 1t on the core, with very short connections to a nanoVNA OSL calibrated from 1-50MHz.

Above, the measurement fixture is simply a short piece of 0.5mm solid copper wire (from data cable) zip tied to the external thread of the SMA jack, and the other end wrapped around the core and just long enough to insert into the inner female pin of the SMA jack. Continue reading Measurement of recent ‘FT240-43’ core parameters

nanoVNA – RG6/U with CCS centre conductor MLL measurement

In my recent article RG6/U with CCS centre conductor – shielded twin study I made the point that it is naive to rely upon most line loss calculators for estimating the loss of this cable type partly because of their inability to model the loss at low HF and partly because of the confidence one might have in commonly available product. In that article I relied upon measured data for a test line section.

I have been asked if the nanoVNA could be bought to bear on the problem of measuring actual matched line loss (MLL). This article describes one method.

The nanoVNA has been OSL calibrated from 1-299MHz, and a 35m section of good RG6 quad shield CCS cable connected to Port 1 (Ch0 in nanoVNA speak).

A sweep was made from 1-30MHz with the far end open and shorted and the sweeps saved as .s1p files.

Above is a screenshot of one of the sweeps. Continue reading nanoVNA – RG6/U with CCS centre conductor MLL measurement

RG6/U with CCS centre conductor – shielded twin study

Some online experts advise the use of synthesised shielded twin instead of ordinary two wire line for HF antennas claiming it is vastly superior.

Now it could be vastly superior for several reasons in all, but let’s focus on just one important parameter, loss under mismatch conditions.

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.

Now rather than depend on loss calculators, most of which don’t reconcile with measurement of CCS RG6/U, I will used measured loss. RG6/U with CCS centre conductor at HF gives a chart of measured loss of a sample of commercial grade CCS quad shield coax.

Above is a comparison of matched line loss (MLL) based on measurement of a length of RG6/U Quad Shield CCS cable and prediction from Simsmith of Belden 8215 (also CCS). The ripple is due to measurement system error, measurements were made quite some years ago with a AIMuhf. Continue reading RG6/U with CCS centre conductor – shielded twin study

nanoVNA – measuring cable velocity factor – demonstration – open wire line

The article nanoVNA – measuring cable velocity factor – demonstration demonstrated measurement of velocity factor of a section of coaxial transmission line. This article demonstrates the technique on a section of two wire copper line.

A significant difference in the two wire line is that we want the line to operate in balanced mode during the test, that there is insignificant common mode current. To that end, a balun will be used on the nanoVNA.

Above, the balun is a home made 1:4 balun that was at hand (the ratio is not too important as the fixture is calibrated at the balun secondary terminals). This balun is wound like a voltage balun, but the secondary is isolated from the input in that it does not have a ‘grounded’ centre tap. There is of course some distributed coupling, but the common mode impedance is very high at the frequencies being used for the test. Continue reading nanoVNA – measuring cable velocity factor – demonstration – open wire line