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Larsen NMO 27B Frequency Change

Discussion in 'Amateur Radio Antennas' started by darrenster, Dec 26, 2010.

  1. darrenster

    darrenster Member

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    Hi All,



    I have a Larsen NMO 27B which I no longer want to use for a CB radio; brand new in its package. The place I brought it from wont take it back. Is there any chance I could re-tune it so it could operate on 144.390MHz?

    The specs are as followed:

    NMO 27B
    Frequency: 27-30 MHz.
    Gain: 0 dBd
    Type: 1/4 Wave
    Power: 200 Watts
    Base: NMO27BCO Black
    Whip: W490B Black
    Height: 53 inches (132 cm)


    Thanks!!
     

  2. VA3ES

    VA3ES Old Buzzard

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    No. The internal coil is too big, i.e. it has way too much inductance. You will never tune it. Now, if you want to operate it on 12M Just get a 65" whip (W640B) and you're good to go on 24.9 more or less
     
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  3. W5LZ

    W5LZ Crotchety Old Bastard

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    If you want to go to the trouble of re-building that antenna, you could use it on whatever frequency you wanted. But, it would definitely require some 'not so small' modifications to it, and probably not worth the expense.
    - 'Doc
     
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  4. glsmith1544

    glsmith1544 W9WDX Amateur Radio Club Member

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    Just a suggestion, I have one & you can't buy a higher quality antenna than Larsen (noted for their commercial antennas). Might come close to getting your money out of it by selling it.
     
  5. VA3ES

    VA3ES Old Buzzard

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    Larsen NMO34 w/64" whip...

    I have exhaustively tested all the Larsen "Low-band" coil and whip pairings and have come to certain conclusions. The Larsen NMO27 antennas, at 49" are virtually worthless.

    If you are looking for an excellent 10M or CB antenna that will rival the Wilson 1000, then you can't do better than the Larsen NMO34 coil with the 64" whip. That combo is the NMO34B coil, and the W640 whip. Depending on your mounting situation, you may have tro add the Larsen spring (SPRINGB). This antenna measures 70" total length from bottom of coil to tip of antenna, (longer with spring). It performs beautifully on 10 - the first time I tried it I ended up working a VK with my 7W PEP QRP portable!!!. Around town, it has superb range; I can work bases and mobiles out 25 and 30 miles away.

    For CB use, I recomment the Larsen NMO30 coil, with the 64" whip. That will put you around 27.200 Mhz, (approximately Ch 20.) with the same results, (depending on your mounting situation; again, you may need a spring).

    These antennas are special order only, but any Larsen dealer can get them easily.
     
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  6. W5LZ

    W5LZ Crotchety Old Bastard

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    Ed,
    I think you may be missing something, the 'conversion' would be to 2 meters, not for HF at all.
    In effect, that would mean changing the internal coil to impedance matching and no, or very little loading for that particular length of whip compared to the loading for use on HF. That sort of modification is certainly possible, the difficult part would be accessing the parts to modify without destroying the antenna's physical usability.
    If I absolutely had to, I think I could do that electrical modification, but I'm not sure about the part about keeping the thing physically usable. I figure you could too if it came to that, or the poster for that matter. The key words in that are "absolutely necessary" and "usable". Your point about that antenna's worth is also a consideration! It seems that swapping it for a 2 meter antenna would be another consideration to keep in mind, and one I'd certainly be more comfortable with, you know?
    - 'Doc

    (And if it came to that point, doing away with the coil at the base, cutting the whip down to something a little longer than 20 inches then tuning the thing would at least give you a 1/4 wave antenna on 2 meters. I really wouldn't recommend that unless you were really desperate.)
     
  7. VA3ES

    VA3ES Old Buzzard

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    No Doc, I got it the first time! I mentioned that the coil had way too much inductance to be easily modified. As well, modern Larsen black coils are sealed, making access to the internal coil difficult without ruining the coil housing.
    Ditto see above.
    Precisely why I said it was a useless endeavour! Better to get a Larsen NMO150B, and be done with it! If he can, he might try selling the NMO27 on eBay... or do as I suggested and add the 64" whip and put the thing on 12M.
     
  8. darrenster

    darrenster Member

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    thanks for all the comments. I think there might be a little bit of confusion here. I'm looking to ditch the CB radio all together and want to convert my current larsen 27b antenna to work on the APRS frequency. From the first reply it looks like it can not be done because of the inductance of the coil that came with my current larsen 27b antenna.

    Now if anyone is interested in buying my larsen 27b antenna or making a trade, send me a pm.

    Thanks!!
     
  9. VA3ES

    VA3ES Old Buzzard

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    For APRS, the Larsen NMO150B works well.
     
  10. AndrewGS

    AndrewGS Sandbagger

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    OK, so 8 years later here I go bringing this thread back.

    I'm thinking about going with the NMO30BCO coil and W640B whip. I plan to use it for CB and between 28.300 and 28.700 on 10M. Would this antenna be wide enough to do that?
     
  11. Mudfoot

    Mudfoot Sr. Member

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    Larsen NMO34 coil with the 64" whip.
    I use the Larsen NMO34 coil with the 64" whip. Works like a champ, but it won't play that wide. Tune it for the upper CB portion, then you should be OK to 28500. I have it stored for later use or I'd check it. It's comparable to a Wilson 1000 in performance, but without the big honking coil base.
     
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