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Antenna design again

Discussion in 'General Electronics Discussion' started by flippineck, Feb 3, 2014.

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  1. flippineck


    Sep 8, 2013
    I've given up with my yagi for my 2100MHz mobile phone / internet unit.

    Firstly, it seems that the tower I have it pointed at, probably isn't in use anymore :eek:

    The UK Ofcom database of cellphone masts appears to be horrendously out of date.

    I tried installing an app on my smartphone called OpenSignal. It's quite nifty in that it shows you a map with all the cell base stations marked on it, and connects a red line from your position to the tower you're connected to in realtime.

    Well when I ran it, I ended up with a list of about 10 different cell towers, and the phone regularly swapped from one to the other.

    Now I did go out in the car to the locations indicated & could not see -anything- at all that looked like a cell tower at any of the locations indicated. One of the locations is slap bang in the middle of a large field owned by a friend, and he's fairly certain he ain't got no cellphone masts in there, just some horsies.

    The phone was pretty consistent in finding and re-finding each of these transmitters though, evidenced by their CID/LAC numbers. So, they must be out there *somewhere*.. maybe OpenSignal tries to deduce their position from the crowdsourced data they run on, and so the indicated locations are only 'average probabilities'? I've tried contacting the mobile operator for cell tower locations but might as well talk to the wall.

    Anyway.. what I did figure was, that all these regularly changing connections, are in different compass directions.

    So, a highly directional yagi would probably actually be a BAD idea in practise! (especially if it's pointed at nothing energised *slaps own forehead* DOH)

    So.. I guess what I'm looking for is an omnidirectional / omniazimuthal antenna.

    A plain vertical 1/4 wave dipole has a radiation pattern shaped like a doughnut laid flat on the table. I understand you can get more gain in a horizontal direction by 'squashing' the doughnut flatter, by using some weird and wonderful antenna variations.

    I've heard of discone, coaxial collinear, biconical designs.. they all seem pretty heavy on the maths and I don't think I could come up with anything workable unless I had exact plans.

    I had a good look at the biquad antennas but maybe they're still too directional. Could you set up 4 biquads at right angles to each other and connect all the feeds together? Or would the maths mess up and signals just cancel out and all go to hell in a handbasket

    What's the easiest and most reliable antenna I could try that has a good bit of squashed doughnut omniazimuthal gain?
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The other option is to use the original directional antenna and point it a other towers. Find the one that you can most easily point it at and which gives you a strong signal.

    This is probably one reason we could see nothing particularly wrong when we looked at the common culprits :)
  3. flippineck


    Sep 8, 2013
    I was wondering about that. Maybe if it 'locked on' to a good mast with a decent signal it wouldn't swap around from tower to tower?

    I did try standing in an upstairs room with the yagi on a broom pole, slowly rotating it through all points of the compass for a good long while.. never 'found' an appreciably better signal in any particular direction though. To do the same with it mounted up outside I guess I'd need a rotator motor of some sort.

    One thing.. one of the handheld smartphone's 'favourite' towers that it seems to like to 'hang onto' more than the others, is on the opposite side of the house to the outside wall I've got the yagi bolted up to. This means that at the moment, not only is the yagi pointing 90 degrees away from the favoured tower, it's line of sight is interrupted by the roof of the house. (The pole it's on gets it up above the lower eaves into free space, it's got direct line of sight to the old switched off tower, but there's still the higher part of the roof in the way, in the direction of the currently favoured tower. Which exists on the map but is an open field with nowt but cows in reality but i digress)

    Problem is, to get it into line of sight for the good tower, and keep a short coax length, I'd have to mount it on the front of the building and the local council may prohibit that. Could go for a chimney mount but that would involve about 12 metres of coax.

    I could probably get away with a taller pole and a less conspicuous antenna, located where it is now. I was wondering about a biconic design with rods approximating the solid cones, don't know if that would be any flatter a doughnut than just a dipole

    Wish I'd bought a phone with an external antenna connector. As it is, only the router unit has external antenna provision. The smartphone (Sony Xperia E) can only be connected to an external antenna via an inductive strap-on type connector.

    Anyone suggest a simple, cheap, current, network unlocked smartphone available in the UK that has an external antenna connector, that 'supports 3G'? (the network, Three, specifies that their SIM will only work in a 3G phone). Android would be good so I could run OpenSignal on it. I can't find anything that fits that doesn't cost gazillions
    Last edited: Feb 3, 2014
  4. davenn

    davenn Moderator

    Sep 5, 2009
    That would be logical :)

    a lot of the gain of the antenna would be lost in the coax

    you would probably be better off with the earlier option ( first in this post)

  5. flippineck


    Sep 8, 2013
    I thought I had a great idea involving arduinos and multiple yagis then I read about something called WiBE (Wireless Broadband Enabler).. seems it's already been done

    4 antennas arranged at 90 degrees to each other covering all 4 points of the compass, with walls in between them in a + shape if you're looking down on it. 4 antennas in 4 quadrants pointing N-S-E-W

    Apparently it uses 'special active algorithms' to switch in, whichever of the 4 antennas is 'pointed' at the currently in-use/best tower

    Nifty but expensive and I'm not even sure if they're still doing them.

    Trouble with the router I've got is that whilst it does have not one but two external antenna ports, i can't run OpenSignal on it like i can on the phone.. so I don't actually know what it's doing, what tower it's connecting to etc.

    I think, I will give this collinear type a go (changed slightly for 2100MHz as opposed to 2400MHz):

    Came to the conclusion half wave length would be 71.5mm and 3/4 wave 107.25mm, with the tip bit being 90.71% of 3/4 wave ie 97.3mm - for 2100MHz
    This site says you can add a ground plane, any idea what I should add for that.. 4 wires flat? 4 wires sloping down? lid of a baked bean can?
    Last edited: Feb 4, 2014
  6. davenn

    davenn Moderator

    Sep 5, 2009
    looks cool :)

    a circular flat disc of aluminium etc with a 1/4 to 1/2 wavelength in radius should be ok :)

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