The updated logperiodic antenna calculator uses a classic calculation algorithm that takes the bandwidth boundaries and the input impedance of the antenna as input.
For the logarithmic structure of the LPDA, the same ratio between the lengths of adjacent elements and between the distances from them to the top of the structure must be observed. This relation is called the structure period τ.
Thus, the dimensions of the elements and their distances from the top of the triangle decrease according to the law of decreasing geometric progression with a denominator equal to τ. The characteristics of the antenna are determined by the period of the structure and the angle at the vertex of the circumscribed triangle α, which is related to the antenna parameter σ - the relative interval. The smaller the angle α (and the larger the period of the structure τ (which always remains less than one), the greater the antenna gain and the lower the level of the rear and side lobes of the radiation pattern. However, this increases the number of elements of the structure and the length of the antenna. Therefore, when choosing the period of the structure you have to make a compromise decision: τ - from 0.8 to 0.98. There is an optimal value of the relative interval σ for a some τ, the calculator determines it automatically. If you wish, you can change it, but this is not recommended. For those who find it difficult to decide on the choice of τ and α and who needs a ready-made design for the UHF range for HDTV, the site has a description of an already calculated log-periodic antenna for digital television.
Schematic representation of the antenna:
INPUT DATA:
Javascript source code:
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Limitations and features of antenna calculations
A two-part antenna boom is a transmission line that collects energy from the antenna elements. The calculator calculates the parameters of this collecting TL made of two square or round crossection tubes. If you use another type of TL (for example, of PCB strips), then it must be calculated separately, taking the characteristic impedance of the collecting line Zf from this calculator as the initial parameter. This parameter is not the input impedance of the antenna, but characterizes the "empty" transmission line, on which the elements are subsequently "hung", as a result of which its impedance is reduced to the required input. The antenna does not require special matching baluns. The feeder, if possible, is passed through the bottom tube of the boom (see figure). The braid is connected to the lower tube, the central conductor - to the upper one. Behind the first element at a distance of λmax/8, a short-circuit stub is installed on the collecting line. The diameters of the elements also decrease by geometric progression when they are shortened, but it is quite acceptable (and this is usually done) to use one diameter. Landmark - the mechanical strength of the antenna.
We offer, written by one of the anonymous authors, a small Windows application that calculates a log-periodic antenna. The application has one nice feature - after creating a model, all data can be output to a .maa file for further modeling in the MMANA program. However, in this case, you need to be careful and check the models. Sometimes the app outputs the distance between the axes of the collecting line tubes is less than the tube diameter, which is nonsense. MMANA does not see this nonsense. In addition, MMANA (as well as 4NEC2) is not at all suitable for calculating a log-periodic antenna with a low input impedance, since in this case the limitations of the NEC kernel are not met. Thus, this app, jointly with MMANA, will help you correctly calculate only a log-periodic antenna with an input impedance of at least 150 ohms.
For owners of Android smartphones and tablets, a free "LPDA designer" app is available in the Google Play store. The application allows you to calculate the log-periodic antenna. You can download it to your mobile device by clicking or scanning the QR code. Don't forget to rate the app...
Link to calculation formulas and the prototype of this calculator: http://www.stroobandt.com/lpda/en/index.html
To be honest, I don't think 1/32" would be that critical.
We have redesigned inch calculation for friendly usability. Now, notation and precision varies depending on antenna mean operating frequency.