## On testing coax cable loss with an analyser / VNA – part 3

On testing coax cable loss with an analyser / VNA – part 2 gave a method of approximating the matched line loss (MLL) of a section of transmission line based on measurements of ReturnLoss with the section terminated in both an open circuit and short circuit. The article demonstrated the method using TLLC to provide expected measurement values.

So, does it work in practice?

Let’s measure a 10m length of Belden 8267 (RG-213) fitted with N connectors using a Rigexpert AA-600 and an instrument grade N(F) short circuit.

ReturnLoss @ 3.5MHz is 0.15dB. Continue reading On testing coax cable loss with an analyser / VNA – part 3

## On testing coax cable loss with an analyser / VNA – part 2

On testing coax cable loss with an analyser / VNA – part 1 questioned a common method of measuring Matched Line Loss (MLL) of a section of an open circuit transmission line section, posing the questions:

The example gives MLL’ (based on half ReturnLoss) of about two thirds cable MLL.

• Why is that?

• What does it say for the measurement technique?

## On testing coax cable loss with an analyser / VNA – part 1

A recent online video provides instruction on how to measure loss of a section of coax cable, loss to mean Matched Line Loss, $$MLL=\frac{P_{in}}{P_{out}}$$ when the cable is terminated in its characteristic impedance Zo, and which can be expressed in db as $$MLL=10 log_{10} \frac{P_{in}}{P_{out}}$$. Note that MLL in dB is ALWAYS a +ve value for a passive DUT such as this.

There is nothing new in the method, it appears in lots of analyser user manuals, and has a built in assist in many analysers.

The video deals with the case of an antenna analyser that has a ‘measure cable loss’ function and using a VNA. Lets use the VNA graphic as it shows more detail of what is happening.

Above is the video’s graphic for the case. The narration says to use the dB magnitude of s11 or ReturnLoss as equivalents. They aren’t equivalent (a hammy Sammy thing), $$ReturnLoss=-20 log_{10}|s11|$$ or $$ReturnLoss_{dB}=-s11mag_{dB}$$ (both wrt the VNA reference impedance). Continue reading On testing coax cable loss with an analyser / VNA – part 1

## 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

## 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

## RF Power Meter 2 (RFPM2) – display update

RF Power Meter 2 is a logging RF power meter based on AD8307 and ESP8266.

The original LCD display was white on blue, but was very difficult to read at some viewing angles, so it had to go. Unfortunately I could not find more displays with that hole pattern, it seems to have been discarded for a newer hole pattern as almost everything I looked at had the same newer patter.

So, the box front needed rework, and there would be visible spare holes… so a dress escutcheon was designed in Freecad and cut on a CNC router.

The escutcheon was designed to be cut from some 3mm black acrylic sheet that was on hand, and it would cover the reworked panel. Continue reading RF Power Meter 2 (RFPM2) – display update

## A desk review of the MiniPa100 kit – #1: characterise the output transformer

This article is one in a series of a desk review, a pre-purchase study if you like, of the MiniPa100 kit widely sold on eBay and elsewhere online.

One of the first questions to mind is whether it is likely to deliver the rated power, so let’s review the MOSFET output circuit design from that perspective.

Sellers mostly seem to need to obscure the MOSFET type in their pics, so essentially you buy this with no assurance as to what is supplied, no comeback if the supplied MOSFET is not up to the task. Online experts suggest the MOSFET is probably a MRF9120 (or 2x IRF640 in a 70W build). The amplifier claims 100W from 12-16V DC supply.

Note that this module does not include the necessary output filter which will lose 5-10% of the power from this module.

In this case Carlos, VK1EA, connected a sample output transformer (T2) core from a recently purchased MiniPa100 kit to a EU1KY antenna analyser. The fixture is critically important, it is at my specification. Continue reading A desk review of the MiniPa100 kit – #1: characterise the output transformer

## Some pretty woolly thinking on measuring Thevenin equivalent source impedance of a ham transmitter

A ham seeking to optimise his station based on some measurements with a VNA and some modelling of a matching network posted the results of a test in the process.

The radio is an Icom IC-7300. I bypassed the built in tuner, transmitted a tone into my external tuner, adjusted it for SWR=1. I then disconnected the tuner from the radio, and measured the impedance looking into the tuner with a VNA. Surprisingly, (to me anyway) the result was a pretty good 53-j3 Ohms at 14 MHz.

What should we / have expected? It is an interesting case to study. Continue reading Some pretty woolly thinking on measuring Thevenin equivalent source impedance of a ham transmitter