Please turn on javascript in your browser to play chess.
Science Forum

Science Forum

  1. 10 Mar '08 17:53
    How the hell can someone make mobile phone antennas so small, when they work at around 2 Ghz frequencies?
    (no super-natural explanations, please)
  2. Standard member sonhouse
    Fast and Curious
    10 Mar '08 21:32
    Originally posted by serigado
    How the hell can someone make mobile phone antennas so small, when they work at around 2 Ghz frequencies?
    (no super-natural explanations, please)
    They have resonators that cut down the size. I am a ham operator, so I can give you an analogy to a low frequency dipole: At about 7 Mhz, the "40 meter" ham band, a dipole is about 67 feet long, cut in the center where the feedline goes. If you want to shorten it, you add inductance say, about halfway, depending on how much you shorten it, you might get away with something like 30-35 feet long. The problem with that scheme is the transmit efficiency goes down, you pay for it with a somewhat smaller transmitted signal, even though you might have a decent VSWR (Voltage Standing Wave Ration).
    You strive for a 1:1 ratio, where the final amp output goes to the antenna and nothing gets bounced back. How it gets bounced back is with an impedence mismatch, so the typical impedence is 50 ohms, and if the transmitter is 50 ohms and the transmitter looks into a 50 ohm antenna, all is good, all the energy leaving the transmitter goes out the antenna and none comes back, a mismatch, say an antenna impedence of 25 ohms means a VSWR of 2:1 and a certain percentage comes right back down the feedline. Now that is not the whole story. The transmitter can think its 50 ohms alright and the antenna can say, Hey, I'm 50 ohms too, so the RF checks out and does not check back in (bad for the transmitter, it gets heated twice that way). The kicker is the antenna transmit efficiency. If you have a pure dipole, you have a certain transmit efficiency, forget the exact # but it is charactoristic of most all dipole antennae. Now if you do like I said in the case of the shortened antenna, some of the energy that would have been transmitted in a high effiency system now goes into heat at the antenna
  3. Standard member menace71
    Can't win a game of
    11 Mar '08 04:35
    My Dad was a Ham radio operator. I remember he had to match his transmitter to his antenna when he turned his Unit on. One time he forgot to do just that and smoked his transmitter. I don't much about it all but I remember that much. I remember him saying that antennas were a science unto themselves. He made the antenna in our backyard. WA6ANR was his call sign.

    Manny
  4. Standard member sonhouse
    Fast and Curious
    11 Mar '08 23:22
    Originally posted by menace71
    My Dad was a Ham radio operator. I remember he had to match his transmitter to his antenna when he turned his Unit on. One time he forgot to do just that and smoked his transmitter. I don't much about it all but I remember that much. I remember him saying that antennas were a science unto themselves. He made the antenna in our backyard. WA6ANR was his call sign.

    Manny
    A California dude. Same as me, I started out as KN6QDT then K6QDT when I advanced out of novice. What part of Calif. was he from?
    I talked about that in my post, the antenna impedence wants to match the transmitter impedence to get the most power out and the least reflected energy back to the transmitter. Why your dad's transmitter smoked is when the impedence mismatch becomes great enough, it doubles the heat load on the transmitter, say it puts out 1000 watts and then goes to a poor antenna and that 1000 watts gets reflected back, its like being heated with 2000 watts which will burn it out if it goes on like that too long. Its not really free energy but it acts like it in the transmitter. BTW, my call is now AI3N. See if you can figure out what state I live in
  5. Standard member nmdavidb
    I Drank What? ©
    11 Mar '08 23:38
    Guys...since I have been working on my degree to quit doing stand up in clubs and get on the radio I have always been fascinated with messing around with the whole HAM radio deal.

    Can anyone tell me what would be a good set up and the price range?

    Oh and trust me I know all about the FCC and all...i do a clean act...which is why most of my gigs lately have been all corporate.

    Dave
  6. Standard member sonhouse
    Fast and Curious
    12 Mar '08 00:26
    Originally posted by nmdavidb
    Guys...since I have been working on my degree to quit doing stand up in clubs and get on the radio I have always been fascinated with messing around with the whole HAM radio deal.

    Can anyone tell me what would be a good set up and the price range?

    Oh and trust me I know all about the FCC and all...i do a clean act...which is why most of my gigs lately have been all corporate.

    Dave
    That depends on how you want to play the game. There are several varieties of amateur radio, entry level to the top level called Amateur extra. The entry level of novice allows you to talk on some bands while more advanced licenses give you more places on the radio dial to talk and other forms like digital mods. Here is a link to the bands allowed and the level of license it takes to get those bands.
    http://www.arrl.org/FandES/field/regulations/allocate.html
    It is a bit daunting to go through I know. But you can talk on some bands with the novice license and the tech license allows talking on some high frequency bands like 2 meters, which is 144 to 148 megahertz, a pretty big band, 4 megs wide. You can also learn morse code and do what is called CW or the dots and dashes like dit dit dit dah is V, and so forth. So the cheapest way to get into it would probably be at a hamfest where you have dozens and maybe hundreds of hams with the back of the van full of gear for sale cheap.
    Another route to at least see what you would like is to get in touch with a local ham club, if you have a license, they may allow you to check out some club gear for a couple of weeks or months to see what you like. Generally, hams are in groups which are devoted to long distance communications, usually on the lower frequency bands, like 40 meters, around 7 megahertz ( you have to compete with foreign broadcasters there) but that band is a world wide band and takes usually at least a dipole antenna about 70 feet long strung up between two trees. The higher the frequency, the shorter the antenna. The band I talked about earlier is a more or less line of sight band, 144 to 148 Mhz, or '2 meters', a lot of repeaters are on that band so you can talk a lot farther than line of sight and you can be a novice or tech and use that band. If you want to confine yourself to that band, there are rigs you can buy for about 100 bucks used that will get you there and handhelds, walkie talkies that will work there and go WAY beyond the bare range of the walkie talkie because of the repeaters, receivers coupled to transmitters that rebroadcast your puny handheld signal and put an antenna up on top of a mountain or high building that really extends your range, much more than you would get from just one walkie talkie to another, which poops out after maybe 5 miles at best but that same rig can literally talk clear across the country because of the repeaters, some of which are linked together to get your voice thousands of miles away, from repeater to repeater. But the longest range is had by the lower frequency band, like on 20 meters once, 14 megahertz, I talked from my house in Slatington Pa directly to another ham in Antarctica and I was using a simple antenna. Its a long involved tale about all this so the best bet is to get to know someone local who can take you to a ham club and they will be more than willing to put you into ham classes and such, take morse code if you want, its no longer an absolute requirement to learn code but it does get you some more bands to use. There is an amazing amount of bandwidth that hams can legally use! I few hundred bucks can get you started with used gear. You can also spend tens of thousands if you are rich! I have invested a few thousand, maybe 3,000 bucks and have some nice rigs, Collins, a classic ham rig company big in the 50's and 60's and a newer incredible rig that does just about everything from a company called ICOM, the venerable 706. It is the size of a CB radio, about 800 bucks new I think and has way too many bands for one radio! Its a great little rig! I should stop while I am ahead, I tend to rattle on for hours if someone doesn't stop me! Good luck, eh.
  7. Standard member nmdavidb
    I Drank What? ©
    12 Mar '08 02:32
    Wow! that was alot of info.

    Maybe pm some links and all and I will check out the local Ham club here in Denver.

    Thanks!

    Dave
  8. Standard member sonhouse
    Fast and Curious
    12 Mar '08 16:29
    Originally posted by serigado
    How the hell can someone make mobile phone antennas so small, when they work at around 2 Ghz frequencies?
    (no super-natural explanations, please)
    One more quick note about those antennas. At a frequency of 300 megahertz, the wavelength is one meter and a dipole for that wavelegth would be half that, being made up of two quarter wave sections, so a dipole at 300 megs would be about 20 inches, 2 ten inch section of equal length with the feed at the center. Now at 3 Ghz, the wavelength is one tenth of a meter or about 4 inches or 100 millimeters. So a dipole at that frequency is going to be only about 50 millimeters or 2 inches so its naturally small at those frequencies, so at 2 gigs, a dipole would still be only about 3 inches long or 75 mm. So taking a 2 gig antenna and cutting the size in half isn't a huge stretch over a plain dipole.