How to make an antenna

Let's talk about two types of antennas. Both are of a kindred relationship, the authors found it logical to discuss the constructions in one review. It's about the j-pole and Franklin aerial. Consider the appearance, the prerequisites for selection, the nuances of the implementation of the preferred concept. Let's figure out how to make an antenna, try to present material collected from numerous sources, with comments.

J-antenna

Already wrote about the mentioned types of antennas. J is Franklin's historical ancestor, although the date of the invention is not given in the exact sources. This type of antenna was invented by the Germans - it is possible, by fascists. It is known that the current form of construction acquired in 1943, was originally created for military airships. Named Zeppelin. J is short for a long word. By the way, Russian readers from specialized forums, the fact of the genesis of the abbreviation is well known, experts call J-antennas airships.

The meaning of the device: at the bottom of the vibrator 5/8 wavelength( see the exact calculation below) there is a matching line. The distribution of currents on the J-antenna is such that the resistance varies from:

• tens of thousands of ohms at the top, where the gap;
• to zero at the bottom, where the line closes on the vibrator.

Due to the change, the prerequisites are created to achieve precise agreement. If for common antenna users it is necessary to produce quarter-wave transformers, then J will connect with the cable directly - with the skills of setting up the product. Experienced people say that it is better to use the KSV-meter, as radio engineers do. Rothammel mentions the use of a low-voltage LED or light bulb for similar purposes. The specified device is connected in parallel to the unloaded loop. At the ideal connection point, the glow is brighter than in other places.

Before matching, the antenna should be excited by an arbitrary transmitter over the air. We remind you that vertical polarization is adopted in broadcasting, position the antenna properly( vertically).For coordination, a short segment of the loop within a meter is sufficient, at the end of which you can already connect a light bulb. It looks logical, but Rothammel is also a clever fellow, we don’t bother to contradict.

J-antenna tuning is carried out in a relatively simple manner, according to the specified Rothammel postulates.

Let's talk about the appearance of the antenna and the parameters. Imagine a letter U with right angles or an inverted letter P. The height is 21502 / f cm, where f is the frequency in MHz. For 300 MHz, 72 cm is obtained. Now we cut one leg to length of 7132 / f cm, about a quarter of a wave is obtained. The antenna is ready. It remains to properly power the cable. At 671 / f cm from the upper edge of the lower horizontal jumper, solder the power cable of the core to the vibrator, shield the quarter-wave segment. About a choice of a place spoke earlier. What is the resistance for these conditions?

J-antenna is designed for communication, vertical polarization. Consequently, the loop has a resistance of 50 ohms. It turns out that the antenna for the TV should be rotated horizontally, the soldering should be carried slightly higher. How to find the exact location of the dock, already discussed. Now about the design.

Both pins are made traditionally from copper tube. A distance of 640 / f cm is given between the nearest edges. Take measurements with a caliper. Material for the manufacture of the antenna with his own hands: tubes from the auto brake system, old air conditioners and refrigerators, and so on. It is permissible to lead the solder, crimp couplings, bend, solder. Pay attention to two points:

1. Not always the J-antenna works exactly as intended. Sometimes you want a little change in frequency. This is true for WiFi channels modems and routers. For this, French-speaking Canadian with Indian roots Pityu Nagy proposes to equip each of the two pins with trimmers. As for the short line, a nut is mounted on the top of the tube, where the locking bolt is screwed. The strength of the screw element depends on the characteristics of the antenna. At the end of the vibrator nut is placed in the wall of the tube. A fair piece of steel elastic wire is drawn from the end. This changes the frequency. At the junction of the wire( like a telescopic structure), a signal reflection occurs. Therefore we will tear up the tube with a small piece of dense plastic, connect the torn ends with a line from the former tube of half the wavelength. The result is a matching device, a kind of bridge to the antenna of Franklin.
2. The J-antenna has a definite plus, besides those indicated for working on the roof of the house. Ground the bottom, you get an excellent lightning rod. This does not affect the operation of the equipment, but it will protect the home receiver from thunderstorms. Catching channels may not interrupt during bad weather.
3. Consider that copper in the conditions of moisture ingress is aggressive in relation to other metals. Electrochemical corrosion will occur. Only galvanized steel withstands copper onslaught. Take note when choosing fasteners.

The majority of readers will be able to make an antenna on their own according to this scheme, coordination is carried out using relatively simple methods. The question remains with the thickness of the tube, but it is not emphasized in the literature, and the trimmers will help to get the maximum effect. Of course, you need a strong source of waves to hold the agreement. And it will not be possible to use a voltmeter instead of a light bulb, the frequency is relatively high. We believe that true radio amateurs will assemble the trimmer with their own hands from an amplifier, rectifier and any indicator, including Chinese multimeters.

J-antenna gain is better than a half-wave vibrator, according to eyewitnesses.

Franklin Aerial

Franklin worked for Marconi, was not the president of the United States. However, inventions at the beginning of the 20th century gave a lot. Among other things - trimmer capacitor. In 1924, he invented the famous antenna, on which Chinese pins for WiFi are being built today. The difference: the equal-arm construction has lost one half, and the matching loops have bent into the coils. Similar constructs ZikValera in the video posted. An example of the calculation is given in the first volume of Rothammel on page 232.

Imagine a wave vibrator, with ends topped with two matching loops, half in each wave. The device ends with another torn vibrator, with shoulders totaling the same wavelength. The result is a Franklin antenna, the signal from the device is taken at the center line, and the radiation resistance depends on the point of entry of the cable. For understanding led a modest picture, which shows the antenna Franklin.

We believe that from the specified image it is already clear how to bend the desired shape from the wire. The sizes are large, it is advisable to use Franklin antennas in the UHF range and higher, although the network is full of structures at much lower frequencies. Get ready to detach a special pillar in the yard for the construction, or create spacers of decent length on the roof.

Franklin's antenna has a good gain( good, size is considerable) 3.2 dB.For the construction indicated in the figure, in the middle of the loop feed point, the radiation resistance will be 300 Ω.If you add a vibrator on each side, it becomes 500 ohms. Gain exceeds 5 dB.We add that the width of the alignment line is not critical. For example, 7 mm at a wavelength of 32 cm.

We note two shortcomings of the design:

1. Power should ideally be led by a two-wire line, although lovers solder the cable as shown in the figure. No one complains.
2. Starting to make the antenna yourself, make sure that you can exactly level the shoulders. They lie clearly along the same line, the matching sections may be slightly bent( the Chinese generally make coils, given the inductance in the manufacture of the antenna).

Calculation of Franklin's antenna is not considered a simple matter, in its original form it is not always possible to use the specified type of devices. It is recommended to copy Chinese models, which will help to measure the caliper. In this case, a homemade antenna is guaranteed to show good performance. Even on the forums, the collinear lines have not been properly studied. This is clearly not the best way to make a simple antenna.