The Arachnio is an Arduino Micro variant with onboard WiFi via an ESP8266. The included antenna is a printed antenna which is good for many applications. However, some applications need something better. This instructable will show you how to attach an external antenna or coax pigtail to the board to do just that.
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Step 1: Tools and Materials
You will need the following tools and materials:
- An Arachnio
- A coax pigtail or an external antenna with a coax connection. This instructable uses a WCP2400-MMCX4 from Laird Technologies, but any small diameter 50 ohm coax will work.
- A good pair of diagonal cutters
- A wire stripper capable of handling the coax
- An Xacto or similar razor knife
- Small diameter (< 0.032") flux core solder
- A good soldering iron with a very small needle or screwdriver tip and controllable temperature.
- Liquid electronics solder flux or a flux pen
- A good pair of electronics tweezers
- Electrical tape or a hot glue gun
Step 2: Trimming and Stripping the Coax
The Laird antenna has an MMCX connector attached. The first step it to cut that off.
Once you have a trimmed bare coax end, strip the outer insulation back about 0.75" (18 mm).
Once you have the shield exposed, slit the shield up one side. Pull the shield braid away from the inner insulator and twist it into a pigtail, as seen in the third photo above. Tin this pigtail with the soldering iron and the solder.
Strip the inner insulator back about half as far as you stripped the outer insulator and tin the inner conductor.
Step 3: Preparing the Arachnio
It is critical to disconnect the existing antenna before connecting another. If both antennas are left connected, the result will be an improper impedance match between the ESP8266 and both antennas, which will result in worse performance than you started with.
Disconnect the antenna by using your Xacto to slice through the feed and ground connections where the labels 'RF' and 'GND' are on the antenna side of the board, as shown in the picture above. Make sure your cuts go all the way through the copper and separate it. I generally make two parallel cuts close together and scrape away the copper between them in order to make sure I have truly separated the two sides.
Then scrape away a little bit of solder mask around the ground connection to expose the copper ground plane with your Xacto knife.
Step 4: Soldering the Wires
Make sure you have a nice amount of solder built up on the inner and outer conductor of the coax as well as RF and ground pads on the Arachnio. Coat everything with a generous amount of flux. Liquid flux is better than paste flux, since it will impede your view of the joint less.
Using your tweezers and soldering iron, solder the center conductor to the RF pad and the antenna end of C13. If you are having trouble with this, get a magnifier to make the joint easier to see and support your wrists to steady your hands. Once it's soldered and cooled, give it a light tug to make sure the joint is good.
Then solder the outer braid to the place you scraped the soldermasks off the ground plane. Give it a light tug to make sure it's soldered securely.
After you have attached the antenna, make sure the Arachnio can still talk to your local WiFi network. If you are using a coax pigtail, you will have to attach it to an antenna for this step.
Step 5: Finishing Up
Once you have confirmed that the antenna is connected and working, you need to add some strain relief to the coax to prevent mechanical forces from breaking your solder joints or pulling the pads off the board.
The picture shows one way to do it, which is to wrap the joint in several turns of electrical tape.
Another way to accomplish the same task is to lay a blob of hot glue over the solder joint and adjoining coax to hold it all in place.