Category: Test equipment

Coax connectors and accurate / repeatable measurements – Keysight’s advice

Keysight publishes an interesting video course “Vector Network Analyzer Basics“.

In lesson 6 they talk of three myths of VNAs, and the second is this:

I learned when I first started using SMA connectors in the early 1970s that consistent measurement required properly tightened (torqued) connectors. My opinion was formed by measuring Return Loss of various types of connectors at 6GHz, and noting the effects of connector tightness and cleanliness on consistency. Continue reading Coax connectors and accurate / repeatable measurements – Keysight’s advice

Last update: 28th October, 2024, 11:23 AM

Conversion of NOELEC style balun board to 1:1 – through measurement

Conversion of NOELEC style balun board to 1:1 left readers with a challenge to measure the through performance of the modified balun board.

The link grounding the centre tap was cut for this test to float the secondary so that one side could be grounded. This will give almost identical response to the case where the centre was grounded.

Above is the test configuration, the yellow thing is a top view of a modified plastic clothes peg which is used to clamp one of the wires from the transformer to the SMA threads. Continue reading Conversion of NOELEC style balun board to 1:1 – through measurement

Last update: 3rd August, 2024, 7:15 AM

Conversion of NOELEC style balun board to 1:1

This article describes a small 1:1 balun for use in measuring field strength using a TinySA Ultra and a small loop antenna.

The balun is also useful for measurements using the NanoVNA (or any VNA), eg for measuring two wire transmission line parameters.

Materials

Some “TC1-1T RF Balun Transformer 0.4-500MHz 1:1CT” transformers were purchased on Aliexpress, 5 for $7. The part number is a Mini-circuits part, but these are likely to be clones. The balun boards also came from Aliexpress, about $4 each. Also needed are compensation caps of 10pF (0805).

Conversion

The boards come with a nominal 1:9 transformer and in my experience a capacitor (though I think the NOELEC board may use a TVS). In any event, it should be removed and the transformer removed. Fit the new transformer and solder a short circuit across the cap pads.

High end compensation

See High end VSWR compensation in a ferrite cored RF transformer for an explanation of compensation.

Connect the board to VNA Port 1 and sweep it to 200MHz, adjusting s11 e-delay so that the trace is a dot at the left of the Smith chart X=0 line. This adjusts the reference plane approximately to the position of the capacitor. Write down the e-delay value for later.

Above is the modified transformer and some calibration parts loads. There are two loads, 200Ω 1% and the other has 2×100Ω 1% in parallel to give 50Ω 1%. The calibration parts are made on ordinary long header pin strips, break three off and pull the middle one out with pliers. Slide the pins to a suitable projection and cut off the other end to uniform length then solder the resistors or shorts to them. This calibration effectively sets the Port 1 coupler Directivity to 46dB (which is quite good). Continue reading Conversion of NOELEC style balun board to 1:1

Last update: 16th December, 2024, 6:06 AM

VNA measurement – small is beautiful

I have written online and in many many emails that a very common failure of VNA measurements of components is the test fixture, and the standout problem is most often the length of connecting wires.

This article works a couple of theoretical designs based on a validated model and experience of building and measuring many baluns of similar or identical design. We will then look at extracts from a Youtube video by ferrite manufacturer Fair-rite and appraise the results.

Validated theoretical choke designs

FT240-43

It is possible to calculate a pretty good estimate of the impedance of a common mode choke wound on a #43 material ferrite core over 1-30MHz. Measurement of a real choke suggests an equivalent shunt capacitance to calibrate the model to measurement. Whilst I have given the generic name to this core, it is based on Fair-rite’s 5943003801 and Fair-rite’s published 2020 #43 mix characteristics. There are imposters, and they may be significantly different.

Let us take a practical example design and calculate the expected choke impedance and from that, the expected |s21|dB in a VNA series through measurement setup.

Above is a SimNEC model of a FT240-43 with 11t winding and 2.5pF equivalent shunt capacitance to calibrate the self resonant frequency. The model calculates and plots choke impedance, and |s21|dB in the series through measurement configuration shown. Continue reading VNA measurement – small is beautiful

Last update: 6th January, 2024, 6:20 PM

VNA fixture for measuring Zcm of a common mode choke – twisted pair wound

VNA fixture for measuring Zcm of a common mode choke – coax wound discussed issues with common ham practice for measuring coax wound common mode chokes.

The article left readers with some homework:

  • Does the same thing occur if the core is wound with twisted pair that is well represented as a uniform two wire transmission line?
  • Are the resistors beneficial?
  • Do they degrade fixture behavior?
  • Then, why are the used so often?

This article addresses those questions.

Does the same thing occur if the core is wound with twisted pair that is well represented as a uniform two wire transmission line?

Let’s treat the common mode choke as a black box with two input terminals at left and two output terminals at right with voltages as annotated above. Continue reading VNA fixture for measuring Zcm of a common mode choke – twisted pair wound

Last update: 5th January, 2024, 6:33 PM

VNA fixture for measuring Zcm of a common mode choke – coax wound

A common online question is what sort of fixture is appropriate to measure the common mode impedance of a common mode choke.

Above is a screenshot from a Youtube video by Trx Lab, probably about 2016 vintage. I see many problems with the fixture, lets start with the resistors. Continue reading VNA fixture for measuring Zcm of a common mode choke – coax wound

Last update: 3rd January, 2024, 11:17 AM

Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #7

Seventh part in the series documenting the design and build of a Guanella 1:1 (current) balun for use on HF with wire antennas and an ATU.

  • This article describes a measurment of common mode impedance Zcm of the packaged balun.

Packaging

The prototype fits in a range of standard electrical boxes. The one featured here has a gasket seal (a PTFE membrane vent was added later).

AtuBalun201

Above, the exterior of the package with M4 brass screw terminals each side for the open wire feed line, and an N(F) connector for the coax connection. N type is chosen as it is waterproof when mated. Continue reading Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #7

Last update: 9th September, 2023, 2:23 PM

MismatchLoss of severely mismatched EFHW transformer – system response

It is easy to become focused on the behavior of a component, but don’t lose sight of the fact that it is but a component of a system where components interact, and the system response is the bigger / more complete picture.

In the article MismatchLoss of severely mismatched EFHW transformer , a caveat stated and restated was:

Transmitters are not necessarily well represented as a Thevenin source, so measurements using such sources (VNA, SA with TG) and application of linear circuit theory are not necessarily applicable.

So, can we estimate a likely system response to a 30+j0Ω load, good 1:49 transformer and modern HF 100W (20dBW) SSB transceiver designed for a nominal 50Ω load?

The following analysis gives a likely solution and it deals with a common implementation where the source is anything but a Thevenin source.

PA VSWR protection

Most transceivers of this type incorporate several PA protection measures, and one of them is commonly to reduce IF gain so that reflected power measured in a directional coupler near the antenna jack is not more than say 4W. This accommodates VSWR up to 1.5 without power reduction due to VSWR protection.

So, with an extreme mismatch, Pref=4W due to the PA protection system.

30+j0Ω load via an ideal 1:49 transformer

The scenario of 30+j0Ω load if that of the quoted measurement in the previous article, used without comment on its merit.

Lets use the calculation from the previous article, the equivalent case of a 50Ω Thevenin source with load of (30+j0)/49=0.6122+j0Ω.

The quantity 1/(1-|s|^2) is the MismatchLoss, \(MismatchLoss=\frac{P_{fwd}}{P_{fwd}-P_{ref}}=\frac1{1-\frac{P_{ref}}{P_{fwd}}}\) and it is 20.92, so we can calculate that if Pref=4w (by virtue of PA protection), that \(P_{fwd}=\frac{P_{ref}}{0.952}=\frac4{0.952}=4.202 \text{ W}=6.235 \text{ dBW} \). Continue reading MismatchLoss of severely mismatched EFHW transformer – system response

Last update: 14th April, 2024, 1:34 PM

MismatchLoss of severely mismatched EFHW transformer

In an social media discussion about loss of EFHW transformers under mismatch conditions, one of the gathered experts said:

It doesn’t even have to be highly complex Z. Just presenting an impedance other than 2450 sends the loss through the roof. The back to back transformer test is misleading unless the antenna presents something very close to 2450 on each band for which it is used.

giving this graphic to quote someone else’s work in support.

Interpreting this graphic is fraught with risks, the author obviously does not understand and accept / follow the conventional meaning of term loss. Continue reading MismatchLoss of severely mismatched EFHW transformer

Last update: 14th April, 2024, 1:34 PM