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Larsen NMO-27-C testamonials?

Discussion in 'CB Antennas' started by Turbo T, Jul 22, 2012.

  1. Shockwave

    Shockwave Sr. Member

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    Things to keep in mind about this model Larsen is they have a small ceramic cap in the base that does not handle watts. This is used because the loading coil does not have a tap to ground in order to perfectly match the 1/4 wave antenna to 50 ohms. The cap fixes that problem while introducing power limitations.


     
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  2. Recon

    Recon NY 881

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    Good info! Thanks! I'm using a stock Stryker 447 and it's fine just the way it is without adding extra power.
     
  3. Recon

    Recon NY 881

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    The Low Band Cutting Chart covers NMO27, NMO30, NMO34, NMOWB40, NMO40 and NMO50. The longest length of 52" is for 40.0 MHz.
     
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  4. Shockwave

    Shockwave Sr. Member

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    In that application, you're going to like this antenna. I've found it to be a near perfect compromise between good performance and a sleek, low profile.

    The coil is also modifiable. The base of the coil has two large, flat blade screwdriver slots to unscrew the coil assembly out of the housing. There is also a hole towards one edge where the ground side of the ceramic cap is soldered to the brass threaded mount. That connection is the tricky part because you need to remove all of the solder before the plastic insert can unscrew.

    You can use a small flat blade screwdriver to "chisel" out the solder so you do not melt the plastic. You will however need a very powerful soldering iron or gun in order to reestablish that ground connection after any modifications. To fit a larger iron, just make the hole a little bigger and fill it with silicon afterwards. The connection on the top of the coil to the whip is made through a finger contact and is not soldered.
     
  5. Recon

    Recon NY 881

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    I am not expecting too much performance because of the location and installation of the antenna. The antenna is mounted on a Third Brake Light Antenna Adapter on my pick-up truck and not in the center of the roof, therefore there is probably little or none ground plane effect.
    How can the coil be modified? I have no radio / electronic experience and knowledge to tamper with modifying the coil. It is what it is.
     
  6. Shockwave

    Shockwave Sr. Member

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    The basic idea is to remove the ceramic cap that goes between the bottom feedpoint on the coil and ground. It is replaced with 4 turns of added copper wire to ground. Adjusting the expansion or contraction of the coil wraps, adjusts the resonance and impedance of the antenna. The top part of the coil adjusts the resonant frequency and the bottom section adjusts impedance.

    It does take some patience, trial and error to get right. You'll need a soldering iron or gun that is at least 275 watts to make the ground connection to the brass base. I didn't solder this until I had a flat VSWR with testing first. Prior to that, I just left the ground wire long enough to come in contact with the grounded base when screwed on and confirmed DC shunt connection with an ohm meter across the connector.

    I actually ordered this antenna for use on a customers Jeep thinking it would have the bottom of the impedance matching coil already intact in order to match the antenna over a poor groundplane surface. Without the ability to adjust a DC shunt section on the matching coil, the best VSWR that could be obtained was just under a 2:1. With the modified coil properly adjusted, this dropped to flat. Because the antenna is relatively short at 27 MHz, it does not have a wide bandwidth.
     
    #66 Shockwave, Dec 25, 2019
    Last edited: Dec 25, 2019
  7. Recon

    Recon NY 881

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    What would I be gaining by tampering with the coil and "trial and error" testing? Wouldn't it be more practical to leave things as-is and just buy a different / larger coil?
     
  8. Shockwave

    Shockwave Sr. Member

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    I do not recommend trying this without previous antenna experience. But, what you can gain is the ability to obtain a flat VSWR in compromised mounting locations where the groundplane is weak. The coil is designed to work over a typical groundplane surface. If that groudplane is lacking, the stock coil will not provide an optimum match although, it may still be acceptable
     
  9. Recon

    Recon NY 881

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    That's me for sure! No previous antenna experience.
    As I mentioned in a previous post, The antenna is attached to a Bullet Proof Diesel Third Light Antenna Mount on my F-350 Super Crew roof which obviously is at the rear of the cab. Obviously the metal roof is not at the same level as the metal bed, therefore I am assuming there is a "break" in the ground plane effect. Starting at Ch. 1, the SWR range is 1.1 to 1.4 on Ch. 40. I'm not at all educated about this subject, but I don't think it is worth the effort to find someone to modify (trial and error) what I have for the sole purpose of obtaining a flat VSWR. I don't want this to become a rocket science project. I don't know of anyone else who has a third brake light mount and I ventured-out and had taken the risk and bought this (very expensive) type of mount. It may not be the best application but the SWR is within tolerances and the radio is transmitting and receiving within a four to seven mile radius and I'm OK with that.
     
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  10. Shockwave

    Shockwave Sr. Member

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    That's actually better results than I might have expected. It also indicates that just trimming the bottom of the whip a little shorter will improve the match on the higher channels. But, at 1.4:1 as the highest on 40, that will work fine. No coil modification required here.
     
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  11. Recon

    Recon NY 881

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    Yeah, I plan to snip-off another 1/8" tomorrow which should reduce the 1.4 down to ????
     
  12. Shockwave

    Shockwave Sr. Member

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    You'll tell us that tomorrow. Just don't take off too much or you'll move the higher VSWR to channel 1. An 1/8 inch at a time is good since there is at least that much adjustment in the set screw. The typical goal is to balance the VSWR between 1 and 40 so that it is lowest around 20.
     
  13. Dmans

    Dmans Sr. Member

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    I'm no expert but would have thought that 40MHz would have been the shortest length??:unsure:

    73
    David
     
  14. Recon

    Recon NY 881

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    26 Dec. @ 1840
    I'm done playing around with the SWR on my NMO-27. About ten-minutes ago I finished with my last attempt of trimming the antenna and the best SWR measurements I can get are; Ch. 1 = 1.2:1, Ch. 20 = 1.0:1 and Ch. 40 = 1.4:1.Considering that I monitor and talk mostly on Ch. 19 AM in my truck, I am satisfied with what I have.
     
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  15. Recon

    Recon NY 881

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    No argument from me. The Low Band Antenna Tuning Instructions I have was in the box with the antenna.
     

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