ANTENNA
EH 160 mt
Introduzione
Queste pagine non hanno alcuna
pretesa se non quella di "descrivere" le mie esperienze con l'antenna
EH;
Non vogliono essere nemmeno una guida pratica alla costruzione della EH:
la mia intenzione e' semplicemente quella di rendere disponibili i risultati
delle mie prove.
L'antenna EH e' stata presentata come un'antenna basata su un nuovo principio
che dovrebbe stravolgere del tutto le attuali conoscenze sulla teoria
delle antenne;
non voglio entrare in merito, per approfondire l'argometo ci si puo' rifare al
web dell'inventore (Ted Hart,
w5qjr) o a quello dell'Arno Elettronica (la ditta che
la commercializza in europa);
Riporto di seguito un estratto dal sito dell'inventore, in cui e' esposta la
teoria su cui si basa la EH:
The Hart EH Antenna consists of two (2) elements having a natural capacity between them. (Think of a fat dipole) When a voltage is applied to a capacitor an E field will be developed. Also, the current through the capacitor (called displacement current) will develop an H field at right angles to (encircle) the electric field. However, when current flows through a capacitor, the phase of the current leads the phase of the applied voltage. Therefore, the phase of the H field leads the phase of the E field and the difference in phase (time) prevents satisfaction of the Poynting Theorem for this configuration.
If the external power applied to the EH antenna is first applied to an inductor between the source and the antenna, the inductor will retard the phase of the current relative to the applied voltage. Therefore, within the antenna the phase of the voltage (E Field) and the phase of the current (which causes the H Field) can be made to be the same. In other words, they occur simultaneously, thus, the name of the EH Antenna. This allows satisfaction of the Poynting Theorem and radiation occurs at the frequency where the reactance of the external inductance causes the phase of the current thru the capacitor to be the same as the applied voltage. This is at a frequency approximately equal to the resonant frequency of the external L and the internal C of the antenna. More complex phasing/matching networks and/or feedback techniques may be used to enhance bandwidth by maintaining the desired phase relationship over a range of frequencies. Greater amounts of radiation also result from more complex networks.
Due to the high efficiency of the integration of the E and H fields within the physical sphere of the antenna, where they are created simultaneously, the antenna need only be a very small fraction (less than 1%) of a wavelength. This is due to the very strong fields. The Poynting Theorem says Radiation = E x H. Since the space between the capacitor plates is only a fraction of a meter, the E field, measured in volts/meter, is large even for small applied voltages. The H field, measured in amp turns/meter, is large but relatively low, since the H field is less than the E field by a ratio of 377, the impedance of free space.
The EH Antenna can be physically configured to allow antenna pattern gain in the E plane in two different ways. One enhancement method is similar to that of a microwave horn, even though the operating frequency is such that the physical size of the antenna is very small compared to the operating wavelength. This is most evident in the Bi-cone version of the EH Antenna, where radiation occurs between and in a very small area at the apex of the cones, and the remaining cone area enhances the gain by shaping the antenna radiation pattern. The other method is to have long cylinders relative to the diameter of the antenna for the dipole configuration.
Due to the necessity of the H field being a closed loop (circle), the bi-cone must be non-directional in the H plane. In fact, all basic EH Antennas are non-directional in the plane orthagonal to the E field. Directive gain in the H plane may be achieved with phased arrays made of active EH Antennas, or special shapes.
Due to the E and H fields being primarily within the physical sphere of the antenna, Electro Magnetic Interference (EMI) is virtually eliminated. Since the E and H fields are contained, the EH Antenna can not be used as a parasitic element in an array.
Since the antenna is not a resonant structure, the frequency of operation is totally dependent on the external-phasing network. Since the typical phasing network only covers a small range of frequencies, the EH Antenna virtually eliminates harmonic radiation.
Since antennas are reciprocal, the EH Antenna offers full performance for both transmitting and receiving. In addition, since the E and H fields are primarily contained within the physical sphere of the antenna, the antenna rejects external E or H fields and receives only radiation. Thus, the EH Antenna is exceptionally quiet, thus producing very high signal to noise ratios in the presence of man made and atmospheric E field or H field noise.
Fatta questa piccola premessa, diciamo che il motivo che mi ha spinto a provarla
e' stata l'affermazione secondo la quale l'antenna EH, lunga solo qualche
centesimo di lunghezza d'onda, avrebbe le stesse prestazioni di un dipolo "full-size".
La cosa mi ha incuriosito a tal punto che ho deciso di costruire un esemplare
per i 160 mt, con le seguenti dimensioni (che mi sono state date direttamente da
Stefano, ik5iir).
Diametro dei cilindri=25 cm
Altezza di cilindri= 37,5 cm
Distanza tra i cilindri= al diametro degli stessi
Sviluppo totale parte radiante= 1 mt
Sviluppo totale antenna = 1,5 mt circa
Capacita' condensatori 128 Pf circa
Numero spire necessarie per L1=19
Numero spire necessarie per L2=16
Ora, se l'affermazione di prima
fosse' vera, questo antennino da poco piu' di 1 metro avrebbe le stesse
prestazioni di un dipolo di 80 Mt lineari...
Le pagine successive sono una descrizione sommaria degli accorgimenti tecnici
adottati; alla fine invece ci sono le conclusioni ed i risultati dei test.
Potra' sembrare tutto un po' confuso, ma questo e' dovuto al fatto che queste
pagine servono principalmente come "quaderno degli appunti" per il
sottoscritto.
73 de iz7ath, Talino Tribuzio