AIM915a was recently pulled from the distribution site and replaced by a new release, AIM916.
AIM916 chokes on some calibration files created with earlier versions, so again historical scan data is rendered worthless. Note the illogical diagnostic message… typical AIM quality.
I cannot recall ever finding a new release that did not have significant defects, commonly inconsistency between displayed values. In the common theme of one step forward, two steps backwards, this version has defects that were not present in AIM910B.
This problem existed in AIM915a, it persists in AIM916.
Let’s review the internal consistency of this part of the display screen.
The article Baluns in antenna systems explores some different dipole and feed line configurations and the effectiveness of common mode chokes at various locations on the feed line.
Models 1, 2 and 3 particularly show the effect of a quarter wave vertical common mode conductor grounded and isolated, and a half wave vertical common mode conductor grounded.
These illustrate that those common mode conductors can be viewed to some extent as a ‘single wire’ transmission line, and the impedance presented at the dipole feed point is low or high in keeping with simple transmission line analysis of a shorted or open line of quarter or half wave length.
The question then arises with the Radcom “cable balun”, does it behave similarly, to what extent does the folding of the conductor affect its quarter wave resonance.
One way to explore this is to construct an NEC model of the structure and a reflection of itself.
This article visits the implementation on a pair of antennas that I built 50 years ago, and are still in use today (albeit with some small preventative maintenance once during that interval). The basic antenna is a four element Yagi for 144MHz copied from an ARRL handbook of the time, probably based on NBS 688. It was designed to deliver a split dipole feed point impedance of 50+j0Ω.
I built them using a Gamma Match, partly to get some familiarity, but mostly to implement a Gamma Match that was reliable, weatherproof and lasting… features that are alien to most implementations I had seen at that point.
Both antennas were constructed and the Gamma Match adjusted for VSWR<1.1 using a Bird 43 directional wattmeter. The dimensions of each (including the key gamma dimensions) are the same, not surprising, but a confirmation of repeatability. See Novel Gamma Match Construction for more discussion.
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 directional coupler at top left contains half wave peak detectors for forward and reflected waves. They are wired to the two compensated op amps at lower right (the connections are not shown on the circuit as the coupler may be remote, follow the terminal designations). Continue reading Should you trust your VSWR meter – linearisation
One often sees newbies ask about their VSWR meter readings, and a common observation is that the measured VSWR is better at low power and as power is increased, VSWR increases.
With the evolution of the ‘shack in a box’, and knowledge and experience to match, the problem is often reported observed with the transceiver’s internal VSWR meter.
Some of these ‘shack in a box’ have some pretty nifty features, for example the very popular Icom IC-7300 not only has an internal VSWR meter for the HF bands, but it can perform an assisted sweep and display the results graphically.