Homebrew outdoor antenna to 2.4 GHz band.
Antenna gain is compareable to panel antenna including Freedom Antenna Set sold in Finland. Comparing is made simple by connecting antennas to same computer with Orinoco wlan card and indicating results in Link Test of the Client Manager. My antenna has found been about 2 dB better gain than the Freedom antenna, which is specificated as 12 dBi antenna. And prices ... o, I need not even mention this. In European here is maximum allowable output power of 20 dBm (100 mW) because of the ETSI standard we use. While Orinoco "red" card is transmitting 8 dBm signal and my antenna gain is about 14 dBi there have to be only 2 dB cable and connector loss to keep output level below 20 dBm. The Orinoco Adapter Cable takes about 1 dB and H1000 antenna cable takes 2 dB/10m. So using 5 metres antenna cable there is no risk to exceed the output power within the ETSI standard. This antenna can also been built without the Ring to decrease antenna gain if needed especially with very short antenna cable.
Dimensioning
The very first point in antenna dimensioning is to count wavelength (the
Lambda character has replaced with L letter):
L/mm = 300 /
(f/GHz) -> at 2.45 GHz L = 122 mm.
Antenna picture without mast fasteners:
With the cover Without the cover
Main parts can been found
even in mama's dish cabinet but maybe safer to by then from department store.
The Reflector is made from aluminium cake pan and the Cover from plastic
microwave bulb, both 240--250 mm dia. The cace pan must be straight and smooth
without profilations and the sides will be orthogonally with the bottom. If you
find a pan made from stainless steel may be that will be more better.
Other parts needed:
A piece of copper water conduit, internal dia
10 mm, length below 40 mm
A piece of brass rod, outer dia 4-4.5 mm, length
< 40 mm
Copper wire about 2 mm dia, length < 70 mm
N-connector,
panel socket
A couple pieces of tinned steel sheet e.g. from tin
can
A piece of PVC coated electric wire 1,5 mm2
Screws and bolts M3,
solder tin etc.
Antenna construction:
The Reflector (aluminium pan) dia = L x 2, the height of the
sides = L / 4.
In the centre of the bottom there is the N connector
to left and the dipole to rigtht. A distance from bottom to the dipole is little
more than L / 4. The Dipole is mounted with an air-insulated coaxial type
foot, which impedance is 50 ohm and length L / 4. Inner wire of the
coaxial is made of 4 to 4.5 mm rod or conduit and shield made from splitted
copper conduit which internal dia is 10 mm. To the right of the dipole there is
a disc which dia is L x 0.4 (= 49 mm) and distance from the dipole is
again L / 4. Disc material is thin aluminium or tinned steel sheet.
On same distance as the disc there is also a ring made from pvc-coated electric wire 1,5 mm2 by binding it to ring diameter about L x 2. It is not neccessary to short the ends of the ring, I left ca. 1 mm space. If you want you can short the ends together as well. Pvc coating on the wire has no magic, you can use e.g 2 mm clear copper wire as well.
I mounted the Disc and the Ring to the plastic cover made from microwave oven bulb. There is needed some piece of plastic sheet and glue to mount the disk. The ring I glued inside the cover, so the diameter of the ring become some smaller than L/4.
Assembling:
The sides of the reflector (cake pan) must be L / 4 or 32 mm, so extra
height must be cut away. In the centre of the pan there come a hole 12 dia and
four 3.5 mm holes according the N-connector. The feet of the dipole is made of
splitted copper conduit and inside it a rod or a pipe. The impedance of the feet
must be 50 ohm which actually means the ratio of inner dia of the outer conduit
and dia of the inner rod be 2.3. It is quite near when inner rod or pipe has 4
to 4.5 mm outer dia and copper conduit has 10 mm inner dia. The exact equation
of Impedance versus diameter ratio is: Z = 138 * Log(D/d)
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If the inner wire is from rod it is good idea to drill axially a 3 mm hole on the end of it so the rod can be solder firmly to the inner tap of the N connector. The length of the rod became about L/4, but it is better first to leave it longer and cut it later after the parts are first pre-assembled.
The copper conduit must split with metal saw as accurate as possible and deburr with file. The end of the splitted conduit is then ought to fasten to the centre of the pan. It is not possible to solder it directly to aluminium but we make a flange from tinned steel sheet to same dimensions as the flange of the N connector. In the middle of the flange is drilled hole equal the outer dia of the conduit and four 3.5 mm holes as in the N connector. The one end of the splitted conduit is then soldered in the centre hole of the flange while keeping width of the slots betveen halves to 1-1.5 mm.
The splitted conduit is then cutted so that the length of the open slots becomes very accurate to L / 4. This the most accurate point in the antenna construction. Among with many roles the slot acts as a band pass filter which rejects other frequencies than nominal hf.
No it is good time to preassembly parts. With four 3 mm screws the N connector with the centre rod and the flange with splitted conduit are assembled on opposite sides of the centre hole of the pan, N connector to the outer side. The centre rod is now cutted to same length as splitted conduit.
The Dipole was made from 2 mm enamelled wire. The one arm of the dipole is
soldered both to the centre rod and to the half of the copper conduit. The
second arm of the dipole is solderes only to the second half of the conduit. The
arms of the dipole are cutted to L/4 from axle so total width of the dipole
becomes L/2.
Before soldering it is good idea to file grooves for dipole
wire to the centre rod and the halves of the conduit.
To the right of the dipole there is the disc and the wire ring which can be assemble to the cover bulb so that their distance from dipole becomes to L/4.
The cover bulb can be locked with three 3 mm screws and sealed with silicone mass as well as the N connector joint.
On the very lowest point of the pan there must be drilled a small hole for condensed water exhaust.
Mast mounting of the antenna
Mast fasteners can be made from exhoust pipe clamps. Above some mounting
ideas. Be carefully not to mount clamps in centre line otherwise the mast
prevents to connect cable to the N connector. The rightmost picture is from back
side.
Lightning protection
When the antenna is mounted above the roof there become risk of lightning damages to the wlan card.
The mast have to ground firmly e.g with 16 mm2 copper rope to the good earthing point. Lightning protection module is recommended between antenna cable in the place where the cable is incoming from the roof.
Antenna cable and connectors
Cable type Belden H1000 is found to be low loss on microwawe band and it is not very expensive. All cable and connector impedances must be 50 ohm. Outdoor connections are not watertight enough as such. So called self vulcanizing tape is best choice to wind over connection and above all there have to be wound black electrician tape to protect the vulcanizing tape against ultraviolet light. See cabling details.
Sources
Antenna construction is from page http://6mt.com/2304tech.htm where is
found an item MICROWAVE ANTENNA YOU CAN BUILD (73 10-82) c56.zip . I have dimensioned it to 2.4 GHz
band and made some addings.
11 July 2001
Martti Palomaki
Ilmajoki, Finland
elepal@saunalahti.fi
