JED - October 2015 - 35

By Ollie Holt

installation locations available on the weapons platform,
the goal is to use as few antennas as possible (ideally you
would only use one antenna) to operate over the total desired frequency range. This lowers the antenna efficiency
(η) at the non-optimal frequencies, which lowers the gain
at those frequencies. In order to make the best trade-offs,
these antennas would be sized to provide the best performance at those frequencies in which the most lethal
threats operate.
Examining the survey results you will see the typical
supplier and part number columns then the next column
defines the antenna type. There are many different antenna types. The type sometimes also defines the antenna polarization, spirals for example are circular antennas with
circular polarization, whereas a linear antenna's polarization is more dependent on its installation (horizontal, vertical or slant). There are other types of antennas, such as
a horn or some types of notch antennas, in which the feed
assembly defines its polarization. So, usually, its best to
both know the type of antenna and its polarization.
The next value is the antenna gain. This is usually defined with respect to a lossless isotropic antenna and listed as dBi. Sometimes it is related to a ½ λ dipole antenna
and listed as dBd. Note that dBd=dBi-2.15dB. The gain is
also related to the antenna beamwidth and operational
frequency range. Don't expect the gain to be flat over either the beamwidth or the frequency range. It will typically vary by a few dB over either. The numbers provided
in the survey are typical values over the frequency range
and beamwidth.
The next column addresses polarization. As discussed
above, receive and transmit polarization of the antenna is
important. Typically if you are not receiving with the same
polarization as the transmitted waveform your receive
power will not be optimal. Theoretically if your receive
antenna polarization is orthogonal to the transmitted signals antenna polarization you will not receive the signal.
Usually that's not completely true because other conditions will effect the polarization and some low level of
signal maybe detected, but your design should not depend
on this happening.
Sometimes the antenna can be arrayed (multiple elements summed together for additional gain). This column
addresses that possibility and notes how many elements
are arrayed together.

The Journal of Electronic Defense | October 2015


his month's technology survey covers antennas that can be used in electronic warfare
(EW) or signals intelligence (SIGINT) applications. The antenna on a SIGINT system or an
RF EW system supports receiving the RF signal
and, in the case of jammers, the transmission
of RF countermeasures responses.
The two main challenges that systems designers face
with antennas are that they need to be big (which we will
address again below) and they need clear access (a clear
field of view) to the RF environment in the direction of
desired operation. This is not always possible. On any tactical platform, trying to find good locations for EW antennas is usually very difficult. Typically the prime antenna
locations are already claimed by moving surfaces or by
antennas for radar and communications systems, thereby
relegating EW antennas to less-than-ideal areas.
These "less prime" areas also tend to drive the size and
shape of the antenna to less-than-desirable dimensions.
Size impacts antenna gain, while shape can impact the
antenna polarization. Signals are transmitted through antennas that have a polarization defined by the transmitting antenna. This can be horizontal, vertical, circular or
even slant. For both circular and slant, if its right or left
circular or right or left slant, this can impact the received
signal strength if the antennas are of different polarization. So you need to evaluate a few major factors when selecting a location for an antenna on a specific platform; 1)
location, 2) desired receive and/or transmit polarization,
3) antenna pattern and 4) antenna size (gain).
The optimum size of an antenna is a ½ wavelength (λ)
and λ=1⁄Frequency. For low frequencies like C/D Band, λ
can be very large compared to the space available for an
antenna. Aperture area impacts the achievable antenna
gain. Antenna gain can be measured by the formula,
G=4πηA/λ2, where η is a measure of antenna efficiency
and A is the aperture area. Antenna efficiency (η), also
known as radiation efficiency, is a factor that includes
all reductions from maximum antenna gain. It is usually
expressed as a percentage and can vary from very small
figures (0.002-0.5 for spirals) to larger percentages (0.0050.93) for some horns.
The other factor that impacts the antenna efficiency
is the desired bandwidth (operating frequency range) of
the antenna. Typically, because there are very few good



JED - October 2015

Table of Contents for the Digital Edition of JED - October 2015

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
World Report
Going Small: Jamming the Mini-Drones
EW Simulation and Testing: Keeping Up With the Threat
Technology Survey: EW and SIGINT Antennas
EW 101
Index of Advertisers
JED Quick Look
JED - October 2015 - cover1
JED - October 2015 - cover2
JED - October 2015 - 3
JED - October 2015 - 4
JED - October 2015 - 5
JED - October 2015 - The View From Here
JED - October 2015 - 7
JED - October 2015 - Conferences Calendar
JED - October 2015 - 9
JED - October 2015 - Courses Calendar
JED - October 2015 - 11
JED - October 2015 - From the President
JED - October 2015 - 13
JED - October 2015 - 14
JED - October 2015 - The Monitor
JED - October 2015 - 16
JED - October 2015 - 17
JED - October 2015 - 18
JED - October 2015 - 19
JED - October 2015 - 20
JED - October 2015 - 21
JED - October 2015 - World Report
JED - October 2015 - 23
JED - October 2015 - 24
JED - October 2015 - 25
JED - October 2015 - Going Small: Jamming the Mini-Drones
JED - October 2015 - 27
JED - October 2015 - 28
JED - October 2015 - 29
JED - October 2015 - 30
JED - October 2015 - EW Simulation and Testing: Keeping Up With the Threat
JED - October 2015 - 32
JED - October 2015 - 33
JED - October 2015 - 34
JED - October 2015 - Technology Survey: EW and SIGINT Antennas
JED - October 2015 - 36
JED - October 2015 - 37
JED - October 2015 - 38
JED - October 2015 - 39
JED - October 2015 - 40
JED - October 2015 - 41
JED - October 2015 - 42
JED - October 2015 - 43
JED - October 2015 - EW 101
JED - October 2015 - 45
JED - October 2015 - 46
JED - October 2015 - 47
JED - October 2015 - 48
JED - October 2015 - Index of Advertisers
JED - October 2015 - JED Quick Look
JED - October 2015 - cover3
JED - October 2015 - cover4