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  • International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 5, May (2014), pp. 111-121 © IAEME 111 BROADBAND HF ANTENNA’S VSWR VARIATIONS DUE TO THE EFFECTS OF THE COMPLEX NAVY SHIP SURROUNDINGS Evangelia Karagianni Hellenic Naval Academy, Sector of Battle Systems, Naval Operations, Sea Studies, Navigation, Electronics and Telecommunications, Hatzikyriakou Avenue, 18539, Piraeus, Greece. ABSTRACT An antenna onboard a warship was struck by lightning and part of the antenna system was destroyed. The RG218 cable had no apparent damage, so it was not replaced. After restoration of the damaged parts, sighs of degraded operation appeared. The VSWR of the antenna’s cable was more than 3:1 at low frequencies. In order to localize the source of faulty operation, special set of measurements were taken during a period of 6 days. During measurements, the GR navy vessel was moored and onboard. In this paper, interference levels caused by the nearby environment are discussed and simulated with the use of a software for simulating 3-D full-wave electromagnetic fields, in order to locate the strongest interference source in a dockyard environment and in open sea environment and to verify that the system is working properly. KEYWORDS: Broadband HF ship antenna, Interference, Lightning, RG218 cable, Standing Waves Ratio. I. INTRODUCTION The overall capability of an electromagnetic radiating system is dependent on its ability to operate effectively in a complex environment, in that its pattern performance can be adversely limited by pattern distortion effects, such as blockage and structural scattering. In many cases these detrimental effects can be minimized by wisely locating the antennas or minimize the effect of a strong interference environment (nearby buildings, ships, other constructions). This task is complicated by the large number of systems that is competing for prime locations on, for example, a modern military ship. Without an efficient means to position such systems one normally attempts to use locations which may be inexpensive but are certainly not optimum. As a result there is a great need for electromagnetic tools that can efficiently evaluate the pattern performance of radiating systems in their proposed environment. INTERNATIONAL JOURNAL OF ELECTRONICS AND COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET) ISSN 0976 – 6464(Print) ISSN 0976 – 6472(Online) Volume 5, Issue 5, May (2014), pp. 111-121 © IAEME: www.iaeme.com/ijecet.asp Journal Impact Factor (2014): 7.2836 (Calculated by GISI) www.jifactor.com IJECET © I A E M E
  • International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 5, May (2014), pp. 111-121 © IAEME 112 II. SPECTRUM CERTIFICATION AND WIDEBAND INTERFERENCE Electromagnetic interference (EMI), or Radio-Frequency Interference (RFI) when in radio frequency, is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The source may be any object, artificial or natural, that carries rapidly changing electrical currents (for example an electrical circuit or the Sun). EMI can be intentionally used for radio jamming, as in some forms of electronic warfare, or it can occur unintentionally, as a result of spurious emissions. It frequently affects the reception of AM radio in urban areas. It can also affect cell phone, FM radio and television reception. Electromagnetic environmental effects are the impact of the electromagnetic environment upon the operational capability of equipment, systems, and platforms. It includes electromagnetic compatibility and electromagnetic interference, electromagnetic vulnerability, electronic protection, hazards of electromagnetic radiation to personnel, ammunition and volatile materials and -last but not least- natural phenomena effects of lightning and precipitation static. Spectrum certification is required by communications equipment, radars, transmitters, receivers, electronic warfare systems, simulators and global positioning system (GPS) equipment. Items not requiring certification include electro-optics devices, nontactical and intrabase radios. In order to locate wideband interference, firstly we have to distinguish between the propagation of signals at low frequencies as opposed to high frequencies and take into account the power network. Also, the “T” intersection of power lines and telephone lines act like half, or quarter, or multiples of, or parts of wavelength stubs and thus the noise will behave accordingly. These intersections are often the location of strong standing waves and radiation, and may be a long way from the interference source. The International Electrotechnical Commission (IEC) promotes standardization in the electrical and electronic fields. For example, standard IEC60601-1-2 is for electrical equipment used in medical practice, electrical equipment that is not medical equipment and telecommunications. The standard IEC61000-3-X is for EMC Limits for harmonic emissions, test and measurement and electrostatic discharge. The International Special Committee for Radio Interference (CISPR) is a subcommittee of the IEC. The standard CISPR 11 is for ISM (Industrial, Scientific and Medical) radio frequency equipment, EM disturbance characteristics and sets international standards for radiated and conducted electromagnetic interference. These are civilian standards for domestic, commercial, industrial and automotive sectors. III. DESCRIPTION OF THE DEVICE UNDER TEST The HF antenna system consists of a Transmitter and Receiver, a matching network (Coupler), a Transmission Line (cable), an insulator and an aerial, as it can be seen in Fig. 1. The power amplifier (PA) is the transmitter. The Peak Envelope Power of the PA is 100 W. In normal conditions, the upper limit of this power is 500W. The signal is Single Side Band modulated and this value of the power is the mean value of an RF pulse at the peak of the envelope. Regarding the PA’s limitations for the reflected power, the manufacturer gives 50W. The power variation is approximately ± 1 dB. The bandwidth of the HF transmitter is from 1.6 MHz to 30 MHz. As regards the block diagram of the transmitter, it consists of a power supply (via the network or a battery) that supplies the PA. The protection circuit for the PA (which follows the PA) as well as the filter that rejects harmonics are fed by the same supply. The harmonic suppression is more than 40dB and IM less than 30 dB/PEP.
  • International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 5, May (2014), pp. 111-121 © IAEME 113 The antenna coupler is an impedance matching device. The ability to match is more important than efficiency when choosing a coupler. Coupler usually does not affect the pattern of the antenna, but only if the ratio of any common mode current on the feed line to the antenna currents remains the same. A coupler placed at the aerial side results in a more efficient system than one placed at the transmitter side, but the impedance at the aerial is different and the coupler might not be able to match it. Unfortunately, we don’t have an aerial to cover all desired frequencies with an acceptable VSWR for our equipment. Fig. 1: The block diagram of the antenna system. Highlighted red are the damaged parts of the antenna system by the lightning Fig. 2: The coupler of the antenna system The multi-coupler is the device which makes it possible for one or two antennas to do the electrical work of many, as opposed to using a separate antenna for each individual wavelength. Ideally, every signal would have an individual antenna perfectly measured to resonate to its frequency. Realistically, there are hundreds of wavelengths, and it becomes impractical to equip a space (especially in a ship) with that many aerials. Technicians, therefore, change their electrical length instead of their physical length. Over the years, many types of antenna couplers have been View slide
  • International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 5, May (2014), pp. 111-121 © IAEME 114 developed. An automatic coupler system can sense when the aerial needs tuning and perform the function itself. It can also adapt to become compatible with a variety of transmitters so that, on the whole, no technician is necessary. Usually, however, an automatic antenna coupler has a manual override function so in case of equipment malfunctions, a technician can fix the problem. To avoid interference, antenna systems are usually turned down to very low signal strength during tuning. The general standard is that an antenna's signal should be less than 250 watts when it is being tuned. The Twin Loaded TX Coupler (TLC) is the circuit that is placed next to the transmitter in our Device Under test (DUT). This special circuit enables one broadband antenna to be used by two different transmitters. The transmitters can work independently into a single aerial with all attendant advantages it brings including cost and space savings. The decoupling between the transmitters is sufficient to ensure that intermodulation between them will exceed ISB requirements. The aerial bandwidth itself will determine feasible operating bandwidth. As an example, one transmitter can provide the upper, the other the lower sideband. For telegraphy operation any two suitable frequencies may be set with receiver selectivity left to determine the frequency difference. The TLC is a passive hybrid circuit that consists of two inputs (ΤΧ1 and ΤΧ2) and two transformers. The one transformer matches this circuit with the aerial. The two 50 impedances are transformed to 25 from the one transformer and converted again from the other transformer to 50 . In case of power or phase mismatch of both connected transmitters, half of the power is dissipated in a resistor group. No power is being dissipated when both transmitter outputs have the same power, frequency and no mutual phase shift. The group of resistors is mounted on a heat sink and the heat generated by the dissipated power is blown outside the units by two blowers which are driven by a DC Voltage, generated by a built-in power supply. The transmission system (transmitter – transmission line – aerial) is efficient as long as the transmitter’s output impedance Ζout transiever is equal to the complex of the transmission line’s impedance Ζin coupler and so on as shown in Fig. 1. When not, then a portion of the transmitter output power is reflected back, resulting in standing waves. In case the forward and the reflected waves add up a composed wave is created neither forwarded nor reflected, instead the composed wave is oscillating hence the name. Measuring the voltage and the amperage at various points of the transmission line, we see that the standing wave alternates between a minimum and a maximum value Vmax and Vmin. The ratio of those values is the VSWR (Voltage Standing-Wave Ratio). Interrupting the cable at the output of the coupler, in order to measure the VSWR at point A and assuming that Ζout coupler >Ζin cable , we can define the VSWR in various ways: coupler out max transmitted reflected cable in min transmitted reflected Z V V V 1 VSWR Z V V V 1 + + ρ = = = = − − ρ (1) where ρ is the reflection coefficient coupler cable out in reflected reflected coupler cable out in transmitted transmitted Z Z V P Z Z V P − ρ = = = + (2) So, we can calculate the reflected power back to the transceiver 2 reflected transmitted VSWR 1 P P VSWR 1 −  =   +  (3) View slide
  • International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 5, May (2014), pp. 111-121 © IAEME 115 Measuring of VSWR shows how well the transmission line (eg cable or waveguide) is cooperating (matched) to the transmitter and particularly to the aerial. If VSWR is high most of our power intended for radiation is dissipated as heat on the transmission line. Special instruments are used for measuring VSWR; these are named as standing wave bridges or reflectometers or power meters. There two types of VSWR measuring devices. Those measuring separately the forwarded from the back warded power and those directly measuring both directions of power. Notwithstanding that VSWR shows how well power is fed to the antenna, it doesn’t show how effectively the antenna radiates the power it accepts. Antenna may be totally ineffective, or the transmission line may of high losses. Also a VSWR measuring device introduces losses. A normal thought when facing a high VSWR is to lower transmitter’s output in order to protect it. That was true particularly for older transmitters but modern transmitting systems are equipped with special protection circuitry that is activated when VSWR exceeds the value of 3:1. The case when VSWR drops abruptly is very unusual; a system never improves its SWR by itself. The most probable cause will be that loses in the transmission line have become excessive, for example due to rust or water ingress. These causes result in dissipating the power output and hence in a lower SWR. Connection points and transmission lines should be regularly checked for their tightness and good condition; also the connection points should be often checked. Summarizing, losses in a transmission line are of two types: those inherent to the line (ie construction characteristics for which we cannot do something to remove) and those induced due to degradation or damages. In order to have the maximum of transmitter’s output power effectively fed to our aerial we should always maintain the good condition of our transmission lines, keeping induced losses to a minimum. IV. MEASURMENTS FOR THE EFFECTS OF THE NEARBY EM ENVIRONMENT ON THE VSWR The basic Aerial under Test is a monopole 14 m height on the ship. This is the left antenna that was struck by a lightning. Damage was diagnosed from the fall of lightning at the locations that listed in Fig. 1 and highlighted with red colour: The two upper sections (2 meters each) of the aerial, plus insulator, plus two resistances of the multi-coupler circuit. All damaged parts are repaired. The cable which was of type RG218 suffered no apparent damage, but also measurements made therein with the help of a generator and an oscilloscope, on day one, showed positive results (Table 1). After repairing the damage the antenna worked in expectancy as regards its operational use. But standing wave measurements showed very high ratio standing waves at low frequencies (1-5MHz). Table 2 shows the measurements of the repaired antenna using the original cable (day two). Table 1: Oscilloscope measurements on the original cable RG-218 with the use of a signal generator Frequency (MHz) 5,35 4,5 3,5 2 1 Output Voltage (V p-p) 20,2 18,4 21,4 20,2 19,2 Input Voltage (V p-p) 23 22,6 22,2 21.4 21 LOSS (dB) 1,12 1,79 0,32 0,5 0,78
  • International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 5, May (2014), pp. 111-121 © IAEME 116 Table 2: Standing waves measurements of the repaired antenna with the use of the original cable (RG218) FREQUENCY (MHz) TRANSMITTED POWER (W) REFLECTED POWER(W) SWR PR/PF(%) 1,7 75,9 21,4 3,3 28,2 2 82,9 23,3 3,3 28,1 2,2 55,9 15,4 3,2 27,5 2,5 82,9 21,1 3 25,5 2,75 119 30 2,99 24,9 3 66,6 17,3 3,1 26,2 3,5 66,9 18,6 3,2 27,9 4 110 30,4 3,2 27,5 4,5 51,9 13,4 3,1 25,7 5 71,4 16,8 2,87 23,6 6 42,1 8,75 2,65 20,6 8 54,1 3,2 1,63 5,9 10 60,3 1,4 1,35 2,3 12 53,3 3,26 1,64 6,1 16 55,8 2,83 1,57 5,1 18 48,9 2,95 1,64 6 20 61,3 5,9 1,9 9,7 Fig. 3: Blue lines are VSWR measurements of Table 1. Red lines are for VSWR measurements on the left antenna of the same moored ship which was not suffered by lightning and has the same type of cathode (RG218). Green lines are VSWR antenna measurements on another moored ship with the same type of aerial and cable
  • International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 5, May (2014), pp. 111-121 © IAEME 117 There is an identical aerial on the right side of the ship and one identical antenna system on another moored ship. On days three and four, VSWR measurements were made using a power meter at the output of multi-coupler, to the DUT and to the other two antenna systems. The chart in Fig. 3 shows comparative measurements between the DUT and the two other identical systems. The ships were anchored in an environment with strong reflections from the surroundings, in strained EM field. The results were disappointing as measurements showed that our cable may be suffered from the lighting. The next step was to replace the cable with a new one, type RG58. These measurements were held on day five. The graph in Fig. 4 presents comparative measurements in the same antenna onboard the ship, but with a different cable. The cable was placed in our DUT was of low standards and the results of the measurements are not so satisfactory. These measurements lead to the conclusion that indeed our original cable is not damaged and the insulation of the cable itself is likely to affect our measurements. Obviously, although the environments are similar, we cannot exclude the existence of different reflections and sources of interference. It should be also taken into account that our cable length is different and the path is different, since it did not pass through the same internal route in the ship which passes the original cable. Fig. 4: Comparative meaurements of VSWR with the original supect cable (RG218) and the new one (RG58) Αs this high VSWR for low frequencies does not affects antenna’s operational use, as mentioned above, the ship was sailed, on the sixth day. When at open sea, new measurements were made and they are presented in the following graph in Fig. 5. This chart shows that the measurements of the AUT when the warship is moored in a strong nearby EM environment and when the ship is at open sea where the only interference may come from natural sources. Our measurements were markedly better and this shows that the presence of interference (external) is strong in DUT. V. SIMULATING THE NEARBY EM ENVIRONMENT Using silicon nitrate cone tubes of 40cm outer radius and 39 cm inner radius for the lower base, 2 cm outer radius, 1 cm inner radius of the upper base, we construct the simulating device. Because of the program restrictions, our antenna is 10 m high. Regarding borders, a 0.5m, 2 m and a 5 m cylinder is placed, radiating only as it is shown in Fig. 6. The results showed a strong effect on the input impedance, especially at short distances. A lumped excitation port is used.
  • International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 5, May (2014), pp. 111-121 © IAEME 118 Fig. 5: This graph shows that VSWR exceeds the limit of 3:1 at 2.5 GHz only. At low frequencies, VSWR improved. We can conclude that when the ship was moored at the dockyard there was strong interference from the surroundings, especially at low frequencies. When the ship was at sea, these influences disappeared Fig. 6: Simulating antenna with HFSS Simulation results as presented in Fig. 7, showed that at low frequencies, the input impedance of the aerial is very low and it is depended on the distance of the radiating path, yielding in mismatching. VI. CONCLUSION Five or more meters of spacing is required to prevent interference among systems at antenna sites, but the problem is when the space is limited. Actual antenna spacing requirements can be estimated using comprehensive interference analysis techniques. Most interference studies only consider intermodulation and harmonics and ignore other effects investigating potential combinations of frequencies. This kind of analysis can be performed using simple software
  • International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 5, May (2014), pp. 111-121 © IAEME 119 programs. In many instances, the vertical spacing among antenna arrays is limited only by physical spacing needed for the antennas. Half a meter of vertical spacing from antenna tip to antenna tip is often sufficient to prevent interference among systems. Comprehensive interference analysis makes it possible to estimate when this is possible to place antennas more closely. (a) (b) (c) Fig.7: Simulation results of the aerial with radiating only boundary at 2 m
  • International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 5, May (2014), pp. 111-121 © IAEME 120 Factors driving interference include the number of active channels at the site, the relative placement of the antennas, the frequency bands used, and the characteristics of the technology and base station equipment. Much of the newer technology and equipment is still under development and has not been extensively tested for compatibility in a shared site environment. In general terms, interference has been defined as follows: “The effect of unwanted energy due to one or a combination of emissions, radiation, or induction upon the reception of a radio system manifested by the serious degradation, obstruction, or repeated interruption in communication. Interference may be generated by sources at a shared site as well as by signals located some distance away from a shared site”. In our case, interference cased by emissions of the nearby environment affect the reflected waves in low frequencies of the broadband system because of the lack of matching, but had no effect on operational use.In RF reflections there are three types of errors: mutual coupling between the probe and the DUT, multipath reflections (walls, scanner and DUT mount), and leakage from the transmitting and receiving systems. The mutual coupling between the probe and the DUT can be minimized by using absorbing material around the probe. The multipath reflections from the walls and the mount can be reduced by using absorbing materials. Leakage from the transmitting and receiving systems can be minimized by proper shielding and cabling, especially the connectors. VII. ACKNOWLEDGEMENTS The author acknowledges that this work was partially supported by the Microwaves and Fiber Optics Laboratory of the National technical University of Athens. The author would also like to acknowledge the Hellenic Navy and especially the crew of the frigate “SPETSAI” namely, Commander J. Retsas (CO), Lieutenants A. Karamitros (EE) and S. Orfanos (CE) for their substantial contribution to this work. REFERENCES [1]. Roger Block, "The Grounds for Lightning and EMP Protection", PolyPhaser Corporation. [2]. Tzyh-Chuang Ma, Sung-Jung Wu, “Ultrawideband Band-Notched Folded Strip Monopole Antenna”, IEEE Transactions on Antennas and Propagation, Vol. 55, No.9, September 2007. [3]. Jin-Ping Zhang, Yun-Sheng Xu, Wei-Dong Wang, “Microstrip-Fed Semi-Elliptical Dipole Antennas for Ultrawideband Communications”, IEEE Transactions on Antennas and Propagation, Vol. 56, No.1, January 2008. [4]. G. Marroco, L. Mattioni, “Naval Structural Antenna Systems for Boroadband HF Communications”, IEEE Transactions on Antennas and Propagation, Vol. 54, No. 4, pp. 1065-1073, April 2006. [5]. Radio Regulations Articles, Edition of 2012, ITU. [6]. Per-Simon Kildal, Xiaoming Chen, Charlie Orlenius, Magnus Franzén, Christian S. Lötbäck, “Patané Characterization of Reverberation Chambers for OTA Measurements of Wireless Devices: Physical Formulations of Channel Matrix and New Uncertainty Formula” IEEE Transactions on Antennas and Propagation, vol. 60, no 8, August 2012. [7]. Majed Koubeissi1, Bruno Pomie1, Erwan Rochefort „ Perpectives of HF half Loop Antennas for Stealth Compat Ships” Progress In Electromagnetics Research B, Vol. 54, 167-184, 2013. [8]. ANSI/IEEE Standard Test Procedures for Antennas, ANSI/IEEE Std. [9]. Antenna Measuring Theory, IEEE, New York, John Wiley Distributors [10]. M. Walter Maxwell “Another Look at reflections”, August 1976 issue of QST magazine.
  • International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Volume 5, Issue 5, May (2014), pp. 111-121 © IAEME 121 [11]. “Navy Electricity and Electronics Training Series Module 10—Introduction to Wave Propagation, Transmission Lines, and Antennas” NAVEDTRA. [12]. Gaurav Belwal, “Circularly Polarized Compact GPS and SDARS Integrated Antenna”, International Journal of Advanced Research in Electronics and Communication Engineering (IJARECE) Volume 3, Issue 4, April 2014. [13]. Amaresh Singh1, Dr. Srivatsun G., “A Small-Fed Monopole Antenna for Wireless Applications” International Journal of Advanced Research in Electronics and Communication Engineering (IJARECE) Volume 3, Issue 5, May 2014. [14]. Quazi Md. Alfred, Tapas Chakravarty, Salil Kumar Sanyal, “Overlapped Subarray Architecture of an Wideband Phased Array Antenna with Interference Suppression Capability” Journal of Electromagnetic Analysis and Applications, 2013, 5, 201-204. [15]. Dr.V.Murali Krishna, Karimella Vikram and Prof. Narasimha, “Broadband Wireless Communication” International Journal of Electronics and Communication Engineering & Technology (IJECET), Volume 3, Issue 2, 2012, pp. 519 - 530, ISSN Print: 0976- 6464, ISSN Online: 0976 –6472. [16]. Neha Goyal, Kirti Vyas and A K Sharma, “Compact Broadband Circular Microstrip Feed Slot Antenna With Asymmetric Bevel on Ground Plane for Wimax and Wlan Applications”, International Journal of Electronics and Communication Engineering & Technology (IJECET), Volume 4, Issue 7, 2013, pp. 216 - 221, ISSN Print: 0976- 6464, ISSN Online: 0976 –6472. [17]. Rahul T. Dahatonde and Shankar B. Deosarkar, “Design of Radiating-Edge Gap-Coupled Broadband Microstrip Antenna for GPS Application”, International Journal of Electronics and Communication Engineering & Technology (IJECET), Volume 3, Issue 3, 2012, pp. 303 - 313, ISSN Print: 0976- 6464, ISSN Online: 0976 –6472.