Category: Measurement

A simple NanoVNA test of a ferrite core and winding to check its suitability in a 50Ω:xΩ transformer

The most common problem of broadband ferrite cored transformer designs for RF is insufficient turns which results in:

  • low magnetising impedance Zmag causing:
  • high InsertionLoss at lower frequencies;
  • excessive core loss at low frequencies, and
  • high InsertionVSWR at low frequencies.

This article give a simple test for a transformer that will have a nominally 50Ω input or output winding

Without going into a lot of magnetic and transformer theory, a through test using a VNA of the core and just one winding configured as a 1:1 (50Ω:50Ω) autotransformer is revealing. If that combination of turns, core, frequency is not adequate, it is very unlikely any transformer

Above is a schematic of the test configuration, the DUT is the central element, everything else is supplied by the VNA. Continue reading A simple NanoVNA test of a ferrite core and winding to check its suitability in a 50Ω:xΩ transformer

Last update: 27th August, 2024, 2:37 AM

Thoughts on the ARRL EFHW antenna kit transformer – improvements?

This is a follow up to Thoughts on the ARRL EFHW antenna kit transformer.

The first point to note is that Amidon’s 43 product of recent years is specified identically to National Magnetics Group H material. It is significantly different to Fair-rite’s 43 mix.

Though the parts list specifies an Amidon #43 core, I note that W1VT posted recently: Continue reading Thoughts on the ARRL EFHW antenna kit transformer – improvements?

Last update: 8th September, 2024, 6:13 AM

A quick and simple, but effective test of coax matched line loss

Background

From time to time, ham radio operators may question whether a section of installed and used coax is still good or significantly below spec and needs replacement.

A very common defect in coax installed outside is ingress of water. The earliest symptoms of water ingress are the result of corrosion of braid and possibly centre conductor, increasing conductor loss and therefore matched line loss (MLL). Any test for this must expose increased MLL to be effective. Continue reading A quick and simple, but effective test of coax matched line loss

Last update: 24th August, 2024, 8:24 PM

Is port extension or e-delay a universal solution?

Several recent articles examined the use of s11 port extension or e-delay in some scenarios that might have surprised.

Recall that s11 port extension adjusts the measured phase of s11 based on the e-delay value converted to an equivalent phase at the measurement frequency.

It is:

  1. an exact correction for any length of lossless line of Z0=50+j0Ω transmission line;
  2. an approximate correction for a very low loss length of approximately 50Ω transmission line; and
  3. an approximate correction for some specific scenarios such as those discussed at Some useful equivalences of very short very mismatched transmission lines – a practical demonstration.

Of course 1. does not exist in the real world, but 2. can give measurement results of acceptable accuracy if used within bounds. Both departures mentioned in 2. occur in the real world, non-zero loss and departure from Z0=50+j0Ω. Provided these departures are small, port extension may give acceptable results.

Let’s analyse some example measurements based on a 10m length of ordinary RG58A/U from 1-11MHz.

Above, measurement of the first series resonance with SC termination. Continue reading Is port extension or e-delay a universal solution?

Last update: 16th August, 2024, 12:58 PM

CMRR and transmitting antennas

Since the widespread takeup of the NanoVNA, a measure of performance proposed by (Skelton 2010) has become very popular.

His measure, Common Mode Rejection Ratio (CMRR), is an adaptation of a measure used in other fields, he states that he thinks the application of it in the context of antenna systems and baluns is novel and that “CMRR should be the key figure of merit”.

Skelton talks of different ways to measure CMRR, but essentially CMRR is a measure of the magnitude of gain (|s21|) from Port 1 to Port 2 in common mode, with the common mode choke (or balun) in series from the inner pin of Port 1 to the inner pin of Port 2.

Note that this is the same connection as used for series through impedance measurement, but calculation of impedance depends on the complex value s21.

Above is capture of a measurement of a Guanella 1:1 common mode choke or balun. The red curve is |s21|, the blue and green curves are R and X components of the choke impedance Zcm calculated from s21. Continue reading CMRR and transmitting antennas

Last update: 11th August, 2024, 10:33 AM

Some useful equivalences of very short very mismatched transmission lines – a practical demonstration

This article presents a simple practical test of the concepts laid out at Some useful equivalences of very short very mismatched transmission lines.

Above is the DUT, it is a short circuit at the end of 102mm of two wire transmission line with VF=1, conductor diameter 0.47mm and 5mm spacing.

The transmission line is not perfectly uniform, but sufficiently good for this demonstration.

We are going to use port extension or e-delay to adjust the reference place to the short circuited end of the transmission line. Continue reading Some useful equivalences of very short very mismatched transmission lines – a practical demonstration

Last update: 11th August, 2024, 11:02 AM

NanoVNA-H4 radio remote trial #6 – HC-05 Bluetooth matured

NanoVNA-H4 radio remote trial #1 – HC-05 Bluetooth described intial tests on a Bluetooth remote connection to a NanoVNA-H4 using an inexpensive HC-05 adapter by hc01.com.

UART connector

For more convenient access to the UART pins, I installed a SIL 6w female header and cut a 3x18mm opening in the back for access.

I have seen reports that the Bluetooth module can be fitted inside the case. At this stage I am reluctant to do that for several reasons, EMC being one, and a convenient means of turning the power off to the Bluetooth module is another (it might be useful if one of the IO pins signaled that the UART interface was selected). Continue reading NanoVNA-H4 radio remote trial #6 – HC-05 Bluetooth matured

Last update: 10th August, 2024, 7:57 AM

Using your NanoVNA-H4 for Zref other than 50Ω

An online expert recently promoted this way of using a NanoVNA on 600Ω lines: Normalizing the NanoVNA for any characteristic impedance.

Essentially, he calls for SOL calibrating the VNA with the L or LOAD being a 600Ω resistor.

This does have the effect of ‘correcting’ all s11 measurements to be wrt Zref=600Ω, but most of the calculations of derived values like R, X, etc are wrong.

Above is an example where the NanoVNA-H4 was calibrated with LOAD=470Ω, and then that resistor measured. Note the |s11| is very small, it is correct. The Smith chart locus is a dot in the middle of the chart, it is correct… but the Smith chart marker legend shows Z=49.96-j0.0206Ω which is a gross error, it should be very close to 470Ω. Continue reading Using your NanoVNA-H4 for Zref other than 50Ω

Last update: 9th August, 2024, 3:16 PM

Use of port extension or e-delay to measure a dummy load

I have an MFJ-264N which does not perform very well.

A measurement of the impedance at the inboard side of the N connector jack would be informative.

So, a NanoVNA-H4  (SOL calibrated at the Port 1 connector) with short SMA(M) to SMA(M) and SMA(F) to N(M) adapter are available, but how to set the reference plane to the inboard end of the N(F) connector? Continue reading Use of port extension or e-delay to measure a dummy load

Last update: 2nd August, 2024, 1:14 PM

Some useful equivalences of very short very mismatched transmission lines

This article explains some very useful equivalences of very short very mismatched transmission lines. They can be very useful in:

  • understanding / explaining /anticipating some measurement errors; and
  • applying port extension corrections to VNA measurements where the fixture can be reasonably be approximated as a uniform transmission line.

Port extension commonly applies a measurement correction assuming a section of lossless 50Ω transmission line specified by the resulting propagation time, e-delay, but as explained below, can be used to correct other approximately lossless uniform transmission line sections of other characteristic impedance. Continue reading Some useful equivalences of very short very mismatched transmission lines

Last update: 9th August, 2024, 11:57 PM