Radio and Television Today (Jan-Nov 1941)

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DIPOLE ANTENNAS (Continued from page 44) wave "arm" is hall' of this value, or 5.09 feet. These lengths are about 95% of the actual wave length in order to correct for the "end effect", which prevents all antennas from being theoretically correct A simple half-wave dipole has directional characteristics which are broadside to the antenna. As the length of the antenna is increased to 3/2 wave lengths, or a greater number of odd half waves, the directional pattern becomes such that it is more receptive to signals approaching at an angle to the axis of the antenna. For example, a three half-wave length antenna has six lobes of maximum reception. The two main lobes are broadside and the four minor lobes are located two on each side of the main lobes and about 30 degrees from the axis of the antenna. GREATER GAIN With such a 3/2 wavelength antenna, stations which are about 60 degrees apart can be received without turning the antenna. Reception of both probably would not be possible with a simple half-wave dipole. If reception in opposite directions is not required, a reflector can be placed behind the antenna to increase the pick-up gain on the front side. The reflector should be about 5% longer than the antenna, and the exact length should be adjusted for maximum reception. The spacing between the reflector and the antenna determines the amount of gain over a plain dipole. The gain is maximum at about 0.15 wavelength or 3.18 feet and is slightly less at 0.25 wavelength or 5.09 feet. The reflector length should be adjusted for best performance if the distance between it and the antenna is changed. Another important factor to consider is the height of the antenna above the ground. The ground reflection of a received wave combines in various phase conditions with the wave received at the antenna. Whether the reflected wave aids, or opposes the direct wave depends upon the height of the antenna, and the angle above the horizon at which the waves arrive. In general, the higher the antenna, the more improvement there is in reception of low-angle signals. Since FM waves travel either on a line of sight or at low angles to the horizon, this low angle reception is to be desired. The actual height of the antenna should be changed slightly during the testing period, since a difference of 1/4 wavelength, or 5 feet may mean that a weak distant signal will be received. It is best to have the antenna as high as possible, but a few feet up or down may greatly improve reception for certain stations. IMPEDANCE MATCH The impedances of the transmission line and the antenna must be matched in every case for maximum results. With the simple half-wave dipole the low impedance line is connected to the center where the impedance is about 70 ohms. If longer antennas are used for better directional effects, matching stubs must be used to secure the proper match. These stubs are short paral lel conductors, usually 1/8 to 1/1 wave long which are connected between the antenna elements and the transmission line. Tlie transmission line is moved along the matching stub until best performance is secured. Pig, L' shows the dimensions of an extended dipole antenna with matching stub for connecting a low impedance transmission line. As the high frequency broadcast services become more numerous, so will the need for service experts who can install correct antennas. N. U. Shop Cards Help Servicemen Three new display cards for service shops are offered by Nal lonal I nion Radio Corp., .".7 State St., Newark, N. •I., lo help build customer confidence and insure an understanding of the shop's policies. One ol Ihe cards stales "We reserve the right to make a nominal inspection charge," with the explanation that the location of radio troubles takes an unpredictable amount of time. The second card states thai a charge of 5c is made for testing each tube, but that the test is free if defective tubes are replaced with one purchased from the service shop. The third card is in the form of a Certificate of Award which is good for a period of one year and states that "up-to-date radio test equipment" is used in the shop. To Si HT&W. esistor Replacements . . • One ONf kvtSss °»t I *ATr " ^ sps.. *Only $6.00 list for two each of these 5 univeri a) sizes. :-m . . ,,«,ntlv used CUMIU-"00"0 . Wire Wound IRC Power wire is— -sr?;»5 plete range from 5 '° ■*! °:h:*trodu:yrRec Coo,-,n8.ogUordogo,„SMhe most common cause otp ,e»istor failure. IflQC Stock . co.pl. o.ch ol *. » . 2soo tance value from 50 onm wattage-low Uw A* low range »«* *£**»• in all cases Resistance re^ ^a upon the full the wattage *atiria. *. length of the "•"»** Adjustable Resis IRC Type ABA lO-wattAd^ge S^ter^anSaanaUstateOceach. POWER WIRE WOUND INTERNATIONAL RESISTANCE CO., 401 N. Broad St., Philadelphia, Pa. NOVEMBER. 1947 49