A 1:1 RF transformer for measurements – based on noelec 1:9 balun assembly

The Noelec 1:9 balun (or perhaps Chinese knock off) is available quite cheaply on eBay and provides a good hardware base for a 1:1 version.

Above is a modified device with the transformer replaced with a Macom ETC1-1T-2TR 1:1 transformer. The replacement is not exactly the same pads, but it is sufficiently compatible to install easily. The track is cut to disconnect the secondary centre tap, essentially allowing the secondary to float and so have higher common mode impedance than with the centre tap grounded.

The most notable departure from ideal of these small transformers is leakage inductance of 50nH give or take. Continue reading A 1:1 RF transformer for measurements – based on noelec 1:9 balun assembly

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