Need help understanding how to make the best antenna choice.

[Jack.Straw]

Hello. I've built a 12v bluetooth speaker out of a mini beer keg. The bluetooth module is located inside the steel can, so i've connected a through-hole SMA female jack for an external antenna. I had an old wifi antenna here, but it seems that it is the RP-SMA... so it won't work with the jack i have. So, i've been looking at purchasing a different antenna and i'm not sure how the different specs will effect things.

The antennas i've been looking at (the elbow style found on many wifi units) list a GHz (or sometimes MHz) rating, and a dBi rating. I've tried doing a bit of reading on the net but the answers have mostly confused me. Will selecting a higher dBi rating give me more range? Is the GHz rating something i need to consider for a project like this?

Finally, does my antenna choice have any effect on power consumption? (the amp will often run on battery power)

Thank you for any advice you can offer.
-Scott

 

[ronsimpson]

Finally, does my antenna choice have any effect on power consumption? (the amp will often run on battery power

No change in power used from the battery.

 

[ronsimpson]

http://www.freeantennas.com/projects/template/

I made some thing like this. I used a flat sheet of brass not window screen. It works for WiFi so the same thing will work for blue tooth.
My size, left-right, was 30% to 50% larger. The height is about the same.
It focus RF on the antenna. AND It also takes power from the antenna that goes the wring direction and sends it in one direction.
I can not find the template I used. It is based on "parabola" for left and right. Up and down it is flat. You should put the Blue Tooth antenna at the focal point. (or the Blue Tooth module)

 

[RadioRon]

Hello. I've built a 12v bluetooth speaker out of a mini beer keg. The bluetooth module is located inside the steel can, so i've connected a through-hole SMA female jack for an external antenna. I had an old wifi antenna here, but it seems that it is the RP-SMA... so it won't work with the jack i have. So, i've been looking at purchasing a different antenna and i'm not sure how the different specs will effect things.

The antennas i've been looking at (the elbow style found on many wifi units) list a GHz (or sometimes MHz) rating, and a dBi rating. I've tried doing a bit of reading on the net but the answers have mostly confused me. Will selecting a higher dBi rating give me more range? Is the GHz rating something i need to consider for a project like this?

Finally, does my antenna choice have any effect on power consumption? (the amp will often run on battery power)

Thank you for any advice you can offer.
-Scott

The GHz rating describes what frequency range the antenna is tuned for. The antenna you buy must be tuned for the frequency band that Bluetooth uses, namely 2.4 to 2.485 GHz, or usually just called "2.4GHz". If you find one with a MHz rating, this is simply GHz times 1000. The GHz rating must include 2.4GHz or the antenna will not work adequately, so this is an absolutely critical spec.

The dBi rating is an indication of how well the antenna works and usually a higher number is better. In theory, a higher dBi rating gives better range. If you want the unit to have good range in any direction, you usually use an "omni" antenna (omni is short for omni-directional or "all directions"), one which radiates equally in all directions towards the horizon (but not straight up). These are long and thin plastic or metal, and usually have dBi ratings between 0 and 6. There are larger base station versions of these with dBi ratings that go higher, but at greater cost. If you don't mind having to aim your antenna, you can use a directional type, such as Ron's example, which can have higher dBi ratings. Directional antennas of various kinds can have dBi ratings up to 30.

Antennas are very sensitive to how they are mounted and oriented. You would be wise to show us a photo of where you intend to mount the antenna so that we can give advice.

 

[Mikebits]

You could just get a RP-SMA to SMA adapter.
https://www.adafruit.com/product/92...Hta5Qi3ThiMMp0OTafsUwThVei_ZAzjgaAkj5EALw_wcB

 

[Jack.Straw]

Thanks guys! That's exactly what i needed to know.

Ron, here are pics including the RP-SMA antenna that currently does nothing. It's far from ideal because it sticks out, but i guess it's a necessary evil since the keg is pretty much a faraday cage. If there was a type that had an even lower profile that would be ideal. As it is, with no antenna... it works, but i can block the signal with my hands and can't get very far from the unit.

Mike, great idea but that would just extend the antenna even further than it already does. I wish i didn't have to have that thing sticking out at all.

 

[RadioRon]

I agree that the antenna on the side is a bit ugly. The ideal place to put an antenna on this sort of thing is centered right in the top. Now, obviously that is a problem because you have the speaker grill there. But bear with me for a minute. What I would do would be to choose a low profile type of antenna (example links below) that is either a helical monopole (a sort of nipple sticking up about 1 inch and only about 10 mm wide) or a puck style (like a miniature hockey puck, a black cylinder about 10 mm tall and width of maybe 30 to 50 mm across) depending on which one is less ugly to you. I'm assuming that your speaker grill is metal, and conductive, this is a critical point. You would mount the antenna in a hole that you drill in the center of the grill with the wire going straight down inside. Below the antenna, you would curl the antenna cable back up to the rim of the speaker so that it doesn't touch the cone. To get the wire past the speaker, you would make a wood mounting spacer ring that lowers the mounting position of the speaker by about 20 mm or so. You would drill or cut a special passageway through the edge of the wood ring to snake the antenna cable around the edge of the speaker and down into the cavity below.

Putting an omnidirectional antenna on the center of the metallic speaker grill is the ideal position for such an antenna, and if you choose a low profile type of antenna, it might actually look pretty cool. Here is a few antennas that I ran across that might be worth considering:
https://www.digikey.ca/product-deta....4-WRT-MON-UFL/ANT-2.4-WRT-MON-UFL-ND/4576776
https://www.digikey.ca/product-deta....4-WRT-MON-RPS/ANT-2.4-WRT-MON-RPS-ND/4576777
https://www.digikey.ca/product-deta...-S-RP-19/TANGO23-2.5M-SMAM-S-RP-19-ND/6096414 (kinda big)
https://www.digikey.ca/product-detail/en/taoglas-limited/WS.02.B.205111/931-1340-ND/4965543
https://www.taoglas.com/wp-content/uploads/2015/04/MA530.A.CG_.003.pdf

Of course you could also put a regular whip antenna like the one in your photo up there at the top too, but it would stick out like a sore thumb. A low profile type is better, even though it will have a lower dBi rating. Let us know if this idea is interesting, and we can look around a bit more. There are many of these types out there.

 

[Jack.Straw]

Wow, great idea! The bottom two would look nice i think. Unfortunately it's like $75. But the idea is sound so perhaps i'll do some more shopping around now that i know that sort of thing exists. I would much prefer that low profile button in the center of the speaker grill to the awkward elbow thing i have now. Thanks for the idea!

 

[Jack.Straw]

I'm assuming that your speaker grill is metal, and conductive, this is a critical point.

Ron, the speaker grill is indeed metal but has a black coating of some sort. Is it critical that these types of antennas come in direct contact with the metal? Will i need to sand off the black coating where the antenna will mount?

 

[RadioRon]

Yes, it is critical that there be contact. However, you risk having some bare metal showing if you are not careful, so to be more precise, I would say that you need bare metal contact in a small ring around the connector, but not beyond that. In other words, if you are using the puck style antenna, you only need to clean the black color in close to the connector, not all the way out to the rim of the antenna.

I also want to add that you should not be splicing the coax cable. It will come with a coax connector already mounted on the end of it and you really can't do a good job of re-attaching that connector if you decide to shorten the cable, so don't try. Buy it with enough coax length, or buy an additional pre-built coax jumper to lenghten it.

 

[RadioRon]

Hi Jack
If you find the puck style antenna too big or too expensive, we can consider another option. That is where you buy an antenna that is intended to be embedded inside a housing and you come up with your own plastic cover. There are numerous low profile antennas available at low prices, but you would have to find a suitable plastic cover to make it look good. I should warn you that these all come with tricky conditions for mounting and so you will need assistance in selection and how to install. I can give some assistance.

Here are some examples:
https://www.digikey.ca/product-detail/en/laird-technologies-ias/MAF94264/994-1035-ND/2392214
https://www.digikey.ca/product-detail/en/laird-wireless-thermal-systems/001-0014/001-0014-ND/4732758
https://www.digikey.ca/product-detail/en/taoglas-limited/FXP72.07.0053A/931-1075-ND/2332702
https://www.digikey.ca/product-detail/en/molex-llc/0479500011/WM5031-ND/2709379
https://www.digikey.ca/product-detail/en/laird-technologies-ias/CAF94505/994-1009-ND/2392188
https://www.digikey.ca/product-detail/en/wiznet/W5I-BO-07/1278-1006-ND/3829643

 

[Jack.Straw]

Yes, it is critical that there be contact. However, you risk having some bare metal showing if you are not careful, so to be more precise, I would say that you need bare metal contact in a small ring around the connector, but not beyond that. In other words, if you are using the puck style antenna, you only need to clean the black color in close to the connector, not all the way out to the rim of the antenna.

I was thinking perhaps i could remove the coating on the underside instead of the top. Looking at the images, it seems like the bottom of the antenna, the threaded shaft, and the nut/washers are all in contact as one ground. That would get the job done, right?

I also want to add that you should not be splicing the coax cable. It will come with a coax connector already mounted on the end of it and you really can't do a good job of re-attaching that connector if you decide to shorten the cable, so don't try. Buy it with enough coax length, or buy an additional pre-built coax jumper to lenghten it.

My bluetooth module doesn't have an type of antenna connector built into it. It has 2 little holes for the antenna and ground wires that i have to push the wires through and solder. I was thinking i would drill a hole between the speaker and the outer edge of the grill housing and install a male connector for the antenna there with a wire that runs down to the bluetooth module. That way i could connect/disconnect the antenna while the speaker is installed. One like the image below that i would solder a length of the coax wire to:

Those new options you provided might be better for me. Do you think i could get away with installing something like that in the speaker area? I have a nice amount of space between the speaker and the grill housing. The top part of the grill goes on the outside of the ring in this picture:

Do you think i could get away with using the "whip" style antenna running in a circle between the speaker and the grill housing? Like this one:

https://www.digikey.ca/product-detail/en/anaren/66089-2430/1173-1134-ND/3903374

Thanks again for all your help!!

 

[RadioRon]

I was thinking perhaps i could remove the coating on the underside instead of the top. Looking at the images, it seems like the bottom of the antenna, the threaded shaft, and the nut/washers are all in contact as one ground. That would get the job done, right?

Yes, thats exactly right. Doesn't matter if there is still a coating on the top as long as the underside is contacting.

My bluetooth module doesn't have an type of antenna connector built into it. It has 2 little holes for the antenna and ground wires that i have to push the wires through and solder. I was thinking i would drill a hole between the speaker and the outer edge of the grill housing and install a male connector for the antenna there with a wire that runs down to the bluetooth module. That way i could connect/disconnect the antenna while the speaker is installed. One like the image below that i would solder a length of the coax wire to:

That sounds reasonable, except that you are adding a bit of cost, but not much. What bluetooth module is it? I would like to look up a picture of it to see what you mean about the holes. Unfortunately, it is not a good idea to attach the ground braid of the coax using a single wire. I strongly advise you to see if you can surface mount the coax braid to the ground of the bluetooth board and then run a very short center conductor connection to the hot pad of the antenna connection. This is very important for best range.

Those new options you provided might be better for me. Do you think i could get away with installing something like that in the speaker area? I have a nice amount of space between the speaker and the grill housing. The top part of the grill goes on the outside of the ring in this picture:


Do you think i could get away with using the "whip" style antenna running in a circle between the speaker and the grill housing? Like this one:

https://www.digikey.ca/product-detail/en/anaren/66089-2430/1173-1134-ND/3903374

Thanks again for all your help!!

Anything you put underneath the grill will not work because the grill is metal and will effectively shut in all the antenna radiation and nothing will get out. The grill plus the metal housing (the keg) is all metal, so the antenna has to be outside of that metal envelope to work.
The item that you linked to is actually not an antenna at all, it is only a piece of coax with a connector on the end. I think this is offered as an accessory for those pcb antennas that don't come with coaxes.
Unfortunately, all of those inexpensive alternatives need to be mounted outside the metal to work.

I am happy to see that you are getting creative with ideas.

 

[Jack.Straw]

That sounds reasonable, except that you are adding a bit of cost, but not much. What bluetooth module is it? I would like to look up a picture of it to see what you mean about the holes. Unfortunately, it is not a good idea to attach the ground braid of the coax using a single wire. I strongly advise you to see if you can surface mount the coax braid to the ground of the bluetooth board and then run a very short center conductor connection to the hot pad of the antenna connection. This is very important for best range.

The bluetooth module is linked below, although i bought it a couple years ago. The 2nd image shows the bottom where the antenna connections are marked. The antenna connection i'm currently using is picture below as well. I just bundled and twisted up the outside ground cable and pushed it through the ground hole. I didn't mean to imply that i used a single strand of it. Is the "very important for best range" part of your reply referring to the very short connection between the bluetooth module and the connector? Unfortunately the bluetooth module is located near the bottom of the keg, and i was hoping to put the actual antenna connector up on top by the speaker. Will i loose range if i do that? It sure would make it easier for me to assemble that way.

**broken link removed**

Anything you put underneath the grill will not work because the grill is metal and will effectively shut in all the antenna radiation and nothing will get out. The grill plus the metal housing (the keg) is all metal, so the antenna has to be outside of that metal envelope to work.
The item that you linked to is actually not an antenna at all, it is only a piece of coax with a connector on the end. I think this is offered as an accessory for those pcb antennas that don't come with coaxes.
Unfortunately, all of those inexpensive alternatives need to be mounted outside the metal to work.

Ahh, bummer. I was hoping all the holes in the speaker grill would be sufficient for the signal to escape. I'll keep looking through antenna options like the samples you provided and see if i can come up with something that looks better than center mounting one of the previous options on the grill.

Thanks!!

 

[RadioRon]

I hope you have viewed this video:

Now, about the antenna connection. I'm not happy with the way that you have prepared the bare end of your coax in the picture. At 2.4 Ghz, the way you have your ground braid pigtailed out should never be done and you will suffer a considerable loss of communication range doing it this way. If you want good range, we must do it differently and this will take some minor surgery for a good antenna cable connection. I hope your soldering skills are up to it.

The linked video begins with a closeup of the board, and I think it is the same as your board. This closeup shows a strange antenna connection that I think is bad. The true antenna feedpoint for the Bluetooth transceiver (the point where a 50 ohm transmission line should be connected) is on the little submodule, the little board with the 8645 chip on it (see image below). It's feedpoint feeds an existing antenna on the submodule of a type called Planar Inverted F Antenna, which is the gold colored trace in the shape of an inverted F. The maker of the board has put a jumper wire from an abitrary point on the inverted F to their J2 connection point, and this jumper wire shows an utter lack of consideration for RF transmission line principles. I suspect it has been done this way because they don't give a damn about optimizing the communication range. In order for your coax connection to work properly, we are going to have to connect it up to the feedpoint, and we are going to have to disconnect the existing inverted-F antenna. You cannot have two antennas on one connection without a proper power splitter, which we don't have here.

As you can see, there are no convenient pads for this purpose, and this is where the surgery comes in. You see that little white chip component at the feedpoint. I suspect this is a capacitor, but we have to check to make sure. Our coax connection will be at the antenna side of that capacitor. This involves using a fine knife (xacto or equivalent) to cut the gold trace as shown in the attached image (second one below). The remaining little patch of gold trace still connected to the capacitor will be the connection pad for the center conductor of our coax. However, don't connect this up just yet. With the really small coax that we are dealing with here, we always must attach the ground braid first and then the center conductor. This avoids stressing the center conductor wire, which is very fine and can break easily. In order to solder the coax braid, we have to surface mount it. This means that we prepare the coax as shown in another attached image, tin the exposed braid with solder, then surface mount the braid down to some exposed ground plane very close to the feedpoint. This will be tricky as there is not much to connect to. I will provide a drawing showing the best place to do this in a later post. Until then, the third image is a rough attempt to show you how the coax will dress. The black is coax, the green color is coax with the outer jacket removed and the braid exposed. The grey is the braid cut away exposing the dielectric, and the yellow is the center conductor wire exposed. Soldering the ground braid down to the board is tricky and should be done with a fine iron.

 

[RadioRon]

I'd be willing to attach the coax for you, but shipping will be an annoying hassle.

An additional problem is that the coax in your picture uses a dielectric that melts fairly easily, I think. It doesn't look like teflon. That makes it tricky to surface mount as recommended. Might have to get a teflon coax instead.

 

[RadioRon]

Your comment about where you have the bluetooth module has me thinking of yet another way to do this and it is indeed because we want to keep the entire coax connection as short as we can. Perhaps it would be best if we used one of those little circuit board antennas, but mount it down where you have your antenna right now. This is not the optimum spot for best range, but it might be ok if you are able to rotate the keg for best signal. The antenna down there will make the whole thing have a directional signal, but still workable.

For that matter, maybe you can live with one of the "helical monopole" type of black plastic antennas, the one that looks like a tall nipple. If they arent' too tall for you (would stick out about one inch), that would look pretty decent and its easy to install, right? With this type, at least you don't have to come up with your own plastic cover.

If you make the proper coax connection at the circuit board as I described above, I think one of these nipple antennas might give you pretty decent range.

I'm thinking of the type that looks like this:
https://www.digikey.ca/product-deta...NT-2.4-CW-RH-SMA/ANT-2.4-CW-RH-SMA-ND/1139575

 

[Jack.Straw]

Ron, wow man... you are awesome! Thank you so much for spending so much time helping with my project!

Unfortunately i did not find that video before i bought this board back in 2016. Considering the problems outlined in that video, your observations concerning the antenna situation, and the fact that i've already had to re-solder the audio output jack to the board after a slight bump broke it off, I think i would prefer to just find a better module. Those modifications you outlined for me (thank you!) don't look like fun, and chances are i'd screw it up and need to buy a new board anyway. I'll look for one that has an antenna connection right on the board to start. I chose this one because of the audio input jack, which is something that I wasn't seeing much when i was shopping around back then. I'm sure they are out there though.

That board doesn't have any kind of screw holes for mounting, so i ended up making something custom to hold it and the other parts. If i get a new module i'll end up redesigning the internals anyway, so i could potentially move the bluetooth board someplace better for a the top-mounted antenna idea. Or maybe i'll go with something like the one you just linked... that wouldn't be too bad.

 

[RadioRon]

The one I linked seems ok, but i like the looks of this type better when the black plastic covers up the gold connector part, so all you see is black. Might be some out there like that.

 

[RadioRon]

I realize that this latest suggestion is contrary to my advice given in post#7 above. Putting the antenna in the center of the top is ideal but it presents other difficulties. The puck antennas available seem to be a bit large, some about 80 mm wide, and somewhat expensive. A coax cable will have to extend roughly 24 inches to reach that location, increasing losses by 1.5 dB.
Keeping the antenna in its present location eliminates installation hassles and minimizes cost. The negative to this location is range. We can expect the keg to act as a reflector and obstruct propagation in the direction through the keg, although not to a large degree due to its shape and size. This is the biggest problem, but we don't yet know the magnitude of this problem. In other words, we do not know what range will be satisfactory and in what direction away from the keg, plus we don't know how much propagation will be impaired by the keg. Since we have the ability to rotate the keg to optimize range, it would seem a good idea to try to optimize the present antenna location first through experimentation before investing in new hardware and drilling new holes. Propagation at 2.4 GHz is typically rich in multi-path, even in residential situations, and strong multi-path makes efficiency of the antenna more important than radiation pattern. We will see how that applies in this case.