What properties does a new antenna change in a router/access point?

Question

If I buy new antennas for my wireless router or for an access point what properties can I expect to be improved and worsen?

I expect the antenna's range to be improved but does the antenna itself has an effect on bandwith for example?

Answer 1

Many properties are affected!

• First and foremost, your regulatory compliance is affected! If you increase the gain of your antenna without a compensating loss (for instance, by putting a lossier coax run in as well), you may end up exceeding permissible limits.
  • In the U.S., this is governed by the FCC
  • Start with https://secure.dslreports.com/faq/wisp/3_Radio_power___other_legal_stuff and then move to the actual FCC regulations, documents like https://www.fcc.gov/document/5-ghz-unlicensed-spectrum-unii and
  • Note that in some cases, directional antennas (mostly dishes, panels, and/or Yagis) used solely for point to point links may have more permissive rules that use more math
  • Note that in some cases, limits change based on frequency (channel) and other factors
  • Also, depending on your regulatory location, you may or may not be allowed to radiate (transmit) after you change the antenna, as the device may not be considered licensed anymore.
• Next, your radiation pattern is affected!
  • This affects your transmissions (when you radiate, where does the energy mostly go?)
    • In any area that gets more power, that's no different technically than simply increasing your transmit power
  • This affects your reception (how much energy is "enough" for your reception, i.e. what is the "signal strength" of other transmitters)
    • This means that with a higher gain antenna, within the main lobes, you can receive data from transmitting units (like phones and laptops) further away than with a lower gain antenna, or if the device is outside the main lobe and therefore getting lower gain due to position.
  • "Range" is a matter of being able to transmit so the other device can receive well enough, and being able to receive the other device's transmissions well enough. One without the other is worthless for using wifi networking normally.
  • Here's a spec on a 2dbi vertical omni http://www.streakwave.com/mmSWAVE1/Video/ANT-DS-S2403BP-0310.pdf
    • See how it's circular in the H-plane, and has big fat lobes on the chart in the E-plane
    • Note the 38 degree vertical beamwidth
    • in other words, you get reasonable signal strength (receiving and transmitting both) if you're out a ways and up or down some, like on a hill, or on a floor above or below but not right above the antenna.
  • Here's a spec on a 15dbi vertical omni http://www.streakwave.com/mmSWAVE1/Video/15-503.pdf
    • See how much narrower and thinner the lobes are?
    • Note the 6 degree vertical beamwidth?
    • In other words, you can be much farther out and get reasonable signal strength, but if you go up or down relative to the antenna, you're going to lose signal very quickly
      • I.e. don't put it on the top of a tall tower on a hill!
      • I.e. do put it in the middle of a flat field, not overly high up
    • Note that 15 dbi is really quite likely to put you out of regulatory compliance unless you reduce your transmit power. Decent radios have options for this; good ones actually measure it for you (e.x. Ubiquiti products)
  • And here's a spec on a 19dbi parabolic dish http://www.streakwave.com/mmSWAVE1/Video/TA-2418.pdf
    • Note how there's only one major lobe, and it's narrow and thin in both planes?
    • Note also the beamwidths of 18 and 20 degrees? The H-plane of the omnis was 360 degrees; this puts almost all the radiation and sensitivity into one direction
      • This can be very good, since you need very little power for the same signal, and even powerful signals out of the lobe are going to interfere very little; for instance, your neighbor's wifi!
    • Note that 19 dbi is really quite likely to put you out of regulatory compliance unless you reduce your transmit power. Decent radios have options for this; good ones actually measure it for you (e.x. Ubiquiti products)
    • With a radiation pattern like this, you'll need to very carefully point the antenna.
  • Just to round it out, a spec on a 16dbi Vagi http://www.streakwave.com/mmSWAVE1/Video/PAWVA_Data_Sheet-v2.pdf
    • Much like the dish, but with larger 25 and 30 degree beamwidths
    • All the same concerns as the dish apply, except you've got a larger useful area.
• Additionally, other properties are affected
  • VSWR and Impedance are commonly affected; if your impedance is mismatched or your VSWR increases, then you end up with less radiated power, which isn't good for your Wifi connection.
    • Note that VSWR changes based on frequency, particularly with high frequencies like 802.11a/b/g/n/ac uses; this is particularly important in the 5Ghz "band" - see Bandwidth, below.
  • Bandwidth of course varies also, i.e. the range of frequencies (channels) with low (or "low", if your whole antenna is ) VSWR.
    • Since the low 5Ghz Wifi Channels are quite far from the high ones; channel 36 is 5.18Ghz, while channel 161 is 5.805 Ghz. It takes an antenna with a wide bandwidth to handle both frequencies with a reasonable VSWR.

Answer 2

consider that if you improve the range, you're also increasing the probability of interference from other wireless devices (because you're suddenly receiving them better as well).

Antennas do affect bandwidth. If the antenna is designed for the device, it shouldn't be an issue. Bandwidth issues you might experience would more likely be from error correction, etc. from the 'interference' mentioned above.