My father lives in the village where no broadband connection is available. To access Internet he used creepy and hell expensive GPRS connection. Once tired by absence of normal connection and impossibility to show him some video on youtube I decided to make something to resolve this nonsense. At that time I was aware few neighbors of him are using 3G USB modems with Yagi antenna attached. After small research USB modem was bought to do the same. Unfortunately first attempts to connect to the network using solely internal antenna was complete failure. But when I was outside house and connected 1/4-wavelength long piece of copper wire into modem’s antenna socket, I was able to connect to the network.
Due to impossibility to make connection from the house, first more or less working solution included wireless router with USB port (at the time one of the cheapest TP-Link TL-MR3220) that was located in attic and this fascinating wire antenna:
Surely I was able to connect modem directly to PC using only external antenna and USB extension cable, but I have reasons to go wireless. To name a few:
- I and my nephew have wi-fi enabled devices (phones, laptops, ipods, etc.) we would like to use them anywhere in the premises. So this hotspot will serve not only standalone desktop PC, but any wireless device in range.
- I didn't wanted use PC as a router as it often turned off because is a little loudly and located in the living room.
- While drilling in the attic was OK, inside cabling will be a pain. Wireless access from desktop is a real saving and relief.
Few weeks after installation I visited father again and found that:
- Iron roof coating does not allow to connect my laptop to Wi-Fi access point from most of the courtyard;
- Summer happened (oops!) and under iron-coated roof heated by direct sunlight daylight temperature is more than 50° C (122° F) and router together with modem become really hot; it was promising nothing good;
- Connection rather unstable and disappears from time to time; though demonstrates not so bad performance, up to 0.5/0.2 Mbit DL/UL.
Inspired by performance measurements I decided to make more reliable solution with better antenna and install it outside of the house to avoid overheating.
I started with a search of the case that will be able to accommodate all required components. But I found that cases in stores either hermetically sealed (which is unacceptable in my design because of airflow is required to cool components) or does not provide adequate protection from harsh conditions and mostly expensive (appropriate sizes starts at around US $50 equivalent for the IP55 rated box). Thus I decided to go with own case design and manufacturing.
First of all I made plastic sheet bender out of nichrome wire strained between two long screws secured in the wood plank and 12V/10A power supply with alligator clips. Heat intensity was regulated by moving alligator clips along the wire. Then I used it to bend two 45cm*70cm big polystyrene sheets into shapes pictured below.
Polystyrene was chosen because of its availability, low cost and good frost resistance as in our region frosts under -20 degrees C are rather usual during the winter.
Plastic sheets were bent according to this drawing to form two parts (base/roof and lid) of protective case. You may refer to the previous image for resulting shapes. All sizes in millimeters.
Take into account that it's impossible to bend relatively thick (mine is 3 mm) plastic to have sharp corners. So you should consider resulting slightly rounded corners when perform bending and maybe introduce some tolerances into design. I used bottom cutouts to make exact fit between lid and the base part.
After bending lid and base was put together, secured, and four 3.2 mm assembly holes was drilled on the sides to keep resulting case in one piece. Later I drilled 6 mm holes on each side to hang the box on two crews screwed into the wall.
To make assembly more convenient I screw M3 screws and nuts together through mounting holes of the base and then melted nuts into base' plastic using hot soldering iron. To completely secure the nuts in the base I used small amount of cyanacrylate momentary glue.
To protect internal space from humidity, ventilation holes are made in the places that are safe from water sprays, but everything else should be securely insulated. I have used self-adhesive insulation ribbon for doors/windows sold in hardware store.
80mm cooler from broken ATX PSU was used for forced ventilation. I had drilled 12 mm vent holes first then glued sparse silkscreen mesh from inside to protect internals from insects. From my experience bugs are more dangerous for electronics mounted outdoors than water and frosts.
To heat electronics during winter frosts I decided to use two ceramic 10W, 10Ohm resistor connected in series. Total power dissipation at 12V will be 7.2W
All ventilation and heating components mounted on the lid. I added two LEDs to indicate when heater or fan is on but it is not really necessary. I often use melting glue gun to mount different parts as it is done in this case: both LEDs and controller glued to the lid with a glue gun.
To control cooling (fan) and heating (resistors) I prepared very simple circuit consisting of comparator, few resistors, 10K NTC thermistor and dual N-Channel MOSFET transistor. Comparator forms inverting and non-inverting Schmitt triggers. I calculated resistors values for this circuit using online calculators on Random Science Tools and Calculators site to turn on fan when temperature is more than 30 degrees C and turn it off if T below 27 degrees C. Heater respectively turns on if T is below about 3 degrees C and turns off when it rises more than 7 degrees C. Numbers are approximate, but tests showed I hadn't missed anything.
Controller powered directly from 12V/1A wall PSU.
Schematics of “HVAC” controller:
- IRF7103 SO-8 dual MOSFET 50V, 3.0A
- LM393 SOIC-8 dual comparator
- 0.1uF C0805 ceramic capacitor
- 1uF C0805 ceramic capacitor
- 3x Molex 2-pin polar connectors 0.1" step
- 10K NTC thermistor R0805
- 33K R0805 resistor
- 39K R0805 resistor
- 330 R0805 resistor
- 2x 10K R0805 resistor
- 56K R0805 resistor
- 22K R0805 resistor
- 330K R0805 resistor
To ensure drainage in case water forced inside, meshed vent holes located on the bottom of the case also were drilled. Here SMA/RP-SMA connectors for Wi-Fi and 3G antennas and cable input for power supply located as well.
To access Internet I bought Novatel Wireless Ovation™ U720 EV-DO Rev A USB modem and data plan from local provider who is working in 800 MHz band using CDMA EV-DO Rev.A technology.
I have ordered few MS-147-to-SMA pigtails for external antenna connector but all received parts had in fact DB9 connector that does not fit on connector. So finally I disassembled the modem and made small mod by connecting commonly available U.FL-to-SMA pigtail directly to PCI-E card instead of wire connector to MS-147. U720 in fact is USB to PCI-E adapter plus PCI-E EV-DO modem.
As reception of CDMA network signal in the area where box will work is very poor, I decided to create external antenna.
- Transmit 824.7-848.31 MHz
- Receive 869.7-893.31 MHz
I made calculation for mean frequency between 824.7 and 893.31 MHz. Then I draw the shape in design software and etched it on FR4 board to ensure correctness of the sizes and shape.
I hadn't 30cm long etching tank so I used paper box and polyethylene film to make temporary etching container. I used direct toner transfer method for PCB artwork preparation and ferric chloride as etchant.
To protect antenna and ease its mounting I put antenna board inside 30cm long 1/2" PVC plumbing pipe and put stubs on both sides. There are SMA connector on one side to connect extension cable I made using connectors and RG-316 low loss coaxial cable bought on eBay. Both stub caps are sealed with melted glue.
Here you can see all router components together. Here goes list of all components and approximate prices in USD:
- 2x 45cm*70cm polystyrene sheets - $15
- Self-adhesive insulation ribbon for doors/windows - $3
- 80mm computer fan - $3 (Mine is scavenged from broken PSU, no cost for me)
- HVAC controller board components and FR4 - $5 (not sure as I bought only thermistor and resistors/caps and this is few cents; everything else were scavenged from old PC motherboards and FR4 I had before and don't recall the price, but it is very small piece and also should cost few cents)
- Heater (2x 10W ceramic resistors) - $1.5
- PVC pipe, caps, pipe clips, SMA connector and FR4 for antenna - roughly $10
- RG-316 coaxial cable (~10'/3m) and connectors - $10
- Cable input and pair of screw clamps - $2
- 2x pigtails (U.FL/SMA, RP-SMA male/female) - $8
- Tripple power socket - $3
- 12V/1A wall PSU - $5 (I have few of them for free from old equipment)
- Novatel U720 USB CDMA Rev.A modem - $40
- TP-Link TL-MR3220 3G/802.11b/g/n wireless router - $40
- Collinear Wi-Fi antenna for 2.4 GHz band - $12
- Melted and cyanacrylate glue, double-sded adhesive tape, nylon cable ties etc. - $5
TOTAL: around $160
In total I had spent about 16 hours in total for design, production and assembly of final product.
There are notes on the flikr to help identify components.
Below you may find photos of assembled box:
Wi-Fi box was mounted on the pediment of the house on Nord-West side of the house. Top Internet speed I witnessed was 0.9/0.4 Mbps DL/UL according to speedtest.net using CDMA EV-DO Rev.A connection. In the area where box is working, connection using solely internal modem's antenna is not possible.