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MFJ-868 SWR/Wattmeter
On Thu, 08 Sep 2011 14:12:20 -0500, John S
wrote: On 9/8/2011 1:30 PM, Jim Lux wrote: And, you can take ESD hits that degrade, but don't destroy, the part. I've got some LNAs in my desk drawer that appear to work just fine, except the NF is a bit higher than it used to be. How do you know it was ESD that caused the degradation? "How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?" Sir Arthur Conan Doyle, The Sign of the Four, chap. 6. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
MFJ-868 SWR/Wattmeter
In article , John S wrote:
I think I blew up my first piece of test gear at the age of 14 or so... tried to measure line voltage, with the VOM control still set on "ohms". POP! Fortunately it was just one resistor... I eventually replaced it, and I still have that old 1000 ohm/volt VOM (a 1965-vintage Lafayette) sitting in a drawer somewhere. So, you only did it *once* in all this time? Every few years I do the same thing, except the fuse catches my error. Oh, no, by no means - that was just my *first* time. As in your case, the fuses tend to save most of my VOM/DVM foulups these days. Reminds me, I've gotta go grab some 250 mA mini-fuses to fix one of my meters and restock my oops box. To paraphrase Foghorn Leghorn, "Fortunately, I always keep a spare set of feathers in my locker, for just such an occasion." -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
MFJ-868 SWR/Wattmeter
On 9/8/2011 12:27 PM, Jeff Liebermann wrote:
On Thu, 08 Sep 2011 14:12:20 -0500, John wrote: On 9/8/2011 1:30 PM, Jim Lux wrote: And, you can take ESD hits that degrade, but don't destroy, the part. I've got some LNAs in my desk drawer that appear to work just fine, except the NF is a bit higher than it used to be. How do you know it was ESD that caused the degradation? Oh.. I don't know on the ones in the drawer that's what did it (because they still work, after a fashion), but on some other ones where we did failure analysis, you can see the damage on the SEM photos. We've got whole books full of pictures of this kind of thing. That's partly why industry has gone from marking just sensitive things to marking everything ESD, including nuts and bolts. It's just too easy to get damage without any obvious signs. Consistent practices reduce the possibility, which increases yield. The concern is the "weakening" or "partial damage without causing outright failure" kind of scenario, especially in a hi-rel situation. There's a lot of interest in figuring out a way to test for this kind of thing, as you can imagine. And there's even evidence that the damage can heal (anneal), much like radiation damage (both are basically deposition of energy in the wrong place and the wrong time, right), but the mechanics of the process is poorly understood. http://www.fainstruments.com/PDF/eosesd93.pdf http://eed.gsfc.nasa.gov/562/ESD_Failures.htm http://trs-new.jpl.nasa.gov/dspace/b.../1/00-1338.pdf |
MFJ-868 SWR/Wattmeter
On 9/8/2011 12:24 PM, Dave Platt wrote:
In , Jim wrote: Well.. Consider a 20 meter long wire hanging 10 meters above the ground (half a 80m dipole) so now you're talking tens of pA, 100 Meg isolation isn't hard to get with clean insulators. That 20 m wire is like a 850 pF capacitor. If assume, say, 25 pA charging current, you get a volt every 42 seconds. After half an hour or so, you're up to 40 volts or so.. Granted it's not a lot of Joules.. (heck, probably not even a microjoule).. So fair weather charging isn't likely to kill your MFJ.. Back to the dust/rain/induction charging, which can certainly get the levels needed. I've observed clear-air charge on a TV antenna (about 30' above ground level) which was sufficient to create a visible spark when I touched the center connector of the coax to a grounded static-drain block. No rain at the time, no appreciable amount of dust (SF Bay area)... just clear air with a mild breeze. There could actually be a fair amount of particulates without you being aware of it. I used to work for a company that made things that created fine particles in air and we tried to remove the particles as well (often using electrostatic techniques). For very small particles, they don't scatter the light very well so you don't see them. Ultimately, you get small enough: molecule sized and you get blue sky, but even fairly large particles (on the order of a wavelength of light in size) are basically invisible as such, but do increase the attenuation, and do scatter some light so you get decreasing visual contrast at long distances (called atmospheric perspective in the computer graphics biz) One way that cinematographers can make a room seem bigger is by putting a very small amount of haze into the room, because that makes distant things less contrasty, which your brain turns into "farther away". Some numbers: At 1 mg/cubic meter of 1 micron diameter particles, you won't see any noticeable haziness. That's about 2E15 particles per cubic meter, and each one can have a max charge of about 1E-13 coulomb. So you can potentially move 200 coulombs with a cubic meter of air. That gentle zephyr blowing by at 1 m/sec can move a lot of cubic meters in a fairly short time. Say you have a 1cm diameter antenna element.. it will intercept 0.01 cubic meters/sec of that breeze.. 2 Coulombs/sec - 2 Amperes... In reality, of course, the particles won't have that much charge (they'll repel each other, for one thing), and not all of the particles will touch your antenna, especially once it starts to charge, and so forth. When I was working with a Vollrath Electrostatic Charging scheme (like a Van de Graaff generator using dust instead of a belt) the best kind of charging current I got was 10-100 microamps with dust flowing in a 4" diameter pipe at several meters/sec. But, the real thing is that you can get particulate charging in what seems to be perfectly clear air. Compressed air system run into this kind of thing... you can have fine oil particles from the compressor, or condensed water particles from a sudden expansion of the compressed air, both of which might be invisible, but which can build up a surprising charge. |
MFJ-868 SWR/Wattmeter
On 9/8/2011 4:18 PM, Jim Lux wrote:
On 9/8/2011 12:27 PM, Jeff Liebermann wrote: On Thu, 08 Sep 2011 14:12:20 -0500, John wrote: On 9/8/2011 1:30 PM, Jim Lux wrote: And, you can take ESD hits that degrade, but don't destroy, the part. I've got some LNAs in my desk drawer that appear to work just fine, except the NF is a bit higher than it used to be. How do you know it was ESD that caused the degradation? Oh.. I don't know on the ones in the drawer that's what did it (because they still work, after a fashion) That's the reason I asked. Thank you. |
MFJ-868 SWR/Wattmeter
On 9/8/2011 4:12 PM, John S wrote:
On 9/8/2011 4:18 PM, Jim Lux wrote: On 9/8/2011 12:27 PM, Jeff Liebermann wrote: On Thu, 08 Sep 2011 14:12:20 -0500, John wrote: On 9/8/2011 1:30 PM, Jim Lux wrote: And, you can take ESD hits that degrade, but don't destroy, the part. I've got some LNAs in my desk drawer that appear to work just fine, except the NF is a bit higher than it used to be. How do you know it was ESD that caused the degradation? Oh.. I don't know on the ones in the drawer that's what did it (because they still work, after a fashion) That's the reason I asked. Thank you. Yeah.. the guy or gal who figures out a good non-destructive, no disassembly required way to detect latent ESD damage is going to be a hero for those of us building one-off high-rel gear. There have been some partial solutions for specific parts, but no generalized way, yet. |
MFJ-868 SWR/Wattmeter
Dave Platt wrote:
In article , Jim Lux wrote: Well.. Consider a 20 meter long wire hanging 10 meters above the ground (half a 80m dipole) so now you're talking tens of pA, 100 Meg isolation isn't hard to get with clean insulators. That 20 m wire is like a 850 pF capacitor. If assume, say, 25 pA charging current, you get a volt every 42 seconds. After half an hour or so, you're up to 40 volts or so.. Granted it's not a lot of Joules.. (heck, probably not even a microjoule).. So fair weather charging isn't likely to kill your MFJ.. Back to the dust/rain/induction charging, which can certainly get the levels needed. I've observed clear-air charge on a TV antenna (about 30' above ground level) which was sufficient to create a visible spark when I touched the center connector of the coax to a grounded static-drain block. No rain at the time, no appreciable amount of dust (SF Bay area)... just clear air with a mild breeze. Were there any elevators nearby? |
MFJ-868 SWR/Wattmeter
Jim Lux wrote:
On 9/7/2011 11:43 AM, dave wrote: Jeff Liebermann wrote: ohms-guesser). It therefore seems possible that the owner had transmitted into the instrument (even though he denies it). Why would anyone connect that antenna analyzer to a transmitter (unless using the BNC?) Oh.. that's an easy mistake to make. Bunch of coax with connectors lying on the floor, hook up the wrong one. Hook a switch up incorrectly. Coax switch without enough isolation (100W transmitter with a 20dB isolation switch is a watt into the analyzer..) Anyone who has never accidentally cooked a piece of test equipment just hasn't been working long enough. I remember frying a scope once measuring audio from a distant studio that was on the opposite mains leg. It was a 50 cents a pound government surplus scope, not my 465. I would never leave an unterminated transmission line connected to my transmitter. You always want 50 Ohms on that line when connected to your XMTR. I also would never "guess", (which is a kind of "assume"), unless I really had to. I know I learned this the hard way. |
MFJ-868 SWR/Wattmeter
"Jeff Liebermann" wrote in message ... snip I carry a neon lamp line tester in my tool box. It's useful for checking AC power, but also for detecting high voltages on antenna terminals. It's amazing what I find at broadcast sites. Yup. My work on USN ships showed me some of that. Within a few feet of any HF transmit antennas, what they call "standing rigging," things that are mounted or attached topside, can be energized with hundreds of volts. The Navy imposes RF RADHAZ and RF BURN zones in selcted topside areas. They're always marked and sometimes cordoned off with physical barriers. |
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