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I have a NMO mount antenna- 1/2 wave on 2 meters. I am going to stick it on top of a PVC pipe which is 5 feet tall.

Since this is a bicycle mobile antenna, I do not have the option of large radials out in every direction. Would it work to have a 1/2 wave radial sticking straight down like a vertical dipole?

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If your antenna is already a 1/2 wave long, it's quite likely already a vertical dipole. Resonant dipoles are a 1/2 wave long. Resonant verticals are a 1/4 wave and make the other half as the image antenna created by the ground plane. In this case, adding another wire probably isn't going to do much of anything, since the antenna was probably designed to work without a ground plane anyway.

If what you have is in fact a 1/4 wave vertical, then adding another 1/4 wave, co-linear wire makes it a dipole. That will indeed work, and it will work very well.

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  • $\begingroup$ Thanks for the answer. It is the NR770HNMO antenna made by diamond, and it claims "Radialless mobile gain whip". I still don't understand how it works without another counterpoise. Are you saying it is an end-fed dipole which is a quarter wave on each section, and then the little coil in the middle separates the dipole? $\endgroup$
    – Skyler 440
    Feb 16, 2015 at 14:33
  • $\begingroup$ @Skyler440 It's probably an off-center fed dipole, like this. A dipole doesn't need to be fed exactly in the center. You can't feed it at the extreme end, but judging by the thicker portion at the bottom it looks like it's fed maybe 1/8th of the way up, and that bottom 1/8th is the "counterpoise". The coil in the middle is probably there to improve 70cm performance, make the whole thing a little shorter, or both. $\endgroup$
    – Phil Frost - W8II
    Feb 16, 2015 at 20:38

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