From: "Sean H. Breheny" > Hi Scott, > > gunnplexer from shfmicro... Thanks for telling me about them, it's a great > company! Your welcome. I visited the them several years ago, great people, kewl stuff. >...you should be able to get a 1:1 signal to > noise ratio in a typical audio bandwidth (a few kHz) with the 10mW source > and the small (about 5 inch wide) horn when pointed at an object of 1m^2 > radar cross section at 50 feet away. In my circuit, the audio amplifier > produces about the same amount of noise as the gunnplexer ... > I am puzzled as to why you didn't get similar performance. My noise increased significantly when the gunn diode was energized. I should correct myself and say that the of the several units I've used, the alarms used a short approx. 1"x2"x2" horn, with a wide field for area protection. I could trigger at 10 feet and hear targets up to 20'. The 20 GHz thumb-sized units were very dissapointing. not over 5 feet. The gunnplexor with the 6" horn just didn't work as well. You may have gotten a new unit, I got the cheaper 'junk'. Got my alarms at a hamfest. Perhaps as the gunn diodes age, they get weak and noisy. > About the electrons vs. ions, I wonder what the RCS of an Hg ion is at > 10GHz? I have actually seen the figure for a free electron, and it's pretty > darn small :-), although if you have trillions of them(or even more!), it > does add up! Your talking about bulk behavior. The atomic cross section would be in electron volts for photons. And the nuclear cross section in Barns. Many orders in magnitude between the bulk (tube) scale, atomic and nuclear scale. Whipping out my handly "NRL Plasma Formulary" (Do a web search and download a PDF or they'll even mail you a free one. I wish I could feel so greatfull more often!) Electron plasma (Langmuir) frequency * 2 Pi = (4 Pi Ne E^2 / Mi)^1/2 where Ne is electron 'number' (charge) or particle density, E is charge of electron and Mi is mass electron cyclotron (gyro) resonant frequencies * 2 Pi = E B / Mi C where B is magnetic field Similar frequencies for ions, but with ions you multiply the numerators by Z (charge-ionization state) and divide by ion mass. So typicaly your electron frequencies are in GHz and ion frequencies are in MHz, because ions (especialy heavy metals!) are massive and vibrate slowly, and are in charge equilibrium with the electron (1000x lighter). As you increase a magnetic field the gyro and plasma frequencies begin to separate. The gyro resonance is like a microwave magnetron type resonance. As the plasma gets hotter, the electrons vibrate faster and are more mobile and the 'Debye' length (the insulator/conductor transition distance) shortens. Much above the plasma frequencies the waves wont strongly interact, until you get to molecular resonance and atomic photon absorption spectral lines. Then (much) further up, nuclear resonances and energy level transitions. At the plasma (and gyro) frequencies, you can get negative resistance - plasma maser (not to confuse with molecular maser) action. Very similar to the Gunn effect in semiconductors! And there's Landau damping - absorption. Of course magnetic fields are anisotropic, and make a plasma look 'capacitive' across the field lines because the electrons must curve. And as they curve, there path is longer, the frequency lower - more magnetic field, lower frequency and the plasma and gyro frequencies diverge. And there are several types of plasma waves - electron plasma and ion acoustic, Alfven - magnetosonic (the plasma vibrates lengthwise along the magnetic field). There are helical waves, spinors, right & left handed along the field, and of course there are hybrid modes. I've lost a few more hairs 8^( If there are thermal and density (acoustic) variation in the plasma, all kinds of kewl stuff can happen as self-interacting electrodynamic, acoustic and thermal shit happens. I'm fascinated with the idea of firing little plasma bubbles (spheromaks) with a microwave accelerator (waveguide accelerator). A plasma-rifle in the 40 joule range 8^) You could probably research plasma, fusion, and (microwave) accelerators on the web for years. Maybe someone will stick the right sized quartz or alumina tube with some washers or coils on it through there microwave oven, and some really kewl stuff shoot out at a few MeV. Or perhaps fire a rocket/jet engine down a weakly oscillating waveguide with plasma in it, and see the field significantly amplified (are HPM's ag'in the Geneva convention? What a great hunting weapon! A gun thats got no bang to scare off prey, no recoil, leaves no environmentaly unsafe lead, no bloody mess, sterilizes and cooks yer bird on site too!). Matlab must be good for something? > I saw an article from a car magazine where they tested various police radar > countermeasures. They said that some of them contained several fluorescent > tubes with wire wrapped around them (like you said) and they had no effect. > This is probably a case of something that would work if you made it very > carefully and energized the tube properly but has little chance of working > if you just throw it together in a plastic case. Maybe its snake oil, or maybe they didn't buy ads in the magazine. Traveling wave tubes are non-trivial devices. I'd just as soon try cooking a semiconductor, cuz its black art. But as far as a plasma intermodulating, any plasma heated with RF should intermodulate AFAIK. I remember a fox hunt I was on, where I used a switched dipole antenna. Just a diode (1N914/4148) switching between a couple 2 meter 1/2 wave dipoles. As a guy using it moved near the receiver, we could hear the switching frequency in a different receiver as the antenna's intermodulated signals . So re-radiating a signal (repeat-back jamming) isn't tough to spoof a dumb digital toy, but designing a negative resistance electron or plasma TWT is rocket science. I heard a can of rusty nails (iron oxide semiconductor) could cause a cell site to drop calls. But there your transmitting hundreds of watts, recieveing microwatts, and dropping calls if the SAT tone is messed up. That's the idea behind passive RFID transponders. Put a diode on a tuned circuit, the diode switched with a very low current micropower PIC, and interrogate it with a radar beam. Suppose any of those food packages we dropped over Afghanistan had similar tags for pilfering looters to signal those Specter Gunships? My I do ramble. Scott -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.