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ON7YD, longwave, 136kHz, antennas Page 1 of 51 ON7YD Antennas for 136kHz About this page : The main object of this page is to provide information. It has been deliberately kept simple, no fancy and flashy tricks, in order to achieve maximum compatibility for the different browsers and to allow fast downloading. Any comments and/or suggestions are welcome at : [email protected] last updated on 8 July 2004 Index 1. Introduction 2. Short vertical antennas 1. Vertical monopole antenna 2. Short vertical monopole 3. Vertical antenna with capacitive toploading 4. Umbrella antenna 5. Capacitive toploading of single-tower antennas 6. Spiral toploaded antenna 7. Vertical antenna with inductive toploading 8. Vertical antenna with capacitive and inductive toploading 9. Vertical antenna with tuned counterpoise 10. Meander antenna 11. Antenna with multiple vertical elements 12. Using a non isolated antenna-tower as LF-antenna 13. Antennas with a long horizontal section 14. Helical antenna 15. Short vertical dipole 16. Why a horizontal dipole is a rather unefficient antenna on LF 17. Safety precautions 18. Bringing a short vertical monopole to resonance 1. Loading coil 2. Coil losses : the Q-factor 3. Variometer 4. Tapped coil 5. Impedance matching 6. Bandwidth considerations 3. Efficiency of antenna systems on LF (short vertical antennas) 1. Antenna system 2. Efficiency 3. Antenna system efficiency, antenna directivity, ERP, EIRP and EMRP 4. Optimizing the antenna system efficiency 5. Enviromental losses 6. Ground loss 1. Type (composition) of the soil 2. Frequency 3. Shape and dimensions of the antenna 4. Radial system and ground rods 4. Measuring ERP on LF http://www.qsl.net/on7yd/136ant.htm 12/19/2006 ON7YD, longwave, 136kHz, antennas Page 2 of 51 1. Electric field / magnetic field & near field / far field 2. Calculated ERP versus ERP measurements 3. How to measure ERP 5. Small loop antennas 1. Single turn small loop as transmitting antenna 2. Efficiency of a loop 3. Enviromental losses of small loop antennas 4. Single turn loop versus multi turn loop 5. Directivity and polarization of a small loop antenna near ground 6. Bringing a small loop antenna to resonance 1. Resonance capacitor 2. Impedance matching 3. Bandwidth considerations 6. Other transmitting antennas 7. Antennas for reception 8. Software 9. Appendices 1. High power applications of toroidal core coils 1. About toroidal cores 2. Designing a ferrite cored transformer (by Jim Moritz, M0BMU) 3. Designing an iron powder cored coil 10. Acknowledgements 1. Introduction The main subject will be transmitting antennas for 136kHz as this often is the most important part of a longwave amateur radio station. The aim of the transmitting antenna is to radiate the power coming from the transmitter. The power radiated by any antenna is determined by 3 factors : z The radiation resistance of the antenna z The antenna current z The gain (directivity) of the antenna Example : Assume we have an antenna with a radiation resistance of 10 , an antenna current of 2 A and a gain of 4 (6dB). This antenna will radiate a power of 10 x 22 x 4 = 160 Watt. The gain of an antenna is always given relative to a reference antenna. Most common references are the 1/2 wave dipole and the isotropic radiator. This last is a virtual antenna that has no directivity at all, it radiates equally to all directions. In general the gain of any antenna relative to a 1/2 wave dipole is given as dBd while the gain relative to an isotropic radiator is given as dBi. Due to its directivity a 1/2 wave dipole has a gain 1.64 (2.15dBi) relative to a isotropic radiator. At first sight the radiation resistance of an antenna has no influence on the radiated power, as long as you match your transmitter to this resistance. But unfortunately the radiation resistance is not the only resistance that is consuming the transmitter power, there are also the loss resistances. These losses occur within the antenna (+ the antenna matching system) and in the environment of the antenna (ground, objects near the antenna). On HF http://www.qsl.net/on7yd/136ant.htm 12/19/2006 ON7YD, longwave, 136kHz, antennas Page 3 of 51 these loss resistances are often negligible as they are rather small compared to the radiation resistance, but on longwave this is certainly not the case. For most longwave antennas used by amateurs the radiation resistance of the antenna is in the range of 10 to a few hundred m while loss resistances are in the range of 30 to 150 . This means that, dependent on the antenna and its environment, about 99% to 99.99% of the transmitter power is not radiated but absorbed in the loss resistances. The two most common transmitting antennas on longwave are the short vertical monopole (Marconi antenna) and the small loop antenna. The short vertical monopole is an electric antenna, it creates an electric field 'on the spot' (near the antenna) while the magnetic field is created 'on the fly'. Opposite to this the small loop is a magnetic antenna, it creates a magnetic field 'on the spot' while the electric field is created 'on the fly'. As a result of this the main source of losses for a short vertical monopole is in the environment (ground, trees, buildings etc.) while for a small loop the major losses are within the antenna. Therefore a small loop is less dependent on the environment for its functionality. But for both types of antennas the goal is to get the ratio of radiation resistance versus loss resistances as large as possible. In practice most amateurs achieve better results with short vertical monopoles, only when environment losses are extremely high a small loop will be superior. Remark : Throughout these pages the terms ERP, EIRP, dBi and dBd will be used frequently. If you are not familiar with these terms I would recommend to read this first. back to top of this page 2. Short vertical antennas 2.1. Vertical monopole antenna Most radioamateurs are familiar with the quarter-wave vertical monopole antenna, often also called a "Marconi antenna". It is a quarter wave long, is fed against ground (eventually improved by a radial system) and has a radiation resistance of 36 . The dimensions of a quarter wave vertical antenna might be suitable from the 40m band upward, some brave hams might even have this antenna for 80m and 160m. But for 136kHz it would be over 500m (1500ft) high, without doubt beyond the range of any ham. Thus at longwave there is no other way than using a vertical monopole that is (very) much shorter than a quarter wave. When a vertical monopole is less than a quarter wave (it's natural resonance) a few things change : z the radiation resistance will drop as the antenna becomes shorter z the antenna gain (directivity) will slightly drop, but this effect is can be neglected (less than 0.5dB) z the antenna impedance will have a capacitive component z the ground loss will increase as the antenna becomes shorter (see footprint theory) http://www.qsl.net/on7yd/136ant.htm 12/19/2006 ON7YD, longwave, 136kHz, antennas Page 4 of 51 The effect of the antenna length on the radiation resistance and antenna gain can be seen on the first picture at the right. So, in contradiction to what many believe, the antenna gain of a short vertical monopole is only 0.4dB less that that of a fullsize quarter wave vertical (even if the short monolope is only a fraction of the wavelength). Nevertheless the performance of a short vertical monopole is -20dB to -40dB below that of a quarter wave vertical, because the efficiency (ratio of radiation restistance and loss resistances) rapidly decreases as the antenna becomes shorter. Example : 1. A Quarter wave vertical has a radiation resistance of 36 and a loss resistance (groundloss) of 10 . The efficiency of this antenna : (36 / (36 + 10)) * 100% = 78.3% (or -1.1dB) 2. A short vertical monopole of 1% of the wavelength has a radiation resistance of 0.04 , while there is a groundloss of 50 and a loss in the loading coil of 20 . The efficiency of this antenna is : (0.04 / (0.04 + 50 + 20)) * 100% = 0.057% (or -32.4dB) As a result the quarter wave vertical will outrange the short vertical monopole by 31.7dB (31.3dB efficiency + 0.4dB antenna gain). The second picture shows "overall gain" (efficiency + antenna gain) of an average antenna as a function of its length. back to top of this page 2.2. Short vertical monopole Assume we have a short vertical monopole with a height H and fed against ground. If H is small compared to the wavelength then : z The antenna will act as a capacitance (CV) in series with the radiation resistance (RA) and the loss resistance (RG) z The antenna current (I) will decrease linear from the feedingpoint to the top of the antenna, where it will reach 0 z The voltage over the entire antenna will be the same The current distribution, that is different from the sinusoidal distribution we are used to, can be explained as follows : The antenna capitance is not located at one single point on the antenna, but is distributed equally over the antenna. As the antenna current flows into the antenna it gradually 'disappears' via the distributed antenna capacitance, resulting in a linear decrease. Another - and maybe more correct - way to look at it is to compare a short vertical with a full size (quarter wave) vertical. The full size vertical has a sinusoidal current and voltage distribution whith a 90 degrees phase shift between U and I.
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