The sign of Return Loss

I was browsing a ham forum recently when I came across a Return Loss plot apparently from a ham grade miniVNA Tiny.

Lets just remind ourselves of the meaning of the term Return Loss. (IEEE 1988) defines Return Loss as:

(1) (data transmission) (A) At a discontinuity in a transmission system the difference between the power incident upon the discontinuity. (B) The ratio in decibels of the power incident upon the discontinuity to the power reflected from the discontinuity. Note: This ratio is also the square of the reciprocal to the magnitude of the reflection coefficient. (C) More broadly, the return loss is a measure of the dissimilarity between two impedances, being equal to the number of decibels that corresponds to the scalar value of the reciprocal of the reflection coefficient, and hence being expressed by the following formula:

20*log10|(Z1+Z2)/(Z1-Z2)| decibel

where Z1 and Z2 = the two impedances.

(2) (or gain) (waveguide). The ratio of incident to reflected power at a reference plane of a network.

Return Loss expressed in dB will ALWAYS be a positive number in passive networks.

Return Loss according to the miniVNA Tiny

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Above, the miniVNA Tiny presents Return Loss as a negative value. Continue reading The sign of Return Loss

Exploiting your antenna analyser #25

Find coax cable velocity factor using an antenna analyser without using OSL calibration

A common task is to measure the velocity factor of a sample of coaxial transmission line using an instrument without using OSL calibration.

Whilst this seems a trivial task with a modern antenna analyser, it seems to challenge many hams.

We will use a little test fixture that I made for measuring small components, and for which I have made test loads for OSL calibration. We will find the frequency where reactance passes through zero at the first parallel resonance of an O/C stub section, this is at a length of approximately λ/2 (a good approximation for low loss coaxial cables above about 10MHz).

We will use a little test fixture that I made for measuring small components, and for which I have made test loads for OSL calibration.

The text fixture used for this demonstration is constructed on a SMA(M) PCB connector using some machined pin connector strip and N(M)-SMA(F) adapters to connect to the instrument.

VfMeasurement01

Above is a pic of the test fixture with adapters (in this case on a AA-600). Continue reading Exploiting your antenna analyser #25

Exploiting your antenna analyser #24

Find coax cable velocity factor using an antenna analyser with OSL calibration

A common task is to measure the velocity factor of a sample of coaxial transmission line using an instrument that supports OSL calibration, an AIMuhf in this example.

Whilst this seems a trivial task with a modern antenna analyser, it seems to challenge many hams.

There are a thousand recipes, I am going to demonstrate just one that suits the instrument and application.

We will use a little test fixture that I made for measuring small components, and for which I have made test loads for OSL calibration. We will find the frequency where reactance passes through zero at the first parallel resonance of an O/C stub section, this is at a length of approximately λ/2 (a good approximation for low loss coaxial cables above about 10MHz).

The text fixture used for this demonstration is constructed on a SMA(M) PCB connector using some machined pin connector strip and N(M)-SMA(F) adapters to connect to the instrument.

VfMeasurement01

Above is a pic of the test fixture with adapters (in this case on a AA-600). Continue reading Exploiting your antenna analyser #24

Transmitter pulse generator for SSB RF PA adjustment

This article describes a pulse generator for adjustment of SSB RF power amplifiers.

The need

Valve RF power amplifiers usually use high voltage power supplies with poor regulation, and typically the voltage may sag by 10% or more on full power CW output, whilst on SSB telephony the voltage may sag a quarter of that.

The effect is that finding PA loading conditions for maximum power output on a key down CW signal optimises the loading for conditions that are significantly different to SSB telephony and not only is the maximum power output likely to be lower for key down CW, but it will be lower when used for SSB telephony than if it were adjusted using a drive that created full output power without sagging the power supply more than speech would.

Additionally, RF PAs intended for the amateur market cannot sustain key down CW for very long before overheating and sustaining damage forcing very short adjustment sessions. Adjustment at continuous maximum power puts great demands on a dummy load if one is being used.

So, to solve these problems, there are three objective:

  • create a drive / load scenario that is similar to SSB telephony conditions;
  • operate at reduced duty cycle to reduce internal heating of valves and power supply;
  • reduce the average dissipation requirements of a dummy load.

Continue reading Transmitter pulse generator for SSB RF PA adjustment

Antenna analysers – the basics – QST Aug 2016

I was browsing Joe Hallas’ award winning article Antenna analysers – the basics in Aug 2016 QST when I saw some welcome news.

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In the notes to this graphic, Joe tells us that you can display any “individual plot” in Antscope. I would dearly like to NOT display the |Z| curve which is apparently there as a concession to the ham audience that doesn’t understand the complex nature of Z and the need to see it in two dimensions. Continue reading Antenna analysers – the basics – QST Aug 2016

Exploiting your antenna analyser #23

Seeing recent discussion by online experts insisting that power relays are not suitable to RF prompts an interesting and relevant application of a good antenna analyser.

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Above is a sweep of an A/B changeover relay intended for HF application at up to 100W and lowish VSWR. The sweep is actually from 1 to 61MHz to be confident that there is not poor behaviour just outside of the HF range that might present on another implementation of the same design. Continue reading Exploiting your antenna analyser #23

Accuracy of AIMuhf system – AIM910B vs several recent versions on a ferrite cored inductor

AIMuhf

At Accuracy of AIMuhf system – AIM910A vs several recent versions on a ferrite cored inductor I reported significant problems reconciling several recent versions of AIM software measuring the same inductor. Continue reading Accuracy of AIMuhf system – AIM910B vs several recent versions on a ferrite cored inductor

InsertionVSWR of Grebenkemper’s Tandem Match

The findings at InsertionVSWR of Revex W560 on HF and the suggestion that the low frequency problem is characteristic of poorly designed Sontheimer couplers (Sontheimer, C & Frederick 1966).

These couplers were popularised by (Grebenkemper 1987)  in his Tandem Match – An Accurate Directional Wattmeter and have appeared in ARRL handbooks over the decades, and may have inspired the many commercial implementations of the coupler.

Grebenkemper claims his meter is ‘good’ down to 1.8MHz, but does not clearly claim any particular InsertionVSWR. There is limited value in an instrument that can measure down to 1.05 when it causes significantly higher VSWR itself.

Screenshot - 24_07_16 , 15_31_41

Lets drill down on Gebenkember’s article, specifically the coupler design.
Continue reading InsertionVSWR of Grebenkemper’s Tandem Match