Showing the manner in which an end-fed Hertz antenna may be fed through a low-impedance line and low-pass filter by using a resonant tank circuit as at (A), or through the use of a reverse-connected pi network as at (B).
Some harmonic-attenuating provision (in addition to the usual low-pass TVI filter) must be included in the coupling system, as an end-fed antenna itself offers no discrimination against harmonics, either odd or even.
The end-fed Hertz antenna has rather high losses unless at least three-quarters of the radiator can be placed outside the operating room and in the clear. As there is r-f voltage at the point where the antenna enters the operating room, the insulation at that point should be several times as effective as the insulation commonly used with low-voltage feeder systems. This antenna can be operated on all of its higher harmonics with good efficiency, and can be operated at half frequency against ground as a quarter-wave Marconi.
As the frequency of an antenna is raised slightly when it is bent anywhere except at a voltage or current loop, an end-fed Hertz antenna usually is a few per cent longer than a straight half-wave doublet for the same frequency, because, ordinarily, it is impractical to bring a wire in to the transmitter without making several bends.
The Zepp Antenno The zeppelin or zepp an-System term a system, illustrated in figure 2A is very convenient when it is desired to operate a single radiating wire on a number of harmonically related frequencies.
The zepp antenna system is easy to tune, and can be used on several bands by merely retuning the feeders. The overall efficiency of the zepp antenna system is not quite as high for long feeder lengths as for some of the antenna systems which employ non-resonant transmission lines, but where space is limited and where operation on more than one band is desired, the zepp has some decided advantages.
As the radiating portion of the zepp antenna system must always be some multiple of a half wave long, there is always high voltage present at the point where the live zepp feeder attaches to the end of the radiating portion of the antenna. Thus, this type of zepp antenna system is voltage fed.
Stub-Fed Zepp- Figure 2C shows a modifica-Type Radiator tion of the zepp-type antenna system to allow the use of a non-resonant transmission line between the radiating portion of the antenna and the transmitter. The zepp portion of the antenna is resonated as a quarter-wave stub and the non-resonant feeders are connected to the stub at a point where standing waves on the feeder are minimized. The procedure for making these adjustments is described in detail in Section 22-8 This type of antenna system is quite satisfactory when it is necessary physically to end feed the antenna, but where it is necessary also to use non-resonant feeder between the transmitter and the radiating system.
22-2 Center-Fed Half-
The center feeding of a half-wave antenna system is usually to be desired over an end-fed system since the center-fed system is inherently balanced to ground and is therefore less likely to be troubled by feeder radiation. A number of center-fed systems are illustrated in figure 2.
The Tuned The current-fed doublet with Doublet spaced feeders, sometimes called a center-fed zepp, is an inherently balanced system if the two legs of the radiator are electrically equal. This fact holds true regardless of the frequency, or of the harmonic, on which the system is operated. The system can successfully be operated over a wide range of frequencies if the system as a whole (both tuned feeders and the center-fed flat top) can be resonated to the operating frequency. It is usually possible to tune such an antenna system to resonance with the aid of a tapped coil and a tuning capacitor that can optionally be placed either
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