Omnidirectional Antennas-dbi increase; what effects?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N8SAN, Oct 14, 2021.

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  1. N8SAN

    N8SAN Ham Member QRZ Page

    Morning everyone.
    I'm a member on CloudyNights (astronomy- more specifically to me, astrophotography).
    There is a device called the ASI Air. It's basically a raspberryPi with some proprietary software that can control the telescope mount, camera, guide came, and a host of other things.

    You can connect to this device via internal wifi antenna.

    The 2nd generation continued this tradition, internal wifi, and it was atrocious. 6-10' max!
    The 3rd generation added an external antenna with much better, but still not as good results.

    One person commented that changing to a higher gain omni antenna, you would gain directivity and lose omnidirectional ability.
    I somewhat argued that because you're not adding elements. I was thinking like a yagi.

    "The problem with increasing dbi is that the signal becomes more narrow. At 2dbi the signal is a big cardioid donut shape. At 10dbi it becomes more of a directional cylinder. So when its aligned with your device you will have great signal but if you go upstairs or in the basement it will drop."

    A different use bought an aftermarket antenna.
    These are the two antennas in question. Stock on the left. (Simply for visual reference)

    This is the comment they had....
    " a few nights use and it is starting to appear that you are correct. Great connection at times, very little at others."
    Not a ton of information to be sure.
    And ONE data point.

    My 2 part question is this....
    1-Does increasing gain on an omni antenna, decrease it's omnidirectional ability?
    2-Why/why not?

    Thought I would come to the antenna masters here on QRZ! ;)

    Thank you!
  2. K6CLS

    K6CLS Ham Member QRZ Page

    Can you show me a picture of the RPi case?

    I have several, of all versions, all of them connect to the house wifi without issues. Through walls, up to 100 feet away. They are all in plastic boxes.
  3. N8SAN

    N8SAN Ham Member QRZ Page

    Well the original one is plastic and I have no issues with it.
    The 2nd gen was also internal BUT it was metal case. Which is why it's awful.
    The 3rd gen is external as shown above. So the case shouldn't matter?? But it's also metal.
  4. K6CLS

    K6CLS Ham Member QRZ Page

    Hmmm. I don't understand what you mean. The photo above is 2 antennas. I was asking about the RPi case.

    Did you try this instead of the omni antennas?

    I guess if you're doing astrophotography you must have dark skies, and few other wifi devices around. No other sources of radio frequency interference on 2.4GHz?
  5. N8SAN

    N8SAN Ham Member QRZ Page

    OK maybe I wasn't clear.
    The antennas are external.
    The case should not matter in this instance.
    I'm wondering what effect on directional ability does an increase in gain have on the omnidirectional ability.

    The photo was simply to illustrate the two antennas in question.
  6. N4UFO

    N4UFO Ham Member QRZ Page

    Omni and directivity are direct opposites. What happens with a higher gain omni antenna is that the pattern is more a cookie (flat disc) instead of a donut shape around the antenna... but still in all directions. (it is presumed they are pointing up when mounted) The text you quote referring to cylinders, etc. is talking about directional antennas... neither of the antennas you display in your post are directional, both are omni.

    What I did once for a wi-fi setup was take a piece of cardboard, paste aluminum foil to it, fold it into the designed shape (parabolic curve with a mounting piece) and slip it down over the omnidirectional antenna. THEN it had directivity. I aimed that 'dish shape' in the direction of the other wi-fi connection and it did increase the signal some.

    The antennas you show above... are you sure they are both for wi-fi? What frequency? The difference could be gain, but could also be that they are for different frequencies other than wi-fi. I have a small antenna like the one on the left, but it's for my home cell phone box, not wi-fi.
    W9IQ likes this.
  7. N8SAN

    N8SAN Ham Member QRZ Page

    OK. So it does technically lose some of it's omni portion and gains directivity.
    These get mounted on a moving telescope. As the scope moves through the night, that will result in nulls in the signal strength.
    While it still radiates in all directions, its flattens from the doughnut to the cookie. But still 360 on "azimuth" but loses "elevation".

    Here is the link provided to the larger antenna.

    The antenna on the left, the smaller one, is what comes with the box to be used on wi-fi.
    KA0HCP likes this.
  8. W9IQ

    W9IQ Ham Member QRZ Page

    An omni directional antenna that has more gain, flattens the toroidal pattern that surrounds the antenna to achieve this gain. If you view this change in gain from an elevation perspective, the gain in the extremes of the elevation are reduced in favor of the line perpendicular to the antenna. This is generally a desirable feature if the base of both antennas are near the same elevation as this concentrates the power toward the other antenna instead of towards the sky or towards the ground.

    If you do not require omni-directional (360° azimuth) coverage then a yagi type antenna will generally yield superior performance. Much like the old TV antennas, this must be pointed directly at the other antenna as its power is concentrated in a narrow beam (both elevation and azimuth) off the front of the antenna.

    Also keep in mind that there may be regulatory issues relative to your antenna choices.

    - Glenn W9IQ
  9. NG1H

    NG1H XML Subscriber QRZ Page

    The most likely explanation is not that one antenna has more gain. More that one antenna has less loss.
  10. W9IQ

    W9IQ Ham Member QRZ Page

    Perhaps that is an OK for a layperson but amateur radio operators should know that if you increase the antenna loss (decrease efficiency) you also decrease gain since antenna gain is directivity times efficiency.

    - Glenn W9IQ

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