Tuesday, February 14, 2017

Interesting DARPA VLF Program

DARPA, as you probably know, is the Defense Advanced Research Projects Agency and they have a respectable record of success in programs that are high risk but high payoff.

So it's with some interest that I hear they're working on a program to miniaturize Very Low Frequency antennas.  Actually, miniaturize is too weak a word.  See, antennas work best when they're a significant portion of a wavelength long (the wavelength times the frequency is always the speed of light: c = f*l).  As a rule of thumb, most amateur antennas are longer than 1/8 wavelength, and some are several or many wavelengths long.

For example, a relationship lots of hams know is that the length (in feet) to start working on a 1/4 wavelength antenna is 234/f where f is in MHz.  That says an AM radio antenna 1/4 wavelength long at 1.000 MHz (near the middle of the AM broadcast band) is 234 ft long.  DARPA wants to miniaturize that antenna to fit in a teacup, with room to spare.  They want antennas 1/10,000 of a wave long.  That turns the 234 foot tall antenna into 1.12 inches tall.
“At these frequencies, free‐space electromagnetic (EM) field wavelengths are measured in tens of kilometers, resulting in very large transmitter structures when employing conventional antenna approaches. Electrically‐small antennas are defined as having dimensions much smaller than the EM wavelength, with examples in the literature of antenna‐sizes as small as 1/10th of the EM wavelength. DARPA is seeking innovation to bring that size below 1/10,000 of the EM wavelength or by at least a factor of 103 smaller than the current state of the art (SOA).”

Such a tremendous reduction in size is impossible to achieve through traditional antenna design so DARPA said it is looking to gather information “in the areas of materials, mechanical actuation, and overall transmitter architectures to address impedance matching, power handling, signal modulation, scalability, and other system level considerations.”
Unlike the world that car commercial writers live in, nobody can “break the laws of physics”, so nobody is going to come up with a way to treat those antennas to make them behave just like a full-sized antenna.  The laws of physics not can't be broken, nor are they up for negotiation.  On the other hand. they can be dealt with.  There may ..may ... be tricks that can be done to make a system work effectively with antennas that small.

The main problem is going to be the impedances required.  An electrically tiny antenna is barely distinguishable from an open circuit; they're a very, very high impedance.  Full sized antennas have much lower impedances, and most transmitter and receiver systems are designed around a 50 ohm standard impedance.  If you go buy virtually any amateur HF base station radio made in the last 50 years, that's what they'll be designed for.  Your cheap, Chinese VHF/UHF radio will be designed around a similar value.  (Cable TV systems, which don't transmit, are designed around 75 ohms for their cable ports rather than 50 ohms; car radio antennas are electrically quite small for AM radio, but not DARPA 1/10,000 wave "quite small", and they're high impedance). 

Half and quarter wave antennas are the impedance they are because of the physics.  They can be thought of as matching the impedance of the transmitter to the impedance of free space which is just under 377 ohms and any antenna has to match its impedance to 377 ohms.   The output impedance of the power device in the transmitter isn't 50 ohms just because the antenna is 50 ohms, and impedance matching is one of the very basic skills of an RF designer (impedance matching been berry berry good to me!).  The impedance of the output stage depends on the circuit configuration and device.  A convenient way to approximate the output impedance is the voltage squared over 2 times the power output ((Vcc^2)/(2*Po))  That means the higher the voltage and the lower the output power, the higher the output impedance. 

In a transmitter, low output power is rarely something desirable (I never, ever, had anyone ask me or anyone else I was ever around for less transmitter power).  Modern power transistors can run at hundreds of volts, but it seems to me that we'd be looking at even higher voltages to get the impedances they're looking for.  High voltage brings its own problems: notably arcing and corona, but high voltage is handled in the business. 

The biggest problems here are the antenna efficiencies.  An open circuit can only be tuned so far to look like an antenna.  It may require so many parts or be so narrowband that it becomes a useless antenna.  Component quality is going to be a big concern; it will likely require huge, heavily silver-plated coils and capacitors with vacuum dielectrics.  I don't know that the antenna can work.  It may turn out that the combination of extremely inefficient antennas, forcing higher powers to be used, forcing ever higher voltages into the circuits is simply insurmountable. 

I always used to say that the company that can turn the land requirements for an AM broadcaster from "many acres" down to "a store in a strip mall" will never run out of money.  The typical AM broadcaster uses three quarter wave tall monopoles (verticals), spaced around a quarter wave from each other, because they need to carefully recreate an antenna pattern they're authorized to use, so they'd need three of these magic antennas.  Once the radio waves leave that antenna, it's still a full-sized EM wave.  I just don't see how they can do it, but that's what DARPA is for.
The "Trideco" antenna tower array at the US Navy's Naval Radio Station Cutler in Cutler, Maine, USA. The central mast is the radiating element, while the star-shaped horizontal wire array is the capacitive top load. About 1.2 miles in diameter, it communicates with submerged submarines at 24 kHz at a power of 1.8 megawatts, the most powerful radio station in the world.  If scaled the way DARPA is targeting, that 979.5 foot tall tower becomes 11.75 inches.  It's currently 1/10 wave long; 11.75 inches reflects shrinking it by 1/1000.


  1. They've been working on this for, oh, a hundred years now?

    Some of the ham radio "lofer" guys that use the 137kHz band have developed smaller antennas that radiate kinda-sorta OK, but yep, the voltages get pretty high, even at the relatively low power they run.

    One problem they run into is SWR bandwidth. If the transmitted signal rate gets too high, the transmitted signal gets too wide, and the antenna is out-of-tune. It's a problem unique to VLF communications, and is the main reason the symbol rate to the submarines is so low. All the basically transmit is "come up to antenna depth to receive an important message", rather than actually sending them any orders to fire or anything.

    1. I noticed that DARPA specifically said they were interested in modulation effects. If your carrier is 30 kHz, you just ain't getting a whole lot of bandwidth out of it.

      Honestly, though, it makes me wonder what they want out of VLF? In the endless search for more bandwidth, that's the last place I'd expect anyone to go. Who says, "yeah, let's go down to 30kHz and transmit 1 WPM BPSK" WTF? The only thing I found reference to was a way to back up navigation if GPS goes down.

    2. "back up navigation if GPS goes down"
      Like the deactivated Loran C wasn't good enough? The Jupiter Fl. station and tower is all gone. In the immortal words of Bugs Bunny "what a bunch of maroons"
      You can tell that is a sore spot.

    3. VLF is the only way to penetrate water and rock to any "useful" depths.

      Hence, the use for talking to subs....

  2. I see they are talking transmitters, but what else uses those low frequency bands if all they wanted to do was listen?

    Using seismographs to detect nuclear detonations, etc.

  3. Re: "I never, ever, had anyone ask me or anyone else I was ever around for less transmitter power".

    Not always so. As an example, lower TX power is much desired in explosive environments. Other situations also desire minimal TX as well. The trick usually shifts over to the RX side: improve sensitivity.

    1. Well, I didn't say it's never necessary to design for low power, I said I never had anyone ask me or anyone I was working with for lower power and that's the truth. By the time we're slinging transistors, the system is designed.

  4. The ability of ships and subs to talk directly to each other would be a game changer. That's why they want this. Well, that, and kilometer-long trailing wire antennas are rather awkward.

  5. Look at this :

    To address the DARPA need for a small low electromagnetic frequency directional antenna, Physical Optics Corporation (POC) proposes to develop a new Transmit-Receive extremely low frequency– very low frequency (ELF-VLF) Directional Antenna (TREDA). This proposed device is based on a compact resonant two-way antenna and an atmospheric plasma waveguide.
    It continues with: ........meets DARPA needs for a low-frequency antenna system for detection (and also generation) of telluric currents to detect potential threats concealed in underground tunnels and facilities from airborne platforms.

    The antenna is to be approx 0.5 meter at it widest. 1/2 meter !!

    1. That's quite a story. Thanks for that link.

      They say it's a resonant antenna, but since it's 1/2 meter, it has to be an enormous coil and capacitor, with a little radiating element. "Atmospheric plasma waveguide" is a set of words that I don't understand when they're put together. I understand each word separately, but not when put together into a phrase. Note that it said single frequency, which if it means what it says, implies they won't be able to modulate the signal beyond very slow on-off keying.