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United We Stand |
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Brainerd Area Amateur Radio Club, Inc. |
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"Ham Tips" Larsen 2m/70cm Antenna Modification |
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Here is a fender mounted antenna on a Chrysler Mini-Van. It is a Larsen NMO 2/70 base loaded antenna. The original center loaded whip had been cut for the center of the two meter band (146.0 MHz). However, the antenna matched best at 149 MHz when it was transferred to the Mini-Van. The antenna needed to be longer in order to work well over the entire 144 to 148 MHz band. If you look closely, you will see the vertical whip is made taller by the addition of a threaded rod and nut to fasten the rod to the original rod atop the base loading coil. The additional 5/8 inch of height brought the best match to 146.0 MHz. The rod and nut were fine thread, not to be confused with metric. A trip to a good hardware store is required to find this hardware. The threaded rod was made from a machine screw with the head cut off. |
From experimentation, I found the Larsen base loading coil has a lot to do with how the antenna matches. It appears that each coil is slightly different and the whip antenna must be cut to length to make the coil/whip antenna system work at the desired frequency. You cannot swap base loading coils without readjusting whip antenna length.
Story by Mark Persons WØMH 
