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I am looking for good antennas for my Access Points, omni-directional type. Surfing through some Chinese websites, I found some.

This 7dBi 220mm antenna costs 0.45 USD: enter image description here

This 12dBi 300mm antenna costs 0.51 USD: enter image description here

And this "paddle" 18dBi 195mm antenna costs 0.71 USD: enter image description here

They are all from the same manufacturer. And they are all designed for 2.4-5.8Ghz. I don't believe the specification (in dBi) which is likely just a random number. And asking the salesman is a waste of time. My questions are:

  1. What is the purpose of the coil springs? Is more coil springs better?
  2. Does the antenna length matter?
  3. What is exactly the "paddle" type of antenna? Does it have any advantages over the other types such as Yagi or panel?
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    The short answer is No, you cannot tell how good an antenna is by looking at it. WiFi antennas don't operate the way most would logically think they do... each antenna will operate in one of the three ways: (1) radiate a 360-degree signal in all directions equally (this is the logical way most would think, but is the least common by default); (2) radiate to a less degree horizontally than vertically (vice versa); OR (3) radiate to a severe degree horizontally and minutely vertically (vice versa). For 2 & 3, imagine a cone-like radiation horizontally and vertically.
    – JW0914
    Commented Jan 11, 2020 at 13:08
  • Random tip of the minute: Are you sure you want omni-directional? In theory, going all directions sounds great. (After all, why limit yourself unnecessarily, if there is no benefit?) So, if that sounds so great, then, why not? Well, the reason is because antennas have regulatory restrictions on how much power they can output, and so focusing the output power in one direction does provide superior communication in that direction (at the cost of not pumping power output into other directions). If you're strongly seeking better performance, is uni-directional something you can benefit from?
    – TOOGAM
    Commented Jan 11, 2020 at 13:15
  • @TOOGAM My Wi-Fi network covers several houses. Except for my 3rd floor, I want all APs to have omni-directional antenna -- I intentionally put them in the middle of the yard or room. Besides, I do not know why panel antennas are much more expensive, usually 4 to 5 times an omni-directional one.
    – Livy
    Commented Jan 11, 2020 at 13:23

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No, you can't tell how good an antenna is just by looking at it.

You certainly can't tell by looking at the external plastic enclosure (radome) like in the example of the paddle antenna. Plastic is RF-transparent; it's only the metal parts that matter.

There are a few obvious things anyone familiar with antenna design basics can judge just by looking, like if all you see is a single dipole design, it's not a "high gain" antenna; it's just going to be a standard ~3dBi gain omnidirectional. If you see stacked dipoles, you can guess someone was going for a high gain omnidirectional. If you see a patch or panel or yagi or parabolic antenna design, you know it's not intended to be an omnidirectional antenna. If you see the size of the antenna, you can make some guesses as to what frequencies it might be good for.

If you have an experienced antenna designer look at someone else's antenna, they would usually be able to tell what the other designer was attempting, and would know what a well-executed design of that type should be capable of, but they wouldn't want to say if the other designer had met their goals without putting it into an antenna testing chamber and hooking it up to lab equipment and actually measuring its various performance characteristics.

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The short answer is No, you cannot tell how good an antenna is by looking at it.

WiFi antennas don't operate the way most would logically think they do... each antenna will operate in one of the three ways:

  1. Radiate a 360-degree signal on plane [omnidirectional] or on all planes [isotropic]
  2. Radiate to a less degree horizontally than vertically (vice versa)
  3. Radiate to a severe degree horizontally and minutely vertically (vice versa)

For 2 & 3 above, signal is a cone, radiated horizontally and vertically, but at different degree slopes
Generic visualization:
generic visualization

  1. What is the purpose of the coil springs? Are more coil springs better?

The purpose of the loading coil is to increase inductance

  1. Does the antenna length matter?

Generally, the longer the antenna the broader the range and stronger the signal

  1. What is exactly the "paddle" type of antenna?
    Does it have any advantages over the other types such as Yagi or panel?

Generally speaking, paddle-type antennas are more aesthetically pleasing and often have cone radiation with a degree slope to increase range either horizontally or vertically.

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  • This answer is sorta on the right track but it seems to conflate omnidirectional and isotropic. ALL of the dipoles in this answer's illustration would be omnidirectional, radiating equally in all 360 degrees (not 365, like the image erroneously states) in a horizontal disk (toroid really). If you want an antenna that radiates equally in a perfect sphere, that's an isotropic antenna (0dBi), not an omnidirectional.
    – Spiff
    Commented Jan 11, 2020 at 19:32
  • @Spiff I didn't catch that typo in the image, thanks for letting me know =] I spent ~10min using google image search and it was the only picture I came across that was large enough to be acceptable, as most were150px or less, and it was only meant as a generic visualization since I know the logical visualization is that an antenna radiates equally in all directions. Would it be recommended to simply remove the generic picture, or correct it's typo and re-upload? I'll also correct the terminology.
    – JW0914
    Commented Jan 12, 2020 at 12:41

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