Open air coil vhf antenna

[nfsus]

What’s the point of the coil? I can’t see where it’s changed the vertical length and it’s not a loading coil. So what is it?

 

[brandon7861]

It is a center-loaded antenna. At VHF, a little coil can do a lot (esp. to the takeoff angle). When an antenna needs some inductance, I believe the inductance always "models better" the higher it is used in the antenna element as current (the thing that does the radiating) tapers off above a coil quickly. If you need a coil, the bottom is the most convenient, but the worst (with respect to efficiency), spot to put it. The middle of the antenna is a tradeoff between radiation efficiency and wind loading. The top would be absolute terror for the magnet mount and the driver of the car behind you.

Whether the coil was for matching or to simply make it look more sophisticated is hard to say from a picture, but one way or another, it is a coil and the RF definitely sees it. I used to run small NMO trunk mount antennas on VHF and the ones with the center coil (that were a couple inches shorter, but probably using the same amount of material) were very different in takeoff angle. The shorter ones with the center coil are definitely better in low areas where there are repeaters up on high hills compared to a straight quarter wave with no loading coil at all.