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Are these points true?
The following is the simplest way of obtaining a practical yagi design using an antenna modeling program: 1 If you use a non conducting boom then you can build a practical model using the element lengths in the model. 2 For a VHF yagi in the clear you can assume freespace. 3 A simple straight wire in your model that is made source is DE and is treated as a simple hertz dipole by the program. So the values given for R and J are the values you would measure having a hertz dipole as DE. This is true for the model and would be fairly accurate if the boom was non conducting and reasonably in the clear in the practical design. 4 It is easy to scale for frequency in antenna programs. 5 It is fairly easy to alter element diameters in antenna programs and to obtain a redesign that maintains the original performance characteristics.* It seems to me that if one is prepared to use a non conducting boom, a hertz dipole as DE, then the model is pretty much the practical design. Not sure about impact of a balun or the feedline. --------------------------------------------- * http://www.cebik.com/32m.html The second kind of answer involves adapting a given antenna design to a new diameter material. Suppose that a designers specifies 0.1875" (3/16" or 4.76 mm) elements. The builder has a stock of either 0.125" (1/8" or 3.18 mm) rods or 0.25" (1/4" or 6.35 mm) rods. Will he or she need to adjust the element lengths of spacings? The answer is "yes." In fact, without making such adjustments, the antenna will not perform as originally designed. There are two major reasons for this result. In general and first, both driver and parasitic elements lengths require adjustment with every change of diameter. The general goal is to arrive at elements whose self-resonant frequencies are the same as in the original array. Second, the inter-element coupling changes for a given spacing of two elements if we change the element diameter. Element spacing does not change as rapidly as the element length for a given level of coupling when we change element diameter. However, it changes enough so that we cannot ignore the effects. One of the simplest ways to accommodate a revised element diameter is to resort to a Yagi optimizing program. We simply plug into the program the existing design and specify the new element diameter. The program then churns out the revised design. More antenna builders have general antenna modeling programs than have optimizing programs. There is a procedure that we can use to reoptimize a design for a new element diameter, although it has a pitfall from which we must guard ourselves. Here is how the procedure works. 1. Create a model of the original design and establish its operational characteristics. 2. Revise the model to use the new element diameter. 3. Find the frequency at which the new model shows the same operating characteristics as the original model did at its initial design frequency. If we are moving to a larger-diameter element, the new frequency will be lower than the old one. If we are moving to a smaller-diameter element, the new frequency will be higher than the old one. 4. Frequency scale the revised antenna model from the new design center to the original design center. Retain the new element diameter: the amount of performance change occasioned by the small frequency movement will usually not require a reiteration of this step. However, when enlarging or shrinking elements by more than a factor of 2, it may pay to make the change in two steps of scaling and checking. At this stage, check the performance of the antenna across the passband used by the original design. In many instances, the model will suggest that we need not make any further changes. However, in some cases, we may need to adjust some element lengths to center the gain, front-to-back, and SWR curves as closely as possible to their original form (assuming that the original curves are the most desirable ones for our application). The driver length will have the greatest effect upon the SWR curve. Juggling the reflector length and the most forward director lengths can smooth out the performance across the passband, although rechecking the SWR curve may be necessary. For a given band-edge adjustment, alter the element that moves gain and/or front-to-back performance values in the desired direction with least adverse affect on the SWR curve. Finally, when reducing element diameters, you may need to increase the reflector spacing from the driver to raise the general impedance level back to that of the larger elements with which you began. The pitfall in this procedure involves stopping at this point. Although the initial detection of the revised design center frequency and scaling that back to the original center produced element lengths that are very close to optimum, the element spacing moved in the wrong direction. The thin-element model increased element spacing, while the fat element model decreased the spacing. However, as element diameter increases, element spacing must increase to maintain the same level of coupling. Because we have adjusted element lengths, the spacing adjustments may not be dramatic, but they will be noticeable. Therefore, we need one more step. 5. If increasing element diameter, increase the spacing among elements by about twice the amount that the initial scaling decreased them. If decreasing the element diameter, do the opposite. Do not use a simple additive method, but instead find a multiplier based on the scaling ratio used in step 4. Take into account any revised positioning of the reflector in step 4. As well, check the driver, reflector, and most forward director lengths to re-establish the performance curves for the array. (From Notes on 6-Element Wide-Band 2-Meter Yagis by L. B. Cebik, W4RNL) |
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