Much is written about antenna system balance, this article looks at balance issues with the very common ATU configuration that uses a Ruthroff 4:1 voltage balun to adapt coax transmitter output to two wire open feed line. This type of balun is employed in most ham market ATUs that contain an integral balun.
Above is Ruthroff’s equivalent circuit, Fig 3 from his paper (Ruthroff 1959).
If one looks carefully at the transmission line form, there is effectively a two wire line wound into a helix (usually on a magnetic core) and connected from the unbalanced source to one half of the load inverting the connection for the necessary phase reversal.
Ideally, Vout of this line is equal to Vin, ie Vout/Vin should be 1∠0°. That is unlikely as it implies a zero length transmission line which provides the decoupling of the phase inverting line.
This article looks at the Ruthroff 4:1 balun balance using the very popular MFJ-949E as an example.
Common practice is to treat antenna systems as a two terminal device in free space.
Pickup most handbooks, and even text books, and antennas and often antenna systems are described in this way.
That model is quite inadequate for many or most antenna systems installed in proximity of natural ground. For example, a two terminal dipole and feed line system representation cannot have feed line common mode current, and it follows that thinking in terms of two terminal models denies a full understanding of the antenna system.
A three terminal model of an antenna system
(Schmidt nd) sets out a three terminal model of an antenna system in presence of ground using quite conventional linear circuit theory.
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