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Ultra low power 3g repeater?

Discussion in 'General Electronics Discussion' started by flippineck, Apr 14, 2014.

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  1. flippineck


    Sep 8, 2013
    I have bad cell signal inside the house but reasonable outside. My phone is a sony smartphone that does not have an external antenna socket.

    After a whole bunch of experimentation I found, using different kit (a mobile broadband router that does have external antenna connector), that an external antenna on the outside wall of the house does work to improve the performance.

    I'd like to do the same with my phone, put say a collinear antenna outside and feed a bit of coax through the wall to it.

    But, how to connect this antenna to the phone?

    First thought was to try some kind of passive inductive type strap-on / cradle affair. But, having cannibalised one from an old thb bury in-car cradle, it doesn't seem to make the slightest jot of difference.

    I looked at the commercially available 'direct connect' amplifiers to go between the external antenna and the inductive coupler. Only thing is there seems to be a lot of advice that these are not just theoretically illegal, they actually can and do cause interference problems for neighbours etc.

    They all seem to be designed to cover a large area, measured in the hundreds or thousands of square feet.

    What I'm thinking is, if all I am trying to do is boost the signal to one phone which is in a permanent fixed position in it's inductive cradle, could I get away with a repeater amp that put out a much, much lower output than the off-the-shelf units? sufficient only to excite a very closely positioned phone's internal antenna?

    I'm thinking of a very minimal circuit consisting of perhaps just one or two transistors and a small handful of passive components plus a power supply (not actually sure where I'd start with the design though).

    Is this a practical goer, or should I ditch the idea? The phone is apparrently working in the 2100MHz area.

    What's actually inside these passive inductive couplers?
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Call your cell provider and ask if they have any femtocell options for people with poor signal.

    My provider started offering these because they have really poor coverage in some areas and didn't want to improve it. They cost $30 per month, but because they were introducing them I got mine for $5 per month. I wouldn't be prepared to pay an extra $30 per month to fix the provider's problem, but $5 (with some added benefits) was fine.

    I'm nowhere near the north of England.

    A slightly more illegal way is to import a femtocell from the US where they are available as retail purchases. My device is locked to my provider and requires a connection to the cell to initialise (thereafter it uses the internet), so its pretty locked down.

    This might be interesting. It sounds like he has the same problem you do.
  3. flippineck


    Sep 8, 2013
    Great link thanks Steve. Trouble is, the femtocell approach seems to rely on you having a stable broadband connection already, probably over a landline or cable or something. My primary access to the net is purely via the mobile network - so femtocell doesn't really help me.

    There *is* a half decent & useable signal outside, a foot and a half in front of where I sit at my desk, beyond a 9 inch cavity brick wall - Just need to catch it, drag it through the wall & then shove it inside the phone.

    I managed it finally with the 4G router because of it's decent external antenna ports, but the phone's harder because of no antenna port. Although I get internet ok now via the 4G router, it's limited to 10Gb a month. The phone's not limited but, the SIM won't work in the router (network restriction) so I *have* to have it in the phone in order to tether to other PCs etc.

    I have considered installing a new window and it might come to that yet, but it's a bit of a pain what with all the regs and red tape. I did notice at one point that although standing in a window does make a big improvement, modern double glazing glass is not very good at letting the signal through - opening a window & holding the phone outside makes a huge, sudden & repeatable difference.

    I thought it might be easier to try to build a small bidirectional rf amplifier for around 2100MHz just sufficiently strong enough to help the phone 'talk' across the inductive coupler - but not so strong as to send out a significant signal into the street or even across to the other side of the room - purely just across a say, max 10 or 15 mm max distance whilst the phone sits in it's inductive coupling cradle. Just connecting an external collinear gain antenna directly through coax to an inductive antenna coupler doesn't seem to do anything,

    The wifi tethering on the phone is sufficiently strong to allow anything in the whole house to connect just fine, when it's in it's fixed cradle upstairs, when the broadband is actually working.

    Best option of all would be a decent smartphone with a proper external antenna port I guess, like the router, but, they just don't seem to do them anymore

    I tried arguing with my provider as to what does and does not constitute 'a phone' but they were having none of it. It's their way or the highway :-( No tethering on unlimited plans on anything other than a phone, and they get to say what a phone is. And my router isn't a phone.
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Some that I've seen seem to be rather smart bidirectional amplifiers. I'll try to find a link for you.

    Something like this: (you need a different frequency range though)

    You may want to google for "cell phone signal booster 2.1GHz 2100MHz"
  5. dh390


    Jul 30, 2013
    Flippineck. Years ago in the early days of cell phones you used to be able to walk into a store and buy what is called a glass mount feed or pass through cell phone antenna.

    There were 2 or 3 styles. ALL were designed and used for use in cars with cell phones that didn't have an antenna jack or didn't want a cord attached. All had an external antenna with like a 3 or 5db gain and a small internal antenna that would transmit the cell phone signal in and out of the car.

    There were either a double sided tape version for rear windshield and a slide on clip on version for a side roll up window.

    These are near impossible to find now. But here in the US if you go to a truck stop you can sometimes find them.

    I am wondering if you could take say 2 hard wired car cell antennas and connect them together. Then place one outside and feed the cable under a window and the other inside. Don't see why it wouldn't work as it is the same idea as the old ones i described.

    Do an E-bay search for car cell phone anrennas maybe magnet mount ones and once you know what connector it uses look for that type coupler like for example a TNC to TNC.

    Probably a bunch cheeper than a cell amp. And no monthly fee like that cell phone company supplied device.

    Just a thought.

    I did something similar in my house. I bought a magnet mount car cell phone antenna and mounted it outside and ran the cable under a window. But my cell phone is old enough to have a jack, all I needed was an adapter to go from the antenna to my phone.

    The 2 antenna thing should work as i have just put the adapter near the antenna on the phone and the signal strength jumped on the phone. Sow with 2 actual antennas coupled on in one out would be better than my adapter near the cell phone.
    Last edited: Apr 15, 2014
  6. flippineck


    Sep 8, 2013
    Thanks chaps

    The link is exactly the sort of thing I've been looking at and really tempted by. The only thing stopping me getting something like that is, I think, they're heavy duty illegal in the UK?

    It claims to be able to cover an area of 300 square metres, so I'm guessing it must be a significantly powerful amplifier, which could quite possibly spill retransmitted signal into the street, neighbouring properties etc, and thus be realistically capable of being detectable and possibly causing real interference outside the house.

    I was just toying with the idea of using something basically identical to that, but with a capability orders of magnitude smaller - seeing as, the cellphone won't be moving around in a large area, it'll be permanently stationary within millimetres of the transmission antenna / coupler. I have absolutely no experience of designing or building RF circuitry but, i could certainly build from a kit with instruction, or from a circuit diagram with a bit of advice.

    Even something orders of magnitude less powerful than the commercial ones would pedantically by-the-lawbook still be illegal, but, would the much much lower power mean that realworld problems would be almost certain to be nonexistent?

    dh390 - yeah I think I have an idea what you mean there; I actually butchered an old THB Bury in-car kit to try and use it's phone cradle passive inductive antenna coupler. That device had a glass mount feed antenna but unfortunately I threw it in the skip not long ago (throwing things in a skip is a really good way to make them desperately wanted a month later I always find!)

    Yep, I will try playing around with a setup like that again. It didn't seem to work well in the past when I was trying to use a yagi type antenna outside but I haven't tried it with collinears.
  7. shumifan50


    Jan 16, 2014
    Have you checked with your phone supplier that it really doesn't have an external antenna connection? Many phones have the antenna connector in their plug for a car cradle.
  8. flippineck


    Sep 8, 2013
    Checked with Sony & didn't trust their negative answer so googled for days; it's pretty definite the only way to get a direct connection to the xperia E antenna is to strip the phone to get at the connectors on the pcb.

    I found this, which is pretty much what I want to do, but.. I have a gut feeling the amplifier circuit inside will use two old conkers and a dead caterpillar or some equally effective arrangement

  9. flippineck


    Sep 8, 2013
  10. flippineck


    Sep 8, 2013
    Shumifan50 - you were right - dug & dug & dug some more & it turns out there IS a connector accessible without breaking open the phone. It's well hidden & has an almost invisible black sticker over it. I don't want to maul about with it too much though,

    It's circular, about 2mm in diameter and seems to consist of a conical 'well' of insulator with a central copper pad at the bottom. The barrel is a gold-coloured tube but so small, I can't see any clipping features that might be moulded into it's profile.
    Anyone identify this type? Phone model is Sony Xperia E C1505

    Found something similar but I figure I have to get exactly the right connector to avoid damaging it.. it looks like it's the type that incorporates a disconnect switch (so, if you break it poking about, you lose the phone's internal antenna for good)

    Similar but not I think, exactly the right family..

    Lot of very good info here but sadly for a different phone.

    Beginning to think the solution is to solder in a completely different RF socket, least I know what I've got then!

    So if this socket *is* a Murata MM8430-2610, I wonder what Sony are testing with it.. are they testing the output of the phone's internal antenna, or are they injecting signal to test the innards of the phone. If they're using it to test the internal antenna, that would mean, it's useless as an external antenna jack right?

    Anyone got an old Sony smartphone I can butcher and take my ohmmeter to lol
    Last edited: Apr 19, 2014
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