AIM 914 produces internally inconsistent results

 

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A new release, AIM914 appeared recently.

In the common theme of one step forward, two steps backwards, this version has defects that were not present in AIM910B.

Let’s review the internal consistency of this part of the display screen.

Most of the values given above are calculated from a single measurement value, and should be internally consistent. That measurement value is translated to different quantities, many based on the stated Zref (75Ω in this case). Continue reading AIM 914 produces internally inconsistent results

Matched Line Loss of generic RG6/U Quad Shield CCS

This article documents a measurement of Matched Line Loss (MLL) of a 35m test section of generic RG6/U Quad Shield CCS.

It has become impossible in recent years to buy low cost RG6/U with solid centre conductor locally, and the imported product with solid copper conductor is prohibitively expensive (~$6/m as against $0.35/m for the CCS).

The CCS cable does have near copper like performance at UHF and above, but what is its behavior at HF?

Above is calculated MLL from a S11 scan of the test section with S/C and O/C termination. There is a little ripple on the response due to measurement error. The graph also has a curve fit, MLL=0.0285f^0.1506 (F in MHz). Continue reading Matched Line Loss of generic RG6/U Quad Shield CCS

KN5L measurement of JSC 1318 windowed ladder line – MLL @ 4.2MHz

John, KN5L, published some interesting measurements he made of a recent purchase of JSC 1318 windowed ladder line. JSC Wire & Cable is now known as Seminole Wire & Cable, and this is their 1318 product.

Product with apparently similar specifications are sold by many ham retailers, they may or may not be sourced from Seminole.

Some sellers specify the % ICAS rating of the copper clad conductor, usually 30%, some just don’t mention it.

John carefully measured the DC resistance of his line section, and found that it reconciled well with the Copperweld datasheet for 21% CCS.

He also used a VNA to measure S11 of the line section with S/C and O/C terminations, and he gives links to the Touchstone files at the top of his page.

The O/C Touchstone file allows calculation of Zin. The O/C line exhibits resonance at 4.2MHz, at Zin=3.7Ω. His fuller set of measurements showed that Zo at 4.2MHz is very close to 400Ω. We can use those measurements to calculate Matched Line Loss (MLL).

Above, MLL is 0.50852dB/100m.
Continue reading KN5L measurement of JSC 1318 windowed ladder line – MLL @ 4.2MHz

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