


High Gain FM Antenna(Single Quad Loop For FM Radio DXing)N.S.HARISANKAR  VU3NSH
The quad antenna have a gain of 1.4 dB over a dipole and also operate over a relatively wide frequency. Quad antenna dipole to form 1/4 lambda each side makes a square. American Radio Amateur (HAM) Clarence C. Moore, W9LZX developed this system in 1939 for the Missionary Radio Station HCJB at Quito of Ecuadaor (South America). The altitude of the station was over 10,000 feet in Andes. The station was operated in 25 m band SW with TX power of 10 KW. The band width of a single dipole is quite narrow. The quad loop is having high gain and less corona discharges etc. The half lambda folded dipole impedance is 288 Ohms (300 Ohms) and this quad loop is having 125 Ohms feed impedance. Due to low impedance of the quad, there is no need of any matching, for a general FM receiver system. Connecting a folded dipole (gain : 2.14 dB, 1.45 m long and 300 Ohms) to a FM receiver of 75 Ohms input, without any matching, its efficiency becomes to 65% (VSWR4) and a 125 Ohms quad at 75 Ohms receiver without matching it will get 95% efficiency (VSWR1.66) with an extra gain of 1.4 dB over a folded dipole. Due to this 3.54 dB gain from a quad loop there will be a terrific FM Radio reception. Using a split dipole, having 75 Ohms feed impedance, there will be correct match of 75 Ohms FM receiver system. But one of the element will be isolated and it makes static and lightning problems. if we ground the cable braid (shield), the entire quad or foled dipole antenna system get grounded and it avoid the threat from static or lightning effects. FM Radio allocation in India is from 88 MHz to 108 MHz. So the mid frequency is 98 MHz. The equation for getting the wave length of the conductor is (300 x 0.95) / 98 MHz. i.e. 2.908163 meters. If we divide this value by 4 we can get the Quarter Lambda length. i.e. 72.70 cm, we can take it as 73 cm or 74 cm. Due to skin effect of VHF frequency the element should be a tube having more than 4 mm dia or use 3/8th tube for getting good efficiency.
References
[1] en.wikipedia.org/wiki/quad_antenna
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