Spiral antenna for portable radio stations. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / VHF antennas

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For small-sized portable VHF radio stations, the use of short helical antennas is fully justified due to their durability and ease of use of radio stations in the standby reception mode. However, their low efficiency forces us to look for ways to increase it. Articles [1, 2] gave me the idea to investigate the properties of shortened helical antennas with an electrical half-wavelength.

About twenty such antennas were manufactured and tested for frequencies of 145 ... 170 MHz. The antennas were tested with ICOM Т22Е and YAESU FT-11 radio stations; level measurements were carried out with an SMV-8 selective microvoltmeter with a whip antenna 30 meters away. The relative measurement of the levels was checked during radio communications at a distance of 10 to 60 km. As expected, antennas with a geometric length close to 0,12 wavelength have the highest efficiency and broadband.

Such antennas are practically not inferior to full-sized quarter-wave pins in gain, losing only in bandwidth, and are 4 ... 7 dB more efficient than standard antennas of FT-11 and T22E radio stations.

Reducing the geometric length of a half-wave helical antenna in the range of 145 ... 155 MHz to 0,06 wavelength leads to a decrease in gain to the values ​​characteristic of "proprietary" antennas, however, their bandwidth is wider, and in the case of a radio station with such an antenna on the operator's belt or in a pocket of clothes, its efficiency is higher by 3 ... 9 dB - depending on the location.

As you know, the use of a counterweight on a portable radio station can significantly increase the radiated power, however, in the two-meter range, such a counterweight has a length of about half a meter and is not very convenient for a "pocket" radio station (I must say that the use of a piece of flexible mounting wire, unlike a rigid counterweight, is not gives a positive effect due to the strong influence of the operator's body and foreign objects).

Therefore, it was natural to want to explore the properties of shortened counterweights, made similar to helical antennas. Quarter-wave resonant counterbalances with a geometric length of 13 cm at frequencies of 144 ... 146 MHz did not give a practical increase in power, and only with a length of 20 cm did the increase in radiated power amount to 2 dB. If the electrical length of the counterweight was half a wave, then with a geometric length of 13 cm at the resonance frequency, the radiation power of the radio station increased by 5 ... 6 dB. Practical tests showed an increase in the signal on the receiving side by almost 1,5 ... 2 points when switching from the standard FA-82B antenna of the 1C T22E radio station to a half-wave helical 13 cm long with a half-wave counterweight of the same length, and by about one and a half points in the case of a radio station FT-11. Thus, the use of a half-wave helical antenna with a counterweight makes it possible not only to improve the quality of communication at limiting distances, but also to increase the communication range at least twice.

The disadvantage of such antennas, as, indeed, of all strongly shortened ones, is the criticality in tuning. The designs of the antenna and the counterweight are the same and represent a polyethylene rod with a diameter of 7 mm (internal insulation of the coaxial cable), on which 1 m of PEV-2-0,4 wire is wound. At the antenna, the beginning of the winding is fed into the connector, at the counterweight - to the terminal lobe of the appropriate size, which is used for installation under the antenna connector (however, the counterweight mounting design may be different). Next, 3 turns are performed with a step of 0,5 mm, then winding is carried out with a step of 4 ... 10 mm (depending on the selected length of the antenna), and 15 ... 20 turns are tightly wound at the end of the antenna (or counterweight, respectively). The antenna is tuned by changing the pitch of the turns of the first and last (third) windings. The most critical part of the helix for tuning the antenna is the first turns.

The counterweight is set up in the same way.

The tuning indicator is a selective microvoltmeter, but you can use the S-meter of the radio station or the resonant field indicator. However, it is desirable to calibrate them in order not to get a gain increase of 15 ... 20 dB. This is amazing, but, unfortunately, absolutely not true.

After the initial adjustment, the antenna and counterweight are placed in protective covers or wrapped with electrical tape. It should be taken into account that in this case, the resonant frequency of the antenna (counterweight) decreases, and the tuning must be repeated, removing and putting on the case again, until the antenna enclosed in the case is tuned to the required frequency. A heat-shrinkable tube was used as the antenna shell, which gives the antenna a finished look and high strength. The SWR of the antenna, measured with a panoramic SWR meter R4-11 at the resonance frequency, did not exceed 1,6.

Literature

1. Kharchenko K. Conductors with shortening in antennas//Radio. -1979. - N8.
2. John S. Belrose (VE2CV). The 300-Ohm ribbon J antenna for 2 meters: A critical analysis//QST. - 1982. - N4. .

Author: A. Pankov (UA1CHJ); Publication: N. Bolshakov, rf.atnn.ru

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