complex high power RF problems ? - Page 1
Forum section: electronics / rf-microwave

author: coppercone2
author_rank: Super Contributor
timestamp: August 09, 2020, 05:43:45 am
content: I just unclogged a toilet and it made me feel pretty crazy (no idea how the hell I did it with the snake, i was on the bathroom floor for like 2 hours messing with it, thank god it got clogged from tomato scraps!), and I am wondering if anyone has any stories about waveguides/hardline in big installations giving people difficulties and technicians getting mad etc. I imagine once things get big/compliated enough, it might go a little nutty.The only one I recall reading is one where there was a hardline / antenna filled with water and it was making a ham club go crazy over-powering the thing like 5 times before they realized its resistive loss. (apparantly it turned into some crazy argument with people deciding to build ultra high power amplifiers to 'overcome' the problem, which was heliax filled with water. IIRC it made someone quit because they decided it was run by morons. The crux being, that the transmitter was a waste of money, it could not be used legally when the thing was fixed anyway, I think.This is meant to be a fun thread.

author: radiolistener
author_rank: Super Contributor
timestamp: August 09, 2020, 05:55:59 am
content: I heard a story about how someone looked into the waveguide of a working transmitter and went blind for a week. I don't know if this is a fable or trueAlso, there is a known story of accident onU-70 (proton synchrotron). In 1978, Anatoli Bugorski stuck his head into the synchrotron to check a piece of malfunctioning equipment, when the safety mechanism failed and he got hit in the head with a proton beam. He reportedly saw "a flash brighter than a thousand suns", although no pain was reported.He got proton beam of energy 70 GeV with cross section about 2 x 3 mm. The radiation dose at the entrance was 200'000 Roentgen, at the exit - 300'000 Roentgen. He still alive.Here is his story in russian:https://www.liveinternet.ru/users/rewiever/post132062905

author: vk6zgo
author_rank: Super Contributor
timestamp: August 11, 2020, 02:58:08 am
content: Quote from: coppercone2 on August 09, 2020, 05:43:45 amUnlike "dunny" plumbing, waveguide & coax feeder paths are determined so as to facilitate work on them.If coax is damaged, it may need replacement, which will obviously be expensive, but people are set up to do such work with the "minimum of fuss".Waveguides are often used in smaller lengths than coax, so a really badly damaged section may be replaceable by simply disconnecting the flanges & any support brackets, & slot a new piece in.Water ingress to waveguides is less damaging than into coax, as the waveguide can be dismantled, the water blown out, & the guide interior dried and/or cleaned.Commercial installations stay in service for decades without trouble.Large diameter cables are usually pressurised with dry air from a "dehydrator", as are waveguides.The hams in your story were idiots, as using high power to overcome greatly increased losses in the feeder is not going to be effective, nor will it "boil" the water out.it would probably be more cost effective to temporarily replace the coax with some cheaper, lossier cable, live with that known loss or make amodestpower increase to compensate.

author: coppercone2
author_rank: Super Contributor
timestamp: August 11, 2020, 06:13:35 am
content: sounds properly staffed, planed, financed and some how not modified to make more money or extend capabilitieswhere is this magic place? Have trouble believing this exists, with all the scandals that occur in even the best facilities (LHC)i.e.https://home.cern/news/press-release/cern/cern-releases-analysis-lhc-incident

author: coppercone2
author_rank: Super Contributor
timestamp: August 12, 2020, 06:42:37 am
content: fort knox

author: WPXS472
author_rank: Regular Contributor
timestamp: September 04, 2020, 02:25:38 pm
content: Assuming this is supposed to be funny, here is one from my long gone Army days. While stationed in Germany during the cold war, I heard the following tale regarding high power tropo microwave equipment. This equipment operated at around 5 GHz according to the story, and at some pretty impressive power levels. One day, while setting up a shot, the operators decided to have lunch, and they wanted to heat up some salami for sandwiches, so they stuck a couple of pieces of it inside the rather large waveguide sticking through the wall of the shack unterminated, and hit the transmit switch for about 10 seconds, thinking they were simulating a microwave oven. When they went to recover the salami from the waveguide, it had been vaporized by the high power RF. Probably an urban legend, but amusing to a young man in his early twenties.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 05, 2020, 03:05:21 am
content: Quote from: WPXS472 on September 04, 2020, 02:25:38 pmI really think the transmitter would have shut down automatically due to the open waveguide, the Boss would have "torn the perpetrators a new one", & normal service resumed.A better idea would have been to find some external (non active) part of the transmitter which got fairly warm to gently heat the salami. (at one of my old jobs, we found that up against the air cooling output duct warmed pies up nicely)

author: coppercone2
author_rank: Super Contributor
timestamp: September 05, 2020, 04:42:04 am
content: its military, maybe they hoped it would still transmit something after the radome is blown up by a missileI don't think they can afford to have a dent or ding shunt the thing down when it was stationed some where that it might have to deal with a jet carrying nuclear weapons. Boss might expect that one to be fixed with a brick layers hammer in the german winter night, it also makes it harder to sabotage since you can't trip safety systems with minor stuff (they might have been worried about the stasi)I imagine tensions would immediately run high if microwaves stopped being radiated, even in a poor manner, on some base on the front line of russia during the cold war (kind of like when the lights go out in the movie 'pitch black'. Someone might like that opportunity.. the espionage equipment there might just to look if its turned on and cannot analyze if its actually doing something (small low power hidden box). )

author: vk6zgo
author_rank: Super Contributor
timestamp: September 05, 2020, 06:27:00 am
content: Quote from: coppercone2 on September 05, 2020, 04:42:04 amAll that is why systems have redundancy.----- that one trips off, the other one stays up.

author: coppercone2
author_rank: Super Contributor
timestamp: September 05, 2020, 06:46:35 am
content: low budget?

author: vk6zgo
author_rank: Super Contributor
timestamp: September 05, 2020, 11:59:38 am
content: Quote from: coppercone2 on September 05, 2020, 06:46:35 amMilitary?

author: coppercone2
author_rank: Super Contributor
timestamp: September 05, 2020, 10:10:43 pm
content: they never get what they actually want, congress is always pulling their plans in. I am pretty sure the south west would look like a hedgehog with all the missiles and radars sticking out of it. And their fleets are always being reduced or consolidated (multi-role things that are worse). I.e. b2 bombers, they wanted 120, they got 20? Congress is like a samurai

author: LaserSteve
author_rank: Super Contributor
timestamp: September 05, 2020, 10:18:29 pm
content: Local USA story. UHF TV transmission site with 700 foot tower and multiple large beacon lights type L-864. Site is out in the country, and local boys decide to have a contest to shoot out the upper lights, using long rifles.. They hit the waveguide just right with several shots and open a large hole with a flap of metal pointing inside the guide.. VSWR goes to hell and a handbasket, there is a pressurization fault, and automated site sends a warning. A former friend gets the call to TDR the waveguide using a really fancy VNA with fault finding. Before he gets out to the site, station tech overides the alarm and puts the TX back on the air.A rainstorm happens before our hero gets there, and IOT tube finds itself having a wet coupling section. Arcing happens...Now if you believe the consultant RF "Engineer", he packs the damaged IOT waveguide junction with conductive silver epoxy FedExed in and saves the day... As PBS station can't afford to have a new IOT tube installed. Your mileage may very, but I'm wondering why there was not a window or drain trap in the waveguideI think part of it is a TALL tale, but I do know the locals like challenges like shooting at lights.That said,I did know a former US Army Radar operator who got exposed and shocked while working on the system, when an interlock was over-ridden.He spent a lot of his day in a wheelchair. RIP old man...Steve

author: coppercone2
author_rank: Super Contributor
timestamp: September 05, 2020, 10:22:21 pm
content: in regards to non restricted military spending, consider albanian defense spending. Run as a paranoid dictatorship, they spent something around 25% of their yearly earnings for 20 years building bunkers.https://en.wikipedia.org/wiki/Bunkers_in_Albania

author: coppercone2
author_rank: Super Contributor
timestamp: September 05, 2020, 10:24:13 pm
content: Quote from: LaserSteve on September 05, 2020, 10:18:29 pmdid it work for a short while before the rain? The power company does this too by the way, if something is tripped, they will reactivate it 2-3 times to try to 'burn away' small problems, before they finally disable the system, in order to keep the grid up time good (why you don't go near downed wires even when there is no power. Seems irresponsible to keep a public TV system running unless.. they had to get a weather advisory out.. but not worth a man being injured.I usually count, if there is a single small power interruption, I ignore it, if there are two in a short period of time, I start positioning flashlights and getting generator ready to go.

author: LaserSteve
author_rank: Super Contributor
timestamp: September 05, 2020, 10:32:03 pm
content: AT 460 KW ERP, I really question the silver epoxy working at 50 kW OF RF. But I do believe he did the TDR>..Steve

author: coppercone2
author_rank: Super Contributor
timestamp: September 06, 2020, 12:40:28 am
content: I glued a center pin with silver epoxy to fix a circulator with a dented connector and it read dead shorthttps://www.mgchemicals.com/products/adhesives/electrically-conductive-adhesives/two-part-epoxy/silver-conductive-epoxy-moderate-cure-extreme-conductivity-8330silver epoxy 0.001 Ω·cmSolder : 0.000015 Ω·cm66 times less conductive.If someone knew the resistance of a 50kW joint, we can see the dissipation of what happens when you remake the joint with epoxy. I think it might be possible because if its a copper waveguide its a giant heat sink so the connection can get hot.But permatex sayshttps://www.permabond.com/resource-center/electrically-conductive-adhesives/If filled with silver, volume resistivity can be as low as 0.0001 Ohms∙cm.So, I guess if you get a good permatex epoxy, its only 6.5 times worse then solder.