RF receiver in metal enclosure - total no-go or possible? - Page 1
Forum section: electronics / projects

author: Corporate666
author_rank: Unknown
timestamp: June 13, 2015, 11:07:08 pm
content: I don't have a lot of experience or knowledge about RF stuff.I am working on a project that would utilize a keyfob remote control at 315Mhz. I want to use a small Takachi brand enclosure, the MX series is the perfect size, but it is all aluminum. Some questions...-If the keyfob receiver is entirely inside the enclosure, will I effectively get no signal at all, or just a reduced range? I've heard it's a total no-go and some metal case manufacturers claim RF works inside the case.-Would drilling a hole in the enclosure and threading the coiled-wire antenna through it work or would the antenna wire passing through the enclosure wall render it ineffective?-One possible solution would be to cut an opening in the enclosure but based on the recommended dimensions and wavelength of 315Mhz signals, it doesn't seem it would work.-Another possible solution would be to eliminate one of the metal end caps of the enclosure and replace it with a similarly sized piece of plastic. Would that be effective at all or is it a waste of time?-Final solution is obviously putting an antenna connector on the outside of the case and using an external antenna, but I'm trying to avoid that hassle and expense.FWIW, the range of the keyfob and receiver combination is probably realistically 100 meters in real-world use. I'd like to achieve 20 meters or more, ideally.Thoughts?

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: June 13, 2015, 11:31:05 pm
content: Quotewhat size dimensions?Well they make some phones with an aluminium case, but obviously plastic is better RF wise. So it's not completely no go.Some RF will get through I am sure, if there is a hole, gap or even irregularities in the box. There can also be a small gap in the seal.But what you want to avoid are multipathing effects, which can cancel the signal entirely.Can you have a diversity antenna at the other end? That could help a lot.But to restate the obvious for RF plastic will be much better.

author: Richard Crowley
author_rank: Super Contributor
timestamp: June 13, 2015, 11:51:42 pm
content: A perfect metal enclosure with no holes will BLOCK 100% of the RF signal.An imperfect metal enclosure with a few casual holes will only block MOST of the RF signal.Unless you are only going for a range of 1~2m I would strongly advise AGAINST using a metal box.Yes there are some cell phones with metal cases (like my one HTC One) but remember that the entire FACE of the phone is glass or plastic and allows the antenna(s) to communicate with the rest of the world.There are quite nice plastic boxes in similar sizes and shapes that would work quite well with RF.

author: wraper
author_rank: Unknown
timestamp: June 14, 2015, 12:01:55 am
content: Quote from: Richard Crowley on June 13, 2015, 11:51:42 pmUsually there is some insulated part on the backside too. Or enclosure is antenna itself like in iphones (you can notice insulated metal parts in the enclosure). As of entire face being glass, not really. Display itself always has some metal shield on the backside. Therefore only top and bottom can be open for RF on the front side.

author: wraper
author_rank: Unknown
timestamp: June 14, 2015, 12:05:10 am
content: These metal parts are insulated not because it is pretty but because of being part of antenna.

author: Corporate666
author_rank: Unknown
timestamp: June 14, 2015, 01:05:07 am
content: Thanks for the replies thus far... to answer some questions...As for the dimensions, I am looking at the MX-2-7 or MX-2-8 size, so we're talking about 70-80mm wide, 80-90mm long and 20-25mm high. Recommendations for antenna cutouts I've seen are a rectangular recess of 1/2 wavelength long and no more than 10% of the wavelength wide. For 315Mhz, it's close to 1-meter wavelength, so such a cutout would be larger than the whole case.As for using a plastic case, I realize that eliminates the RF issue, but it introduces several other more difficult issues, so I'm focused on making this work with the MX case rather than trying to use a plastic case. The end panels on the case can be aluminum or plastic... any idea how much effect there would be in using a plastic end cap on one end of the case? The antenna could be directly behind the end cap... but I have no idea how effective that would be.Of course I can buy a bunch of cases and test all this, which I will have to do before committing to the final design, but was just looking to get a general idea of if these solutions might work or if I should just assume I will need an external antenna.

author: Circlotron
author_rank: Super Contributor
timestamp: June 14, 2015, 01:38:27 am
content: Using a metal box should be just fine if you poke your antenna wire through a hole in the side of the box but don't lie the antenna wire flat along a metal surface, have it in free air if it all possible.

author: Dragon88
author_rank: Regular Contributor
timestamp: June 14, 2015, 01:50:43 am
content: A slit in the metal enclosure may allow you to get enough signal through, but watch out for directionality. It will not perform equally well 360 degrees around the case if the only egress is a slit on one side. It's impossible to predict the performance more specifically without seeing your assembled device and the antenna placement.There are plenty of options, including mounting an RF connector on the outside of the box. A little SMA panel mount connector would allow you to hook up a good antenna and significantly increase the performance of your project. However, since those key fob devices cost a few dollars themselves, I'm guessing you want to keep it cheap. Just run a new wire antenna through a grommet or something and replace the crummy coil antenna. A simple 1/4 wavelength monopole should do the trick. Make sure to run it straight up from the enclosure. The metal enclosure will act as the ground plane.

author: bitwelder
author_rank: Super Contributor
timestamp: June 14, 2015, 06:14:42 am
content: Looking at the images returned by google for Takachi MX2 boxes, it seems they have front and back panels in ABS.If that's correct there should be ample space for RF to escape the box, and things can be probably optimized by orienting the internal antenna and/or the PCB in the proper way.

author: dom0
author_rank: Super Contributor
timestamp: June 14, 2015, 08:10:58 am
content: Quote from: wraper on June 14, 2015, 12:01:55 amTFT screens would be acceptable shields, if they wouldn't make a ton of noise themselves...