Part 1: From Customer Request to Product Line
In 2006, a pressing need emerged within the US military: a portable, high-power RF system capable of testing vulnerabilities in electronic systems. The stakes were high, as adversaries could potentially exploit such technology. This urgent requirement led the Department of Defense to APELC, setting in motion a journey of innovation that would reshape portable RF technology.
Applied Physical Electronics L.C. (APELC) was founded in 1998 and began as an OEM for compact Marx generators with unique capabilities not readily available to government customers. At that time in the company’s history up to 2006, customers would either purchase a Marx generator for a specific end-use, with the integration of the Marx and load occurring on the customer’s end or APELC would be awarded an SBIR or similar contract to develop a pulsed power-based system that did not yet exist and required seed-funding for new R&D. In 2006, things began to change with a call from one of our DoD customers.
The Requirement: Developing a U.S.-Made Wideband Radiator to Counter Emerging Threat
Having previously worked with APELC under an SBIR contract, our customer was aware of our compact Marx generator technology and had a specific request for a system they wanted to purchase from us: A suitcase/briefcase-sized, battery-powered wideband radiator capable of ~100 kV/m at 1m with a center-frequency around 400 MHz. We learned from them that a German contractor called Diehl was selling this exact thing, and with a concern over such a system making it into the wrong hands and being used against US assets, they wanted a US manufacturer to offer an equivalent source for testing purposes. Having previously driven ultra-wideband and wideband antennas with Marx generators, we felt more than capable of taking on this opportunity to expand our technology portfolio.
Figure 1 Diehl DS-110 Suitcase HPEM system
How Wideband RF Bridges the Gap
Wideband RF had recently become interesting in the T&E community not only because it represented a potential threat, but because it had similarly shown promise as a compact means of effecting electronics with implications for counter-IED efforts (this was still early in the Iraq/Afghanistan wars) as well as vehicle-stopping. Wideband RF is characterized as having a ratio of center-frequency to bandwidth of <100%, and differs from narrowband high-power microwave (HPM) systems in both power-spectral-density (PSD) and center frequency. Wideband (a.k.a. “Mesoband”) RF systems are typically characterized by center frequencies <1 GHz, pulse durations in the 10s to 100s of nanoseconds, and bandwidths comparable to their center frequency (e.g. a 400 MHz center frequency source with a 100 MHz bandwidth).
As discussed in our previous blog entry, Increased EMP Threats and Testing Standards- Part 1: Understanding the Threat, there are significant differences between types of high-power RF systems that make them useful for different applications. A brief summary is provided below to help illustrate where wideband fits into the larger picture of directed energy:
- Ultra-wideband (UWB) has the widest spectral content and the potential for very high field strengths, but has demonstrated few effects given the lack of energy at any specific frequency.
- Narrowband HPM is a fantastic choice when you know a specific target’s vulnerability. In this case, a great deal of energy can be located in-band for that particular vulnerability, while the higher frequency allows for greater directivity/gain and therefore greater stand-off distance.
- Wideband RF provides a potentially useful balance between power spectral density and wide bandwidth that may be useful when targeting unknown or even multiple vulnerabilities. The major tradeoff is most often range, given the inability and/or impracticality of creating an antenna capable of high gain within a reasonably sized footprint.
- Narrowband HPM is great for “the kill”, while wideband is more useful for an upset.
Wideband RF provides a source that can potentially couple well into wiring harnesses, power systems and engine electronics as opposed to an HPM source, which would typically be designed as a “front-door” attack utilizing in-band spectral content specific to the target’s RF front-end (antennas etc…), while taking advantage of the high gain afforded by microwave antennas. While wideband systems lack the directivity and stand-off distance provided by narrowband systems, they are often simpler in their construction, and because they do not require complex vacuum hardware, are often much more compact. As of 2007, the Joint Non-Lethal Weapons Directorate (JNLWD) was funding wideband vehicle and vessel-stopping programs at ~$600k [1]. While this was a relatively small amount of funding compared to the tens of millions going toward the Active Denial System (ADS) or other HPM programs, it did result in enough interesting effects data that the program was continued for several years.
Breaking New Ground: Developing a Wideband RF System Without a Blueprint
Our customer supplied APELC with an example waveform from the Diehl DS-110 system and we immediately recognized the damped sinusoidal wideband signal. Armed with our prior knowledge of generating high-power RF, APELC began designing and testing methods of conditioning the double-exponential pulse from our Marx generator into wideband frequency content around 400 MHz center-frequency, and using some fundamental antenna theory, attempted matching a radiating antenna to the pulse-conditioner, which we began referring to as “the resonator” given its basis in an L-C “tank circuit” tuned to the desired center frequency. As with many R&D engineers/scientists, APELC likes to build hardware as soon as possible to learn what does and does not work. Some of the early prototypes provided the frequency content we were looking for but lacked the desired field strength.
As well as collecting RF data from prototype antenna/resonator combinations, APELC utilized a vector network analyzer (VNA) to corroborate simulation results and better understand the interactions between the Marx generator, resonator, and antenna.
The customer did not provide APELC with much more than a waveform, so our design was completely created from first-principles onward without any knowledge of how it had been done before by others. Because of this, we were able to learn a lot of lessons that would become invaluable to future wideband system development.
Figure 2 Early prototype antenna for the APELC wideband suitcase
The First Fully Functional System
After many iterations, APELC finally settled on an antenna and resonator design that provided a comparable waveform to the DS-110, with a slightly higher field strength and a pulsed power/prime power system capable of higher repetition rates (10 Hz). While far from perfect, APELC was able to design, prototype and construct a completed system that met the customer’s requirements in only a matter of months. The first system delivered to our customer is shown in the figure below along with an overlay comparison of the APELC suitcase system and the Diehl DS110.
Figure 3 APELC’s first complete 400-MHz RF suitcase
Figure 4 Comparison of the first APELC RFSC-400 and the Diehl DS110
From Prototype to Turn-key Products
The APELC 400 MHz suitcase began to generate enough interest in the T&E community that various DoD customers placed several follow-on orders. With each additional system purchased, APELC continued to improve the design of the RFSC-400 suitcase RF source, resulting in a mature/high-TRL system that the customer could easily use with little maintenance. The RFSC-400 sold by APELC today is shown in the figure below. Additional information on the RFSC-400 unit can be found on our website.
Figure 5 Present version of the APELC RFSC-400
Lessons Learned & Innovations Gaines
While many of the lessons learned along the way are proprietary, a generalized summary is provided below:
- Isolation of the Marx from the antenna/resonator.
- Protection of on-board electronics from the intense fields generated by the device.
- Optimization of the antenna to account for loading from the resonator and Marx connection.
- Insulator and pressure vessel design for high-voltage and high-pressure.
- Reliable control and power electronics
- Utilizing a commercially-available rechargeable battery.
APELC has switched all of our portable units over to utilizing COTS rechargeable drill batteries. Not only does this provide a tested and safe battery alternative that can be removed and charged separately, but also allows the customer to purchase additional batteries and chargers from their local hardware store- something many of our customers have greatly appreciated. This particular lesson has since been applied to our full line of portable battery-power systems for high-power RF and HEMP pulsed-current injection (PCI) testing. Additionally, many of the electronics hardening lessons learned from the effort were applied to protect almost all of our portable pulsed power systems.
Scaling APELC’s RF Technology to Meet Diverse Testing Needs
Similar to the developmental arc described in our article on the creation of APELC’s RS105 HEMP test system, the lessons learned from this initial suitcase effort over 15 years ago have led to the development of an entire line of high-power RF test systems.
One of the first to emerge was our RFFL-400, which made the addition of a reflector to the same antenna used in the RFSC-400. Additionally, larger air tanks were added for longer run-time. The result was a portable system with a >150kV/m field strength (at 1m), less back-lobe radiation, and a nearly 8-hour continuous run-time.
Figure 6 APELC RFFL-400 footlocker system
A similar reflector design was later applied to APELC’s Wideband RF Test-Stand. The Diehl system’s one advantage over APELC’s suitcase RF system was tuneability. APELC traded this capability on our suitcase system for durability and maintainability, as this was something highly valued by our customers. However, realizing that our recently developed quick-disconnect connector could be used to swap out a suite of different antennas/resonators onto the same Marx generator would result in a system that could cover a bandwidth of 50 MHz to 500 MHz with an easily interchangeable antenna, we created an incredibly versatile testing asset for the T&E community. Moreover, by using our well-characterized MG15-3C-940PF as the basis Marx generator for the system, higher field strengths (>200kV/m) and higher repetition rates (~100 Hz) were made possible.
Figure 7 APELC Dipole RF Test-stand
Stand by for Part 2 of this story, “Taking Compact High-Power RF to the Next Level” in which we describe additional advancements we made using this technology for other unique and challenging customer requirements.
You can learn more about our full line of high-power RF sources on our website at https://apelc.com/high-power-rf/.
References
- Force, “Directed Energy Weapons,” 2007. [Online]. Available: https://dsb.cto.mil/wp-content/uploads/reports/2000s/ADA476320.pdf [Accessed: 12-Sep-2024].
