Helical antenna for portable radios
Was manufactured and tested about twenty of these antennas for frequencies 145...170 MHz. The antenna was tested with ICOM radios THE and YAESU FT-11, measurement of levels was carried out selective microvoltmeter SMV-8 with whip antenna, remote on 30 meters. Relative measurement levels tested during QSO at a distance of from 10 to 60 km. As expected, the greatest efficiency and have broadband antenna with a geometric length, which is close to 0.12 wavelengths.
Such antennas are not inferior to full sized quarter-wave pins on strengthening, losing only bandwidth, and more effective than 4...7 dB standard antennas radios FT-11 and TE.
The reduction of the geometrical length of a half-wave helical antenna in the range 145... 155 MHz to 0.06 wavelength leads to a gain reduction to the values characteristic "signature" of antennas, but their bandwidth is wider, and in the case of placing the radio with the antenna on the operator's belt or in a pocket its effectiveness is higher by 3...9 dB, depending on location.
As is known, the use of a counterweight on a portable radio lets you to noticeably increase the radiated power, however, in a two-meter range this counterweight has a length of about a foot and a half is not too convenient for "pocket" radio station (I must say that the use of a segment of flexible circuit wires, unlike the rigid counterweight, does not give positive effect due to the strong influence of the operator's body and foreign objects).
So natural was the wish to explore the properties of a shortened balances performed similarly helical antennas. A quarter-wave resonant balances the geometric length 13 cm at frequencies of 144...146 MHz gain practical power is not given, and only at a length of 20 cm, the gain radiated power was 2 dB. If the electrical length of the counterweight were polvani, when the geometric length of 13 cm at the frequency of resonance radiation power stations rose 5...6 dB. Practical tests have shown an increase of the signal at the receiving side almost 1.5... 2 points using standard antennas FA-82B radio 1S THE on a half-wave helical length of 13 cm with opposed half-wave of the same length, and about half of the score in case radio station FT-11. Thus, the use of half-wave helical antenna with a counterweight can not only improve the quality of communication at maximum distance, but also to increase the communication range is at least twice as.
The disadvantage of these antennas, as, indeed, all very short, is critical to set up. The design of the antenna and counterweight are the same and represent a plastic rod with a diameter of 7 mm (inner insulation of the coaxial cable) on which the winding 1 m of wire sew-2 to 0.4. At the start of antenna winding is supplied to the slot, the counterweight is on the terminal lobe of the appropriate size that is used for installation under antenna connector (however, the design of the attachment of the counterweight may be different). Next are 3 coils with a pitch of 0.5 mm, and then is wound with a pitch of 4...10 mm (depends on the length of the antenna), and at the end of the antenna (or counterweight) tightly wound 15...20 turns. The antenna tuning is carried out by changing the pitch of turns of the first and the last (third) winding. The most critical settings for the antenna portion of the spiral - the first turns.
The counterweight is configured similarly.
Indicator settings is selective microvoltmeter, but you can apply the S-meter radio or resonant field indicator. However, it is desirable to calibrate, not to gain gain 15...20 dB. It is amazing, but, unfortunately, absolutely not true.
After initial configuration, the antenna and counterweight are placed in protective covers or wrapped with electrical tape. Be aware that in this case the resonant frequency of the antenna (counterweight) is reduced, and the setting should be repeated, removing and re-putting on the cover, while the antenna is encased in a case, will not set the required frequency. As the shell of the antenna was used termousazhivayuscheysya tube that attaches to the antenna finished look and high durability. Antenna VSWR measured panoramic SWR meter P4-11 at the resonance frequency did not exceed 1,6.
Author: A. Pankov (UA1CHJ); Publication: N. Bolshakov, rf.atnn.ru