Naked Science Forum
General Discussion & Feedback => Radio Show & Podcast Feedback => Topic started by: thedoc on 03/12/2008 11:51:06
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The Naked Scientists hit Hollywood for a special show all the way from California. We meet the cream of Californian scientists from the University of California at LA and at San Diego to find out why designing ocean-going robots is like giving birth; what some dust-covered 50-year old experiments can reveal about the origins of life, and why your friends and even their friends, can affect your weight. We discover the work of the Population Institute, who use radio soap-operas to inspire better family planning. Plus, we find out why plants get jetlag, how nanotechnology can help foil terrorist attacks and we visit the Naked Cafe to discuss the Big Bang over coffee. In Kitchen Science, Ben and Carlos Camara generate x-rays from sticky tape!
Listen to this Show (http://www.thenakedscientists.com/HTML/podcasts/show/2008.11.30/)
If you want to discuss this show, or ask a question, this is the place to do it.
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Hey, Guys (and Ladies). I love the show (http://www.thenakedscientists.com/HTML/podcasts/).
But. I have this to offer about the Yagi antenna question:
TV antennas -- at least in the US -- are typically "log periodic"
antennas, not Yagis. The difference is that a Yagi has just one element that is electrically connected to your receiver. There is one "reflector" behind that, and one or more "directors" in front of it. The reflector does exactly what one would think. The directors help send or receive the signal more or less in one direction.
A log periodic antenna can have nearly any number of elements, and all are connected to the receiver in a clever way that I won't bother with here. The gain is somewhat less than that of a Yagi of the same length, but the log periodic works over a very wide frequency range. My Ham Radio club once built one out of wire that covered 7mhz through 30mhz. It was 135 feet long and 60 feet wide with 14 elements!
(OK, nobody needs to know all that. But it was a chance to show off. :)
John Blinke (KI8Y) in Rochester, Michigan, USA
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You have to start with a basic 2 element array of two fed dipoles. The dipoles are 1/4 wave apart and they are fed with coaxial cables which differ in length by a 1/4 wave. In one direction the signals from the two will combine because they arrive in phase. In the opposite direction they cancel out because they arrive in antiphase.. 1/2 wave (180 deg) out of phase. So all to do with coherent sources, phasing and interference patterns.
Mr Yagi and Mr Uda found that if you just have one dipole the second element, now a continuous element, could be powered from the first parasitcally. It will be in the 'near field' of the first. They found that multiple elements usually with a tapering of length could be all excited parasitically from the fed dipole and from each other and it would produce a fairly sharp forward beam.
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Max Roberts suggested:
I have an easier way to do the Glowing Tape Experiment.
I don't have the patience to sit in a dark room getting rid of light pollution!
Just go to sleep with a roll of tape on your nightstand, and when you wake up in the middle of the night to go to the loo, rip out some tape as you go! (Well not as you "go" but I hope you know what I mean!)
If your eyes aren't sensitive, perhaps try putting the tape under your sheets. Stick your head under when you wake up and rip away.
This is how I stumbled on my technique!
I couldn't sleep tonight. Without turning on the light, just now I was tearing open a paper bag to get to something to help me sleep, and noticed blue light where I was tearing. Astonished, I continued, seeing bright blue light where the paper was coming apart. I fumbled around looking for other paper, but couldn't get anything except the original paper bag to spark like that. It now lies in shredded pieces on the ground around me.
I fired up the computer to write you this email, and now that I have almost finished, I have ruined my night vision and can't see it sparking any more.
I'm sure Dave might be interested to hear that paper was sparking; but let me tell you that I couldn't sleep because of neck pain, and what I was opening was a paper packet of a kind of traditional Chinese medicine. This stuff is on a kind of a patch that you stick onto your skin. When you do so, your skin gets gradually hotter... anyway, I'm just telling you this because I'm sure that there is some kind of glue that keeps the patch attached to your skin, and i suspect that some of the glue has transferred to the paper, and that is what is sparking.
Keep up the good work everyone!
Max in Shanghai
What do you think?
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You describe what I would call a “phased dipole array.” Dr. Yagi certainly built many kinds of antennas. The one that his name stuck to -- at least on this side of the Atlantic -- is the design with one driven dipole element and and two, or more, parasitic elements. (I’ve used aluminum tubing and, alternatively, copper wire -- with equally good results.) The dipole “driven element” is of resonant length. The director is 5% shorter than the driven element. The reflector is 5% longer. The resonant length for the driven dipole element is given by:
468 divided by the frequency in Mhz -- (to get an answer in feet).
There are many resources that will let one design and build custom antennas. One is the ARRL Antenna Handbook from the American Radio Relay League. Ham radio magazines have run many articles on antenna building. Wikipedia has something to say, as well:
<http://en.wikipedia.org/wiki/Yagi_antenna>
I seem to recall a recommended element spacing of .15 times the free space wavlength (Wikipedia says .25 wavelength. My antenna book is in a box, somewhere, so I can’t look at it just now). In practice, one can tinker with the spacing and length of the parasitics to achieve more signal gain at the expense of reduced front-to-back ratio and unwanted side lobes. These can be acceptable, depending on what the operator wants to accomplish. On a crowded band, it can be more important to have a clean forward gain lobe, rather than maximum signal gain with QRM coming in the "back door."
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I only mentioned a phase array so as to lead on to how a Yagi works.
I was arguing elsewhere that it isn't really possible to make Yagis with huge bandwidths such as the whole FM band.. 88 - 108 and big chunks if not the whole 470-860 UHF TV band. How can you have 5% longer or 5% shorter when 108 is 23% higher than 88? They must be very much of a compromise.
I have built Yagis . German Amateur, Guenter Hoch, DL6WU has done a lot of work optimising Yagis and they are quite narrow band. Optimised for about 500 KHz at 144 MHz.. American, L.B. Cebik, W4RNL is another expert.
http://www.geocities.com/guninda/Guenters_page
http://www.antennex.com/Sshack/long-boom/lbyagi.html