## Center-Fed Dipole : elements length for a Z=200 +/- 0j ohms

A chap asked online for dimensions of a 50MHz dipole with a feed point of 200+j0 to suit 50Ω feed line and a 1:4 coax half wave balun. The “+/- 0j” is hammy Sammy talk from an ‘Extra’.

This type of balun, properly implemented, is a good voltage balun, and it is quite suited to a highly symmetric antenna.

A good voltage balun will deliver approximately equal voltages (wrt the input ground) with approximately opposite phase, irrespective of the load impedance (including symmetry).

Where the load is symmetric, we can say a good voltage balun will deliver approximately equal currents with approximately opposite phase, irrespective of the load impedance.

It is an interesting application, and contrary to the initial responses on social media, there is a simple solution.

## One solution

Let’s take a half wave dipole and lengthen it a little so the feed point admittance becomes 1/200-jB (or 200 || jX). We will build an NEC model of the thing in free space.

Above is a sweep of the dipole which is 3.14m long (we will talk about how we came to that length later), and the Smith chart prime centre is 200+j0… the target impedance. Continue reading Center-Fed Dipole : elements length for a Z=200 +/- 0j ohms

## End Fed Half Wave matching transformer – 80-20m – model and measurement

Reviewing consistency of measured and model data, the first posting was based on an incorrect model parameter (aol), the article is now revised for the correct value, apologies.

End Fed Half Wave matching transformer – 80-20m described a EFHW transformer design with taps for nominal 1:36, 49, and 64 impedance ratios.

Keep in mind that this is a desk design of a transformer to come close to ideal broadband performance on a nominal 2400Ω load with low loss. Real antennas don’t offer an idealised load, but this is the first step in designing and applying a practical transformer.

The transformer comprises a 32t of 0.65mm enamelled copper winding on a Fair-rite 5943003801 core (FT240-43) ferrite core (the information is not applicable to an Amidon core), to be used as an autotransformer to step down a EFHW load impedance to around 50Ω. The winding layout is unconventional, most articles describing a similar transformer seem to have their root in a single flawed design, and they are usually published without meaningful credible measurement. Continue reading End Fed Half Wave matching transformer – 80-20m – model and measurement

## U.FL connectors – hints

This article expands on discussion at nanoVNA – that demo board and its U.FL connectors.

Before looking at the specifics of the Hirose U.FL connector, clean connectors work better and last longer. That should not be a revelation.

A can of IPA cleaner and a good air puffer are invaluable for cleaning connectors. The air puffer show  has a valve in the right hand end, it doesn’t suck the dirt and solvent out of the connector and blow it back like most cheap Chinese puffers, this one was harder to find and expensive (\$10!). Continue reading U.FL connectors – hints

## NanoVNA-H4 – inductor challenge – part 7

One method described online on YouTube and in social media is the 90° method as I will call it.

The reason why people make measurements at +/- 90 degrees on the smith chart is because the measurement accuracy using the shunt configuration when trying to measure the nominal value of an inductor or capacitor is highest at 0+j50 ohms (or 0-j50 ohms… OD).

To be clear, this is the phase of s11 or Γ being + or – 90° as applicable.

Is there something optimal when phase of s11 is + or – 90°?

Does the software / firmware / hardware give significantly more accurate response under such a termination?

Above is a diagram from a HP publication, slightly altered to suit the discussion. Continue reading NanoVNA-H4 – inductor challenge – part 7

## NanoVNA – measuring Q of an inductor using s21 – fails?

There is a fashion of seeing s21 measurements as the answer to all things, and amongst the revelations is an explanation of measuring inductor Q using s21 shunt through configuration.

Let’s explore the use of s21 shunt through to directly find the half power bandwidth of a series tuned circuit and calculate the Q from that and the resonant frequency (as demonstrated by online posters).

To eliminate most of the uncertainties of measurement, let’s simulate it in Simsmith.

The simulation has a series tuned circuit resonated at 3400kHz, and the source and plot are set to calculate |s21| in dB. Though the model specifies Q independent of frequency, the D block adjusts Q for a constant equivalent series resistance (ESR) which simplifies discussion of resonance and Q. Continue reading NanoVNA – measuring Q of an inductor using s21 – fails?

## NanoVNA – Port 1 port extension

A VNA is usually calibrated by the user at some chosen reference plane using standard parts, commonly an open circuit, short circuit, and nominal (50Ω) load. As a result of this OSL calibration, the VNA is able to correct measured s11 to that reference plane, and display its results wrt that reference plane.

There are occasions where it is not possible, or not convenient to locate the DUT at the reference plane. This article discusses the problem created, and some solutions that might give acceptable accuracy for the application at hand.

The discussion assumes the VNA is calibrated for nominal 50+j0Ω.

Above is a diagram of a configuration where the unknown Zl is not located exactly at the reference plane, but at some extension. Continue reading NanoVNA – Port 1 port extension

## NanoVNA – DiSlord NanoVNA-D v1.1.00 & NanoVNA-App-v1.1.209-OD10 calibration

This article explains the interworking of DiSlord NanoVNA-D v1.1.00 firmware and NanoVNA-App-v1.1.209-OD10 with respect to calibration.

This applies to the specific combination of versions of firmware and software client, do not assume it applies to other combinations.

DiSlord NanoVNA-D v1.1.00 firmware supports a scan_bin command where bit 3 of the outmask field is used to request raw measurement data, ie uncorrected measurements.

NanoVNA-App-v1.1.209-OD10 supports exploitation of that capability when it recognises that firmware version and command support.

Above, NanoVNA-App-v1.1.209-OD10  has a dropdown list to choose calibration mode. Continue reading NanoVNA – DiSlord NanoVNA-D v1.1.00 & NanoVNA-App-v1.1.209-OD10 calibration

## NanoVNA – trying the DiSlord built in cable length measurement feature

A recent discussion online on the use of this facility in some model or other NanoVNA/firmware combination quickly ran to over 100 posts, and you might think it is really difficult, or plain does not work.

Let me say I am leery of built in features that invite users to perform something they do not understand, and may misinterpret the outcome.

Lots of the discussion ran to explaining why measurement of a sample of coax  would be out by 5% or more, lots of pseudo tech discussion about age related, contamination related, quality related explanations for the measurement, things which might cause the measurer to condemn the sample, to discard it.

Well, you would want to be pretty confident in yourself to make that call, given that the explanation might well be measurement error.

I don’t use this facility, so I am quite unfamiliar with it, and there is no documentation, so one make make an informed guess as to how to use it.

## NanoVNA phase confusion

One sees online discussions and videos where phase from a NanoVNA display is central to the subject, and more often than not, the use is quite confused.

Let’s look at some examples.

## Example 1

A poster advising on how to measure inductance using a NanoVNA posted a .s1p file of his measurements of a SM inductor of nominally 4.7µH from 1-5MHz and discussed the use of phase in determining the inductance.

Above is a plot of the data in the VNWA PC client. Four values are plotted: Continue reading NanoVNA phase confusion