VHF antennas with J-matching
The J-antenna (Fig. 1) has long been and deservedly popular among radio amateurs. Its design is simple, it is easy to set up and matches the feeder of any resistance. However big sizes(total length is 0.75λ) makes it difficult to use on HF bands. But in the VHF bands it is widely used. As can be seen from Fig. 1, it is a vibrator with length λ/2, powered from the end through matching device, made in the form of a quarter-wave open line, closed at the lower end.
The high input impedance of a half-wave vibrator when fed from the end (several kOhms) is easily transformed to the cable resistance by choosing the distance from the power point to the closed end of the line. Using an open line as a transformer ensures low losses at high transformation ratios. J-antenna gain - +0.25 dBd, i.e. slightly exceeds the dipole gain due to the radiation of a two-wire line. The vertical J-antenna, due to incomplete symmetry, has little radiation with horizontal polarization (Fig. 1a).
We modify the J-antenna by bending the quarter-wave line by 90 degrees (Fig. 2).
By slightly adjusting the dimensions, it is not difficult to achieve good matching and 0 dBd gain. However, with this version of the antenna, a noticeable part of the radiation is already horizontally polarized (Fig. 2a). It is caused by a common-mode current in a two-wire line, which plays the role of a counterweight (pantograph) in the J-antenna.
Let's add another half-wave vibrator, connecting it to the free end of the two-wire line (Fig. 3).
The design is now completely symmetrical in the vertical plane, there is no common-mode current in the two-wire line, as well as radiation with horizontal polarization (Fig. 3a).
This option is a collinear antenna of two half-wave vibrators fed through a quarter-wave line closed at the end. This antenna is described by SM0VPO (1) on his website in the article "6 dB collinear VHF antenna by Harry Lythall - SM0VPO". Its gain (about 2.4 dBd) is obtained by narrowing the radiation pattern in the vertical plane. In the horizontal plane, the radiation diagram is circular. The antenna is structurally very simple and can be made from one piece of rod or tube. To maintain its symmetry, it is advisable to connect the power cable through a balun transformer. SM0VPO uses a balun transformer in the form of a U-elbow; you can limit yourself to several ferrite rings placed on the cable near the antenna feed point. For brevity, let's call it a Super-J antenna.
What further modification of this antenna is possible? By adding reflectors to it, we get a 2-element Super-J antenna (Fig. 4). This is already a directional collinear antenna. Its gain is +5.8 dBd.
By adding directors, we get a 3-element Super-J antenna (Fig. 5). Gain - +8 dBd.
An attempt to add a second director noticeably increases the length of the antenna, but gives an increase in gain of only 0.8 dB. What is the advantage of these antennas over multi-element Yagi? With the same area, their gains are approximately equal, but the advantages of Super-J antennas are the short boom length and the associated small turning radius, and ease of matching. The disadvantages include the need to use a dielectric mast, at least its upper part. Figure 6 shows photographs of a 3-element Super-J antenna for the 2-meter range, made of aluminum rod with a diameter of 8 mm.
Fig.6. General view of the 3-element SuperJ antenna.
A dielectric mast (for example, fiberglass) and an insulating spacer can be placed in the spaces between the elements (they are shown in bolder lines in Fig. 7).
It is better to route the power cable horizontally behind the reflectors and return it to the mast in a wide loop, away from the ends of the reflector. In the area near the antenna, it is advisable to place ferrite cores on the cable every 0.5 m.
Fig.8 View of a 3-element Super-J antenna on a mast
The design dimensions of the 3-element Super-J for frequencies of 145 MHz and 435 MHz are shown in Fig. 9 and in table 1.
Dimensions are given in centimeters and between the axes of the conductors. Input impedance at the power point is 50 or 200 ohms. If a U-elbow is used for balancing, it transforms the feeder resistance to 200 ohms, so the connection point to the two-wire line will be slightly further from the closed end. In this case, the dimensions of the matching loop change slightly (see Table 1).
Table 1.
|
Frequency |
Rin, |
||||||||||
|
52,5 |
34,5 |
||||||||||
|
52,5 |
34,5 |
41,5 |
|||||||||
|
14,7 |
17,5 |
17,7 |
16,3 |
11,5 |
0,25 |
||||||
|
14,7 |
17,5 |
17,3 |
16,3 |
11,5 |
13,8 |
0,25 |
* -- the size is specified during setup.
D is the diameter of the aluminum or copper conductors from which the antenna is made.
For ease of setup, it is recommended that the matching device be made with two “sliders” (moving contacts): one that closes the two-wire line is used for tuning to resonance, the second that connects the feeder is used for matching to the minimum SWR level. This allows you to quickly configure the antenna, but after selecting the positions of the “sliders”, you must ensure reliable contact (by soldering or bolts). The efficiency of the antenna greatly depends on the contact resistance. It is worth remembering that copper-aluminum contact is inadmissible and that the contact is protected from moisture. The requirements for contact resistance at the open end of the J-leg, on the contrary, are not strict, since the current there is minimal. An antenna for an average frequency of 145 MHz was made from an aluminum rod with a diameter of 8 mm. It was attached to a fiberglass tube with a diameter of 23 mm, used as a mast. A ferrite tube placed on a cable near the antenna feed point was used as a balun. First, a single element Super-J antenna was tested (Fig. 3). It was noticed that when the antenna is placed on a wooden table parallel to the ground and when it is positioned vertically, the settings do not match. Therefore, the antenna must be tuned by installing it vertically. It is enough that the distance from the lower ends of the vibrators to the ground is about 0.5 m. By moving the shorting jumper along the two-wire loop and moving the cable connection points (these adjustments are interdependent), it is quite easy to match the antenna to SWR<1,1 на желаемой частоте. полоса частот по уровню ксв<1,5 превышает 5 мгц. затем к мачте и активным вибраторам были прикреплены бумы, также выполненные из алюминиевого прутка диаметром 8 мм, поскольку не имелось под рукой диэлектрических трубок необходимой жесткости. в средней точке вибраторов напряжение близко к нулю, поэтому проводящий бум слабо влияет на характеристики антенны, что подтвердило предварительное моделирование. на бумах были установлены рефлекторы и директоры, длины которых выполнялись по расчету модели с помощью программы mmana. пассивные элементы резко снизили входное сопротивление антенны. однако слабо выраженный минимум ксв был найден. передвигая перемычку, и сдвигая точки подключения кабеля, нашли положение, когда минимум ксв соответствовал частоте 145 мгц и уровень ксв не превышал 1,2. длины вибраторов не регулировались. по сравнению с настройкой одноэлементной антенны настройка трехэлементной антенны значительно более острая и критичная. полоса по уровню ксв<1,5 составляла около 3 мгц. длина шлейфа оказалась несколько меньше, а расстояние от замкнутого конца шлейфа до точки питания кабелем с сопротивлением 50 ом несколько больше расчетных значений. работа антенны предварительно оценивалась в городских условиях (кругом были высокие здания, полностью закрывавшие горизонт) при расположении ее оси над землей на высоте всего 1,5 м. по сравнению с четвертьволновым автомобильным штырем она давала прирост сигнала на 2-3 балла при связях на расстояниях 10-50 км. направленность в горизонтальной плоскости была ярко выражена. общее впечатление - антенна работает. более аккуратные оценки работы антенны были сделаны на открытой местности в дачных условиях при подъеме антенны на мачту высотой 7 м. сравнивались антенна рис.6 и четырехэлементная антенна "квадрат" с вертикальной поляризацией (рис.10). антенны устанавливались на одной и той же стеклопластиковой мачте в одном и том же месте. использовался один и тот же кабель в качестве фидера и один и тот же трансивер. оценивалась работа по открытию и слышимости репитеров, расположенных на расстояниях от 30 до 100 км и оценкам корреспондентов при проведении qso в прямом канале на расстояниях до 70 км.
Fig. 10. The “4 square” antenna with which the antenna in Fig. 6 was compared.
In most cases the estimates were very close. If you've heard "square", you've also heard SuperJ. The four-element "square" had a narrower radiation pattern in the horizontal plane, so it had to be aimed more accurately at the correspondent to get maximum rating; the Super-J was almost not turned. The general impression is that the antennas have approximately equal gains and good back-lobe suppression. The antenna under test is two times lighter than the “squares” and has a significantly lower torque and windage. Figure 11-14 shows the antenna design elements.
Fig. 11. Short-circuit jumper, cable connection unit and balun ferrite choke.
Fig. 12. Mounting unit for a two-wire line to a mast.
Fig. 13. Mounting unit for booms to the mast.
Fig. 14. Unit for fastening elements to booms.
Attached are files for modeling the described antennas: MMANA files
RU3ARJ Vladislav Shcherbakov, [email protected]
Photos by RW3ACQ Sergey Filippov, [email protected]
_________
(1) SM0VPO in his article for some reason gives the antenna gain relative to some quarter-wave whip (apparently a car antenna), where its 6 dB comes from.
V. Markov
Radiohobby 6/2003
HF antennas
The J-antenna, its circuit and design have been described repeatedly in print. This antenna is mainly used on the VHF bands and high frequency HF bands. If you make a matching quarter-wave loop from a coaxial cable, taking into account its shortening factor, then it can also be used on long-wave HF bands. The diagram of such an antenna placed horizontally is shown in Fig. 1.
To fasten cables, wires and guy wires, strips made of insulating material, for example PCB, 2-3 mm thick are used (Fig. 2).
The connection point for the power cable is located at 1/8 of the part (A-B in Fig. 1) of the matching cable (loop), counting from the shorted end, for a 50-ohm cable and at 1/7 of the part for a 75-ohm cable. Moreover, the matching cable and power cable are of the same type. At point B, the braid at the end of the cable is wrapped at a distance of 1 cm onto the outer insulation of the cable and wrapped with PVC tape. The length of the radiator L of the antenna is first calculated for the desired range, and then specified when tuning using GIR (GRID-DIP-METER).
After making the antenna, all solders and open parts of the braid are coated with plasticine or sealant. The table shows the antenna dimensions for all KB bands (the length of the cable is given for a cable with polyethylene insulation). The antenna fabric and the matching loop must be on the same straight line.
|
Range, m |
Vibrator, |
Matching loop, l, |
Distance A-B, m |
|
|
50 ohm cable |
Cable 75 0m |
|||
For the range of 20 m and above, I recommend a vertical version of the J-antenna. The author has manufactured several such antennas for the 20 and 10 meter bands. Moreover, telescopic fishing rods 6 and 7 meters long are used as a mast, the uppermost section of which is removed. At the top of the fishing rod, an “asterisk” is fixed, which serves as a capacitive load for the antenna. It is made of 6 L-shaped, horizontally arranged with a long side of 30 cm, wires (bimetal, aluminum with a diameter of 2-3 mm). The vertical 5cm sections of these wires are spaced evenly around the top, wrapped with tinned copper wire and soldered together. The outer ends of the horizontal star conductors are tinned at a distance of 1 cm and connected with a copper wire with a diameter of 1 mm, as shown in Fig. 3.
The antenna sheet is made of double copper wire 01 mm in general “noodle” type insulation with a distance between the wires of 2-3 mm. Both wires at the end are stripped at a distance of 1 cm, twisted and soldered together, and then soldered to the “asterisk”. After this, the “noodles” are wrapped around the fishing rod, preventing it from twisting.
For an antenna on the 20 meter range, the wire is first wound in increments of 2 cm at a distance of 180 cm, then in increments of 4 cm at a distance of 80 cm, then in increments of 10 cm - 60 cm, and then a wire 240 cm long is simply placed along the fishing rod. When winding, every 50 cm the wire is reinforced with PVC electrical tape.
Then the fishing rod with the wound emitter is placed vertically on the ground (the roof of the house), the GIR is connected to the ends of the “noodle” soldered together in the same way as near the “star” and fPE3 is measured.
If the resonance is lower in frequency, the wire is shortened; if it is higher, it is lengthened, achieving fPE3 = 14100...14120 kHz. After raising the antenna to its working height, the resonance practically does not go away. Thus, a wire with an electrical length of X/2 is wound around a fishing rod 5 m long and an antenna is implemented in the form of a shortened half-wave dipole.
Now connect a matching cable to the antenna sheet according to the dimensions from the table with a tap for connecting the power cable. The fishing rod is attached to the mast at a distance of 20...30 cm from the end and does not require guy wires. The SWR of the antenna was 1.1...1.2 at 14000 and 14350 kHz, and at 14120 - 1.05.
Advantages of the J-antenna: broadband, simple design, does not require counterweights.
The disadvantage is that it is single-band.
For radio amateurs who have LW with a length of 81-84 m, it can be recommended to match it using coaxial loops with dimensions taken from the table. But before this, you should make sure with the help of a GIR or an impedance meter that LW has the maximum input impedance in the required ranges, i.e. its electrical length is a multiple of X/2.
The publication is dedicated to the blessed memory of the departed Avdeevka radio amateur Nikolai US5IMU, who at one time kindly provided the author of these lines with the material for the manufacture of this antenna.
Recently, the situation on the VHF amateur radio market has changed for the better for us radio amateurs. Today, an FM radio station on the 2-meter band has become available to everyone. In view of this, the question is which antenna to choose for a radio amateur who is mastering this interesting range for the first time? You can hear many answers, but today we will focus on an omnidirectional whip antenna, the appearance of which resembles the English letter J. This is an antenna for a beginner, for a summer residence, for local communications on VHF.
We will not consider the physics of operation of this antenna in detail. Anyone who wishes can familiarize themselves with it at . Let us just note that the antenna is matched to the transmission line using a quarter-wave loop, which is equivalent to an inductor and capacitance.
So, let's move on to the practical part. A schematic view of the antenna is shown in Figure 1.
Rice. 1. Schematic representation of a J-antenna.
Using the formulas given in Figure 1, or using a ready-made calculator, we obtain the antenna dimensions A, B, C and D.
For frequency 145.5 MHz:
A = 148.29 (cm)
B = 49.19 (cm)
C = 4.63 (cm) (for Rfeeder=50 Ohm)
Material - copper or aluminum, tube or wire. Whatever is at hand. I used round aluminum wire with a diameter of 9 mm. The only thing you need to remember is the shortening coefficient k, which relates the electrical length of the antenna web to its geometric length. The thicker the conductor, the greater this difference. In order not to make a mistake with the antenna length, it is recommended to make size B a little larger, and then bite off the excess during the setup process.
The antenna was tuned using an SWR meter. In my case, I used the RS-40 SWR meter shown in Figure 2.
Rice. 2. SWR meter readings in transmit mode.
We attach the central core of the cable on crocodiles to the long element (A), and the braid to the short one (B). And we begin to alternately switch on transmission, looking at the SWR meter and moving the crocodiles, achieving a minimum SWR at the operating frequency. We turned it on, looked at the SWR meter, turned it off, and moved the crocodiles. There should be a minimum SWR in the area of 4-6 centimeters from the jumper. If you can’t achieve an SWR close to 1.1-1.2, then you should play around with length B, biting off a few millimeters at a time. During measurements, it is recommended to place the antenna between two backs of chairs, away from the floor, surrounding objects, and especially metal.
After setting, clamp the cable onto the bolts with clamps, check if the setting is wrong, and then fill the contacts with automotive or plumbing sealant.
A few centimeters from the connection point, it is recommended to wind a filter, which is 4-5 turns of the same cable on a frame, for example, from a 10 cc syringe. This will somewhat reduce the flow of RF currents onto the cable braid and reduce possible TV interference.
Any 50 ohm cable can be used. In my case, this is a small piece of 3-4 meters of thin RG-58U from the antenna connection point to the balcony, and then through the connector about 25 meters of thick RG-8. I note that the thicker the cable, the lower its attenuation coefficient, as a rule. The thinner it is, the greater the loss of the useful signal. The situation is similar with the length of the cable; the longer the cable from the antenna to the transceiver, the greater the loss of the useful signal. In other words, to minimize losses in the cable, we try to adhere to the rule “the thicker and shorter the cable, the better.”
A photograph of my antenna is shown in Figure 3. It has been installed for two years now and has survived all hurricanes, gusts and icing.
Rice. 3. External view of the j-antenna on the mast of a 5-story building. Photographed from below.
Literature.
1. Karl Rothhammel: Antennas. Volume 2. Edition 11. Publishing house Light LTD., 2007, p. 103.
Alexander US6IGL
Exclusively for RADON magazine
One of the radio amateurs in the neighborhood (our region) called and asked why he couldn’t hear Oscar -7, although according to calculations it was flying directly over Goncharovsky. Since this is not the first time this question has arisen, I think it will be necessary to repeat it. I gave a good review for GUHOR reasons on Hammaniya. I think that there is no need to duplicate this material, and therefore I will answer on this specific situation. There are several logical “I”s here that led to the fact that he probably won’t hear the satellite in the future.
Gosha is an alien ;-)
When reading an author, it is sometimes difficult to imagine him. For example, in my youth I read a lot of Alexander Greene, who actually wrote a lot of things besides “Scarlet Sails”. But when I saw his portrait I couldn’t believe my eyes. The only exception is Mayakovsky, as he writes and looks the same. So that no one doubts what Gosha the radio operator looks like, Sasha Litvinenko UR5RP sent a photo. "Gosha the radio operator writes to the site." And whoever did not believe my good forecast of the HF bands for these three days is to blame: only in the last half hour in the telegraph Jamaica, Lesotho, Senegal and the Dominican Republic. In RTTY Something I didn't have before.
OQRS: QSL from the Internet
I have already written before about how to get a card via the Internet. The method caught on and now almost all DXpeditions run this service because it makes the exchange easier and cheaper for both parties. For those who find it difficult to understand English terms and abbreviations, I will “decipher” the description of how to do this. Well, first you need to have before your eyes all the data for connections with this expedition. In my case it will be 7O6T. To do this, we go the standard way, starting with QRZ.COM, then follow the links until we see a picture of online check of your QSOs. Below there is a REQUEST QSL button. We click on it. 
Now the work is more difficult: In the left column there is initial information about connections,
SDR and LAN
It is clear that life without wires is better. I'm talking about Wi-Fi and so on. wireless technologies. Here's another plus. My first VHF SDR was sealed in a metal box, with a good shielded antenna input (who remembers the photo from earlier). And now I have a tuner in its original case (photo two posts below), rubberized, with a convenient latch connector instead of a thread or bayonet. It works well, but I decided to watch a 3D movie on my new box. The film, of course, is on the laptop, and the TV reads it via Wi-Fi. But it slows down from time to time. I decided that it was slowing down due to Wi-Fi radio interference, the braking was painfully sporadic, rarely, that is. I turned on a regular LAN router, plugged in the Ethernet cable and was horrified: my SDR receiver had shut up.
SDR panorama in VHF transceivers
Sergey UA0ADX
Working via satellites, in particular SSB and CW, I encountered a problem: the orbit is short, the range is quite wide. Sometimes there are a lot of correspondents, but you won’t always find them when you search. Either they switch to reception and you run past, or you work on a general call and don’t hear anyone who works lower or higher. The satellite flies by quickly, sometimes without results. This circumstance made me think about the SDR panorama. The first step was to purchase an SDR receiver. The choice fell on the most budget-friendly of the advanced ones)) - SDRplay RSP-1, there are several other good RTL SDRs, which I unfortunately learned about after purchasing the RSP-1. Next, I had to figure out how to “hook it up” to the same antenna or antennas on which I am working, respectively, in order to see the real picture, while avoiding any switching, detours, etc., due to the unreliability of which more than one device could burn out)) .
SV2AGW Packet Engine
Live and learn! :-) I just learned that a widely used packet network and node emulator on an audio card can convert signals from dual-band transceivers to a single logic converter. I mean KISS AGW by DK3WN. A little introductory information for those who don’t like satellites as much as I do. What our Windows satellites transmit in telemetry lines is displayed on the screen in the form of acceptable screen characters of one or another layout (Greek, Cyrillic or Latin). In order to correctly recognize this information and use it to display real telemetry data, the resulting strings must first be converted to ASCII strings (files), and only then decoder programs “chew” it. So, DK3WN uses SV2AGW sound modem as a modem for its converter (as one of the KISS AGW variants). In its settings you can use both of the stereo channels of your audio card.
Lighthouse on Arduino part 2
Modules are usually connected via five wires: VCC - power, GND - ground, CLK - clock pulses, STR - strobe and DATA (IO). All modules have pin designations on the module side, and the pin on the Arduino side is assigned in the program. For example, a temperature sensor does not require clocking and its output is connected to analog input A1. A clock, for example, has data to transmit, so the connection is five-wire. The assigned pins can be found in the body of the program. The same goes for the button and display board. With simple signals such as PTT, CW keying, connecting an additional antenna or turning on an additional fan, only one pin is enough. They are also assigned in the program and connected through optocouplers to actuators: transceiver, switch, fan, etc. In the diagram it is all transparent. Pin 10 of the Arduino is used to provide permission to the beeper and connects directly to the BUZZER. Since modern transceivers all have self-monitoring in the telegraph, it is not included in this model. But, if you want to turn on, for example, this beacon in FM mode, you will need this signal.
Let's talk about two types of antennas. Both are related in some way, the authors considered it logical to discuss the designs in one review. We will talk about the j-pole and the Franklin antenna. Let's consider the appearance, prerequisites for choice, and the nuances of implementing the preferred concept. We will figure out how to make an antenna, we will try to present material collected from numerous sources, with comments.
We have already written about the mentioned types of antennas. J is Franklin's historical ancestor, although sources do not give the exact date of invention. This type of antenna was invented by the Germans - perhaps by the Nazis. It is known that the design acquired its current form in 1943 and was originally created for military airships. It was called Zeppelin. J is short for a long word. By the way, the fact of the genesis of the abbreviation is well known to Russian readers from specialized forums; experts call J-antennas airships.
The meaning of the device: at the bottom of the vibrator with a length of 5/8 waves (see the exact calculation below) there is a matching line. The distribution of currents along the J-antenna is such that the resistance varies from:
- tens of thousands of ohms in the upper part, where the gap is;
- to zero at the bottom, where the line closes to the vibrator.
Through change, the prerequisites are created for achieving precise coordination. If for antennas common among ordinary people it is necessary to make quarter-wave transformers, then J will connect to the loop directly - with the skills to configure the product. Experienced people say that it is better to use an SWR meter, as radio engineers do. Rothhammel mentions using a low-voltage LED or light bulb for similar purposes. The specified device is connected in parallel to an unloaded loop. At the ideal connection point, the glow is brighter than in other places.
Before coordination, the antenna should be excited by an arbitrary transmitter over the air. We remind you that vertical polarization is used in radio broadcasting; place the antenna properly (plumb). For coordination, a short piece of cable within a meter is sufficient, at the end of which you connect the light bulb. It seems logical, but Rothhammel is also a smart guy, we won’t dare to contradict him.
Tuning the J-antenna is carried out in a relatively simple way, according to the indicated Rothammel postulates.
Let's talk about the antenna's appearance and parameters. Imagine the 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, the result is 72 cm. Now we cut one leg to a length of 7132/f cm, which results in approximately a quarter of a wave. The antenna is ready. All that remains is to properly power the cable. At 671/f cm from the upper edge of the lower horizontal jumper, we solder the main power cable to the vibrator, with the shield on a quarter-wave segment. We talked about the choice of location earlier. What is the resistance for the specified conditions?
J-antenna is designed for communication, polarization is vertical. Therefore, the loop has a resistance of 50 ohms. It turns out that for a TV, the antenna should be turned horizontally, and the soldering should be done a little higher. How to search for the exact docking location has already been discussed. Now about the design.
Both pins are made according to tradition from a copper tube. A distance of 640/f cm is given between the nearest faces. Take measurements with a caliper. Material for making the antenna yourself: tubes from the brake system of a car, old air conditioners and refrigerators, etc. It is permissible to solder, crimp with couplings, bend, solder. Please note two points:
- The J antenna does not always work exactly as intended. Sometimes you want to change the frequency a little. This is true for WiFi channels of modems and routers. To do this, the French-speaking Canadian with Indian roots, Pityu Nagi, suggests equipping each of the two pins with trimming elements. For the short line, a nut is mounted on top of the tube into which the locking bolt is screwed. The characteristics of the antenna depend on the force of screwing the element. At the end of the vibrator, a nut is placed into the wall of the tube. A large piece of elastic steel wire is pulled out 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 the tube with a small piece of dense plastic, and connect the broken ends with a line from the previous tube of half the wavelength. You will get a matching device, a kind of bridge to the Franklin antenna.
- The J-antenna has a definite advantage, in addition to those indicated for working on the roof of a house. Ground the lower part, you will get an excellent lightning rod. This does not affect the operation of the equipment, but will protect the home receiver from thunderstorms. Canal fishing may not be interrupted during bad weather.
- Please note that copper, when exposed to moisture, is aggressive towards other metals. Electrochemical corrosion will occur. Only galvanized steel resists the onslaught of copper. Take note when choosing fasteners.
Most readers will be able to make an antenna themselves according to the indicated scheme; coordination is carried out using relatively simple methods. The remaining issue is the thickness of the tube, but it is not emphasized in the literature, and adjusting elements will help to obtain the maximum effect. Of course, you need a strong wave source to carry out coordination. Moreover, 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 a trimmer with their own hands from an amplifier, a rectifier and any indicator, including Chinese multimeters.
The amplification of the J-antenna is better than the half-wave vibrator, according to eyewitnesses.
Franklin worked for Marconi and was not President of the United States. However, he produced a lot of inventions at the beginning of the 20th century. Among other things - a tuning capacitor. In 1924, he invented the famous antenna, which is used today to build Chinese WiFi pins. The difference: the equal-arm design lost one half, and the matching cables were bent into coils. ZikValera constructs a similar one in the posted video. An example calculation is given in the first volume of Rothhammel on page 232.
Imagine a wave vibrator, with the ends topped with two matching loops, each containing half a wave. The device ends in another broken vibrator, with arms that add up to a wavelength. The result is a Franklin antenna, the signal from the device is taken along the center line, and the radiation resistance depends on the entry point of the loop. For understanding, a modest drawing is shown showing the Franklin antenna.
We believe that from the above image it is already clear how to bend the wire into the desired shape. The dimensions are large, it is advisable to use Franklin antennas in the UHF range and above, although the network is full of designs for much lower frequencies. Get ready to install a special pole in the yard under the structure, or create a decent length of spacers on the roof.
The Franklin antenna has a good gain (fortunately, it is a considerable size) of 3.2 dB. For the design shown in the figure, at the point where the cable is connected in the middle, the radiation resistance will be 300 Ohms. If you add a vibrator on each side, it becomes 500 ohms. The gain will exceed 5 dB. Let us add that the width of the matching line is not critical. For example, 7 mm at a wavelength of 32 cm.
We note two design flaws:
- The power should ideally be supplied via a two-wire line, although amateurs solder the cable as shown in the figure. Nobody is complaining.
- When starting to make your own antenna, make sure you can accurately align the shoulders. They lie clearly along one line, the matching sections can be slightly bent (the Chinese generally make coils, taking into account the inductance when making the antenna).
Calculating a Franklin antenna is not considered a simple matter; it is not always possible to use the specified type of device in its original form. It is recommended to copy Chinese models, which can be measured using a caliper. In this case, a homemade antenna is guaranteed to show good performance. Even on forums, collinear lines have not been properly studied. This is clearly not the best way to make a simple antenna.