It is well known that the widespread of Amateur radio vertical antenna "quarter-wave pin has an input resistance of approximately 37 Ohms. Since most of coaxial cables used for power antennas have an impedance of 50 or 75 Ohms, then the Amateurs sometimes problems arise when negotiating feeder with the radiator. There is a simple and very convenient from a constructive point of view, the solution to these problems.
Increasing the length of the radiator, you can increase the value of the active component of the input impedance to the desired value of 50 or 75 Ohms. Such elongated antenna has a noticeable reactive component of the input impedance that is inductive. To fix it's easy enough to include in series with the emitter capacitor (see figure). Its capacity is chosen such that, together with the inductive component of the input impedance it forms a series resonant circuit, the resonant frequency of which corresponds to the operating frequency of the antenna.
If the feeder has a characteristic impedance of 50 Ohms, the length of the radiator of the antenna is L should be approximately 0,28/l (a is the wavelength, corresponding to an average frequency Amateur band). At wave 75 Ohm L=0,31 l. Note that in order to minimize losses caused by the imperfection of ground - system balances feeder is better to use a 75-Ohm coaxial cable. Each element of the system of checks has length A=0,25/l. To achieve a sufficiently high efficiency of the antenna, the number must be at least 15 (efficiency will be more than 50 percent).
Here is the source data for the manufacture of "long pin" on various Amateur bands. For each of the range given: length in metres for 50 Ohm and through fraction for 75 Ohm feeder, the length of the element system of checks in metres and the maximum capacitance of the coupling capacitor in picofarads.
Range 7 MHz: 11.64/ 12,86 - 10,4 - 250.
The 14 MHz Band: 5,82/ Of 6.46 - 5,2 - 150.
Range 21 MHz: 3,85/ 4,27 - 3,49 - 130.
28 MHz: 2,86/ 3,16 - 2,62 - 100.
The antenna tuning is carried out by adjustment of the coupling capacitor for minimum SWR or by the maximum values of the field strength.
Author: V. Uzun (ex UB5MCI); Publication: www.cxem.net