Jeff, 2E0CIT, sent me a Rigexpert AA-170 measurement file of his test of Insertion VSWR of a commercial balun.
Insertion VSWR is the VSWR looking into the balun with a matched load (termination) on its output, it is a measure of imperfection of the balun. It ought to be a specification item for low Insertion VSWR baluns, but it rarely given.
A broadband low Insertion VSWR balun must be wound with a transmission line of the nominal impedance, 50Ω in this case, and in the case of 50Ω , it is most likely to be coax.
At long last, some PTFE rod arrived to permit assembly of the transformers.
For reasons discussed in an earlier article, the transformers use a larger core than the original VU3SQM. They need to stand above the board, and whilst that compromises the mechanical strength of the assembly, it should have better performance. Continue reading VU3SQM directional wattmeter build – #4
VU3SQM offers an interesting directional coupler based on a Sontheimer coupler, and using AD8307 power sensing for a nominally HF coupler. I must say that I am not a fan of Sontheimer couplers… but that is what the board uses.
This article lays out a preliminary design review to assist in selection of appropriate toroids, and ordering of the needed parts.
Before trusting measurements made with any instrument, its behaviour should be validated, and this article documents issues discovered in one thread of tests. The developer does not like the term “defects”, he prefers “issues”, a soft denial of “problems”.
So, the test scenario is the VA5 measuring the impedance looking into a 35m length of RG6 coax with an open circuit at the far end. The VA5 has been SOL calibrated with the higher quality loads sold by SDR-kits, and the test is a 3.5MHz. The firmware is the latest, v1.08 (about 3 months old).
The screenshots are taken with a camera, there does not seem to be a method of uploading screenshots to a PC.
Whilst preparing A first test of the FA-VA5 antenna analyser, issues were noticed with the user interface design / implementation. I stated in a later article that The matter of the clumsy / unproductive user interface will be explored more at a later time.
This article introduces a short video demonstration of the frustrating / unreliable user interface (UI) in firmware v1.08 where buttons do not seem to operate intuitively and consistently.
The parameters above capture the most basic operation of RFPM2 as a power meter directly displaying dBm with bar graph in fixed 2dB increments to 16dBm FSD. These values serve as a basis for some other applications as they capture the basic intercept and slope of the AD8307 module in this instance.
Current probe calibration
Alternative config files can be loaded on the fly from the webserver interface, for example http://192.168.0.86/config?filename=/dBA.cfg will load the dBA config file for a certain current probe. Continue reading RFPM2 – calibration files
The RF Power Meter 2 (RFPM2) stores calibration constants in a file located in the (SPIFFS) file system in the microcontroller flash.
The WiFi credentials are stored separately at the default location in the flash.
The AD8307 outputs a voltage from zero to about 3V for inputs from around -90 to +15dBm. The nominal output has a slope of 25mV/dB and intercept of -84dBm.
A starting point for RFPM2 calibration constants is intercept=-84 and slope=0.129.
Modules such as that used here may have pots to adjust the gain and offset of the output. Adjust the gain so that the maximum output voltage is a little lower than 3.3V (the maximum ADC input), say 2.7V.
The response of the AD8397 has some ripple in the transition between log amp stages. The log cell ranges are 14.3dB, so min error repeats every 14.3dB. My own practice is to calibrate at -62 and -5dBm input as they fall approximately on the mid line of the ripple trend (4 cycles of the error curve). Continue reading RFPM2 – calibration
This article describes a current probe for use with a power meter calibrated in dBm (eg RFPM1 and RFPM2).
For use with RFPM1 and RFPM2, both of which read to 16dBm max, it is convenient that the scaling factor for the probe is 0dBA/dBm, ie that those meters read dBA directly, implying a current range of -75-16dBA or 0.186mA-6.3A. In fact for low measurement noise, the effective range would be more like -65-16dBA or 0.56mA-6.3A.
The probe comprises a ferrite cored transformer that is clamped or placed over the conductor(s) of interest, and uses a 10t secondary which has a low value resistive load, across which the power meter connects.