Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

TURNSTILE ANTENNA 187the input resistance is equal to Z and the main feed line is matchedin its characteristic impedance. The following table shows the values ofR which would be chosen for a number of layers of Turnstile:Table INumber of LayersR (ohms) when(n) R/Z. Z„ is 70 ohms1 0.5 352 1.0 703 1.5 1054 2.0 140While we could theoretically extend this procedure to many morelayers, it was found that the antenna dimensions became quite criticalwhen more than four layers were used. In order to obtain high parallelresistance, the radiators had been shortened to a point where theresistance changed a great deal with a slight change in antenna length.A practical feed system used for six or more layers is shown inFigure 9. Here the main feed line enters at a point midway in theantenna structure. To match the main feed line, the radiator parallelresistance must satisfy the conditionR = nZ /S (8)The table below shows the appropriate value of R:Table IINumber of LayersR (ohms) when(n) R/Z. Z. is 70 ohms6 0.75 52.58 1.0 70.010 1.25 87.512 1.5 105.0In order to provide a design which would be installed in the fieldwithout adjustment, tests were made on typical arrangements toassure ourselves that close tolerances on dimensions were not necessary.Our objective was to adjust and test an antenna of, say, fourlayers at some frequency in the f-m broadcast band, learn what scalefactor should be applied to each dimension in order to build an antennafor any frequency in the band, and provide charts so that any futureantennas which were to be provided could be constructed from thesecharts.