Wifi (802.11) over coax - Page 1
Forum section: electronics / projects

author: staze
author_rank: Frequent Contributor
timestamp: March 24, 2015, 05:13:45 pm
content: All,Quick question I'm hoping someone can answer relatively easy. I recently bought a wifi connected garage door opener, that sadly is Wifi only (and worse, 2.4ghz only). I've got it working again, but the last couple days, it kept dropping off the network, seemingly because of interference and channel 11. If it was wired, this would be easy, as I have my router literally 2 feet from the device.This got me thinking... I don't use the wireless capability of that router (I have a WAP installed elsewhere in the house)... could I enable wifi, install an SMA connector on the garage door opener, then just run a piece of coax from the router to the opener? I've seen some people running 802.11 over their house RG-6, but not really point to point. Would I have to attenuate the signal at all (wouldn't want to blow out the opener with too strong a signal)? For the unused antenna connections, rather than leave them hanging unpopulated, is it useful (needed) to terminate them?Thanks for any input!

author: mathias
author_rank: Regular Contributor
timestamp: March 24, 2015, 05:34:58 pm
content: It's strange that you experience interfence when the two devices are placed close together.You could run a piece of coax without any problems, but indeed attenuate. Just of the top off my head, try 50dB. I put out 15dBm and receive about -40dBm, i.e. a loss of 55dB, and have good connection.

author: staze
author_rank: Frequent Contributor
timestamp: March 24, 2015, 05:40:23 pm
content: Sorry, I should clarify. Currently the opener gets wifi from my mail WAP, which is elsewhere in the house, I don't have 802.11 turned on on the router at all. The unit receives at -62dB, so it shouldn't be an issue, but the only thing that makes sense is channel 11 being overcrowded in that specific corner of my garage.I can adjust the signal output on the router but only by mW (which, makes sense), so I doubt that would be a whole lot of help without a SA to actually look at the signal strength.okay, I'll hunt for an attenuator. Why they couldn't just put an ethernet jack on the device is beyond me...

author: timb
author_rank: Super Contributor
timestamp: March 25, 2015, 03:17:56 am
content: You can make an attenuator with three resistors. You could put it internal before the SMA jack.The resistors would hook like so:------| | | |== ==The signal goes through the top one and the bottom two connect to ground.Do a Google search for Pi Attenuator and you'll find a calculator for the resistor values.Sent from my Smartphone

author: staze
author_rank: Frequent Contributor
timestamp: March 25, 2015, 04:37:10 am
content: Huh, cool. So does 50, 50, and 25k sound reasonable based onhttp://chemandy.com/calculators/matching-pi-attenuator-calculator.htmfor 60dB?Still makes me wish I had an SA for testing this.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 25, 2015, 04:51:15 am
content: Just use two 50 ohm resistors (assuming your hardware is 50 ohm - some are 75 ohm) and adjust the coupling by adjusting the distance between the two ends.

author: staze
author_rank: Frequent Contributor
timestamp: March 25, 2015, 05:29:31 am
content: Wait, so you're saying use capacitive coupling (so, input 50R to ground, free space (air), 50R to ground, output)?

author: kolbep
author_rank: Frequent Contributor
timestamp: March 25, 2015, 05:56:20 am
content: Just thinking here (maybe I misunderstood)You say the router with disabled wifi is near to the GDO?If that is the case, then why not enable wifi on that router, set it to a random channel number (Eg 4), or some routers can scan and see which channels are quiet.Then if you can, set the Tx power on that router right down.Then just use the normal built-in antennas (or omni, whatever they come with)Because they are nearby to each other, it should drown out other devices (even on the same channel),and the low TX level means that you will not be pumping out too much interference.

author: staze
author_rank: Frequent Contributor
timestamp: March 25, 2015, 06:21:24 am
content: Quote from: kolbep on March 25, 2015, 05:56:20 amYou're not wrong... I just hate running unnecessary wifi. Figured hardwired would just solve the issue... albeit, with modifying the GDO. =P*sigh* And here I wanted to do something largely undocumented. =PI'll give that a shot...

author: kolbep
author_rank: Frequent Contributor
timestamp: March 25, 2015, 06:26:14 am
content: Ok, now I get it.....Btw why would you use a lan connected Gdo???I give you need cred for that, but can't see too many uses, enlighten me

author: staze
author_rank: Frequent Contributor
timestamp: March 25, 2015, 06:31:03 am
content: Quote from: kolbep on March 25, 2015, 06:26:14 amcan open/close from phone. no more need to carry an opener when I go on walks... =)Also, I had a really bad habit (OCD'ish) of leaving the house and thinking "did I shut the garage door?"... now, rather than turn around and drive back a few blocks, I can just pull over and check my phone. =)

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 25, 2015, 06:40:43 am
content: You can also wrap the antennas in black ESD foam to attenuate the signal.

author: Psi
author_rank: Super Contributor
timestamp: March 25, 2015, 07:42:57 am
content: You could build/buy a 2.4ghz directional antenna (patch/helical/yagi) for your wifi AP and point it towards the garage door opener.That would give you quite a significant signal gain.They cost ~$15 from aliexpress/ebay. Just check you get the right connector. SMA vs RPSMA etc..

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: March 25, 2015, 08:45:15 am
content: Or just buy 20m of the worst RG174 money can buy.You will make it hard for hackers by hard wiring your wifi, but visitors will have a problem when you connect them to your network.

author: dom0
author_rank: Super Contributor
timestamp: March 25, 2015, 10:45:02 am
content: He doesn't need the Wifi of the router, because he has another AP.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: March 25, 2015, 10:46:50 am
content: Sorry should read thread before posting.

author: Neganur
author_rank: Unknown
timestamp: March 25, 2015, 02:52:44 pm
content: Quote from: timb on March 25, 2015, 03:17:56 amthis will radiate all over the place, I'd suggest spending the 10$ on inline SMA attenuators likeeBay auction: #320847633150

author: Chris Jones
author_rank: Regular Contributor
timestamp: March 29, 2015, 05:56:32 am
content: Quote from: staze on March 25, 2015, 04:37:10 amNo this will not work. Due to parasitic capacitance, resistors above a few hundred ohms will not behave anything like they should at these frequencies. For example imagine that there is 0.25pF of parasitic capacitance across your resistor. That is -j260 Ohms in parallel with the 25k Ohms. If you want the attenuation to be anything like what you calculate then you need to keep the resistors in the range of maybe 50 to 200 Ohms. This means that you will need to make a 50dB attenuator out of maybe 4 x 12.5dB attenuators in cascade.Also, you probably won't get anything like expected behaviour if you use resistors with wire ends. At 2.45GHz you will want to use surface-mount resistors and microstrip construction, e.g. a 50 Ohm trace is about 2.5mm (0.1") wide on 1.6mm FR4, with the other side of the board being a complete ground plane. Remember that every millimetre of wire has about a nanohenry of inductance, so it has an impedance of j 15 Ohms, neglecting capacitance. Therefore very compact construction would be required. You would also need some edge-launch SMA connectors to get the signal from the cable to the microstrip.You can find SMA attenuators on e-bay occasionally. I would suggest buying some rather than making your own, unless you have access to a VNA (Vector Network Analyser). Whilst you could check the attenuation with just a spectrum analyser, that wouldn't tell you what the input and output match is like, and it could be pretty bad if the constructon is not just right.Chris

author: staze
author_rank: Frequent Contributor
timestamp: March 29, 2015, 02:54:20 pm
content: All makes perfect sense. Was largely looking at those SMA attenuators anyway.Thanks!