GP type antenna (amateur radio slang) — stationary antenna designed as an unbalanced grounded radiator (Ground Plane) with an artificial ground in the form of counterpoises. If the size of the antenna is not specified, it is implied that the length of the GP antenna rod is 1/4 lambda (wavelength).
The antenna has a whip-type design, convenient for installation on both building rooftops and vehicles. It is simple yet sufficiently efficient, which has led to its wide use. For normal operation of a Ground Plane antenna, it is equipped with three counterpoises. The length of the counterpoises is chosen to be equal to or 2.5% longer than 1/4 wavelength. The input antenna resistance depends on the angle between the counterpoises and the mast: the smaller this angle (counterpoises closer to the mast), the higher the resistance. To obtain an input resistance of 50 Ohms, the angle is set between 30–45 degrees. The radiation pattern in the vertical plane has a maximum at an angle of 30 degrees to the horizon. The gain of the antenna is approximately equal to that of a vertical half-wave dipole.
A disadvantage of this design is the absence of a direct connection between the rod and the mast, which requires additional lightning and static electricity protection measures. The simplest protection method is the use of a short-circuited quarter-wave stub made of cable connected to the feeder via a T-connector. The stub connects the feeder’s central conductor to the grounded braid for DC without affecting antenna matching. The length of the stub is calculated considering the velocity factor of the cable used.
Related articles: stationary antenna, Pistolkor’s vibrator, SWR, DF, Wave polarization, Aperture antenna
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