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martes, 10 de mayo de 2011

A small TV aerial (UDA(*) YAGI)

Looks like a Barbie (tm) TV aerial, but it seems to work.
http://users.skynet.be/chricat/antennes/yagi8s.jpg hi http://users.skynet.be/chricat/antennes/yagi8p.jpg http://users.skynet.be/chricat/antennes/yagi8p2.jpg
It is based on calculations from the Java applet on http://fermi.la.asu.edu/w9cf/yagipub/index.html . I started from one of the examples (70 cm, 12 elem) and scaled it to 2400 MHz as explained. I used 1.5 mm² domestic wire. I drilled small holes in the wood to put the wires in and I glued them in. One can see pieces of matches glued to fix the 'N' connector. I just used 8 elements for a first test because my wooden stick wasn't any longer...
I am not sure whether it is a good idea to use wood, wires, folded dipole, etc... (the antenna should be adapted to the coax ?300 Ohms - 50 Ohms, ?balun) But the idea is worth a try. I just tested it in my apartment and it seems to be as good as a tin can or a BiQuad. ( http://www.saunalahti.fi/~elepal/antenna2.html, http://trevormarshall.com/biquad.htm, http://www.saunalahti.fi/elepal/antenna4.html, http://martybugs.net/wireless/biquad/).
I made two longer ones (66.5 cm, 20 elements). One with domestic electrical tubing (gray) that does not work and another one with a wooden stick (7x7 mm²) that gives me three dB's more (I expected five). Maybe, it is because the wood is thicker? I don't know yet what is important and what is not. What would make a good boom? Is it reproductible?... At first sight, it is easy to build, cheap and gives many dB's.
Branislav M. asked me how I did wire the dipole on the 'N-Connector'. Well, it is probably not the way to do... (I should use a Ballun, do it more carefuly,...). But it is the way I did it. And it seems to work (at least with a 2.5 meters pigtail). I have no idea how better it would be if it was 'well' done.
The 20 elements has been tested on 9.5 km (with a CardboardHorn on the other side) (Liege20020615) :
Here is the comparison with other antennae :
Beware of polarization!. It matches a parallel Marconi (Lambda/4 piece of copper)(all antenna wires in the same plane). Horizontal ones would go well with vertical Slotted Wave Guides (http://trevormarshall.com/waveguides.htm)
The original page is on http://ReseauCitoyen.be/?UdaYagi (sorry, it is in french, but there are other DIY antennas on http://ReseauCitoyen.be/?HomeMade and some experiments on http://ReseauCitoyen.be/?LongShots ; note that we use hierarchical lines to ease the navigation within our wiki (something like >Hardware>Antennas>Yagi) ).
Elem    length     distance (mm)
 1       61           0       (reflector)
 2       60           19      <- driven elem, it is a 'folded dipole'...
 3       56           26
 4       55           40
 5       53           60
 6       53           84
 7       52           112
 8       52           144
 (9      51           178)   (my stick is shorter,
 (10     51           215)       for a first test, it is good enough)
 (11     51           255)
 (12     50           296)
NB: I don't know where to link it on this wiki. If somebody can help. Thanks! (photos are still in Belgium; a local copy would be welcome)
  • -- xof, Liege, Belgium

See also :
This one is based on the Frisko design :
Elem    length     distance (mm)
 1       58           0       (reflector)
 2       53.5         20      <- driven elem, it is a 'folded dipole'...
 3       52           35
 4       51.5         50
The wire is 0.8 mm brass (model-making)

During the Belgian Microwave Roundtable 2002, I got the opportunity to 'measure' it (with a Network Analyser, thanks ON4AOD). The S11/SWR plot is on http://users.skynet.be/chricat/antennes/UdaYagi-swr-300h.png. (But don't ask me what it exactly means for the moment. It seems it is not so bad somewhere in the ISM band...;-)

Another crazy idea is to use Tetra Brik (Milk or Juice carton) as a wave guide antenna. It contains enough aluminium to reflect waves. See http://ReseauCitoyen.be/?BoiteDeLait . It is also easy to add a horn and get more dB's. :-)
http://users.skynet.be/chricat/antennes/JusDeFruit.jpg http://users.skynet.be/chricat/antennes/JusDeFruit2.jpg
Lg = 1 / SQRT(1/(L0*L0) - 1/(Lc*Lc))
and Lc = 2 * width
width is here 9.5 cm -> Lg is ~16.6 cm at L0=12.5 cm (2400 MHz); The rod is thus at ~4 cm from the bottom of the brick. The rod is Lambda/4, about 3 cm. It is 'fragile', but so easy to build... (and you can already go far away, probably more than 2 km with two of them).
For further adventures with horns, see CardboardHorn
07sep2002, another Tetra Brik horn was used with a SlottedWaveguide (this one) on a ~15 kilometers link in Belgium (Louvain-La-Neuve -- Waterloo). The bigger CardboardHorn (see the picture) allowed 300 KB/sec.

For the skeptics, I just ran some tests. I compared my antennae about 30 meters away from a (vertical) Lambda/4 inside my apartement (it goes through a window and there are a few tree leaves in the path, so don't extrapolate the distance).
Ups and downs show the polarization effect (vertical/horizontal). This (short) Uda-Yagi is the best :-). Of course, a longer one will still be better.
Measurement are done with the iwspy of the wireless tools on Linux (http://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Tools.html and plotted with http://www.gnuplot.info/). I also use a front-end (http://ReseauCitoyen.be/scripts/dynspy-0.07.txt (a perl script, rename -> .pl) found on http://ReseauCitoyen.be/?TestTools to log the measurements and see the dynamics.

*Note: The Yagi-Uda provides an object lesson in the importance of good communication skills. This antenna was invented in the early part of the 20th century by a pair of Japanese engineers named Uda and Yagi. Uda san was the brains of the operation and had the key idea, but had very limited english. Yagi, however, was fluent enough in english to take care of the publication. As you've seen, he has been immortalized (at least among radio weenies), while Uda languishes in obscurity.) (ref)

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