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Mobile SWR readings vary depending on meter used

Discussion in 'CB Antennas' started by CBA179, Sep 13, 2020.

  1. CBA179

    CBA179 Member

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    I recently installed a Predator 10-K (K-1-22 Competition Plus-BB) - Bad-To-The-Bone, on my pick-up truck. I tuned it using my Dosy TR1000 meter that is installed in the truck and the SWR is 1.3-1 the meter on my Galaxy DX959 is 1.5-1 ( I know CB radio meters are not the best, but use it for quick reference) I recently purchased a MFJ-870 meter just for double checking my Dosy meter. The MJF is reading 3.0-1. Not sure why the big difference.
    I disconnected the Dosy TR1000 I have on my base station cb and checked the truck antenna SWR, same reading as the other Dosy meter.
    I tried both Dosy TR 1000 meters and the MJF on my base cb and they all read the same 1.1-1
    I then tried all three meters on my Jeep which has a Uniden Radio and all three read the same 1.1-1
    Not sure why the MJF is reading so different on the truck. I have multiple grounds from frame to body and antenna mount is grounded. All coax is Astatic and in good shape. The antenna is mounted on the rear wall of the truck bed and the coils are about 6" above the cab roof. I know this is not the ideal location but prefer to have it in this position. The antenna that the Predator replaced was in same location and evrything else the same. It also had an SWR of 1.3-1 with the Dosy meter. Not sure whats up with the MJF meter. Any suggestions appreciated. Thanks


     

  2. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    My fear in this?
    upload_2020-9-13_17-4-8.png

    Your coax may not be "mating / meshing" with SO-239's - can cause the SWR to triple if you are not able to make the coax shields properly terminate at that SWR meter - it (the Meters Case) needs to bond to the coax to properly show SWR - open coax shielding from the meter unable to make a good contact, makes the Meter look bad while everything else works.

    In case of bad typos...
    Done using Microsoft EDGE and it's spell checker
     
  3. sp5it

    sp5it Master of puppets

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    Take 100 Ohm resistor instead of antenna and measure SWR. What is the reading?
     
    tecnicoloco and r carl like this.
  4. CBA179

    CBA179 Member

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    Thanks for the replies. I don't think coax mating is a problem since I took readings on two other radios and got readings that were consistent with the other meters. Is it possible the MJF is more sensitive to to antenna location ? How do I go about testing with a resistor, not familiar.
     
  5. sp5it

    sp5it Master of puppets

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    Take 100 Ohm resistor 4W or more. Not wirewound.
    Plug instead of antenna, measure SWR, come back with result.
    Meter should show SWR 2:0. Further from that value, bigger crap meter is.
    Mike
     
  6. CBA179

    CBA179 Member

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    I will give it a try. Thanks
     
  7. 1iwilly

    1iwilly Active Member

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    what andy was trying to point out that the dosy might not have the same male connector as the mfj look at arrow for a better understanding

     
  8. w9cll

    w9cll W9WDX Amateur Radio Club Member

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    Dosey meters tend to be way off normally, while I am not a huge fan of MFJ meters I would believe that one over the Dosey. Secondly, I would dump the coax and buy LMR or Wireman stuff, Astatic is not Astatic of old and the coax is most likely cheap Barjan crap.
     
  9. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    I was simply pointing out, that when all other facts remain the same...

    Home Base, the Coax and it's jumper - show ok...
    Another meter, the coax and it's jumper - show ok...

    MFJ shows ok on HOME coax and setup...

    But not the Vehicles. In fact it acts like it's "OPEN line" +3 SWR..

    Ok, the only thing that I see changing, is the connections and their coax used for them - the Radio acts ok, SWR from RADIO is ok, Antenna SEEMS ok, but Meters do not agree...

    Ok...What gives?

    Check the connection - whether it is CENTER to CENTER - or SHIELD TO SHIELD - any combination of the setups - a variable can occur, the CONNECTION is changed.

    So if the "Hole" (Center) is not mating (Meshing) or the notch to notch has too much clearance, the SHIELD may be ok, but CENTER may not, or SHIELD is loose, but CENTER is ok.

    So looking again at the facts...

    My thoughts are with the coax Center to Center - Shield to Shield - or the kind (Brand not type - READ: Quality) of connector used on the COAX in the vehicle is different and not making the best connection - but when attached to the radio direct - it's fine. Base antenna confirms this.

    So speaking on behalf of MFJ, I can't say the MFJ is truly at fault, but if the CENTER is elongated, or the Notch can't be tightened to mesh with the notch on the coax side of the connector - then a clearance issue and hence the SWR's "open line" appearance.

    The other problems are with the meters hardware, if the SO-239 is not sticking out far enough, then the ring-knurl for the Shield can't truly rest/press against - the SO-239's outer thread - and in that event - can place the Center stem in a not-seated fully - position. So you'd have to push in or twist the coax end at the SWR meter until the connection is made.
     
  10. CBA179

    CBA179 Member

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    Thanks, I will make sure coax is making a proper connection and try again.
     
  11. Rwb

    Rwb Sr. Member

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    Also look at the pl 259 nut or shell screw it on meter,does it snug up to meter but not snugging up the center? I had this happen on a few pl 259 nuts,took dremmel n cutter blade n shaved off 1/8" of the end of the nut.now it snugs the center up n stops tight just short of the so 259 connector
     
  12. CBA179

    CBA179 Member

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    I checked all coax connections and made sure that the notches were matching and making good connections. I got the same 3.0 on the MFJ and 1.3 on both Dosy TR1000. Radio meter is 1.5. I think the readings on the MFJ are being influenced by something with the truck or antenna location since my two Dosy TR1000 and the MFJ all read the same in my Jeep and base radio.
     
    Handy Andy likes this.
  13. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Glad you checked, because it rules out all variables then - so the MFJ is sub par on this.

    That is a realm of repair, or an RMA for return -
     
  14. Rwb

    Rwb Sr. Member

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    IMG-2477.JPG
    Notice the diffrence IMG-2474.JPG i got some pl 259 ends somewhere and the barrel nut is too long. It bottoms out n center seems snug but isnt.
    You go back to 1 vehicle n get odd reading on mfj but not in others . I read n reread your posts. Connecter might be sloppy fit in mfj n not the others.
    I used dremmel cut some off barrel nuts so they would tighten center up. Saved 1 in case refrence is needed
     
  15. CBA179

    CBA179 Member

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    I will look at that possibility Thanks
     
    Rwb likes this.

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