Article by FRARS G4RFR
Page last updated: Wednesday, 3 July 2002
Mobile 2.45Ghz antenna
A prototype 2.45Ghz antenna for mobile use. I have used this antenna for surveying signal strength using a variety of free wlan tools.
This antenna is very simple to construct, requiring just a piece of wire, an N socket and a 100mm diameter (approx) tin lid.
The 100mm diameter lid has a hole drilled in the middle to accomodate an N type chassis socket. The socket can be pop rivited or soldered into place. The pre-shaped wire is then soldered to the centre pin of the N socket. The wire is about 1.5mm to 2mm in diameter - an old metal coat hanger was used in the prototype. The coils are 11mm inside diameter. Measuring from the groudplane, the first coil starts at 20mm, finishes at 40mm. The second coil starts at 118mm and ends at 138mm. The top of the antenna extends to 218mm. If you wish to add two more sections to this antenna, then leave 78mm of wire between the top of the second coil, and the bottom of the third. Then repeat for the forth coil, and leave the top of the forth coil extending up 80mm.
Use a pair of pliers and an 11mm drill to bend the coils. Try to ensure that the coils all follow the same turning direction although it's not really critical. If your finding it difficult to wind the whole antenna from 1 piece of wire, then make 4 seperate antennas with the phasing coil, and solder them all together.
If you cannot find a suitable tin lid to make the ground plane from, then make wire radials that solder to the N socket. If you start with 8 radials at 45 degree spacing that should provide a good ground plane. You could also wind a spiral of wire starting at the N connector and spiraling to the outside with about 10mm spacing between turns. This will create a good ground plane and make it mechanically rigid.
For the short antenna with 2 phased elements, the gain will be around 5dBi, whilst the 4 element version will have 7 to 9 dBi of gain.
So to summarise:
|
| 80mm
Z phasing coil
|
| 78mm
Z phasing coil
|
| 78mm
Z phasing coil
|
| 78mm
Z phasing coil
| 20mm
+-------- ground plane
N connector
The SWR of this antenna has been checked and it's below 1.2:1 therefore a good match to 50 ohms. The design is of the phased element type which is in common use in cell-phone antenna's.
Look at the hi-gain jpg's a,
b, to see the additional elements I have added to the original design. Using an SWR meter you'll be able to tweak the antenna's overall length to find the point at which the SWR is lowest.
C:\WIFICAN\WIFIWHIP\>cd..