Transmission line measurements – learning from failure

Introduction

A recent article questioned the accuracy of measurement of Matched Line Loss (MLL) for a modified commercial transmission line. The published results were less than half the loss of an equivalent line in air using copper conductors and lossless dielectric, when in fact there would be good reason to expect that the line modification would probably increase loss.

How do you avoid the pitfalls of using analysers and VNAs to measure line loss?

Lets walk through a simple exercise that you can try at home with a good one port analyser (or VNA). Measuring something that is totally unknown does not provide an external reference point for judging the reasonableness of the results, so will use something that is known to a fair extent,

Experiment

For this exercise, we will measure the Matched Line Loss (MLL) of a 6m length of uniform transmission line, RG58C/U cable, using an AIMUHF analyser. The AIM manual describes the method.

If you need to know the cable loss at other frequencies, enable the Return Loss display using the Setup menu and click Plot Parameters -> Return Loss and then do a regular scan of the cable over the desired frequency range with the far end of the cable open. Move the blue vertical cursor along the scan and the cable loss will be displayed on the right side of the graph for each frequency point

Note the one-way cable loss is numerically equal to one-half of the return loss. The return loss is the loss that the signal experiences in two passes, down and back along the open cable.

Our measurements will show that this is a naively simple explanation, and to take it literally as complete may lead to serious errors. Yes, it IS the equipment manual, but it is my experience that the designers of equipment, and writers of the manuals often show only a superficial knowledge of the relevant material.

Datasheet

Above is an extract of the datasheet for Belden 8262 RG58C/U type cable, our test cable should have similar characteristics. Continue reading Transmission line measurements – learning from failure

Cheap Chinese wristwatch #1

I bought an inexpensive Chinese manual wind wristwatch for my grandson. It is a skeleton style watch based on the communist Tongji movement.

Above, the watch looking pretty flashy in gold coloured finish. The gold plate wore off the band in just a few days to reveal a brass band tarnishing by the hour. The bezel is probably base metal and will corrode in no time. Continue reading Cheap Chinese wristwatch #1

Inferring Matched Line Loss from Half Return Loss measurements – Zo error

At On Witt’s calculation of Matched Line Loss from Return Loss I discussed the common but flawed thinking that Matched Line Loss (MLL) can be calculated as half of the Return Loss of either a S/C or O/C section of transmission line.

The article discusses Witt’s calculation (half the average of Return Loss for S/C and O/C conditions) and notes that it can be a good approximation where the actual Zo is very close to the Zo on which the Return Loss measurement is based, and that the line loss is low.

This article looks at a case study of a section of low loss nominally 75Ω line is measured on a 50Ω instrument to illustrate sensitivity to Zo error.

A 3.1m section of RG6 was measured with O/C then S/C termination using a 50Ω VNA, and HalfReturnLoss (HRL), |S11| and phase of S11 is plotted.

Above, the O/C termination. Continue reading Inferring Matched Line Loss from Half Return Loss measurements – Zo error

Feasibility study – loop in ground for rx only on low HF – small broadband RF transformer – discussion of ferrite material choice

At Feasibility study – loop in ground for rx only on low HF – small broadband RF transformer using medium µ ferrite core for receiving use – 50:200Ω I laid out a design using Fair-rite a #43 ferrite smallish binocular core. #43 is a medium permeability NiZn ferrite.

I have been asked by several correspondents why I used #43 when the consensus of online experts is that #75 is a clearly better choice for the application.

Let me say that almost all such articles and posts:

  • are absent any quantitative measurement of their proposed design;
  • they tend to use medium to large toroids; and
  • the few that expose their design calcs treat permeability as a real number that is independent of frequency.

#75 mix is a high permeability MnZn ferrite and subject to dimensional resonance in the frequency range of interest for this application, a problem exacerbated by using larger cores.

Permeability is a complex quantity that is frequency dependent and any analysis that pretends otherwise is not soundly based. Continue reading Feasibility study – loop in ground for rx only on low HF – small broadband RF transformer – discussion of ferrite material choice

Feasibility study – loop in ground for rx only on low HF – SDR for measurement?

A series of recent articles developed a Loop In Ground antenna system design.

To test the prototype, I thought it an interesting exercise to use a low end rx only SDR for the instrumentation, providing a graphic quantitative measure of performance that is within the reach of most hams.

The first device trialled was a RTL-SDR v3 dongle with Sdrsharp (SDR#) software under windows, a very low cost option ($40). I was unable to find meaningful NF specifications or end user measurements for the thing in direct sampling mode. Continue reading Feasibility study – loop in ground for rx only on low HF – SDR for measurement?

Feasibility study – loop in ground for rx only on low HF – small broadband RF transformer using medium µ ferrite core for receiving use – 50:200Ω

A simplified design for small broadband RF transformers using medium µ ferrite core for receiving use. The specific application is an impedance transformer for a nominally 200Ω antenna to a 50Ω receiver input. Intended frequency range is from 0.5 to 15MHz.

The characteristic of typical medium µ ferrite mixes, particularly NiZn, are well suited to this application.

This article continues with the design discussed at BN43-2402 balun example, but using a BN43-202 with 5t primary and 10t secondary for a nominal 1:4 50:200Ω transformer (though at high ratios, the transformation is only nominal).

Lets consider a couple of simple starting points for low end and high end rolloff. Continue reading Feasibility study – loop in ground for rx only on low HF – small broadband RF transformer using medium µ ferrite core for receiving use – 50:200Ω

Feasibility study – loop in ground for rx only on low HF – trial topology selection

* * * D R A F T * * * – a working document.

This article documents the selection of the trial loop in ground configuration as a development from the loop on ground antenna (KK5JY).

Baseline

The baseline is a minor variation of a design by KK5JY, a 15′ square loop 20mm above average ground, with 9:1 transformer and 50Ω load middle of one side.

Above is a plot of feed point impedance when the loop is driven. At 3.6MHz, the source impedance for a rx system is 43+j852Ω, and the mismatch loss to a 450Ω load is 11.0dB, a direct contribution to Antenna Factor (AF). Continue reading Feasibility study – loop in ground for rx only on low HF – trial topology selection

Feasibility study – loop in ground for rx only on low HF

* * * D R A F T * * * – a working document.

This article documents a feasibility study of a smallish loop on or in ground as a rx only antenna for 160-40m, possibly with advantage in high noise environments.

Various ‘on ground’ antennas are discussed online etc, but there is a distinct lack of supporting scientific evidence though subjective anecdotal evidence abounds.

The approach used here is to determine the degradation of S/N resulting from a low gain antenna system in the context of expected ambient noise as per ITU P.372-13. The analysis leans to the conservative side. Continue reading Feasibility study – loop in ground for rx only on low HF

VSWR meter trap for the unwary

From time to time one sees discussion online about consistency of ‘measured’ VSWR at different power levels (on the same instrument).

A question often asked is:

I tune up at 10W and achieve VSWR=1.5, and when I increase power to 100W, the VSWR increases. Which should I believe?

The first thing to note is that good antenna systems SHOULD be linear, VSWR should be independent of power, it is if the system IS linear.

For the most part they are linear, even though many antenna systems contain elements such as ferrite cored inductors that may exhibit some small level of non-linearity in ‘normal’ operation.

Non-linearity caused by for instance saturation of magnetic materials, loss of permeability where the temperature of ferrite cores reaches Curie point, arcing of capacitors or other insulating materials is NOT normal linear operation of a GOOD antenna system. If high indicated VSWR at high power is caused by any of these effects, it is flagging a problem that requires attention.

That said, a significant non-linear element may be the VSWR meter itself.

A common, if not most common way to make these meters is to use a half wave detector to convert the direction coupler RF outputs into DC to drive an ordinary moving coil meter. These meters commonly assume that the detector DC output voltage is exactly proportional to the RF input voltage.

Lets look at the accuracy of that process.

Above is a plot of the detector output vs RF input voltage for a commercial 200W VSWR meter. The measurements cover input power from 10 to 100W.
Continue reading VSWR meter trap for the unwary

Online calculator of ferrite material permeability interpolations

Many of my articles call for finding the complex permeability of a ferrite components from manufacturer’s data.

Let’s explore an example used in a recent article, Another small broadband RF transformer using medium µ ferrite core for receiving use – 50:450Ω.

The core used was a Fair-rite ferrite core of #43 material, and the magnetising impedance of a 5t winding needed to be found.

Above is a chart from Fair-rite’s catalog. Permeability is a complex quantity and is frequency dependent. One could scale from the graph, the values for µ’ and µ” at the frequency of interest. Continue reading Online calculator of ferrite material permeability interpolations