Measuring coaxial cable loss by transmission measurement with a directional wattmeter

The article Measuring coaxial cable loss with a voltmeter discussed some pitfalls of that measurement method, starting with the influence of theoretical error in actual Zo at lower frequencies.

You might expect that using a directional wattmeter has exactly the same problems because as many online experts advise, at the end of the day they are just a voltmeter.

They are wrong, a Bird 43 might use a half wave detector driving a d’Arsonval meter and you might regard that to be a voltmeter, but the RF signal it measures is a combination of samples of forward and reflected waves wrt to its calibration impedance (usually 50+j0Ω) and we will see that makes a difference.

Where a directional wattmeter is calibrated for a purely real impedance (ie X=0), then the relationship \(P=P_{fwd}-P_{ref}\) holds true (On the concept of that P=Pfwd-Prev).

Lets take an example to explore the theoretical answer. We will use 10m of Belden 8359 (RG58A/U) @ 3.6MHz.

Lets model the scenario in TLLC. We will select the “Use Lint” switch for a better model of this specific cable at 3.6MHz and take the “Long” output.

Above is the input form. Continue reading Measuring coaxial cable loss by transmission measurement with a directional wattmeter

Measuring coaxial cable loss with a voltmeter

A question asked online about measuring terminated coax cable loss with an RF voltmeter and whether to condemn it based on comparison with specs raises an interesting case to discuss.

The subject raises some immediate concerns:

  • the accuracy of the termination;
  • the accuracy of the voltmeter;
  • the extent to which the voltmeter disturbs the thing being measured; and
  • assumptions about matched conditions.

Lets take an example to explore the theoretical answer. We will use 10m of Belden 8359 (RG58A/U) @ 3.6MHz.

Lets model the scenario in TLLC. We will select the “Use Lint” switch for a better model of this specific cable at 3.6MHz and take the “Long” output.

Above is the input form. Continue reading Measuring coaxial cable loss with a voltmeter

Simsmith bimetal line type

This article discusses various measurements and models of Wireman 551 windowed ladder line, including adapting Simsmith’s bimetal line type to bear on the problem.

Measurements

A starting point for characterising the matched line loss (MLL) of the very popular Wireman 551 (W551) windowed ladder line is the extrapolation of measurements by (Stewart 1999) to 1.8MHz. Since the measurements were made at and above 50MHz where the W551 has copper like performance, this is likely to underestimate actual MLL and such wide extrapolation introduces its own uncertainty. Nevertheless, the datapoint is MLL=0.00227dB/m.

Dan Maquire recently posted a chart summarising measurements of these lines.

For the purposes of this article, let’s tabulate the MLL at 1.8MHz in dB/m. Continue reading Simsmith bimetal line type

A thinking exercise on Jacobi Maximum Power Transfer #4

The article A thinking exercise on Jacobi Maximum Power Transfer #3 discussed Kurokawa’s power reflection coefficient as in indicator of mismatch at a system node.

Above is a demonstration circuit in Simsmith, a linear source with Thevenin equivalent impedance of 50-j5Ω. The equivalent voltage is specified by useZo, which like much of Simsmith is counter intuitive (as you are not actually directly specifying generator impedance):

Vthev and Zthev are chosen so that ‘useZo’ will deliver 1 watt to a circuit impedance that equals the G.Zo. Zthev will be Zo*.

Continue reading A thinking exercise on Jacobi Maximum Power Transfer #4

The transmitter matching problem

In the article The system wide conjugate match stuff crashes out again I worked through an example proffered in an online discussion to show that Walter Maxwell’s teachings on system wide simultaneous conjugate match do not tend to occur in practical systems.

Why are hams so obsessed with conjugate matching?

The answer is on the face of it quite simple. Continue reading The transmitter matching problem

nanoVNA-H – recovery

I often see reports that a nanoVNA has been ‘bricked’.

The STM32F072 chip  used on the original nanoVNA has some features that make the firmware update process simple and robust, and difficult to mess up.

The normal way of doing a firmware update is using the DFU protocol from a PC over the USB interface. To use this, the device has to be “put into DFU mode”, this means that the chip is reset and started executing the bootloader in permanent system memory.

The concept of DFU is that normal client programs used with the device can easily be extended to include the DFU function as just another menu function of the client software. I am not aware of any nanoVNA client that does this.

So, you need to use a programming client, and for Windows a good choice is ST’s DfuSeDemo. You may need to convert the distributed file format using Dfu file manager from the same distribution, not all developers distribute a .dfu file.

There is a pin on the board, BOOT0, that must be held high during reset to enter the on-chip bootloader. Later firmware versions also provide a menu option to enter the bootloader, but if an attempted upgrade messes up the menu, you may need to use the BOOT0 pin bridged to the adjacent VDD pin while you power cycle the nanovna.

Above is the rear view of the board, and a jumper using pogo pins to bridge BOOT0 to VDD. Continue reading nanoVNA-H – recovery

A thinking exercise on Jacobi Maximum Power Transfer #3

At A thinking exercise on Jacobi Maximum Power Transfer #2 I posed the question of a metric for the mismatch at the L2L1 junction in the following network where the calculated values L2L1_lZ is the load impedance at the L2L1 junction (looking left as Simsmith is unconventional), and L2L1_sZ is the source impedance at the L2L1 junction (looking right). The left three components are the fixed antenna representation.

Common practice is to speak of a “source VSWR” to mean the VSWR calculated or measured looking towards the source, and very commonly this is taken wrt 50+j0Ω which may be neither the source or load impedance but an arbitrary reference. Continue reading A thinking exercise on Jacobi Maximum Power Transfer #3

A thinking exercise on Jacobi Maximum Power Transfer #2

At A thinking exercise on Jacobi Maximum Power Transfer I posed an unanswered Q2:

Keeping in mind that C2 and L2 are an adjustable matching network, usually adjusted for minimum VSWR as seen at the source G. So, the questions are:

  1. Does the system take maximum available power from the source G when the load impedance seen by source G is equal to the conjugate of its Thevenin equivalent source impedance (ie C2.Z=G.Zo in Simsmith speak)?

  2. Does that ‘matched’ condition result in maximum power in the load L?

Above for reader’s convenience is the model conjugate matched at the GC2 interface. The calculated Po figure (lower right) is the power in the load L to high resolution. Continue reading A thinking exercise on Jacobi Maximum Power Transfer #2

nanoVNA-H – rework of v3.3 PCB to v3.4?

nanoVNA-H v3.4 is out, and I don’t yet see significant problem reports.

When I compare the circuit with v3.3 (which I have), apart from new battery charger IC etc, the changes are in three areas:

  1. decoupling power to the mixers;
  2. increasing the drive to the mixers; and
  3. higher attenuation of input on the rx port. Continue reading nanoVNA-H – rework of v3.3 PCB to v3.4?

A thinking exercise on Jacobi Maximum Power Transfer

At The system wide conjugate match stuff crashes out again I discussed the failure of Walt Maxwell’s teachings on system wide simultaneous conjugate match using an example drawn from an online expert’s posting.

The replicated scenario with matching with an L network where the inductor has a Q of 100, no other loss elements is shown below. (Quality real capacitor losses are very small, and the behavior will not change much, the inductor loss dominates.)

Above is a model in Simsmith where I have adjusted the lossy L network for a near perfect match. I have used a facility in Simsmith to calculate the impedance looking back from L1, often known as the source impedance at a node but in Simsmith speak the calculated L1_revZ on the form, (ie back into the L network)  from the equivalent load. Continue reading A thinking exercise on Jacobi Maximum Power Transfer