Introduced readers into the course of the question of designing half-wave vibrators, the topic is so extensive, at times new aspects will appear. The show, so beloved by the audience, which discusses how the antenna is assembled from cans with its own hands, is a new variation that excites the old theme. The structure formed by two tins arranged in a row on a stick forms a dipole - the first active antenna made by mankind. We will discuss the aspects of design, it will be useful for readers to learn: not only beer cans are good for the house, coffee and other tins will help. Readers who have seized the idea, can immediately go to study the graphs below, the rest will be interested in reading the theory.
Antenna Shortening Ratio
The shortening coefficient is commonly understood as a number indicating how many times the line wavelength is shorter than in free space. However, today we touch on another area. It has long been noted: the half-wave vibrator is equipped with ideal characteristics only with a minimum wire thickness( we drop the antenna suspension height).In reality, the wire is provided with finite dimensions that change characteristics. Concerns the frequency. Remember, the dimensions of a half-wave vibrator are determined by the wavelength.
When deviating from resonance, the reception will begin to fall, in practice they try to avoid the effect. With increasing diameter of the vibrator rod, the frequency goes up. Means: should be slightly shortened shoulders for accurate contact with the channel. To give the process a kind of scientific, introduced the term coefficient of shortening. The antenna shortening factor shows how much a percentage of the half-wave vibrator will need to be shortened to impart optimal properties to the system.
The graphic shows: the antenna shortening factor depends on the ratio of the wavelength to the doubled diameter of the wire from which the half-wave vibrator is made. Invented a different form of graphics, the meaning remains the same. For calculation it is more convenient from the point of view of minimization of calculations. Given the frequency of 100 MHz, the diameter of the copper tube, which served as the material, is 25 mm. We find the desired length of the half-wave vibrator using the formula.
Determine the wavelength: 299792458/100000000 = 2.998 m. We find the shortening coefficient using the graph, dividing 2.998 by 0.025( wire thickness), it turns out 120. The figure shows: K = 0.482.We conclude: the total length of a half-wave vibrator will be 1.445 meters. Immediately make a reservation, the calculated schedule strongly differs from the one below( theoretically assumed to be identical).In the second case, the length is smaller, the antenna is shorter than shortened to shorten. The fact speaks in favor of the last schedule: recommends a forum of radio amateurs, people gathered with a practical aim, therefore, you can trust. In any case, it is worth trying both.
How to determine the antenna shortening factor for beer cans
We hope, of course: we will assemble a half-wave vibrator from the cans only for that wavelength, which is approximately equal to the four can lengths. The design of the vibrator implies: both shoulders are equal, you should not try to take a tin of different drinks. High jars with low can not be the shoulders of one antenna. The distance at the break is typically 10–20 mm. Now you need to get a ruler, you can say which frequency is suitable for a regular beer can.
The necessary was found in an official document called GOST R 51756-2001 - Deep-drawn aluminum cans with easily opened lids. Technical conditions. Sizes are normalized, preliminary calculations will spend at home. The standard introduces four sizes of beer cans, indicated by the table. For the convenience of readers, the information given above( first figure).Anyone will find the standard text on the Internet.
We offer readers to do. .. There is a graph of the coefficients of the relative shortening of the half-wave vibrator, in the presence of the sizes of cans. Known distance between the shoulders, amounting to 10 - 20 mm. With information in hand, let's calculate the wavelength taken by the future homemade half-wave vibrator. The 0.5 liter can is provided with dimensions:
- Height 168 mm.
- Outer diameter 66.4 mm.
The dependence on the graph resembles a parabola, the logarithmic scale horizontally. Since there is no exact formula on the hands, it is proposed to determine the range, then, if possible, find the resonant frequency.
Double the height of the can, the gap between the shoulders is totally 350 mm. Half wave, not counting the shortening factor. It turns out a wavelength of 70 cm, the frequency of digital television. Let's find more precisely: 299792458 / 0.7 = 428.3 MHz. Now it becomes clear why some clever people use beer cans to catch television channels.
Calculate the shortening factor for the first( top) graphic, it turns out: λ / d = 0.7 / 0.0664 = 10.54.At the left of the scale, K = 0.4625.We find the frequency under which the half-wave vibrator is suitable by multiplying by a factor, only doubled.396 MHz. For comparison, the first multiplex of Moscow is broadcasting at 560 MHz. Banks take shorter. Approximately 560/396 = 1.41 times, gives the height of the tank 118 mm. Length according to the standard beer can 0.33 liters. According to our own calculations, it turns out: if you make an antenna out of cans with your own hands, the first multiplex of Moscow catches. Unknown involved in the selection of frequencies, but the figure( 560 MHz) makes you wonder.