MADE A OWN WIRELESS ANTENNA
1.)
2.)
Overview
This model is from
picture http://www.gbonline.com/~multiplx/wireless/pics/tincanant.jpg where is not found any
theoretical principles but same construction is used in waveguide tube to coaxial
adapters.
The antenna is constucted only of a cylindrical can and a N connector with centre tap
lengthened. Just point the open end of the can to the ISP station and begin surfing.
Oh, of cource there is needed some cable between the antenna and wlan card,
see: Cabling details .
Dimensioning
Creech Lambda is replaced to L letter.
The length of the can is as is but best length maybe 3/4 Lg or more. The centre tap of
the N connector is lengthened with 4 mm dia brass rod to Lo/4. Lo depends only on
nominal frequency. Lo = 122 mm @ 2.45 GHz so Lo / 4 = 31 mm.
Lg depends on dia of the tube, here are some values:
Inner dia of tube D /
mm
90
95
100
105
110
Lg function of tube diameter @ 2.45 GHz
Standing wave length Lg /
Lg / 4
mm
202,7
51
186,7
47
175,7
44
167,6
42
161,5
40
For a N-connector there is needed a 12 mm dia hole which distance is Lg / 4 from
closed end. According the flange of the connector there are needed also four 3.5 mm
holesi. The inner tap of the N connector is lengthened to Lo/4 or 31 mm with brass
rod about 4 mm dia. Actually the length of the tap is not needed to be very accurate.
I've proved several lengths from 25 to 40 mm and not found big differences although
the impedance of the antenna is depending of the tap length. It's good idea to drill a
3mm hole axially to the end of the brass rod where the centre tap of the connector
tightly goes. So the rod soldering becomes very firm.
N connector is fixed with four 3 mm screws so that bolts are pushed from inside of
the tube and nuts screwed outside, so there becomes inside so less as possible extra
taps, which can disturb the antenna function. The jointing of the connector and tube is
sealed watertight with silicone mass. In the very lowest point of the tube there have
be drilled a small hole for condensed water outlet.
The open end of the can needs a cap. The plastic material needed here must
pass microwave oven test .
Antenna mounting to the antenna mast conduit can be made e.g. with some kind of
band round the cylinder to prevent the can flatten or dent.
Improved model
If the bottom of the can is not smooth a
extra bottom can be added inside the can.
It can be made from tinned steel or
aluminium which is cutted according
inside diameter of the can. How it is
mounted inside the can there are
countless means. It is not neccessary be
tight, microwaves are not passing through
narrow slots. Between the extra bottom and original bottom there becomes a space
with no need.
More effective version
The waveguide antenna can be equipped with a funnel which increases the sensitivity
of the antenna simply by collecting hf signal from larger area. This adding increases
the gain of the antenna by twice or 3 dB.
The right hand picture shows how the funnel is cutted from smooth tinned steel.
Dotted lines are showing margins needed to joints. I made this antenna by air
conditioning conduit with dia D = 100 mm where I added a bottom from tinned steel.
The antenna dimensions are then: D = R1 = 100 mm, D2 = R2 = 170 mm, Lg/4 = 44
mm, Lo/4 = 31 mm, 3/4 Lg = 132 mm.
I used this antenna during about a week with good results until I got a even more
effective antenna ready.
I haven't tested if it is possible to increase the outer dia D2 even more. The idea of the
funnel is from the satellite receiver horn found in ARRL antenna book.
The open end of the funnel is closed with a microwave proof plastic cap. Tighting of the
joint of the N connector and condenced water hole are similar as in the basic model.
Theory of the Waveguide Antenna
There are three different wavelengths in the waveguide tube. Here they are marked as
Lo, Lc and Lg.
Lo is the wavelength of the hf signal in open air or Lo/mm = 300 / (f/GHz).
Lc is the wavelength of the low cut frequency which depends on tube dia only Lc =
1,706 x D
Lg is standing wavelength inside the tube, it is function of both Lo and Lc
A waveguide which is closed on the other end acts similar as a short circuited coaxial
cable. The coming hf signal reflects from ending point and there forms so called
standing wave when incoming and reflecting signals in different places are either
weakening or amplifiering each others:
If there is a measuring
probe which is moving in
axial direction inside the
tube there can be found
some minimum and
maximum points in certain
intervals. At the closed end
the signal is zero and so
will be in halfwave intervals. The first maximum point is quarterwavelength from the
closed end. This will be the best place to outlet signal to coaxial line. You can notice
that maximum area is quite flat. So the place of the outlet must not be very accurate.
It is important to notice that the standing wavelength Lg is not the same as
wavelength Lo counted from hf signal. Large tubes are near as open air where Lg and
Lo are almost same but when tube diameter becomes smaller the Lg increases
effective until there becomes a point when Lg becomes infinite. It corresponds the
diameter when hf signal doesn't come to the tube at all. So the waveguide tube acts as
a high pass filter which limit wavelength Lc = 1.706 x D. Lo can be calculated from
nominal frequency: Lo/mm = 300/(f/GHz). Inverse values of Lo, Lc and Lg forms a
right angled triangle where becomes the equation of Pythagoras:
(1/Lo)2 = (1/Lc)2 + (1/Lg) 2
which can be solved
Lg = 1 / SQR((1/Lo)2 - (1/Lc)2)
In the antenna the N connector is situated in maximum point or length of Lg/4 from
the closed end. Total length of the tube is selected so that the next maximum place
hits on the open end of the tube or 3/4xLg from the closed end. The latter is only
supposed by my own and found be not the worst decision.
A Fiction
I cast here a model I have thinked. Why not use waveguide tube instead of
antenna cable too. The tube shall be so height that the lower end reaches to
near the wlan card of the computer. The tube shall be made of 100mm air
conditioning conduit with a curve and a funnel. The construction is very
resistance against lightnings too I believe. There it will be like a horn of a
steamboat. If someone will build this please feedback results.
Sources:
ARRL Antenna Book
ARRL UHF/Microwave Experimenter's Manual ISBN: 0-87259-312-6
Circular waveguide antenna calculator – JavaScript
See Fig1.1 for how to use values - most of them are unnescessary and slightly confusing.
•D is the interior diameter of the can
•Lo is wavelength in open air = 0.122 metres
•Lc is wavelength at lower dominant mode cut off frequency
•Lu is wavelength at higher dominant mode cut off frequency
•Lg is standing wavelength inside can
Lc = 1.706D
Lu = 1.306D
Lg = 1 / (sqr_rt{(1/Lo)2 - (1/Lc)2})
Ideally for the usual operating range of 802.11b:
•Lower cut-off frequency should be lower than 2400 MHz
•Upper cut-off should be higher than 2480 MHz
85
D in
Calculate
Lower cutoff frequency in MHz
Upper cutoff frequency in MHz
Lg in
Lg / 4 in - needed to make can
3Lg / 4 in - suggested minimum length
Lo / 4 in - needed to make can
Fig 1.1 - Circular waveguide antenna showing design values, click to enlarge.
Table 1.1 - wavelengths and frequencies against diameter
See Fig1.1 for how to use values
D
in
D in
inches
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
2.874
2.913
2.952
2.992
3.031
3.07
3.11
3.149
3.188
3.228
3.267
3.307
3.346
3.385
3.425
3.464
3.503
3.543
3.582
3.622
3.661
3.7
3.74
3.779
3.818
3.858
3.897
Lower cut off
frequency
in MHz
2407.236
2374.706
2343.043
2312.214
2282.185
2252.926
2224.408
2196.603
2169.485
2143.027
2117.208
2092.003
2067.391
2043.352
2019.865
1996.912
1974.475
1952.536
1931.08
1910.09
1889.551
1869.449
1849.771
1830.502
1811.631
1793.145
1775.033
Upper cut off
frequency
in MHz
3144.522
3102.028
3060.668
3020.396
2981.17
2942.95
2905.697
2869.376
2833.952
2799.391
2765.664
2732.739
2700.589
2669.187
2638.507
2608.524
2579.214
2550.556
2522.528
2495.11
2468.28
2442.022
2416.317
2391.147
2366.496
2342.348
2318.688
Lg
1/4 Lg
3/4 Lg
1/4 Lo
752.281 188.07 564.211 30.716
534.688 133.672 401.016 30.716
440.231 110.057 330.173 30.716
384.708 96.177 288.531 30.716
347.276 86.819 260.457 30.716
319.958 79.989 239.968 30.716
298.955 74.738 224.216 30.716
282.204 70.551 211.653 30.716
268.471 67.117 201.353 30.716
256.972 64.243 192.729 30.716
247.178 61.794 185.383 30.716
238.719 59.679 179.039 30.716
231.329 57.832 173.497 30.716
224.81 56.202 168.607 30.716
219.01 54.752 164.258 30.716
213.813 53.453 160.36 30.716
209.126 52.281 156.845 30.716
204.876 51.219 153.657 30.716
201.002 50.25
150.751 30.716
197.456 49.364 148.092 30.716
194.196 48.549 145.647 30.716
191.188 47.797 143.391 30.716
188.405 47.101 141.304 30.716
185.821 46.455 139.365 30.716
183.415 45.853 137.561 30.716
181.169 45.292 135.877 30.716
179.068 44.767 134.301 30.716