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-NOVEMBER NEWS LETTERIsotron 80/40 I have produced these articles since 2013. Up until now there has been no technical descriptions of how the Isotron Antennas work. Well, here you go.
THE ISOTRONS TECHNICAL DESCRIPTION The purpose of the design was to make a compact, convenient design that would perform well. The design is an electrical copy of a 1/2 wave dipole. I will describe electrically what takes place. Radiation patterns and formulas are not necessary for analyzing the tuning characteristics. A 1/2 wave dipole reacts as a SERIES RESONANT CIRCUIT. All electrical formulas you want to apply to this circuit will also apply to the antenna AT THE FEEDPOINT. The feed point is what determines how the antenna will match the transmitter and measurements are taken at the connector of the antenna. Therefore, whether the circuit is on your radio board or hanging as an antenna all formulas and theory apply. What is the difference between a series resonant circuit on your radio board and the same circuit that comprises an antenna? The circuit in your radio is made of small surface areas. Therefore, it develops a very low radiation resistance, just a fraction of an ohm in most cases. This is like having a shorted circuit. High current, low voltage. This keeps the radiation within the circuit. This is what is needed in the radio circuitry, but not good as an antenna. As you increase the physical area of the circuit it starts to develop a resistance and works at a higher voltage. Here is what starts to take place. At the feed point the dipole or Isotron will resonate. This is when the Capacitance Reactance (XC) and Inductive reactance (XL) are equal (value in ohms). However, when this takes place, because the antenna is a radiator it will also develop a pure RESISTANCE (R) when resonant or Radiation Resistance. This R is dependent on the antennas grounding, feed line and environment. This R value is what establishes your SWR measure at resonance. Once an antenna is resonant it becomes a working device with a radiation resistance of 25 ohms or greater. 50 ohms normally. An antenna that can develop naturally a Radiation Resistance of 25 ohms and higher (no specific limit) it becomes an efficient radiator. When the antenna has a higher XC value than the XL the antenna is off resonance. The resonant point is at a higher frequency than where the transmitter is currently set. If the XL is greater than the XC the opposite is true. Off resonance the Impedance (Z) of the antennas increases. It will be at its lowest value at the resonant point. (Z and R are not the same). Testing an antenna at the feed point without the feed line is fine for design work, but is not practical for tuning it to your entire set up (radio, feed line and grounding). The antenna is an infinite extension to your feed line. They both must be tested together to have reliable results. The Isotrons have been built physically to have proper capture area to the atmosphere. Enlarging the antennas more will have little affect on performance. The Isotrons are not a loop. Therefore the radiation pattern is omni-directional and polarization is random. This offers excellent efficiency on transmit. Reception is noticed as soon as the receiver is connected. Low noise, good sensitivity and the affects of changing polarization is lower. Granted it looks a little strange compared to what we are use to. However, the advantages are plain to see. Our 40+ years of service to the HF community is a testimony to the performance of the Isotrons. 73, Ralph WD0EJA BILAL COMPANY 137 MANCHESTER DR. FLORISSANT, CO. 80816 U.S.A PH/FX: 719/687-0650 OUR EMAIL LIST IS ACQUIRED BY PERSONS WHO HAVE CONTACTED US IN THE PAST. IF YOU DESIRE NOT TO BE CONTACTED PLEASE EMAIL YOUR REQUEST. |
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