tag:blogger.com,1999:blog-1592992209402300549.post6061765000122808460..comments2024-03-29T09:08:47.702-04:00Comments on The Silicon Graybeard: Interesting DARPA VLF ProgramSiGraybeardhttp://www.blogger.com/profile/00280583031339062059noreply@blogger.comBlogger10125tag:blogger.com,1999:blog-1592992209402300549.post-63918466177364669592017-02-18T20:43:57.811-05:002017-02-18T20:43:57.811-05:00That's quite a story. Thanks for that link.
...That's quite a story. Thanks for that link. <br /><br />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. <br />SiGraybeardhttps://www.blogger.com/profile/00280583031339062059noreply@blogger.comtag:blogger.com,1999:blog-1592992209402300549.post-32930612929064555002017-02-18T19:45:55.619-05:002017-02-18T19:45:55.619-05:00Look at this :
https://www.sbir.gov/sbirsearch/d...Look at this : <br />https://www.sbir.gov/sbirsearch/detail/8641. <br /><br />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. <br /> 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. <br /><br />The antenna is to be approx 0.5 meter at it widest. 1/2 meter !!<br /><br />JAQUEBAUERhttps://www.blogger.com/profile/17379882377455382796noreply@blogger.comtag:blogger.com,1999:blog-1592992209402300549.post-44246521069377418462017-02-16T08:51:19.663-05:002017-02-16T08:51:19.663-05:00The ability of ships and subs to talk directly to ...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.McChuckhttps://www.blogger.com/profile/10243337792601085456noreply@blogger.comtag:blogger.com,1999:blog-1592992209402300549.post-91262217350672938932017-02-15T21:59:19.555-05:002017-02-15T21:59:19.555-05:00VLF is the only way to penetrate water and rock to...VLF is the only way to penetrate water and rock to any "useful" depths.<br /><br />Hence, the use for talking to subs....drjimhttps://www.blogger.com/profile/05647484115197408897noreply@blogger.comtag:blogger.com,1999:blog-1592992209402300549.post-59680034444678794202017-02-15T20:14:21.379-05:002017-02-15T20:14:21.379-05:00Well, I didn't say it's never necessary to...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. <br />SiGraybeardhttps://www.blogger.com/profile/00280583031339062059noreply@blogger.comtag:blogger.com,1999:blog-1592992209402300549.post-77513572706109715912017-02-15T17:37:35.862-05:002017-02-15T17:37:35.862-05:00Re: "I never, ever, had anyone ask me or anyo...Re: "I never, ever, had anyone ask me or anyone else I was ever around for less transmitter power".<br /><br />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.<br />QAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-1592992209402300549.post-76065453855704645432017-02-15T14:07:29.487-05:002017-02-15T14:07:29.487-05:00I see they are talking transmitters, but what else...I see they are talking transmitters, but what else uses those low frequency bands if all they wanted to do was listen?<br /><br />Using seismographs to detect nuclear detonations, etc.Arthurhttps://www.blogger.com/profile/09289639279717534292noreply@blogger.comtag:blogger.com,1999:blog-1592992209402300549.post-48047481620916271512017-02-15T11:04:21.864-05:002017-02-15T11:04:21.864-05:00"back up navigation if GPS goes down"
Li..."back up navigation if GPS goes down"<br />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" <br />You can tell that is a sore spot.Terryhttps://www.blogger.com/profile/02693579680266086557noreply@blogger.comtag:blogger.com,1999:blog-1592992209402300549.post-17805343935972493512017-02-15T07:32:53.108-05:002017-02-15T07:32:53.108-05:00I noticed that DARPA specifically said they were i...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. <br /><br />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. <br />SiGraybeardhttps://www.blogger.com/profile/00280583031339062059noreply@blogger.comtag:blogger.com,1999:blog-1592992209402300549.post-88378326593953595402017-02-14T23:28:37.357-05:002017-02-14T23:28:37.357-05:00They've been working on this for, oh, a hundre...They've been working on this for, oh, a hundred years now?<br /><br />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.<br /><br />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.drjimhttps://www.blogger.com/profile/05647484115197408897noreply@blogger.com