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Thread: Question on mounting 2m antenna on plastic big rig

  1. #11
    Master Navigator W5BRM's Avatar
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    Update: Double checked all my measurements. 39.5 inches stainless steel whip, 19.25 inch 14ga wire counterpoise attached to mount. 10ft rg58au coax. Tested continuity on the coax. No shorts. Mfj-259c shows swr of 12. Impedance is R=153 X=217 AT 146MHZ. Coax loss is 0.7db.

    Checked other frequencies for resonance. Have swr 1.1 r=47 x=3 at 73.0mhz.

    Cannot figure out why this wont tune. Half wave antenna with counterpoise and not a point of resonance anywhere near the desired frequency. Don't think the mount is bad or wouldn't get resonance at 73.0mhz.

    Frustrating!!

  2. #12
    Orca Whisperer PA5COR's Avatar
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    1/2 wave on 2 is a 1/4 wave at 73 MHZ....
    Is the matching network at the feed point still alright? high impedance on 2 meters for a 1/2 wave....
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  3. #13
    Master Navigator W5BRM's Avatar
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    Quote Originally Posted by PA5COR View Post
    1/2 wave on 2 is a 1/4 wave at 73 MHZ....
    Is the matching network at the feed point still alright? high impedance on 2 meters for a 1/2 wave....
    This is where my theory gets fuzzy. As the antenna is not a "shortened" or otherwise modified antenna, there shouldn't need to be a matching network?? I could see that if I were trying to make a half wave with a non resonant shorter whip . Add a coil, tap it, and I'd be in business. I don't get why a proper length balanced antenna won't tune. Obviously I'm missing something but what....

  4. #14
    Orca Whisperer PA5COR's Avatar
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    Feeding a half wave dipole at the end is feeding it at the high voltage point i.e. high impedance as you can see in your measurements.

    http://www.ked-wireless.com/document...%20-%20QEX.pdf

    Just as with the 5/8 verticals you need to match that high impedance point to 50 Ohm, mostly done with an added coil.
    See PDF, that will answer most of your questions ;)
    "If the Republicans will stop telling lies about the Democrats, we will stop
    telling the truth about them." - Adlai Stevenson (1900-1965)
    “I’m not liberal/conservative, I’m anti-idiotarian.”
    At some point in the last 20 years, the left moved to the center, and the right moved into a mental institution

  5. #15
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    For use as a groundplane, it needs to be 1/4 wave, or about 19.5 inches on two meters. A standard jpole is a halfwave antenna, too - the whole point of the 'J' part of it is to act as an impedance matching device. I'm surprised 39" isn't too high on your truck.

    On his fiberglass tonneau cover for his pickup, a friend of mine uses metal tape on the underside of the fiberglass as a two meter ground plane. I've done this trick, too, and it works.

  6. #16
    Master Navigator W5BRM's Avatar
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    Quote Originally Posted by PA5COR View Post
    Feeding a half wave dipole at the end is feeding it at the high voltage point i.e. high impedance as you can see in your measurements.

    http://www.ked-wireless.com/document...%20-%20QEX.pdf

    Just as with the 5/8 verticals you need to match that high impedance point to 50 Ohm, mostly done with an added coil.
    See PDF, that will answer most of your questions ;)
    Thanks! That does indeed! Much appreciated!

  7. #17
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    Another solution: Mount a vertical dipole for 2M. They're not too big, and doesn't need a ground plane.

    You can get creative with the mounting. I figure a host clamp somewhere will work.
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  8. #18
    Orca Whisperer PA5COR's Avatar
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    http://home.planet.nl/~gmwan000/jwx.html
    antenna to clasp on your windew, scroll a bit down from the top
    http://home.planet.nl/~gmwan000/jwx3...nne%20test.pdf
    PDf with piccies and test.
    "If the Republicans will stop telling lies about the Democrats, we will stop
    telling the truth about them." - Adlai Stevenson (1900-1965)
    “I’m not liberal/conservative, I’m anti-idiotarian.”
    At some point in the last 20 years, the left moved to the center, and the right moved into a mental institution

  9. #19
    Master Navigator W5BRM's Avatar
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    Got it all working. Bought a Diamond NR770ha. Took about 30 seconds to tune it. Got the swr at 144.5 at 1.6. Ohms were a bit high at 73. 146.0 swr 1.3. Ohms at 60. 147.9 swr 1.5. Ohms at 71.

    Not sure what to do about the ohms. Im using 50ohm coax. Tge antenna is tilted a bit forward so that may be the problem. No time left for today to experiment.

    Listening to 146.52 in Columbus Ohio. Madhouse. Have about 7 or 8 hams mobile and base working. Nice to hear the activity.

    Had a brief qso with n8ymt i think was the call. (Didnt write it down). He gave good audio report and strong signal about 4 miles away..

    Still got some things to do to get the radio situated.

    All in all, nice to be active again.

  10. #20
    "Usual Suspect" WZ7U's Avatar
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    That is great! I wouldn't sweat the ohm readings, the impedance is close enough to be effective. Go enjoy it man, you earned it

    Like that post was...
    Moving on, my posts are not helpful

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