Larsen NMO 2/70B and an amp

Discussion in 'Mobile Radio Systems' started by KK4YIV, Mar 20, 2017.

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  1. KK4YIV

    KK4YIV Ham Member QRZ Page

    I currently have a Larsen NMO 2/70 antenna and am considering getting a 160 watt amp. I have seen this antenna advertised as a 100 watt antenna and also as a 200 watt antenna. Will it survive the use of a 160 watt amp?
     
  2. N2PQW

    N2PQW Ham Member QRZ Page

  3. KK4YIV

    KK4YIV Ham Member QRZ Page

    I should have looked there. Thanks.

    I will have to replace my antenna or limit my power to 100 watts. Neither option sounds good.
     
  4. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    One simple thing to remember. The rating Larsen gives that antenna is a continuous duty one. I've run ≈300 watts through mine, and have for over 15 years, with no issues.
     
  5. KK4YIV

    KK4YIV Ham Member QRZ Page

    Thanks Allen. I was hoping that you would chime in.
     
  6. KK4YDR

    KK4YDR Ham Member QRZ Page

    If you are very concerned just drive the amp so that the power doesn't exceed 100 watts of output but the larsen is highly underrated and should handle 160 without even a touch of heat. Of course make sure it is resonant before womping down the power :p
     
  7. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    Allow me to give you a few pointers about running high power (>100 watts) on VHF.

    For safety reasons, roof mount only!!!! And it isn't just for radiation exposure either. Rather the amount of induced current into the wiring, primarily the defroster grid, will cause you more problems than you can imagine. And believe it or not, you can actually cause the defroster grid to burn into! Common mode becomes an issue too.

    If you're operating FM (phase modulations mostly nowadays), the amount of power it takes to double the distance is about 10 dB depending on the antenna and the measurement angle. And you need to be able to hear better, which isn't always possible to accomplish.

    When using SSB, the extra peak and dynamic power does increase the distance rather significantly. However, trying to use a vertical antenna on SSB is a waste of time. If that's your goal, the spend the bucks, and but a couple of halo, and stack them up.

    I'll close with this: If you cannot make a repeater using 30 to 50 watts out, you need a better antenna, not an amplifier!
     
  8. KK4YIV

    KK4YIV Ham Member QRZ Page

    Thank you for the pointers, Alan.

    I have a high top Dodge Sprinter van with the antenna in the center of the roof. I need a little more power primarily for FM simplex. I really can't tolerate a longer antenna.

    Should I use ferrite beads on the coax at the base of the NMO mount?
     
  9. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    Just one Mix 31, and only one turn. If you use mix 43, it takes two turns. You might get by without any, but it is always best to error on the plus side.
     
  10. KK4YIV

    KK4YIV Ham Member QRZ Page

    Thanks again.
     

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