2.4GHz Antenna Design? (For BT Speaker) - Page 1
Forum section: electronics / rf-microwave

author: ssashton
author_rank: Frequent Contributor
timestamp: November 24, 2018, 02:05:36 pm
content: Hello Guys and Gals!I have designed a loudspeaker that uses a CSR8645 based bluetooth audio receiver module.>Datasheet Here<I find the reception is not that great so would like to integrate a better antenna.I have tried simply soldering a bit of longer wire on to the existing PCB trace antenna and it does help, but I'm hoping to do better.Does bluetooth have the same requirements as a 2.4GHz wi-fi antenna given the same operating frequency?I've seen antenna on a PCB like thisDo you think this would work for me, are they directional? I know zero about antenna design so some help to understand what is going on there would be appreciated! I could possibly build an antenna like that on to my own PCB.Also I'd love to know what this part is?!

author: cdev
author_rank: Super Contributor
timestamp: November 24, 2018, 02:47:39 pm
content: The zig zag structure on the left - that is an antenna, and the chip your arrow is pointing to - what pin on the device is it connected to? That will give you the best idea of what it is.The pictured antenna on the top is a size reduced broadband monopole, its polarized vertically if that board is facing such that the pigtail, with its u.fl connector goes downward. Its vertical is the direction to the left. That should be facing upward. Then it will be omnidirectional with the strongest signal at the horizon and possibly above, which is likely what you want, Otherwise it would be horizontally polarized and directional off of the two broad sides.The chip pointed to by your arrow? Maybe its for RF, or quite possibly it is not. Is it in the signal path for the device its next to? Its datasheet will likely tell you what is likely connected to that pin.You can build your own antennas into your design - or build in a connector and make or buy them.Yes, the frequencies are the same for 2.4 GHz wifi so you could use any wifi antenna.Or use a piece of straight wire extending up into free space 1/4 wavelength long. That and a ground plane for the RF ground would be a good starting point.Best to know a bit about antennas so you can ascertain what kind of antenna would work best in your applications. The wikipedia page on antennas is a pretty good start.https://en.wikipedia.org/wiki/AntennaOne thing to keep in mind is that the loss in a thin coaxial cable such as a pigtail at 2.4 GHz is going to be significant for any long transmission line.That why they tend to be short.Another thing you should consider is - If your receiver is a device which may have some RF noise associated with it (possibly from its power supply, if it uses dc-dc conversion) or similar setups. If so using an antenna thats physically separated from it a bit may improve reception.You can easily buy add on antennas with pigtails (like the ones in your picture) but to use them you will need to add an antenna connector to your design and possibly match it to your output appropriately. Two kinds of antenna connectors are the most popular / best choices at 2.4 GHz devices, u.fl (which you pictured) and SMA.u.fl connectors cant be repeatedly plugged in or unplugged and remain viable, figure they can handle maybe at the most 20 plugging/unplugging cycles, so only use them if your antenna is unlikely to be changed often.Use an SMA if you plan on using different antennas and changing them as needed.2.4 GHz antennas are small and generally fairly easy to make, and there are a lot of different plans you can build that work well online. For nearby you likely want anomnidirectionalantenna. But whatpolarizationwill you need? A verticalquarter wave monopoleat 2.4 GHz would be one option and very simple, it could be as simple as a wire that extendsupfrom a PCB. But what if you dont know how the PCB will be used? (which way the user will face up?)Probably the most popular option is similar to what you showed, amicrostrip antennawith what is called ameander linefor size reduction. You could also wind your antenna into a helical shape to reduce its size a little bit.There is a lot of information on the web to get you started. Many books on subjects like microstrip antennas can be found online. The complete design process can be found fully explained in the form of theses from students seeking their degrees. For example, I just found "Small Antennas Design for 2.4 GHz Applications" by Ibrahim Turki Nassar which shows the math.Or you can put an antenna connector on your board and experiment. Thats what I would suggest if you want to learn about antennas.

author: EVS
author_rank: Contributor
timestamp: November 24, 2018, 03:00:01 pm
content: Quote from: ssashton on November 24, 2018, 02:05:36 pmBPF 2.4GHz BF1608-L2R4DAA

author: ssashton
author_rank: Frequent Contributor
timestamp: November 24, 2018, 03:08:41 pm
content: Oh, thank you cdev!The SMD part I arrowed is connected directly to the antenna trace via what seems to be a 0ohm link. There is another pad for a 0ohm link that would connect the mystery component to one of the breakout castellation presumably for an external antenna. It looks like the part they call 'U2' in the datasheet but say nothing further about.EDIT: Ahh okay its a band pass filter! Presumably it needs to see a very specific impedance load to work correctly, then? Again I know zilch about RF but do understand audio filters.I do know the PCB orientation - it will be kind of half-space towards the front of the speaker because very few people will try to listen from behind the speakers (and the back panel is grounded metal). This image will show it I think. BT module is hidden in the top left corner of the main rear PCB.

author: cdev
author_rank: Super Contributor
timestamp: November 24, 2018, 03:14:11 pm
content: Looks nice! Keep in mind though, that the metal ground on the back may block signals coming from the back from getting to an antenna if its placed somewhere in front of it (and possibly increase the signals coming from the front, acting like a reflector). OTOH, if you place your antenna near the top facking upward and use the metal on the bottom as part of your ground plane, you likely will get good reception from all directions. You would need to reduce its size a bit so that the metal does not cover the entire back, leaving some room at the top for the antenna to rise above the metal. You can ignore the non-metallic portions of the case as long as the antenna isnt attached directly to something hard. Use some foam tape or similar to separate the two. Just a tiny bit of distance should be enough to prevent the antenna from being detuned by the wood-like material. The antenna can be on the inside but at 2.4 GHz might work significantly better (at least 3-5 db!) if somehow you managed to put it on theoutsideof the wood box.

author: ssashton
author_rank: Frequent Contributor
timestamp: November 24, 2018, 03:19:34 pm
content: Yes I have found the metal panel improved the BT reception a bit.I'll try to read more but so far I already have a question - The BT module (and the CSR8645 IC) only has a single input pin for the antenna. Does that mean I can only use a monopole, or can I just connect one phase of a dipole to ground and still get benfits?

author: cdev
author_rank: Super Contributor
timestamp: November 24, 2018, 03:22:04 pm
content: The two are the same thing. As long as the antenna wire can go up (you can just use a wire of the right length, which is around 1.9 inches, which you can bend in an L-shape if you need to- or wind in a helix, just not too tightly, the more it goes up the better it should perform.) and the ground plane of the circuit which is connected to the back plate, can still be 'below' it, yes.You should get quite decent reception at a good distance, if you can use a monopole, better than you would with a chip or PCB resident antenna.You could use a separate RF board and place it near the top, but not right at the top. So you could have a wire in the clear going upward. The RF performance may well depend on how much you are able to have the antenna be in the clear, away from metal and dense fake wood objects, going upward.

author: ssashton
author_rank: Frequent Contributor
timestamp: November 24, 2018, 03:41:22 pm
content: Sorry, when you say UP, do you mean from the PCB face, or up towards the top of the speaker box?Is it best to have a ground plane as close to the base of the antenna as possible so any reflection is not phase shifted?sodium aluminum sulfate in food

author: cdev
author_rank: Super Contributor
timestamp: November 24, 2018, 03:51:57 pm
content: I meant up towards the top of the speaker box. If you need to bend it into another shape, try just an L shape, try to have the beginning part going as much upward as possible. Try to have as much of the RF board PCB as possible below and its antenna going up and away from the rest.There is no substitute for experimenting to find out what works best.If I were you I would not have the whole back of the speaker covered with metal, I would leave the top part free of metal to give the antenna a better shot off to the rear if needed, and to give the antenna which would go up a better omnidirectional pattern. have the antenna go up from the PCB and if you can place the "RF in" near the edge of the board so that the antenna does not have to go horizontal very far, you're more likely to get good reception from all directions. It doesnt have to be fancy, just a piece of wire will work best if its not enclosed by metal and is away from other wires for at least a few inches, ideally with its length mostly going straight up.

author: ssashton
author_rank: Frequent Contributor
timestamp: November 24, 2018, 04:00:09 pm
content: Okay thanks!I guess I'll just have to set up some experiments and see how far away I can get before signal dropouts.

author: cdev
author_rank: Super Contributor
timestamp: November 24, 2018, 04:03:00 pm
content: You might be surprised how well it works if you do that. Most Bluetooth devices/antennas are severely space-constrained.

author: ThomasDK
author_rank: Regular Contributor
timestamp: November 24, 2018, 05:45:15 pm
content: Quote from: ssashton on November 24, 2018, 02:05:36 pmYou have added components and traces next to the antenna... And by the looks of it you have a ground plane below it.That's a big NO NO, the antenna needs to be completely free of any nearby metal.In essence the gnd plane converts the antenna into a transmission line, and prevents it from radiating.

author: ThomasDK
author_rank: Regular Contributor
timestamp: November 24, 2018, 05:52:24 pm
content: Proper way to mount it:Notice the cutout below the antenna.

author: cdev
author_rank: Super Contributor
timestamp: November 24, 2018, 07:58:49 pm
content: The cutout with the antenna should extend beyond and above the piece of metal you've put on the back. I hesitate to use the word ground plane, although it is acting as one because ground plane when you want signals to go around you to the horizon should be beneath the antenna as in up down. Really a better word is back plane.You should place the radio board so the part with the antenna is at the top sticking up and well over the highest point on the metal backplane.I would try that first and see how it works. If that change isnt enough to make it work at the distance you want it to, I would replace it with a 1.9 inch length of stiff wire going up as much as possible. Try to avoid sharp bends.If you want to use the 1.9 inch wire as your antenna (it would be a better antenna, almost certainly, if you get it up into the clear) you should sever the trace to the other antenna with an x-acto knife. Do it cleanly at a spot where you could add a solder blob to reconnect it if you want.Then have the wire go straight up as much as you can, so high that if there was no wood box, it would be easily visible from all directions. You could even drill a tiny hole in the box to let it get that extra 2 or 3 cm higher.If you need still more gain, you might look into a 5/8 wave end fed 2.4 GHz antenna. You could make it very inconspicuous by using stiff black insulated wire. It would be almost invisible. Make sure to put a small closed loop at the end (count that as a straight wire, close the loop with a dab of solder) so there is no sharp point with its danger to eyes, etc.) That will likely give you around 3-4 db over a dipole in gain.If you need still more gain, at least 6db over a plain rubber duckie dipole, I would look into making a collinear 2.4 GHz antenna and having that stick straight up from your speaker. (if you didn't use the backplane you could likely get away with mounting it on its back) This would have to be enclosed in some kind of radome to last a long time without getting bent. In my experience (I have two of them) it works really well but any major deformation from the pictured shape causes a big decrement in performance. The antenna works best against some kind of metal object at its base to decouple the feed line. This kind of antenna has also been used successfully for ADSB - Adjusted for size, it makes a really good, fairly durable, roof-mountable, easy to make ADS-B antenna. (1090 MHz) You can add extra sections for more gain at the horizon, it reaches a point of diminishing returns at around four or five sections..You could expect at least three times the range in an open space from the two loop, three section collinear compared to the built in antenna. Maybe more.You might be able to extend it another 50% by adding two more loops and three more sections but at that point it would start to be ridiculously long and unwieldy for a bluetooth speaker. It would be good as a wifi antenna for a community network.Check out these two web sites for the collinear design.http://cheatthezone.tripod.com/wireless/colinear.htmlhttps://martybugs.net/wireless/collinear.cgi

author: ThomasDK
author_rank: Regular Contributor
timestamp: November 24, 2018, 08:16:24 pm
content: Quote from: cdev on November 24, 2018, 07:58:49 pmTake a closer look - it looks like a 4 layer pcb with an internal ground planedirectly below the antenna

author: cdev
author_rank: Super Contributor
timestamp: November 24, 2018, 10:06:48 pm
content: ThomasDK, yes, of course, you're right,Even if the bottom of his add on Bluetooth module is free of a GP, (meaning as you show, its meant to stick out from the body of the device) his putting the module right above a metal backplane negates the benefit of having the antenna by making it work against another conductor parallel to it and very close.Which isnt what it was designed to do at all.For the same reasons that putting ones traces right above a ground plane shields them from external signals, doing that also prevents his receiver from receiving well.Solutions?1.) Moving the module so that it is oriented differently with the antenna allowed to have some space around it?2.) severing the antenna trace with a razor blade or knife edge and soldering a quarter wave piece of wire to the RF input in its stead, then orienting that wire so that it receives omnidirectionally3.) removing some of the 'backplane' or4.) moving the module position so that it is offset beyond the edge of the metal plate, so that the antenna area is positioned well above and away from or well past the edge of the 'backplane'.

author: ssashton
author_rank: Frequent Contributor
timestamp: November 26, 2018, 03:10:06 pm
content: Hi!Thanks for all the advice. I spent some time playing with this yesterday.I found an app that shows the signal strength for discoverable bluetooth devices (but not for connected ones... strangely). So this made it very easy to test if I was getting improvements.I set my phone and BT module at a fixed distance. At first I tested with the back panel assembly reversed on the speaker box for ease of access.With the BT module built-in antenna I was getting around -65dB.I added a longer wire to the existing antenna and got about -60dB.The orientation fo the wire made almost no difference, surprisingly.Adding the wire as an external antenna without the built-in one connected.. also no difference.In fact almost everything I did with the antenna made no difference.I then found that making a coiled end on the wire and spacing it away from the centre of the back panel helped a lot. Increasing spacing up to about 12cm kept getting a better result. I got it as good as -45dB!However, the antenna coil really needed to be aligned in the centre of the rear panel, moving the antenna to one side got me back to about -60dB. I guess this is making a kind of satellite dish? Is there a technical name for this kind of antenna?I was feeling pretty good about that, but as soon as I placed it in the (half) speaker box with the internal baffles and things in the way, the performance totally disappeared.Then I read your comments about the ground plane. Yes, the main PCB has a ground plane on the back (it's only 2-layer). When designing the PCB I really wasn't sure if this would help to reflect signal back at the antenna (since I don't need rear pick-up) or if it would screw it up. Well now I know!I spaced the BT module away from my main PCB with about 25mm of wire and the reception improved about 4-5dB!Interestingly the best orientation was with the module facing forwards as it currently was on the PCB. If I turned it sideways it got a few dB worse. I guess the area of the antenna facing the transmitter is important - oriented sideways the PCB trace is very thin from the transmitters POV.I found increasing the spacing even more so the BT module was past the internal cabinet baffles got further improvement too.Swapping to a straight wire antenna, didn't seem to help, in fact it was worse. Again I suppose this is due to having less area face-on to the transmitter compared to the built-in antenna oriented well.The end result was about -52dB. That's an 8dB improvement!Does RF drop at 6dB with doubling of distance, like sound does? So 6dB will give me double the range?I'm still yet to try one of the external PCB antenna, but I have ordered one. It would be easier to keep the BT module on my main PCB (with gnd plane removed) and run a small coax to an antenna mounted in a better location, rather than run wires to move the whole module to a new location.I'll keep you posted!

author: ssashton
author_rank: Frequent Contributor
timestamp: November 26, 2018, 03:13:10 pm
content: Quote from: CDEVAh, this is part of the fact currents in the ground plane will mirror the positive path at radio frequencies, right?

author: cdev
author_rank: Super Contributor
timestamp: November 26, 2018, 03:22:27 pm
content: Looking at your image with the wire. Your white wire is way too long for 2.4 GHz.If you want to try a plain wire, use a quarter wavelength long wire. But you likely don't need it if all you are looking for is typical Bluetooth speaker performance. (not worse than average)What you said about the orientation of the stock antenna is true. It likely is all you need to do simply to move the module away from the other board, and turn it so the stock antenna is near the top and in the free and clear.That's how bad for your performance having your antenna right next to the backplane was.I would try both 'vertical' and 'horizontal', (end relative to it's starting point) orientations.You can find more about that kind of antenna and what kind of pattern you might expect with a search like "meander line" AND microstrip. Also you should look up whatever was submitted to gain US FCC approval. (the "FCCID") You can look that up on the FCC.gov web site.Also, I deleted the part about a ceramic chip antenna. As you had guessed, I was confusing your post with the one in another thread.

author: ssashton
author_rank: Frequent Contributor
timestamp: November 26, 2018, 03:24:41 pm
content: Sorry, I'm not sure what you are saying about ceramics?You mean try a ceramic SMD antenna?Ahh! I think you have mixed up this thread and the other one currently running about a PCB antenna.

author: Lord of nothing
author_rank: Super Contributor
timestamp: November 27, 2018, 06:15:19 pm
content: I dont want argue around but when you build you own System with a BT Antenna you might need to check if you are confirm with the regulation in therm of output power.It could be cheaper = better to buy some "ready to use" Board who the company declare the comfirmation.

author: ssashton
author_rank: Frequent Contributor
timestamp: November 29, 2018, 02:41:27 pm
content: I tested a pre-made external antenna... it wasn't as good as the built-in one!

author: cdev
author_rank: Super Contributor
timestamp: November 29, 2018, 03:09:35 pm
content: As the included antenna seems adequate to your requirements and if not, you can make excellent 2.4 GHz antennas yourself in minutes, there is no need to buy one unless you have some oddball size or space requirement you can't address yourself.

author: ssashton
author_rank: Frequent Contributor
timestamp: December 31, 2018, 03:10:05 pm
content: Hello again!I'm going to change the BT module to the latest CSR8675, however this module does not have an antenna included on the PCB. So I need to add it on my own PCB.I did get a sample which had an inverted F style antenna (pictured) but amazingly I found it works better if I cut the ground leg!When I design my own PCB I can either just do the same - a simple L stick antenna (no ground leg) and I know it works pretty well but.. I also found a nice looking antenna design from TI (DN007) and I was thinking about giving this a go.I wanted to ask if there is anything I need to consider when applying this to the CSR8675 bluetooth module? I note the TICC2430DB Eval Boardthis was designed for, has a capacitor 5.6pF and inductor 1.8nH in the line which my CSR chip has a little SMD band pass filter. Are these doing the same thing? I also note the CC2430 has another kind of delay line and or something.

author: cdev
author_rank: Super Contributor
timestamp: January 03, 2019, 12:47:03 am
content: just use a3 cmlong piece of wire.

author: ssashton
author_rank: Frequent Contributor
timestamp: January 03, 2019, 03:12:11 pm
content: You don't think this antenna design is worth using? Greater range would be good of course.

author: ThomasDK
author_rank: Regular Contributor
timestamp: January 03, 2019, 04:20:00 pm
content: Quote from: ssashton on December 31, 2018, 03:10:05 pmNo it's not the same, the L+C is for fine tuning the antenna impedance to 50 ohm at 2.4GHz. If you change the board geometry they need to be retuned. You will need a VNA or a Spectrum Analyzer with tracking generator to tune it.Since you are intending to sell these, go for a pre-certified module with onboard antenna to make your life easier. You still need to pass EMC+RED (+FCC if you intend to sell in the US), but a certified module takes the Bluetooth specific part of the testing out of the equation.

author: ssashton
author_rank: Frequent Contributor
timestamp: January 05, 2019, 01:05:40 am
content: I found datasheet info on what those parts do - they are actually to bring the output of the chip to 50 ohm, rather than tuning the antenna impedance which is already designed to be 50 ohms. Apparently the output of the CD2430 chip used on the example PCB from TI has a differential high impedance output.Full in here Page 27CD2430