WW1WW’s matching transformer for an EFHW

At PD7MAA’s BN43-202 matching transformer for an EFHW I gave an estimate of the core loss in PD7MAA’s transformer.

An online expert questioned the analysis and later measurements, and proposed his own transformer design as evidence.

Notably, his transformer uses #61 material and a larger binocular core, a Fair-rite 2861006802 with 2t for a nominal 50Ω primary, giving loss measurements at 7MHz of 0.08dB. Note that the confidence limits of that loss measurement because of the way in which it was obtained (eg a 1% error in the 1120Ω load resistor contributes 0.043dB error to the result), but the measurements do suggest that the loss is probably very low.

Though the loss is low and Return Loss is high at 7MHz, the limits for ReturnLoss>14dB (VSWR<1.5) is 5-18MHz. With compensation, that range may be changed.

Prediction

Lets apply the method laid out at PD7MAA’s BN43-202 matching transformer for an EFHW.

The best Fair-rite data I can find quickly is a chart of the impedance of a one turn winding.

Scaling from this graph, Xs is close of 35Ω at 7MHz, so lets used that to derive some basic parameters for the core. Continue reading WW1WW’s matching transformer for an EFHW

AIM 914 produces internally inconsistent results

 

AIMuhf

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

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