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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING VHF antenna with J-matching. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / VHF antennas This antenna has long and deservedly been popular with radio amateurs. Its design is simple, it is easily adjusted and is consistent with the feeder with any wave impedance. However, its large size (total length is 0,75λ) makes it difficult to use on the HF bands. But in the VHF bands, it is used quite often.
The antenna (Fig. 1) is a vibrator with a length of λ/2, fed from the end through a 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 powered from the end (several kiloohms) is easily transformed to the characteristic impedance of the cable by choosing the optimal distance from the power points (X1, X2) to the closed end of the line. The use of an open line as a transformer ensures low losses at high transformation ratios. The J-antenna gain is +0,25 dBd, which is slightly higher than the dipole gain (due to the two-wire line). The vertical J-antenna, due to incomplete symmetry, has a small radiation with horizontal polarization (Fig. 2).
We modify the J-antenna by bending the quarter-wave line by 90 degrees (Fig. 3). With a little refinement of the dimensions, it is not difficult to obtain a good match and a gain of 0 dBd. However, in this version of the antenna, a noticeable part of the radiation is already horizontally polarized. It is caused by a common-mode current in a two-wire line, which plays the role of a counterweight (current collector) in the J-antenna.
Let's add another half-wave vibrator by connecting it to the free end of the two-wire line (Fig. 4). We get a completely symmetrical design in the vertical plane. There is no common-mode current in the two-wire line, as well as radiation with horizontal polarization. This option is a collinear antenna of two half-wave vibrators fed through a quarter-wave line closed at the end.
Such an antenna is described by SM0VPO 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 pattern is circular. The antenna is structurally very simple and can be made from a single piece of aluminum rod or tube. To maintain the symmetry of the antenna, it is desirable to connect the power cable through a balancing transformer. SM0VPO uses a U-bend balun. You can limit yourself to a few ferrite rings worn on the cable near the feed point of the antenna. Let's call this design the Super-J antenna for short. And what is its possible further modification?
By adding reflectors to the design, we get a two-element Super-J antenna (Fig. 5). This is already a directional collinear antenna with a gain of +5,8 dBd. And if we add directors, we get a three-element Super-J antenna (Fig. 6) with a gain of +8 dBd (Fig. 7). Trying to add a second director results in a gain of only 0,8 dB, but noticeably increases the length of the antenna...
What is the advantage of these antennas over multi-element Yagi? With an equal area, their gains are approximately equal, but the advantages of Super-J antennas are the short boom length, the associated small turning radius, and ease of matching. The disadvantages include the need to use a dielectric mast, at least its upper part. On fig. 8 shows a photograph of a three-element Super-J antenna for the 144 MHz band, made of an aluminum bar with a diameter of 8 mm.
A dielectric mast (for example, fiberglass) and an insulating strut are located in the gaps between the elements. On fig. 9 they are shown with thicker lines. It is better to take 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 this section (near the antenna) every 0,5 m, it is advisable to put on the cable tubular ferrite magnetic cores (from monitor power cables).
A similar three-element Super-J antenna can also be made for the 430 MHz band. In the table and in fig. 10 shows the required structural dimensions for frequencies of 145 and 435 MHz. The dimensions of the elements and the distance between their axes are indicated in centimeters (D is the diameter of the aluminum or copper conductors from which the antenna is made). The input impedance at the feed point is 50 or 200 ohms. If a U-bend is used for balancing, it will transform the feeder resistance to 200 ohms, so the connection to the two-wire line will be slightly further from the closed end. In this case, the dimensions of the matching cable change slightly (see table).
Table
The dimensions of elements marked with an asterisk are specified during configuration. For ease of setup, the matching device is recommended to be made with two movable contacts (sliders): one that closes the two-wire line is used to tune into resonance, the second, connecting the feeder, is used to match to the minimum SWR level. This allows you to quickly tune the antenna, but after choosing the positions of the sliders, it is imperative to ensure reliable contact (by soldering or bolting). The efficiency of the antenna is extremely dependent on the contact resistance. It is worth remembering the inadmissibility of copper-aluminum contact and the protection of contact from moisture. The requirements for contact resistance at the open end of the J-leg, on the contrary, are not strict, since the current is minimal there. Initially, the antenna was made according to Fig. 4 for an average frequency of 145 MHz from an aluminum bar with a diameter of 8 mm. It was attached to a fiberglass pipe with a diameter of 23 mm, used as a mast. A ferrite tube was used as a balancing device, put on the cable near the antenna feed point. Her tests showed that when the antenna is placed on a wooden table parallel to the ground and when it is placed vertically, the settings do not match. Therefore, the antenna must be tuned by placing it vertically. It is enough that the distance from the lower ends of the vibrators to the ground was about 0,5 m. By moving the closing jumper along the two-wire loop and moving the cable connection points (these adjustments are interdependent), it was quite easy to match the antenna to SWR<1,1 at the desired frequency. The operating frequency band of the antenna in terms of SWR<1,5 exceeded 5 MHz. Then booms were attached to the mast and active vibrators, also made of an aluminum bar with a diameter of 8 mm, since dielectric tubes of the required rigidity were not at hand. At the midpoint of the vibrators, the voltage is close to zero, so the conductive boom has little effect on the characteristics of the antenna, which was confirmed by preliminary simulations. Reflectors and directors were installed on the booms, the lengths of which were calculated by the model using the MMANA program. The two-wire line and booms are fixed to the mast by means of 10 mm vinyl plastic plates and U-shaped brackets. The antenna elements are attached to the booms with duralumin U-shaped brackets and bolts. Passive elements dramatically reduced the input impedance of the antenna. However, a weakly expressed SWR minimum was found. By moving the jumper and shifting the cable connection points, we found a position where the minimum SWR corresponded to a frequency of 145 MHz and did not exceed 1,2. The lengths of the vibrators were not regulated. Compared to the tuning of a single element antenna, the tuning of a three-element antenna is much sharper and more critical. The SWR <1,5 bandwidth was about 3 MHz. The length of the loop turned out to be somewhat less, and the distance from the closed end of the loop to the power point with a cable with a resistance of 50 Ohm is somewhat larger than the calculated values. The operation of the antenna was previously evaluated in urban conditions (among tall buildings that completely covered the horizon) when its axis was located above the ground at a height of only 1,5 m. Compared to a quarter-wave automobile pin, it gave a signal increase of 2 ... distances of 3...10 km. Directionality in the horizontal plane was pronounced. The general impression is that the antenna works. More accurate assessments of the performance of the Super-J antenna were made in open areas in summer conditions when the antenna was raised to a mast 50 m high. Its performance was compared with that of a four-element "square" antenna with vertical polarization. The antennas were installed alternately on the same fiberglass mast in the same place. The same cable was used as a feeder and the same transceiver. The work on the discovery and audibility of repeaters located at distances from 7 to 30 km and the assessments of correspondents when making QSOs in the direct channel at distances up to 100 km were evaluated. In most cases, the scores were very close. If you've heard "square", you've also heard Super-J. The four-element "square" had a narrower radiation pattern in the horizontal plane, so it had to be more accurately directed at the correspondent to get the maximum score, Super-J almost did not turn. The general impression is that the antennas have approximately equal gains and good back lobe suppression. The tested antenna is two times lighter than the "squares" and has a significantly lower torque and windage. Files for modeling the described antennas in the MMANA program can be downloaded from ftp://ftp.radio.ru/pub/2017/01/ant86_30.zip. Author: Vladislav Shcherbakov (RU3ARJ)
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