## Measuring a 1/4 wave balanced line – nanoVNA

A question was asked recently online:

I am about to measure a 1/4 wave of 450 ohm windowed twinlead for the 2m band using my NanoVNA. My question is, since I will be making an unbalanced to balanced connection, should I use a common mode choke, balun or add ferrites to the coax side to make the connection, or does it really matter at 2m frequencies? The coax lead from my VNA to the twinlead will be about 6″ to 12″ long. I will probably terminate the coax in two short wires to connect to the twinlead.

It is a common enough question and includes some related issues that are worthy of discussion. Continue reading Measuring a 1/4 wave balanced line – nanoVNA

## Measure transmission line Zo – nanoVNA – PVC speaker twin – loss models comparison #3

Measure transmission line Zo – nanoVNA – PVC speaker twin demonstrated measurement of transmission line parameters of a sample of line based on measurement of the input impedances of a section of line with both a short circuit and open circuit termination. From Zsc and Zoc we can calculate the Zo, and the complex propagation constant $$\gamma=\alpha + \jmath \beta$$, and from that, MLL. ## Measure transmission line Zo – nanoVNA – PVC speaker twin – loss model derivation

The article Measure transmission line Zo – nanoVNA – PVC speaker twin demonstrated measurement of transmission line parameters of a sample of line based on measurement of the input impedances of a section of line with both a short circuit and open circuit termination. From Zsc and Zoc we can calculate the Zo, and the complex propagation constant $$\gamma=\alpha + \jmath \beta$$, and from that, MLL.

## Measurement with nanoVNA

So, let’s measure a sample of 14×0.14, 0.22mm^2, 0.5mm dia PVC insulated small speaker twin. Above is the nanoVNA setup for measurement. Note that common mode current on the transmission line is likely to impact the measured Zin significantly at some frequencies, the transformer balun (A 1:1 RF transformer for measurements – based on noelec 1:9 balun assembly) is to minimise the risk of that. Nevertheless, it is wise to critically review the measured |s11| for signs of ‘antenna effect’ due to common mode current. Continue reading Measure transmission line Zo – nanoVNA – PVC speaker twin – loss model derivation

## CirroMazzoni Baby small transmitting loop – a desk study Above, the Baby loop is a small transmitting loop with a novel remotely controlled loop tuning capacitor and tuning mechanism, and gamma match. Continue reading CirroMazzoni Baby small transmitting loop – a desk study

## Coil former made of cast acrylic sheet

This article describes a coil former made from 3mm cast acrylic, cut on a CNC router, part designed in Freecad. The project is really an exploration of manufacture of one on my CNC router. Above is a 3D view of the part design. Continue reading Coil former made of cast acrylic sheet

## An interesting study in the effect of fixture on impedance measurement

A chap posted a pic and some mini VNA measurement results of a resistor which he reported has a DC resistance of 80Ω. Above is part of the pic, focusing on the ‘fixture’. The chap reports that the VNA was OSL calibrated, and we might assume that was at the SMA(M) connector (it is difficult to explain the results if the reference plane was at the VNA jack). Continue reading An interesting study in the effect of fixture on impedance measurement

## A common scheme for narrow band match of an end fed high Z antenna – surely it is a 1:9 transformer? A reader of A common scheme for narrow band match of an end fed high Z antenna commented:

…if the coil is tapped at 1/3, surely then the coil is a 1:3^2 or 1:9 transformer and the capacitor simply ‘tunes out’ the coil reactance, what is the input impedance when it has a 450+j0Ω load?

That is very easy to calculate in the existing Simsmith model. Above, with load of 450+j0Ω, the input impedance at 50MHz is 8.78+j34.36Ω (VSWR(50)=8.4), nothing like 50+j0Ω. Continue reading A common scheme for narrow band match of an end fed high Z antenna – surely it is a 1:9 transformer?

## A common scheme for narrow band match of an end fed high Z antenna

This article discusses the kind of matching network in the following figure. A common variant shows not capacitor… but for most loads, the capacitance is essential to its operation, even if it is incidental to the inductor or as often the case, supplied by the mounting arrangement of a vertical radiator tube to the mast. Continue reading A common scheme for narrow band match of an end fed high Z antenna

## Stacking two ferrite cores of different permeability for an RF inductor

One of the magic ham recipes often proposed is to stack two ferrite cores of different permeability for an RF inductor, but an explanation is rarely offered, I have not seen one.

## An explanation

Starting with some basic magnetism…

The inductance of an inductor is given by $$L=N\frac{\phi}{I}$$.

For a closed magnetic circuit of high permeability such as a ferrite cored toroid, the flux is almost entirely contained in the core and the relationship is $$\mathcal{F}=\phi \mathcal{R}$$ where $$\mathcal{F}$$ is the magnetomotive force, $$\phi$$ is the flux, and $$\mathcal{R}$$ is the magnetic reluctance. (Note the similarity to Ohm’s law.) Continue reading Stacking two ferrite cores of different permeability for an RF inductor