Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How to calculate antenna length for specific frequency, to be used in remote area?

  1. May 2, 2012 #1
    I would like to build an antenna that will be used in a very remote area to improve our cell signals. We have access to the GSM 850 (824.2 - 849.0 Uplink | 869.2 - 894.0 Downlink).

    I need help in calculating the antenna length based on the frequency and need advice on the most effective design. What seems to make the most sense is to install this at our base camp atop a tall tree and perhaps run a wire down toward the cabins to allow all of us to gain some cell signal strength.

    Any suggestions, advice etc would be greatly appreciated. (If I had the funds to purchase an out of the box system I would, so if it's possible, I'd like to go with a DIY approach).
     
  2. jcsd
  3. May 2, 2012 #2

    berkeman

    User Avatar

    Staff: Mentor

    Re: How to calculate antenna length for specific frequency, to be used in remote area

    Welcome to the PF.

    Do you know where the other antenna is that you want to have better reception with? Can you aim a directional antenna at it reliably? If so, then a Yagi directional antenna would be a good choice.

    http://www.zdacomm.com/images/stories/categories/ZDADJ800-13YG.jpg
    ZDADJ800-13YG.jpg
     
  4. May 2, 2012 #3
    Re: How to calculate antenna length for specific frequency, to be used in remote area

    Thank you for the welcome and for the reply!

    I had looked at the Yagi antenna, but I'm not sure how well it would work (this isn't my area of expertise). Our camp is located up a mountain side and there are many tall, old growth trees, which makes signal strength sporadic. I consulted a map for the cell towers, and see there are 2 (city located) towers about 30 kilometers (or 18 miles) directly North. There is also another tower about 45 kilometers (or 27 miles) to the South-East. I'm assuming the South-East location is what we've been able to connect to, as we are more to the South-East of a mountain side, and I think the Northern towers would be inaccessible. This however is all conjecture, and I'm at a loss for how to proceed.

    Are there any other alternatives, or additions (to the Yagi antenna) that would help? I'm fully capable of constructing a system (even welding), if need be. The location however, is currently inaccessible (due to snow packs), so I have no real way of testing the system until it's been built, therefore I'd like to gain as much knowledge and confidence in the methods before committing to a plan.
     
  5. May 2, 2012 #4

    davenn

    User Avatar
    Science Advisor
    Gold Member

    Re: How to calculate antenna length for specific frequency, to be used in remote area

    a commercial yagi like berkman has shown are dirt cheap I paid ~ US$25 for the last one I bought, they are probably cheaper now.

    If you dont have testgear etc for tuning your own construction, then spending just a few $$ is the way to go. At 800MHz and up tuning and lengths of elements of an antenna get quite critical and it would be near impossible to match a commercial unit.

    yes you need to find out where the local cell towers are and aim a yagi in the direction of one of them. Trying for the best, and clear line of sight to the tower as possible. Any other form of antenna with decent gain is going to be much bigger and more difficult to manage .... say like a corner reflector or a dish.

    Dave
     
  6. May 3, 2012 #5

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Re: How to calculate antenna length for specific frequency, to be used in remote area

    Does your cellphone have a connector for an external antenna?
    If not, how were you planning to couple the signal into it?
    I might suggest using a dipole of about 35cm length (two 17cm lengths of copper wire attached to inner and outer of the bottom end of your coax). This should couple quite effectively to a nearby cell phone on the ground, whilst the Yagi up a tree would improve on both transmission and reception.
    I am not aware whether anything like this set up is available to buy. I would have thought it could sell well in some areas.
     
  7. May 3, 2012 #6
    Re: How to calculate antenna length for specific frequency, to be used in remote area

    Thanks Dave. I'm going to look around again. The last one I saw was over $100. I don't mind paying the money, but I also don't want to worry about it getting taken since we're not the only ones using the backwoods.
     
  8. May 3, 2012 #7

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Re: How to calculate antenna length for specific frequency, to be used in remote area

    You can buy a cheap Yagi for TV reception for less than £20 in the UK. Even a massive postage charge would make it worth while ordering one from over here.
     
  9. May 3, 2012 #8
    Re: How to calculate antenna length for specific frequency, to be used in remote area

    Thanks for the reply.

    Unfortunately no, my cellphone does not have an external antenna. I'm still at the research stage of this project and trying to ascertain what I need and then how to make everything work, and since this isn't my area of expertise, it's a learning process...

    In our group, we have a few cellphones between us, and if we can get ONE to work effectively, then I'm happy. It's not imperative that all our cellphones work (although that would be nice if we could).

    Do you think there's any benefit to locating the antenna on my cellphone and making a wire out that could then work with a dipole antenna? (Please don't laugh if I'm completely lost here). :)
     
  10. May 3, 2012 #9
    Re: How to calculate antenna length for specific frequency, to be used in remote area

    Thanks. I'll definitely look into that. I know a few people in the U.K. so I'm sure I can get someone to send it to me (if need be).
     
  11. May 3, 2012 #10

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Re: How to calculate antenna length for specific frequency, to be used in remote area

    I think you'd be very lucky indeed if you managed to get inside your phone and make a suitable connection. That's why I suggest using a local dipole (at the bottom) which should couple nicely with the internal antenna of the phone or any other phone(s) you might want to use - even at the same time.
    At the distances you are describing, you may have a pretty weak signal unless the cell base station is specifically designed for wide area coverage. Most base stations have an individual coverage range of less than 10km. This implies that Your antenna needs to have a reasonably high gain if it is to work at 30km distance. An elevated yagi (10m at least, or above the trees - whichever is the greater) should perform much better than an internal phone antenna on the ground but you need to use a reasonably low loss downlead for operating at 900MHz.
    It should be worth a try and you could feel pretty smart if you could actually communicate on your link. Smarter and a lot cheaper than the alternative of a satellite phone!
     
  12. May 3, 2012 #11
    Re: How to calculate antenna length for specific frequency, to be used in remote area

    We actually looked into getting a satellite phone, but they aren't cheap! So, I'm hoping to work out this alternative (and feel a sense of satisfaction when it works).

    I can't find any further info about the cell tower, and what it's range may be, but I'm assuming it must be greater than 10kms, since we can get a fairly good signal (in some areas). It's just one of those situations where you must stand on a stump with your arm extended and fingers crossed, and then low and behold it works. ;)
     
  13. May 3, 2012 #12

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Re: How to calculate antenna length for specific frequency, to be used in remote area

    Well, you are not far from a solution, in that case. My 10km figure probable refers to reliable reception at ground level and even in a building.
    A while ago, there was a thread here about a similar gizmo that attached to windows and was supposed to deflect mobile signals into a room where reception was poor. This is all my proposed antenna system is doing, effectively. A few metres of cable and a light-weight cheapo TV antenna wouldn't take up a lot of room in the rest of all your equipment. Definitely worth a try. And all your friends could use it too!
     
  14. May 3, 2012 #13
    Re: How to calculate antenna length for specific frequency, to be used in remote area

    I've seen those systems online, and they don't look incredibly complex (although some include an amplifier, which I understand can actually inhibit the signal versus improve it?)

    Do you suppose an old-school T.V. antenna (that someone would've had on their roof), would work for this purpose? Since everyone had to convert to a digital T.V. signal, I would bet there are a number of thrown away antennas that I could get my hands on. That was my initial thoughts, but since the science behind this is new to me, I wasn't sure if it would work or not. Then I read that you need to calculate the antenna length based on the frequency you want to reach, and that's where I figured I would have to either purchase or build the antenna. Any thoughts?

    Installation wouldn't be too much of a problem either as we already have a lookout platform on one of the camp trees about 70 feet up the tree. (But I might have to get someone else to put it up there, because at that height my knees start shaking!)
     
  15. May 3, 2012 #14

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Re: How to calculate antenna length for specific frequency, to be used in remote area

    That old antenna would probably do the job if the main, driven element (the one just in front of the reflector) is about 35cm long.
     
  16. May 3, 2012 #15
    Re: How to calculate antenna length for specific frequency, to be used in remote area

    Great! Thanks for all your time and help.
     
  17. May 4, 2012 #16

    vk6kro

    User Avatar
    Science Advisor

    Re: How to calculate antenna length for specific frequency, to be used in remote area

    One wavelength at 850 MHz would be 35.3 cm long in air so a half wave would be about 17.6 cm (6.94 inches)

    So the driven element would be a little less than this. say 16.5 cm.

    The reflector would be 10 % longer or about 17 cm long and spaced 0.25 wavelengths behind the driven element (or 8.5 cm).

    And the first director would be 10 % shorter than the driven element (14.5 cm) and spaced 8.5 cm in front of the driven element.

    Spacings are left variable so that they can be moved for matching purposes.

    I found a website that has a calculator for this design.
    Here are the results for an 11 element yagi at 850 MHz:
    Lengths of elements (in mm) :
    170, 165, 145, 143, 141, 138, 137, 135, 134, 132, 131
    Cumulative spacing:
    0, 70.5, 97, 160,236, 324, 423, 529, 640, 756, 878 mm.
    (so, the reflector is at 0. The driven element is at 70.5 etc)

    Element diameters all 6 mm.
    This is the website:
    http://www.k7mem.150m.com/Electronic_Notebook/antennas/yagi_vhf_quick.html

    Antennas at this frequency are extremely difficult to make and get working properly. Matching the feedline to the driven element requires some serious equipment.
     
  18. May 4, 2012 #17
    Re: How to calculate antenna length for specific frequency, to be used in remote area

    Thank you so much! That helps a lot.

    Whether I'm successful in making a working Yagi or not, I'd still like to learn as much as I can and give it a try. In the end, I may have to buy an antenna, but at least I'll know more about how it works.

    Thanks again, especially for the link! ~ Have a great weekend.
     
  19. May 5, 2012 #18

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Re: How to calculate antenna length for specific frequency, to be used in remote area

    That was stupid of me. You want 1/2 wavelength and not a full wavelength for the elements (and the dipole at the bottom).

    Seriously, though, your biggest unknown is with how to get the signal into and out of the phone. Unless you can find specific information about your particular phone, you should avoid getting in there with a soldering iron! A wireless coupling is what you should go for. Once you have erected your antenna on the tree and are at the bottom with your cable end then it's easy to play around with bits of wire, two metal plates or even a loop and see which gives you more bars on your signal indicator. If you need to stand on one leg for best reception then so be it. Life will be just the same as it used to be with a TV set-top aerial in a bad reception area!
     
  20. May 5, 2012 #19
    Re: How to calculate antenna length for specific frequency, to be used in remote area

    Thanks for the clarification.

    What I'd like to do is get this working so that any and all our cellphones have improved signals. If that's not successful (or feasible), then I had considered getting an alternate cellphone for the camp that we could connect to the antenna, and have it as our "main line". With the use of alternative power sources we've established, we could then leave the phone on all the time in case someone needed to get in contact with us, or we needed to call out.

    This is definitely a work in progress, and I imagine there are several hurdles to overcome.

    Thanks for the advice!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: How to calculate antenna length for specific frequency, to be used in remote area?
Loading...