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dual band antennas — Parallax Forums

dual band antennas

xanaduxanadu Posts: 3,347
edited 2013-05-24 11:46 in Accessories
If I had a "regular" dipole whip antenna with no markings would there be a way to figure out if it is 2.4ghz, 5.8ghz or dual? I can measure signal strength but it doesn't seem to help, or the antenna is dual band. I have a few of these and they got mixed up with other access point antennas and I don't want to just guess which went where. Thanks.

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2013-05-23 20:07
    You couldn't tell whether something is a 2.4GHz or dual band antenna because all the guts is embedded in what looks like the antenna. For that matter, you can't really tell what's what enyway. Antennas are effective based on their overall length and structure. You can have a 1/4 wave whip, a 5/8 wave whip, a 1/2 wave whip and all sorts of variations with various loading coils or loading capacitors on the ends of the whip, all to match the antenna to the transmitter, yet have the most antenna for the space available. I think a full wave at 2.4GHz is about 5 inches and at 5.8GHz about 2.5 inches. That doesn't leave much for a 1/4 wave antenna. You can stack multiple 1/2 wave antennas end to end to make an antenna structure. So, given a particular physical overall length including molding around the whole thing, what's the wavelength?
  • Duane DegnDuane Degn Posts: 10,588
    edited 2013-05-23 23:27
    I've been surprised several times to find the actual antenna inside plastic shell to be much shorter than the plastic shell.

    I have a similar problem with some 900MHz and 2.4GHz antennas. They both look the same but I don't know which antenna is which. I suppose I could attempt some sort of range check.
  • LeonLeon Posts: 7,620
    edited 2013-05-24 02:08
    I've never heard of a "dipole whip antenna"!
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2013-05-24 07:58
    Leon wrote:
    I've never heard of a "dipole whip antenna"!

    Imagine a vertical dipole formed from a length of coax by rolling the shield back onto the jacket by 1/4 wave, thus exposing 1/4 wave of center conductor. Of course, that's not how they're really made, and everything is molded into plastic or rubber, but it's the electrical equivalent.

    -Phil
  • xanaduxanadu Posts: 3,347
    edited 2013-05-24 10:49
    The antennas are 7.9" total and use rp sma connector. Now the whole dual band thing has me confused, two antennas using two wires? Even if they shared the same ground that still only leaves one wire. Never thought about it before. Lots more reading to do I guess haha. Neither of my enter keys are working in the forum right now lol... Duane I know what you mean because I've seen cheap access points that say they have a 2dbi antenna and the antenna is 10" long. That's messed up but I also think a lot of consumers wouldn't like the larger antenna and get the integrated ones. Still no enter keys. As always thanks for the information!
  • Mike GreenMike Green Posts: 23,101
    edited 2013-05-24 11:14
    If you have two (or more) antennas for very different frequencies, you can connect them in parallel. Only one of them will have a low impedance at a given frequency and the other(s) will have a very high impedance at that frequency and almost all of the power will go to the antenna tuned to the frequency involved. Multiband amateur radio antennas often work this way. You might have an antenna for each band (3.5MHz, 7MHz, 14MHz, 21MHz, and 28MHz for example) and you'd connect all of them in parallel at the top of a mast with only one coax feed down the mast. You can transmit on any one band and the signal will go to the right antenna. Downside of this is that spurteous signals are often harmonics and this sort of antenna complex will radiate the harmonics as well (very effectively).
  • xanaduxanadu Posts: 3,347
    edited 2013-05-24 11:34
    That makes perfect sense Mike, thanks. It seems like the 5.8ghz access point doesn't like the dual band antenna as much as the 5.8ghz antenna, and that is why this is confusing. Someone took down 20 access points, some were dual band, some were 2.4ghz and some were 5.8ghz. All of them had removable antennas which were brought to me separately in a big box. Looking at the picture online of the access point I was able to find some small differences that lead me to believe which antennas are which. Oddly, the dual band do not seem to work well at all for the 5.8ghz APs. They work great on the dual band radios though. Unless I have the antennas mixed up again, oh boy never ever let a moving company take down a network and box it up with no guidance from IT.
  • xanaduxanadu Posts: 3,347
    edited 2013-05-24 11:46
    The manufacturer offered to sort the antennas if I ship them. They won't provide an official statement saying the radios can be damaged by using the wrong antenna. The boss for this job is under the assumption that the radios could be damaged by using the wrong type because I mentioned it as a disclaimer. So I guess I'm mailing these antennas and moving on to project eight bazillion million seven thousand and sixty three, give or take. Thanks again for all the help. Hopefully this thread will be of use to others wondering the same thing about dual band antennas.
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