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High SWR Problem

Discussion in 'General CB Services Discussion' started by 530, Feb 20, 2014.

  1. 530

    530 Member

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    I finally got all my stuff together to install my new radio set up in my car (2000 Grand Marquis LS) and something weird is going on with my SWR. I have an Optima MKIII radio brand new on a 3ft/meter/9ft coax set up with a stud mount and a 102 inch whip centered on my trunk. My SWR's are around 3 when I have the trunk closed. When I opened the trunk, about a half inch and checked, it dropped. I then matched it on 1 and 40 and got 1:2 SWR with the trunk open. Then I closed the trunk and it was at 3 again. Things I've tried... Changing coax lengths also tried 3ft/meter/12ft and 3ftmeter/21ft. I also tested for trunk ground on a meter and it's fine as well as any shorts in all coax, connectors, meter, radio, mount, everything. I also tried with and without ground straps. I even tried coiling the 3 diff configurations of coax. I also tried disconnecting the electronic trunk switch to see if that was causing any problems. No matter what I do the only thing that brings the SWR's down is opening the trunk a little. Any help? Thank you.


     

  2. RatsoW8

    RatsoW8 Supporting Member, W9WDX ARC Member - WD8T

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    I'd say the "trunk closed" reading is most correct. Is there any variance in the swr between channels 1 and 40? What I would do first is try adding a 4" spring or quick disconnect to lengthen the antenna a bit to see if that gets you back in the ball park. The whip on my truck is right at 106" and 1.2:1 swr across all 40 channels. Then I just remove the 4" quick disconnect to use the same antenna on 10 meters.

    What you want to avoid is tuning the antenna system by using different lengths of coax. That can cause the coax to become part of the radiator and that's not a good thing.
     
  3. 530

    530 Member

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    Hi

    I forgot to mention I have a 12 inch adjuster between the stud mount and antenna and the antenna is all the way to the top. The reading is matched between 1 and 40 with or without the trunk open, it's just high when it's closed. The antenna is prob around 106 inches now and i am using a 3ft/meter/9ft coax set up. I just tried other combos to see if it changed anything.
     
  4. GnG8d

    GnG8d Well-Known Member

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    I'm inclined to think it's something mechanical, but what did you use for ground straps?
     
  5. W5LZ

    W5LZ Crotchety Old Bastard

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    Since there's a significant difference with the trunk open/closed, then I would suspect that the problem is with the antenna's 'stud' mount and/or the trunk lid's bonding with the rest of the car. Is that stud mount assembled correctly?
    - 'Doc

    Anything else you forgot to mention?
     
    1 person likes this.
  6. KM3F

    KM3F Active Member

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    Some comment first, the 102 "whip plus about 3' make it close to 11 feet off the ground.
    While the long whip normally would be the most efficient it's really not a good setup from the stand point of hitting things as you travel.
    the trunk lid has two hinges and a latch as the only DC connections to the rest of the body.
    The DC connections is not the important parameter in your case.
    The RF paths are dominate and is why your having an issue.
    Simply stated the whip has to be tuned for the specific setup you want to run.
    Just bolting it on and expecting a match is never guaranteed.
    You either have to adjust the length of the whip to Resonate with the body or use a matching network.
    What you need to understand is the whip is "only half" of the whole antenna system. The car body is the other half.
    Since bodies take on different shapes and sizes and configurations you have to do some matching work to get a match you can accept.
    What is happening when you lift the trunk lid is the capacity of the antenna to the body and just under it is changed.
    When this happens the impedance of the whip at the feed point changes closer to the coax value of 50 ohms hence the SWR comes down.
    It could have just happened to work the other way where the match got worse when you opened the lid but that was not your luck.
    To get this system into match, first get as much bonding of the trunk lid to the body as practical then either change the length of the whip or use a matching network to transform the antenna impedance at it's feed point to the feedline impedance.
    This is the way Ham radio does it.
    IMO you should consider a good base loaded whip that has an adjustable whip height.
    Going this route accomplishes all the same things mentioned above for obvious reasons.
    The base loading coil is already designed to effect a resonance and a match along with the whip length for 11 meter use as opposed to just plopping a big whip on and having a hassle with getting it tuned and matched.
    This is why you don't see a lot of long whips used due to the length and matching problems with a lot of different mounting situations.
    I'm sure all this is not what you wanted to hear but it can be done if you understand what needs to be done and work to accomplish it as a lot of trial and testing.
    The way I would approach it is to use an antenna analyzer to see what the impedance is now, then make the changes to get it to where it needs to be.
    An SWR meter can be used but your working half blind with it, not seeing anything but he results of a guess driven change.
    I wanted this reply to be both educational and offer two solutions for you.
    Good luck.
     
  7. 530

    530 Member

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    I am using 2 1 inch wide 3ft long braided with ring connectors on the trunk deck on each side where the trunk is connected to the hinge arm, to car body. I checked grounds from the bottom of the mount to the coax ground side then the bottom of the mount to the trunk, then the coax ground side to the braid and any other one I could have checked, all good.
     
  8. 530

    530 Member

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    I checked everything with a meter, all are where they should be as far as readings. Someone else on another forum did mention about a stud mount.
     
  9. RatsoW8

    RatsoW8 Supporting Member, W9WDX ARC Member - WD8T

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    Can you post a picture of the mount install top and underside?
     
  10. 530

    530 Member

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    My antenna was 102 inches -4 had been cut off. I have it in a 12 inch antenna adjuster on a stud mount. The antenna bottom is about 4 inches from the top of the adjuster, making it at about 106 inches total length right now from the mount to the top of the antenna.
    I'm not to worried about hitting anything round here.
    I have 2 1 inch grounding straps with ring connectors one on each side at the arm hinge to trunk deck screw and then screwed in to a plate at the body.
    The antenna was matched exactly on 1 and 40 while in the 3 SWR range with the trunk closed.
    3 Different types of antennas where on this trunk. Including a wilson 1000, a 7 foot tunable Firestik, all with the same results. Matched on 1 and 40 at a little over 3 SWR, open the trunk and they were matched on 1 and 40 at 1:3 SWR
     
  11. 530

    530 Member

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    I can, but it will have to be Sunday. I'll be posting pics of everything then, if it's still not solved. I'm no expert but usually my setups have gone smooth and I have done whips before. I drilled a whole in the center of my trunk I was so confident, lol. Believe me that stud is installed correctly if that's you're query.
     
  12. KM3F

    KM3F Active Member

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    If other antennas had a best match of 3 to 1 that isn't a good match under any conditions for any antenna.
    The match has to be under at least 1.5 to 1 to be called good at the radio..
    Here's the point as mentioned in my first reply, the whole deal has to resonate not just a good match.
    Another point is an antenna can 'resonate' at any impedance not just 50 ohms.
    Commonly a vertical base impedance is in the 36 ohm range that has to be matched to 50 ohm radios by some manner.
    Take a normal ground plane base antenna that has radials is an electrically complete antenna.
    Your setup has about the right length whip as one half but the other half is the vehicle you have no idea about.
    The trunk lid as an area, sets mainly on a rubber gasket.
    The hinges and your straps are unknowns at this point and act like added circuit inductance.
    Raising the lid hugely changes the feed impedance due to this situation.
    When the lid is 'down' the capacitance between the lid and the body greatly increases around the outside edge but still acts like a larger value series capacitance to the body.
    This may be way out there to believe but some metallic paints present a very lossy surface medium to RF current.
    All the RF current flows mainly on outside of the vehicle not on the inside.
    As you can see the straps are not doing anything for you.
    To solve this you either have to get lucky or use an analyzer to see what you have and work from that point to change it or the antenna will never be able to present a correct feed impedance to the feed line at the radio.
    The length of the feed line being about 9 feet is about 1/4 wave length and not by coincidence.
    Another thing is the mismatch tries to send common mode current down the coax. This in itself will upset the meter reading to the point you don't know what you have . A way to get an idea about this is to put a hand tightly on he coax from the antenna. If it affects the meter reading you have common mode interference form the mismatch adding to the hassle.
    A possible way to get a handle on this is this is to use clamp-on Ferrite cores to stop the common mode but it's a cheating way to overcome a mismatch.
    Good luck to you. I can't offer anymore but to explain some of what is likely going on and how to address it..
     
    #12 KM3F, Feb 21, 2014
    Last edited: Feb 21, 2014
  13. M0GVZ

    M0GVZ Sr. Member

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    It could actually be a faulty stud mount if you're using this type of mount:

    [​IMG]

    The 3/8 body mounts have a brass bit press fitted into the bottom of the threaded part. Over time they get loose and the first indication is the background noise level changing/SWR jumping up and down as you drive down the road. I used to get about 6 months out of one. It would certainly explain why when you open the trunk the SWR goes down. I bet you'll find if you pushed forward slightly on the antenn you'd get the same result.

    If you want a decent mount get a Breedlove mount. They do all kinds of variations and they're built like tanks. Not cheap but you won't need to replace them every 6 months either.
     
  14. 530

    530 Member

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    [​IMG]
    That's the mount I'll be able to get some pics up on Saturday or Sunday. It is set up correctly however if that's your query.
     
  15. 530

    530 Member

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    KM3F. FYI I am not that great at doing this stuff. It's hard to find and CB service shops around here in Secaucus NJ. I quickly looked up the antenna analyzer and they go for 100+$ and I am not sure if I would know what to do with it, even if I did buy it. I understand what you are saying about most things you wrote, can you give me some suggestions to try w/o that analyzer? Do you think it's better to just take the ground straps off or re-position them poss.? Do you think a ground plane you can buy for a mobile antenna might help? Do you think another mount might help as i have read on puck mounts about "caring for the space between the hot and ground on their mount" and not only that I have read several people had problems with stud mounts before on the trunk and they changed it and it worked. I have seen these whips on police cars just as i have it and i have a mercury grand marquis similar to the crown vics. I don't want to "cheat" when doing this. Any testing I could do here first as a backyard mechanic where i don't have to drop 100+$ on a analyzer? thank you.
     
    #15 530, Feb 21, 2014
    Last edited: Feb 21, 2014

: high, problem, swr

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