Static and lightning protection of antenna connected to PC - Page 1
Forum section: electronics / projects

author: Hobgoblin612
author_rank: Contributor
timestamp: January 19, 2016, 12:38:58 pm
content: Hi all,I am building a wireless sensor network and I need a base station/ collector node. The antenna and RX/uC needs to be mounted outside on on a high tower (An old hot water tank stand. Metal and about 8m tall) and have a serial connection to a PC inside the house.My current plan is to use RS485 over Cat5 cable and use the remaining 3 pairs to send 5v from the PC to power it like so:I want to make sure that the PC is protected from nearby lightning strikes and general static from the antenna/Cat5 cable.Is there anything I can do apart from grounding the antenna?I have considered using isolated RS485 converters and an isolated 5v DC-DC module to galvanicly isolate the whole thing but I'm not sure this would do much at the extremely high voltages involved?Thanks, Timothy.

author: rrinker
author_rank: Super Contributor
timestamp: January 19, 2016, 01:44:03 pm
content: Do use isolated converters regardless of what other protection is applied to the antenna. That will protect you from low level transients that wouldn't necessarily be caught by the high power protection at the antenna, and also avoid potential ground loop problems.

author: Marco
author_rank: Super Contributor
timestamp: January 19, 2016, 01:48:08 pm
content: How about using POF for the data and putting a bunch of hipot isolated DC/DC modules in series for the power?

author: AF6LJ
author_rank: Unknown
timestamp: January 19, 2016, 02:06:05 pm
content: The easiest way to protect your receiver front end from damage from static discharge and nearby lightning strikes is to use an antenna that presents a DC short at the connector.Protecting against direct lightning hits gets really expensive.

author: Unknown
author_rank: Unknown
timestamp: January 19, 2016, 03:28:52 pm
content: Not much you can do to protect from a direct or even close strike, but you could use isolators to protect from general static and such. I would also power the receiver from a seperate supply (like a 5v phone charger) rather than your PC which eliminates that path.

author: Hobgoblin612
author_rank: Contributor
timestamp: January 20, 2016, 12:09:31 pm
content: Thanks for your suggestions guys,I think I will use an isolated driver and a separate wall wart. If I use a double insulated power supply do I still need a DC-DC as well?Thanks rrinker, I had not thought of ground loop problems.

author: Jeroen3
author_rank: Super Contributor
timestamp: January 20, 2016, 01:20:08 pm
content: Quote from: Z80 on January 19, 2016, 03:28:52 pmFor starters, make sure your signal wire is not the lowest resistance path to earth.Then you could at least add spark gaps. Maybe common mode surge filtering on the supply side.

author: dcarr
author_rank: Regular Contributor
timestamp: January 21, 2016, 06:11:50 pm
content: There are relatively low cost RS-485 to fiber converters. These would provide excellent protection for your PC and then you only have to worry about power protection which can be a much easier problem.Here's one example:http://www.ebay.com/itm/Honeywell-Northern-Computers-Access-FC485-Fiber-Isolated-RS-485-Converter-/301504113527?hash=item46330bab77:g:1jcAAOSwk5FUxR4mDavid

author: _Andrew_
author_rank: Regular Contributor
timestamp: January 21, 2016, 06:50:28 pm
content: Take a look at ametur / ham radio resources there is lots of useful information to be foundhttp://www.arrl.org/lightning-protectionwww.bwcelectronics.com/articles/WP30A190.pdfwww.novaris.com.au/uploads/1/Principles_of_Lightning_1.pdfYou should also check the regulations in the country you are in as there maybe specific requirements you will have to comply to.

author: KJDS
author_rank: Super Contributor
timestamp: January 21, 2016, 06:54:42 pm
content: POut a short circuit quarter wave line on the antenna feed as a primary arrester. Add in spark gaps and GDTs to cope with the secondary level that comes out of the short circuit.

author: HAL-42b
author_rank: Frequent Contributor
timestamp: January 21, 2016, 07:03:38 pm
content: +1 on asking the HAMsSeems they already have material on the topic.

author: bcbeck96
author_rank: Newbie
timestamp: January 22, 2016, 07:33:59 pm
content: I would also look at isolating the actual USB with an off the shelf, quality, USB optoisolator. I have had one on a scanner which was hit by lightning, and my computer lives on.

author: Ian.M
author_rank: Super Contributor
timestamp: January 22, 2016, 07:42:12 pm
content: It might be worth making the base station solar powered and using WiFi or fibre back to the PC. Done right, that might even survive a strike to a lightning rod on the tower, and 40m of separation means your PC would be no worse off than if the tower was struck now. If the tower has any remaining metal piping running back to the house, that should be ripped out as it could direct a large ground current into the house earthing system if the tower was struck.

author: rsee
author_rank: Contributor
timestamp: January 23, 2016, 09:15:00 am
content: Quote from: Ian.M on January 22, 2016, 07:42:12 pmIt'd probably be pretty cheap to use a raspberry pi for that part of it.