HF Antenna VSWR Variations Due to Ship Surroundings

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)
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© I A E M E

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.

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

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)

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

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

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.

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

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

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.
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