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How to best mount and wire 2.4GHzantenna on a metal box?

Discussion in 'Radio and Communications' started by Triode, Feb 23, 2017.

  1. Triode

    Triode Member

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    Hi, I'm unsure about this because I know antenna placement can be a bit complicated. I understand the length calculation in open space, but the metal case confuses the issue a bit.

    I have a RC radio receiver that is to be mounted in an aluminum box. The antenna can be mounted on the outside of the case. I only need about 1" of wire to make it from the mounting point of the receiver to the outside of the box. It's a 2.4GHz radio system, the transmitter is a Flysky FS-T6, and the receiver is the corresponding R6B.

    Questions:
    Is mounting the antenna sticking straight up at 90 degrees from the aluminum surface going to cause any problems with picking up the signal? I'm not sure if it would hurt or maybe help.

    When calculating the length of the antenna, I don't count the part inside the box right?

    When you have an extension wire to an antenna, what prevents that from counting as part of the antenna length? I've seen several systems where you have this precise antenna and then a 1-meter extension cable. It seems like the electrons wouldn't care where we say the antenna begins and the extension cable ends. Is it that the extension is shielded? I'm thinking it is, and thus the part of my wire inside the box would also not count, but this is just my best guess.

    Since the wire coming from my receiver is meant to be an antenna can I use it to go to the bulkhead where the new antenna enters the box, or should I replace it with a shielded cable that is better suited to extend an antenna?

    Finally, is there any reason a 2.4GHz antenna with a plastic housing that is designed for 2.4GHz wifi wouldn't work? It seems like it would be ideal.

    Thanks!
     
  2. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    How big is the metal box? That determines if the box is a suitable "groundplane". Ideally, it should be a min of 1/2λ x 1/2λ. If so, then a 1/4λ monopole mounted at ~right angles to the box surface will be optimal. Some WiFi antennas are actually longer than 1/4λ. If so, they are an end-fed 1/2λ dipole, and do not have to be mounted on a groundplane.

    Note that such antennas suffer from "cross-polarization", meaning the transmitting antenna should be the same orientation (parallel to) the receiving antenna and vice versa...
     
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  3. Triode

    Triode Member

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    Thanks!

    The box is 7.5 x 5 x 1.5 inches, roughly 0.125 inches thick, with the large surface facing up. This is sitting on top of an aluminum robot chassis about 8 inches by 6 inches, elevated by the 1.5 inch thickness of the box of course.

    I'm glad you prompted me to look up ground planes in reference to antennas. I didn't realize the importance of it. If I understand properly it reflects signals back onto the antenna, and so has to be λ/4 the size, so that the full length of the wave can reflect onto the antenna assuming its at the center. I'm going to infer that then I want to make sure the antenna is at least this far from any edge?

    I'm not dead set on this configuration of antenna. If it would help I could use two antennas, a "clover" antenna, or a dipole, or mount the antenna further up, though this would extend the wire needed to reach it.

    To give a little more detail, the most common use of my rover will be on sand, sometimes wet sand, the dunes are typically less than 2 or 3 feet tall, and the height of the track rover only has the antenna about 8 inches off the ground. The rover is either controlled by a computer or a hand held transmitter, typically about 4 feet off the ground. Under normal use the rover may tilt as much as 20 degrees, and it will usually be within 100 feet of the control source.
     
  4. dave

    Dave New Member

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  5. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Sounds like your GP is big enough. For a mobile robot, you may want to try two antennas to produce some diversity (kinda like a WiFi router with two antennas sticking out of it).
    Tilting +-45degrees won't matter much ; the antennas have to be almost at right angles to each other to experience the -20db cross-polarization loss.
     
  6. Triode

    Triode Member

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    Would diversity require a special receiver? I thought that meant that you had a system which could choose the better of two signals, or use a controlled mixer of some sort. Can I just connect the center wires from both together and into the antenna connection on the PCB?

    Another very beginner question; It looks like these connectors will end up in contact with my metal case, is that ok/intended? Should my case be connected to the ground for my electronics as well?

    Thanks!

    [​IMG]
     
    Last edited: Feb 23, 2017
  7. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    The coax feedline shield is supposed to connect to the metallic ground plane at the base of the antenna, so the threaded outer part of the SMA female does that intrinsically.

    You are right: true diversity reception requires two receivers, but I doubt that is what is going on inside a router. I think they just sum the two antennas in an attempt to fill-in some holes in the antenna pattern that might otherwise occur due to proximity of metallic stuff placed near the router. If your (single) antenna is the highest thing on the robot, then the second antenna will just confuse things...

    When installing VHF radio in a car, a vertical whip antenna at the center of the roof is better than putting it anywhere else on the car...
     
  8. JimB

    JimB Super Moderator Most Helpful Member

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    Use a coax cable from the receiver to a suitable antenna socket on your ali box.
    Look at this pic taken from the interwebs:

    R6A-B receiver.jpg

    I guess that your receiver looks like this.
    Make the coax connections as short as possible, as shown in the picture.
    It is even better if you connect half the coax braid to the groundplane on one side of the connection, and the other half of the braid to the groundplane on the other side of the cable.
    Like this, but shorter if possible:

    Short connection.JPG

    Make sure that they make good connection, it is essential.

    In situations like this the antenna feeder is a coax cable, and the length of the coax does not affect the antenna length.

    JimB
     
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  9. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    It doesn't really matter that much, too small a ground plane will simply reduce the range somewhat - exactly like pretty well every walkie talkie ever made :D

    You're obviously restricted by the physical limitations, and very often need to 'use what you have'.

    Years ago I used to have a 2m portable, a Kenwood 2200 (if I recall correctly), and this used an internal speaker and a plug-in microphone on a curly lead. You could increase range substantially by simply standing the transceiver on the roof of a car, where the greatly improved ground plane really made a difference.
     
  10. Triode

    Triode Member

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    Thanks for all your help! I soldered the connections to the board like JimB showed, and bolted the antenna base directly to the box as close to the center as I could get it. It works fine. I can't tell what the range limit is, but I can drive the robot as far away as I can see well enough to control, about 400 feet, and it doesn't have any jitter or dropout. If I put an FPV camera on it I can see if it goes a lot further than that.

    For now, the problem is that the tracks keep coming loose, but that's a totally different kind of issue. I'll make a spring tensioner.
     

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