Spiral GP for the LF bands
The vertical antenna to low-frequency ranges are of considerable height, which complicates their installation in Amateur conditions. The proposed option bring the GP height to acceptable levels is not devoid of originality and will help hams for a new approach to the design of antennas.
The antenna, which will be discussed in this article, is an intermediate option between a full-sized vertical pin and its shorter version - helical antenna with normal radiation. Here the term "normal radiation" indicates that the antenna radiates perpendicular to its axis, like the regular GP (in unlike spiral antennas with axial - along the axis of the radiation). Such antennas, despite their relatively low efficiency, are widely used in wearable УSW radio stations and shortened SW/УSW antennas for mobile means connection. Unlike classic spiral GP is that offer the antenna has only one spiral turn, allowing about twice to reduce the height of the GP, without losing efficiency significantly. In practice, volumetric spiral for LW bands to make difficult, but you can replace the "broken helix" (Fig. 1).
These ideas were tested on the antenna range of 80 meters (Fig. 2). Length the selected emitter 22 meters. His painting represents one round of "broken spiral" with a step of 0.1 and winding nominal diameter 0,04 0,06.... The antenna and its two counterweight (21 meters each) made of improvised cord diameter The 2.5...3 mm (multiple twisted wires sew 0.5 mm).
The antenna is installed directly at the earth's surface. Her chart orientation in the horizontal plane is nearly circular. The angle of maximum radiation in the vertical plane is 25&No. 176;. Form isolated from land balances (their length is not less than 0.25.) can also be curved. When this may somewhat alter the radiation pattern.
The antenna is fed with 75-Ohm cable length 11 m, laid over the earth at a height about 2 M.
To get close to one of the CWS between the power line and the transmitter included matching device. Three section variable capacitor with air-dielectric - from the old tube radio. Switch slide type 15 provisions.
The coil has 30 turns of wire sew-2 1.1 mm, is wound with a pitch of 1 mm on the frame with a diameter of 45 mm. the Taps of the coils are made every two turns. It the device has been successfully used by me for several years with various types of antennas for all Amateur radio bands.
The antenna tuning is carried out by correction of the length of the helical radiator and balances, but when you use a matching device that may not to do.
The bandwidth of the antenna with matching unit when the CWS is not worse than the 1.9 got about 60 kHz and no worse than 1.2 to about 30 kHz. When you move on range with a step of 60 kHz and reconfiguring these matching device the settings are saved in the frequency band from 3500 to 3750 kHz.
The antenna performance was evaluated in comparison with the "Inverted V".
The test results showed that on the slopes above 1000 km "spiral GP" more effective than the "Inverted V". On routes of up to 1000 km there was no difference in estimates of correspondents. On the slopes up to 2000 km, the difference was estimated to be 0.5-1 point on the volume. At distances of 4000...5000 km, the difference was estimated to be 1-2 points. And finally, on tracks with a length of 8000 km or more, the difference was in average 2 points and above.
Author: E. Osminin (UA4ANV)