2.4 GHz antenna for 5GHz?

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LOM
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PostPosted: Mon Aug 23, 2010 14:52    Post subject: Reply with quote
looki, looki here:

http://www.moxa.com/product/ANT_WDB_ANM_0502.htm

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PostPosted: Mon Aug 23, 2010 15:37    Post subject: Reply with quote
Yes, but what they actually use is more likely a 1/4 wave whip consisting of a piece of wire, because it's cheap and works well. If they have two frequencies, they might use two pieces of wire in a single housing, tied together at the feed end, with insulation on at least one of them so they don't make contact along their length. The one for 2.4 GHz would be about an inch and a quarter long, the one for 5 GHz about half that. These lengths would actually be determined experimentally, I guess, because they would be affected by capacitive effects. Of course they could be using other lengths -- but they won't be using even multiples of a half wave. They could probably even find a single length that would work OK for both frequencies, perhaps 3/4 λ on one freq and 3/8 λ on the other. My telephones, which use both frequencies (the handset transmits on one freq and the base transmits on the other) use separate antennas, with a capacitive top hat on the one for the lower frequency so the two can be about the same length.

Antenna design is partly analytic and partly experimental.

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luis55
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PostPosted: Tue Aug 24, 2010 0:34    Post subject: Reply with quote
What about the inverse question. What would happen if I use a 5Ghz antenna with a 2.4Ghz radio? Can I damage something?
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h4x0r
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PostPosted: Tue Aug 24, 2010 0:42    Post subject: Reply with quote
luis55 wrote:
What about the inverse question. What would happen if I use a 5Ghz antenna with a 2.4Ghz radio? Can I damage something?

You definitely will not damage anything, as for performance, I do not know. If you are talking about laptop internal antennas, then it doesn't seem to matter. (I recently upgraded my netbook with a dual band n wireless card (Intel 5300 half size) and added two antennas, because the laptop had only one, which is ridiculous. I did not replace the original antenna though) But the laptop antennas seem to have a different design that those in desktop adapters and routers (except for routers with integrated antennas where the antennas are exactly the same as in laptops)
socal87
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PostPosted: Tue Aug 24, 2010 2:40    Post subject: Reply with quote
My few cents on this...

As a radio technician in the Marine Corps, most of the antenna systems I have come across are 1/4 wave...meaning they're approximately 25% of the full wavelength of the operating frequency. Now, keep in mind that we obviously don't use just one frequency, so for instance...VHF communications are from 30 to 87MHz. 1/4 wave at 30MHz is about 8.2 feet, while at 87MHz it's 2.83 feet. So, we have to find a compromise, and either use a tuner to change the antenna's electrical wavelength, or one that is appropriate for a fairly broad range of frequencies.

Let's assume that most manufacturers use half-wave antennas, since they lack a suitable ground plane to simulate a half wave with a quarter wavelength antenna. Explanation: With antennas that are the length of half the wave, you get the best radiation of power (so I've been convinced anyway) but obviously at lower frequencies this is impractical as the antenna would have to be very long. So, using a quarter wave antenna with a ground plane allows the quarter-length wave to reflect off said plane, in effect simulating a half wave. In order to visualise this, imagine a circle, such as a plate, cut in quarters. Then imagine one of these pieces placed against a mirror. The image of the piece plus its reflection appears to be half of a plate.

Keep in mind that every time you DOUBLE the frequency, you HALVE the wavelength. So, the 802.11bgn band, from 2.412GHz to 2.462GHz (in North America) would have to use a 2.44 inch antenna on channel 1, with 2.39 inches at channel 11. Not much difference, so a 2.41 inch antenna would suffice.

However, with 802.11an, you're looking at 5.180GHz (1.14 inches) on channel 36, and 5.825GHz (1.01 inches) on channel 165. Again, not a huge difference, but as aforementioned you get the best performance using an antenna that's tuned to the particular frequency you're using.

Thus, seeing the difference in wavelength between a 2.4GHz and 5GHz antenna, you can see why you can't really mix the two. Though, I suppose you could use a 5GHz antenna as a 1/4 wave with groundplane for 2.4GHz, but that's neither here nor there, and I imagine everyone's brains are fried by now.

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PostPosted: Tue Aug 24, 2010 2:56    Post subject: Reply with quote
socal87 wrote:
My few cents on this...

As a radio technician in the Marine Corps, most of the antenna systems I have come across are 1/4 wave...meaning they're approximately 25% of the full wavelength of the operating frequency. Now, keep in mind that we obviously don't use just one frequency, so for instance...VHF communications are from 30 to 87MHz. 1/4 wave at 30MHz is about 8.2 feet, while at 87MHz it's 2.83 feet. So, we have to find a compromise, and either use a tuner to change the antenna's electrical wavelength, or one that is appropriate for a fairly broad range of frequencies.

Let's assume that most manufacturers use half-wave antennas, since they lack a suitable ground plane to simulate a half wave with a quarter wavelength antenna. Explanation: With antennas that are the length of half the wave, you get the best radiation of power (so I've been convinced anyway) but obviously at lower frequencies this is impractical as the antenna would have to be very long. So, using a quarter wave antenna with a ground plane allows the quarter-length wave to reflect off said plane, in effect simulating a half wave. In order to visualise this, imagine a circle, such as a plate, cut in quarters. Then imagine one of these pieces placed against a mirror. The image of the piece plus its reflection appears to be half of a plate.

Keep in mind that every time you DOUBLE the frequency, you HALVE the wavelength. So, the 802.11bgn band, from 2.412GHz to 2.462GHz (in North America) would have to use a 2.44 inch antenna on channel 1, with 2.39 inches at channel 11. Not much difference, so a 2.41 inch antenna would suffice.

However, with 802.11an, you're looking at 5.180GHz (1.14 inches) on channel 36, and 5.825GHz (1.01 inches) on channel 165. Again, not a huge difference, but as aforementioned you get the best performance using an antenna that's tuned to the particular frequency you're using.

Thus, seeing the difference in wavelength between a 2.4GHz and 5GHz antenna, you can see why you can't really mix the two. Though, I suppose you could use a 5GHz antenna as a 1/4 wave with groundplane for 2.4GHz, but that's neither here nor there, and I imagine everyone's brains are fried by now.


Nice explanation. I once made a TV (Longwave) dipole antenna that was composed of two 1 metre pipe connected to coax. It was exactly the right length for one channel as I was sick of seeing snow on the screen. I considered threading the ends of the pipe, and using bolts that I could thread out to extend the length if I wanted to get other channels. I think the next channel down needed to be one inch longer on each end...

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