Robert Romanofsky, Senior Scientist, Antenna and Optical Systems Branch
- Tuesday, 02 February 2010
NTB: What makes a cellular reflectarray antenna different than the parabolic reflectarray antenna systems that most people are familiar with?
Dr. Romanofsky: Actually, the cellular reflectarray and the ferroelectric reflectarray are considerably different. The ferroelectric reflectarray is electronically steerable. It can essentially form a collimated beam anywhere in the hemisphere in front of the antenna. The cellular reflectarry is intended for a commercial application. We were approached by a company called Alienworks a few years ago and they were seeking a replacement for a parabolic reflector for the following reasons. Next generation Direct TV is going to be operating at much higher frequencies than we’ve had in the past and the antennas will be much larger, and there is a concern that with the larger antennas and the higher frequencies leading to much finer beam width, wind loading will cause disruptions in the service which, of course, is not tolerable. So they were hoping that a flat antenna could be provided to eliminate most of the wind loading problems. In addition, aesthetics is always an issue with commercial antennas. A lot of people don’t want to have a 30-inch diameter antenna sitting on their roof; they’d rather have something that would blend in, or even be conformal, if that was possible.
A cellular reflectarray does just that. Basically you would tell the provider of the antenna what your zip code is, and I’ve got a program that maps out a reflectarray for that latitude and longitude and all the installer needs to do is make sure the antenna is aligned to magnetic north and that it’s flat, and it will place a beam on the satellite. That’s one of the other big issues with Direct TV installation; every time they’ve got to make a run to do an install, it’s something like a $200 charge. If you’re looking at tens-of-millions of subscribers, that’s an enormous chunk eating away at your profits, so this goes a long way to minimizing the installation costs.
NTB: Another project you’ve worked on is the design of large inflatable antennas that could be deployed aboard relay satellites in deep space. Tell us about that project and some of the unique challenges it presents.
Dr. Romanofsky: We were instructed by NASA Headquarters, by the Space Communications and Navigation project in particular, to initiate a technology development effort of large deployable antennas approximately four or five years ago. Actually, it was initially intended to be part of the JIMO – the Jupiter Icy Moons Orbiter project – and there was a desire to have a large relay station at the Jovian distance to relay enormous data rates back to the Earth and it necessitated, perhaps, a 10-meter diameter antenna. Well, you can’t fit a 10-meter diameter antenna in, say, a 4-meter diameter spacecraft fairing, so you need a way to deploy an antenna. In addition, mass is always an issue, so we started working on gossamer antennas and other types of deployable antennas, and very suddenly the thrust switched from JIMO to exploration.
Of course, there was renewed interest in going back to the Moon and then on to Mars. A similar situation arose, and that is the communications that are going to be coming back from Mars in the future will be at much higher data rates than we’ve had historically. If you do the analysis, you would again need a 5 to 10-meter diameter antenna, possibly in an areostationary relay situation, to get hundreds of megabits per second back to the Deep Space Network. So the same limitation exists – you’ve got to get a very large antenna in a fairly small fairing, and you want to reduce mass as much as possible. So we’ve been developing inflatable membrane antennas, shape-memory polymer antennas, and we’ve helped a few companies with the most popular type of deployable antenna out there now, which is a mesh-type deployable.
NTB: Speaking of technical challenges, what would you say has been the most challenging project you’ve worked on to date at NASA?
Dr. Romanofsky: All of them really have their own challenges and twists. I think the most difficult project I had early on was actually my first space experiment. We were developing a hybrid superconducting, semiconducting receiver. It was a joint effort with NASA Glenn and JPL. It was delivered to the Naval Research Laboratory in the mid-90s and my responsibility was to develop a mixer for that receiver that would work with a starved local oscillator. Starved meant we had a very tight thermal budget because of the limitations on the cooler power, so we needed the mixer to work with as little RF energy from the local oscillator as possible. That, coupled with the fact that it had to be space qualified and we had very hard deadlines, was quite a challenge, and we got it done, and it was quite exciting. It was actually my first nonlinear circuit, and back then I guess I would still consider myself as being a kid, and designing your first nonlinear microwave circuit is sort of a rite of passage I suppose.