HowTo: TDA7492/CSR8635 – External antenna mod to improve Antenna amplification/range

After a few days of using the Bluetooth retro speaker, I soon realized that the integrated antenna had a somewhat modest wireless range.
Mostly was after about 2m as the crow flies. A connection through one or even two walls was out of the question. With the phone disappear on the toilet while the music is running was impossible.

After a few more days, I decided to get to the bottom of the problem. Finally, I put the lack of reception quality on the tiny PCB antenna on the CSR8635 Bluetooth module. So only a new antenna with a better gain helped to get rid of this issue.

How you can do this is described in this article.

Hints for our lovely english readers: Basically, many of the articles on Nerdiy.de are translations from the original german articles. Therefore, it may happen here and there that some illustrations are not available in english and that some translations are weird/strange/full of mistakes or generally totaly wrong. So if you find some obvious (or also not obvious) mistakes don't hesitate to leave us a hint about that in the comment section. 
Also please don't get confused, that instead of a "dot" often a "comma" is used as decimal separator. 🙂


Safety instructions

I know the following hints are always a bit annoying and seem unnecessary. But unfortunately, many people who knew it "better" from carelessness lost their eyes, fingers or other things or hurt themselves. In comparison, a loss of data is almost not worth mentioning, but even these can be really annoying. Therefore, please take five minutes to read the safety instructions. Even the coolest project is worth no injury or other annoyance. https://www.nerdiy.de/sicherheitshinweise/


Requirements

Helpful Articles:
Before you start with this article you should have dealt with the basics of soldering. Information on this can be found in the following article.
Electronics – My friend the soldering iron

Required material:

In the following list you will find all the parts you need to implement this article.

Required tools:

In the following list you will find all the tools you need to implement this article.


Overview – What is where?

On the following photos the problem and solution is explained a bit.

After opening the case back, the amplifier board should be easily accessible.
This is what your unprocessed amplifier board should look like. On the blue Bluetooth module, the integrated PCB antenna is very easy to recognize.
This photo shows the trace between the Bluetooth module and the PCB antenna. This will be removed later to no longer use the integrated PCB antenna.
The aim of this mod is to replace the integrated (bad) PCB antenna with an external Bluetooth antenna. This can also be an old W-Lan antenna. Since Bluetooth and W-Lan work in the same frequency range, both antennas can be interchanged.
The external antenna can then be mounted outside the housing with a screw thread.
For this conversion you need a connection cable to the antenna. Make sure that both fit together. Special RP-SMA plugs/sockets have been introduced for the W-Lan antennas. As a result, they do not mechanically match the normal SMA plugs.
So make sure that the socket of your antenna …
… matches the plug of your antenna cable.

Disconnect the old PCB antenna from the Bluetooth module

Before you can connect the new antenna, of course, first the old antenna must be removed. To do this, the connection between the Bluetooth module and the PCB antenna must be disconnected at the appropriate place.

To disconnect the old PCB antenna from the Bluetooth module, the small trace between the Bluetooth module and the PCB antenna is interrupted with a sharp knife. If the knife is sharp enough, you can easily separate the track and bend it up a bit. This clearly shows that the track is broken. To be on the safe side, you can also check with a multimeter/continuity checker if the track is actually interrupted.
Here is the track between Bluetooth module and PCB antenna again in a close-up view. The red circled area is the part you should interrupt.
Closeup of disconnected connection between bluetooth module and pcb antenna. You can see the upturned track well.
In a strong enlarged shot you can …
… see that the track is clearly interrupted.

ATTENTION: In this condition – without the antenna connected – you should not turn on the Bluetooth module under any circumstances. This can damage the module.


Solder the antenna cable to the Bluetooth module

Now it’s time to solder the new antenna cable to the Bluetooth module.

To do this, if necessary, remove the plug from one end of the antenna cable.
This plug is unfortunately very small but only crimped and thus reasonably easy to separate from the antenna cable.
Rear view of the IPX connector used in devices with limited space.
To disconnect the plug from the antenna cable you must first …
… bend the holding arms crimped on the cable to the outside as shown. These are responsible for the mechanical seat on the cable.
The closer to the plug attached holding arms also make the electrical contact to the shielding of the antenna cable ago. These…
… you can now bend to the outside.
Now it should be possible to pull the plug carefully in the direction of the antenna cable …
… and let go of this.
As a result, the two lines in the antenna cable come to light. The inner and outer conductor or shielding must now be separated from each other and prepared for soldering.
Carefully remove approx. 1.5cm of the insulation.
The outer conductor is laid in the form of a wire mesh around the antenna cable and can be fixed with a needle or tweezers …
… easily “aufdröseln” and separate from the inner conductor.
So that the two conductors can be soldered better later, it is now advisable to tin both inner and outer conductors with a small amount of soldering tin.
Now you have to connect the prepared cable to the antenna contacts of the Bluetooth module. The inner conductor of the antenna cable must be connected to the green marked and the shield of the antenna cable (the outer conductor) to the red marked contact of the Bluetooth module.
The whole thing should look like that when soldered.
Check after soldering again that no short circuits have been caused by the solder. You should check both the solder joint of the first …
… and second contact.
Closeup of the soldered antenna cable.
After you have tested the newly installed antenna and everything works, you should secure the spot where you have soldered the antenna cable with some hot glue. This protects the filigree solder joint against mechanical stress.

Installation of the antenna connection in the rear wall of the housing

So that the external antenna can later be connected outside of the housing, now the screw connection must be attached.

By measuring the thread thickness of the antenna connector you will find the right drill bit. In this case, you will need a 6mm drill bit for later through drilling and …
… a (rounded up) 10mm drill for the blind hole. The blind hole is necessary because the thread is shorter than the thickness of the speaker back panel. If you do not know what a blind hole is: do not worry, that will be explained in the following steps.
However, before any large holes are drilled, you first look for a place where your external Bluetooth antenna will be attached later and drill a hole with a diameter of 3mm.
Now the small, pre-drilled hole is first drilled to the larger diameter with the 10mm drill (the one for the blind hole). Watch out! Do not pierce. Drills first only about 5mm deep. This hole should only make room for the over-wide “collar” of the antenna thread.
If the hole drilled to 10mm you can now …
… pierce with the 6mm drill bit.
In the resulting hole, the antenna plug should be plugged.
Unfortunately, the blind hole (the hole with 10mm diameter) was not deep enough after the first drilling. If the thread does not look far enough out of the back wall of the case, as in this picture, you have to drill the hole a bit more with the 10mm drill bit. The following applies: Always drill in small steps and check whether the plug fits. This will prevent you from being accidentally piercing through the backpanel.
Have you found the right depth for your blind hole you can …
… secure the antenna connector with a union nut.
From the inside it all looks like this.
And from the outside – with attached antenna – so. Please note that you can never turn on the Bluetooth module without the antenna. This could damage the Bluetooth module.

Once you have completed these steps, you can close the back of the housing. Make sure that you do not pinch the very sensitive antenna cable.

Before starting up, you should now follow the tips from the article Electronics – Commissioning a new circuit.

And then enjoy an increased wireless range of your self-made Bluetooth box. 🙂


I hope everything worked as described. If not or you have any other questions or suggestions, please let me know in the comments. Also, ideas for new projects are always welcome. 🙂

Fab

P.S. Many of these projects - especially the hardware projects - cost a lot of time and money. Of course I do this because I enjoy it, but if you appreciate it that I share these information with you, I would be happy about a small donation to the coffee box. 🙂

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