I will touch on ATU design in the next post.Hey folks – summer is upon us here in the great green North… the past few days we have been under a heat warning, which happens here any time the mercury climbs above 30 degrees Celsius, or about 85° F.
Smith chart hd trial#
Broadbanding even a simple one tower AM antenna can require quite a bit of time and some trial and error. It varies on system design, but things like switches, contactors, mating connectors, ATU enclosures, etc can also add VSWR and asymmetry. That means that final bandwidth observations will need to be made at the transmitter output terminal or in some cases, the input to the matching network. When working with AM systems, the bandwidth of the entire system needs to be examined. After looking at the plotted Smith chart, my first inclination would be to reduce the rotation, more tower +75° as a first step in tweaking. This will likely be close, but will need to be tweaked a bit to find the optimum bandwidth. Thus, the best phase rotation to start with is +79°. The above example, the line is fairly shallow, which is typical of a skirted tower. The cusp is where the direction of the line changes, which in this case is the carrier frequency, 1430 KHz. To determine phase rotation, the cusp of the plotted graph is rotated to face either the 3 o’clock or 9 o’clock position (0° or 180°). Having the proper phase advance or phase retard rotation will distribute the sideband energy symmetrically about the carrier. The antenna tuning unit will match the line impedance to the load impedance and cancel out the reactance. One of the first principles behind broadbanding an AM antenna is to distribute the sideband energy evenly and have symmetrical VSWR. Plotted on a Smith Chart: 1430 base impedance plotted on a Smith chart The base impedance is not too far out of line from what is expected for a tower this tall. Thus this tower is slightly taller than 1/4 wave length. A 1/4 wave tower (typical for AM) is 90 degrees tall. An AM tower that is expressed in electrical degrees is denoting wave length. The tower is skirted, 125.6 degrees tall. To explain this simply, I will use an example of a single tower non-directional antenna.īelow is a chart of base impedance from a single tower AM antenna on 1430 KHz. It is a complex topic, especially where directional antenna systems are concerned, as there are several potential bottle necks in a directional array. I touched on the black art of AM antenna broadbanding before. More on basic Smitch chart usage information on this video: If the load impedance is 85 ohms and the reactance is +j60, then the normalized Smith chart point would be. Thus, if the input resistance is 50 ohms and the load impedance is 50 ohms j0, then the normalized Smith chart point would be 50/50 or 1. To normalize, the load resistance and reactance is divided by the input resistance. In the center of the Smith chart is point 1, which expresses a perfect match. Impedance and reactance are expressed as ratios of value units like VSWR. The first thing to understand about a Smith chart is normalization. pdf version available here: smith-chart. They can be very useful for AM antenna broadbanding. Smith charts offer a great way to visualize what is going on with a particular antenna or transmission line. They look complicated, but are really pretty easy to understand and use, once you get around all those lines and numbers and stuff. I have been fooling around with Smith Charts lately.
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