Screen capture of the nanovna etc using a Python script

I find it convenient to capture the screen from my tiny devices directly to a computer over the USB connection.

I use a script derived from Ho-Ro’s nanovna-tools.

The script emits a png of the screen, and a png of the screen with the colours inverted (for presentation and printing).

Above is a screen shot from a tinysaultra.

The script is published on github: https://github.com/owenduffy/tinydevicecapture .

RFPM2 – current probe – #4 – TinySA Ultra

I have been asked about compatibility of the RFPM2 current probe with the TinySA and variants.

The current probe was intended for use with a broadband 50Ω RF power meter, but could also be used with a Spectrum Analyser with 50Ω input or an oscilloscope with 50Ω input.

RFPM2 – current probe described a current probe for use with a power meter calibrated in dBm (eg RFPM1 and RFPM2). RFPM2 – current probe – #2 exposed some of the build details. RFPM2 – current probe – #3 showed the implementation.

Above the current probe with RFPM2. Whilst this is not of a clamp-on design, the aperture in the core is sufficient to pass a DIN 7/16 connector through. Continue reading RFPM2 – current probe – #4 – TinySA Ultra

RF compensation of power relays – bigger relays

RF compensation of power relays referred to a video I have recently posted RF compensation of power relays.

Above, the example relay.

So, does this technique work for bigger relays?

Firstly, small is beautiful… it is easier to get good compensation of smaller relays over a wider frequency range.

Above is an example relay by K5UJ for discussion. I do not have measurement data for this relay box, but experience tells me that at HF, the compensation technique discussed above is likely to give good results for its intended purpose as a HF T/R relay. Continue reading RF compensation of power relays – bigger relays

DSO – dead time after a trace

Oscilloscopes have always had some dead time after completing a trace when they were not able to be retriggered.

I noticed a hand held DSO that I was evaluating to produce a different result on a train of pulses that repeated every 10m, different to a budget Tek TDS2024C DSO.

So, I have conducted an experiment comparing 7 DSOs that I can put my hands on.

The test waveform is a burst of 1MHz 50% square wave of adjustable number of cycles n, and the burst repeated every 100ms.

Setting the DSO timebase to 1µs/div and NORMAL trigger, and with zero time at mid display, the initial display with n=10 is a flat line up to mid screen, then pulses (5 or 6 depending on the display width) to edge of screen. This is rewritten every 100ms with every new burst. Continue reading DSO – dead time after a trace

Antenna assessment using NanoVNA – learning from a user example

A recent online posting provides content for learning. K3EUI posted a NanoVNA-Saver screenshot of his antenna described as:

Set out a horizontal loop wire antenna for possible NVIS paths
Wire is about 140 ft length with an outside CLC tuner, fed with 50 ft RG213. …

Here were the Nano VNA graphs of this new loop antenna, measured from inside the shack (50 ft RG213)…

Can we learn something from this? Continue reading Antenna assessment using NanoVNA – learning from a user example

IC-7300 VSWR protection

A ham consulting the experts on QRZ asked:

On 30 meters, my SWR reads 3:1 to my antenna (an EndFed 53 feet long wire up about 25 feet). Reading a chart I have, I see that at 80 watts output, my reflected power should be 20 watts. I verified this by looking at my Diamond SX-200 Meter which also indicates the reflected power is 20 watts. My questions are these: does the 20 watts reduce my 80 watts output to 60 watts at the antenna? I have a choke on my feed line in my shack (near my transceiver) & the SX-200 Meter is between the choke & transceiver….

The OP later explained that the transceiver is a IC-7300 and it appears that the internal tuner is in use above… so let’s proceed on that basis.

Analysing the OP’s report, his SX-200 indicates VSWR=3 Pf=80, therefore Pr=20, and P=60W. Note that \(P=P_f-P_r\) is valid because Zref is real, so the answer to his question about power to the antenna is 60W, he is quite correct.

He went on to ask where the 20W reflected goes to… but I will leave that to Walt Maxwell devotees to discuss energy sloshing around and re-re-re reflections… the stuff of ham lore.

Understanding the IC-7300

As an example of what might be expected of the IC-7300 with a mismatched load, I did a series of measurements at 7MHz with a sample variably mismatched load.

Above is a plot of power output vs VSWR for a sample mismatched load. Also plotted is the measured reflected power and the calculated power output based on the ham lore \(P=P_f (1-\rho^2)\). Continue reading IC-7300 VSWR protection

Transmission lines – forward and reflected phasors and the reflection coefficient

Let’s consider the following transmission line scenario:

  • Lossless;
  • Characteristic Impedance Zo=1+j0Ω; and
  • load impedance other than 1+j0Ω, and such that Vf=1∠0 and Vr=0.447∠-63.4° at this point.

The ratio Vr/Vf is known as the reflection coefficient, Γ. (It is also synonymous with S parameters S11, S22… Snn at the respective network ports.)

Above is a  phasor diagram of the forward and reflected voltages at the load. Continue reading Transmission lines – forward and reflected phasors and the reflection coefficient

Measurement of various loss quantities with a VNA – a worked example

This article documents a worked example of the matters discussed at Measurement of various loss quantities with a VNA.

Above is an air cored solenoid of about 20µH connected between Port 1 and Port 2 of a NanoVNA-H4 which has been calibrated. The whole lot is sitting on an inflated HDPE bag to isolate the DUT from the test bench. Continue reading Measurement of various loss quantities with a VNA – a worked example