Alaris Antennas is often tasked with supplying monitoring and/or direction-finding antennas for naval applications. To perform well in these applications, such antennas are typically designed to be as sensitive as possible to extremely low-powered radio signals that may be originating from hundreds of kilometres over the horizon. As one could imaging, a huge radar transmitting 5kW in close proximity to any COMINT antenna is not exactly a friendly neighbour
MIL-STD-461 is the military standard that is often used to define the requirements for equipment that operate in such harsh environments. Radiated susceptibility refers to the ability of a device to deal with large electric fields that may propagate through the area where such COMINT equipment is expected to work according to specification. Typical electronic devices will usually deal with this problem by ensuring proper shielding is implemented, to shield against such high electric fields, but herein lies the conundrum for antenna houses such as Alaris Antennas. Antennas are specifically designed to do exactly the opposite, i.e. be very effective receivers of electromagnetic energy.
How bad can it possibly be?
The RS103 section of MIL-STD-461 states that equipment used in naval applications should be able to operate with incoming electric fields of up to 200 V/m.
To get a better idea of why this is problematic for COMINT antennas, let’s consider an example; An antenna element has a gain of 0 dBi at 100 MHz, by using the gain and frequency, we can calculate the antenna would need to deal with about 48 dBm or 63 W of received power at its feed point. To put this into context, consider that most RF components are typically only designed to survive up to about 20 dBm or 100mW.
Customizing the solution for the specific operational environment allows for the antenna to be hardened in a way that allows for optimum performance over most of the band within that environment, while providing the appropriate amount of hardening and protection where it really matters.
Targeted desensitizing of the antenna elements
The first and most effective strategy in protecting the antenna and subsequent RF devices from within such environment, is to prevent such fields from coupling to the antenna elements in the first place. It is sometimes possible to desensitize the antenna purposefully where high field strengths are expected.
Applying radiating structures as filters are very effective, as it effectively prevents any part of the antenna or downstream circuitry from being exposed to high voltages or currents but could be challenging and comes with additional trade-offs.
Designing and building robust components
Even with the use of clever antenna element designs, there is still likely to be coupling with such antenna elements. A typical antenna element will often include some sort of balun and or matching network. Since these baluns and matching networks are fundamental parts of the antenna element, they are usually implemented as physically close to the feed point of the element as is practically possible, and hence it is not practical to implement additional filtering to protect these components. The balun and matching network of these antenna elements are thus directly subjected to any power that are received by the antenna element.
Alaris makes sure that such components will be custom designed to handle significant amounts of RF power, and often include the need for heat mitigating strategies to keep the components within reasonable operating temperatures.
Lastly would be to add additional filtering to protect the down-stream RF electronics. Such filtering could include band stop filters (as an example), assuming the frequency band of the offending transmitter is known, and/or limiters that will clamp the voltage to within reasonable values.
Figure 1. Example of the reduction in sensitivity caused by an IFF filter installed on one of the Alaris DF antenna arrays.
The use of limiters could cause non-linear effects when they are operated near the clamping voltage, which may cause harmonics to be generated. For this reason, limiters are typically installed as a last-resort safeguard.
As we have shown, using antennas in high-field environments like those typically present on many naval vessels is rather challenging. Alaris Antennas prides itself in working closely with our customers to tailor designs suitable for the specific Naval applications. Working closely with our customers allows us to create a solution that not only provides the necessary protection against such harsh environments, but still giving the best possible sensitivity and DF performance.