VHF antenna magic

Discussion in 'Instruments / Avionics / Electrical System' started by PrzemekMac, Jun 10, 2014.

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  1. Jun 10, 2014 #1

    PrzemekMac

    PrzemekMac

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    Hi. I have this plane (photo) with RAMI VHF antenna mounted on top. Antenna is mounted to grounded (connected do frame) cooper plate. Antenna is solid-statne (not tuneable).
    The problem is. Last time I saw that I have a little bit low range. Second, when I was in my friend aerodrome with 118,875 frequency i saw that I can't transmit because I have interferences in my headsets.


    I checked the antenna. On 122.300 i have SWR=1:2,5 on 136,900 i have 1:1,5 and on 118.000 i have 1:3 !!
    I cleaned all groundings, no effect. I replaced my antenna for COM antenna from beechcraft. Still I have high SWR. I replaces antenna cable (h155) - no difference.
    I tried another radio, no effect.


    Why solid-state antennas on my plane don't work?
     
  2. Jun 10, 2014 #2

    PrzemekMac

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    Sorry, photo.
    DSCF2318.jpg
    Antenna is on top.
     
  3. Jun 10, 2014 #3

    Himat

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    Why the solid stat antenna doesn’t work?
    There could be multiple reasons.

    - First, noise/interference in the headset is a sign that electronic noise enters or propagates within the radio system.

    - Next, loss of range could be loss of signal strength or bad signal to noise ratio. As for bad signal to noise ratio, see above. Low signal strength can be caused by impedance mismatch between the radio, cable and antenna.

    - The placement of the antenna in combination with the grounding plate and aircraft structure can divert the radio waves away from the wanted direction. The aerodrome radio might end in a “shadow” when in some positions relative the airplane.

    The difficult thing with electrics/electronics when you get to radio frequency is that a ground and resistance is not necessarily so. A direct current short might be a radio frequency open circuit and a direct current open circuit might act as a radio frequency “short”. That is the beginning and from there on the fun starts.
     
  4. Jun 10, 2014 #4

    PrzemekMac

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    Yes, i know. But it's hard to understand why I have so high SWR on two solid state, oryginal antennas.
    I replaced cable (50ohm) and I tested with another radio. Still SWR is high and sensitivity is poor.
    I tried with engine on and off. I have just high attenuation on antenna line.
    I ever tried to connect SWR metter and radio directly to antenna (avoiding aeroplane cable) and the same.
     
  5. Jun 10, 2014 #5

    Aerowerx

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    Is the SWR meter designed to work at that frequency?

    Some of the lower quality units give misleading results when used at VHF.

    Try disconnecting the antenna while listening. Does the noise level change? If it does not, then the problem is most likely in the receiver.

    Is this a new antenna? Has it worked before?

    Do you know any radio amateurs in your area? They would be able to help you with this problem.
     
  6. Jun 10, 2014 #6

    PrzemekMac

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    Hi.
    Is the SWR meter designed to work at that frequency?
    Yes, its up to 200MHz

    Try disconnecting the antenna while listening. Does the noise level change? If it does not, then the problem is most likely in the receiver.
    Yes, there is a difference. I checked this antenna on my SL40 and FTA-750. The same result.

    Is this a new antenna? Has it worked before?
    I tried two. First I bought with plane (i not sure if it's working) second removed as working from beech byron.

    Do you know any radio amateurs in your area? They would be able to help you with this problem.
    I'm probably only one.

    Tomorrow I'll try to make my own 1/4l antenna and tune it on plane.

     
  7. Jun 11, 2014 #7

    Aerowerx

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    FB OM! N8EE here.

    From the SWR values you gave it sounds like your antenna is too short.
     
  8. Jun 11, 2014 #8

    PrzemekMac

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    Yes, on higher frequency (shorter wave) it's better but why two oryginal antennas are too short?
    I just got parts for 1/4L, I'll try today.

    SQ1GQP
     
  9. Jun 11, 2014 #9

    PrzemekMac

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    Ok. It's absolutly magic.


    Today I put in old place on top of the plane, on cooper plate under fabric new antenna.
    It's just simple 1/4 GP made by myself.
    The same antenna I have on my car.


    I cut 70cm rod (a little bit longer) and start cutting for best SWR. I made tune and I had 1:1,5 on 118MHz and 1:1,5 on 136MHz and about 1:1 on 122MHz. Excellent.
    After this i cleanup plane, tight wires and check again. Suprise... on 118MHz i have ~1:3 on 136MHz 1:1,5. Terrible!


    I made another test. I put just on the plane (in place I have com antenna) my magnetic car 1/4gp antenna. On the car I have abour 1:1,2 on all range. On my plane I have the same, terrible SWR. When I moving this antenna back (a little bit more to the tail) SWR falling.


    I have no idea, what can be wrong with this plane. Magic? I thought about gps antenna (it's near com) - maybe any collision? Maybe my wings make illegal reflector or sth?
    I can't find the moment when I lost my 1:1 SWR. Cables and plugs are new, checked, soldered, well made. Just after nothing my swr raise to red range..
     
  10. Jun 12, 2014 #10

    djschwartz

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    When you measure the antenna SWR are you inside or very close to a metal hangar? Remember that SWR is a measure of reflected voltage. The SWR bridge cannot tell whether that voltage is reflected off of a mismatch or from an external reflector.
     
  11. Jun 12, 2014 #11

    TFF

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    How big is the ground plane?
     
  12. Jun 12, 2014 #12

    PrzemekMac

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    Hmm, The hangar was about 10m from aeroplane. Maybe? I'll check today.
    Ground is cooper plate under the fabric. Is about 30cm width and 40-50 cm long. There is not much space for if (i have GRS outlet).
    I connected this plate by short pigtail to aeroplane frame and by second pigtail do antenna ground ring (fabric is isolator).
    Everything was cleaned.

    I have small GPS blade antenna very near (about 10 cm) VHF antenna. But it could make so much difference? Especially I caught good SWR a few times but it raising up in inexplicable conditions (just).
     
  13. Jun 12, 2014 #13

    fredoyster

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    No it is not magic. Wings, hangar, gps, probably not part of the problem. Your 30x40 cm ground plate is not a ground plane, it needs to be at least 1/4 wave in all directions. It doesnt have to be solid, try adding a few radials at least 62 cm long, at least 3 or 4 of them (62 cm from the center of the plate where the antenna connects, to the top of the radial.) Regular wire is OK. Then tune for SWR in the plane with the radials you have -- they are part of the antenna. With a ham radio background and good SWR meter you are far ahead of most guys with this problem.

    73 de WA6NMF
     
  14. Jun 12, 2014 #14

    Himat

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    Ok, there was maybe some important detail.
    You state that the ground plane is connected to the frame with a short pigtail and another short pigtail from the ground plane to the antenna ring. I would like to see pictures of those two details, this might be the problem.

    Area, shape and length of these two pigtails can make them work as inductors or open circuits at radio frequencies. From your description I do think either your antenna have not proper contact to the ground plane, the ground plane not proper contact with the airframe or both. That is on VHF radio frequencies. Check with a multimeter and the connection is just fine. The SWR meter on the other hand does show this.

    Two things you can try:
    - Change the pigtail from the antenna ground plane to the airframe to a braided strap like those often used in cars/boats to connect the battery to the car body.
    - Connect the antenna directly to the ground plane. If this is not possible, remove the pigtail and try without it.

    Edit, after fredoyster posted while I was typing.
    With proper straps connecting the ground plane to the airframe, the airframe becomes part of the ground plane if it is made of an electric conductive material. And your airplane has a metal tube frame?
     
    Last edited: Jun 12, 2014
  15. Jun 12, 2014 #15

    PrzemekMac

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    I forget to make new photo but in this antennas are good visible.
    I know that gps is very near but I shouldn't make so much difference.
    Mainaly I have problem to hear handheld radio on aerodrome (random radios) and my friend flying near me can hear it for 5.

    Zrzut ekranu 2014-06-12 o 09.43.27.jpg
     
  16. Jun 12, 2014 #16

    PrzemekMac

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    [​IMG]

    Plane has metal mesh.
    I'll try to make photos today. It's not easy access. It's probably very hard to disassemble or replace this cooper board. Probably fabric damage.
    I checked resistances between plate and airframe and cable shield, it's ok.
     
  17. Jun 12, 2014 #17

    PrzemekMac

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    It could be a way... Unfortunatelly i can't remove this copper plate but I can add radials and connect them in the center to the antenna..
     
  18. Jun 12, 2014 #18

    Himat

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    Did you check resistance with multimeter?
    Then DC connection is OK.
    But SWR meter indicate that connection might be bad at VHF radio frequencies.
     
  19. Jun 13, 2014 #19

    PrzemekMac

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    New photos. Now I'm looking for this and I think that plate is too small. It can be a reason?
    I'm not sure I'm able to replace it (I have only small access hole).
    Maybe it would be good to swap places of gps and com antennas. VHF would be more at center.
    I have GRS deployment cover left side of antenna. It will be good to glue to cover some aluminium reverly and connect it to ground?
    DSCF2823.jpg DSCF2815.jpg
    DSCF2820.jpg
     
  20. Jun 13, 2014 #20

    Himat

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    I have made a cut out of one of the pictures. I assume the green wire in this picture is the pigtail connecting the VHF antenna screen to the copper plate. If so, this pigtail can act as a choke, an inductor, at radio frequencies that mess up the antenna matching. I have not worked much with radios, but some a bit with electromagnetic compability, EMC, and suppression of electromagnetic noise/interference. Such a pigtail is then a no no, as it does not “earth” noise at all frequencies and might even radiate noise (radio waves) at other frequencies.
    DSCF2815Pigtail.jpg
    With the antenna bolted to the bracket and the copper plate bolted/riveted to the bracket, is the pigtail needed to get electrical contact between the antenna and bracket?

    The ground plane plate is small, but it looks like it sits on a bracket bolted to the metallic tube airframe. If the copper plate is bolted or riveted to this bracket all of the airframe should become part of the antenna earth plane. I am not sure if the size of the plate is that critical then.

    But you said that the SWR changed with where on the plate you put the antenna and that the antenna was OK if you put it on your car roof. It can be that your ground plane act funny then and adding radials as fredoyster suggest in post #13 will help.
     

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