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I'm looking for a dual band base station antenna, and find that many of them have short radials around the base.

Due to the specifics of my installation, I need to remove them. However, it is being mounted in a way that the antenna penetrates the flat metal roof of the structure (a solid, single sheet of metal about 20'x8'). I can mount it so the roof is at the same location as the radials would have been. I might still attach the radials, in fact, and have copper mesh on the lower side of the roofing metal sheet to contact the radials.

Will this impair performance? Do the radials need to be left alone, or are they simply the grounding mirror for the antenna?

Can I get rid of them, or do I need to have the sheet metal contact the antenna at the base where they attach?

The radials would be on the bottom, and are grounded to the mounting point, and presumably to the coax shield. I'm looking at the Diamond X50A at the moment, but there are many with essentially the same design. The instructions don't provide any help.

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  • $\begingroup$ Slightly related: ham.stackexchange.com/questions/1901 $\endgroup$ – Kevin Reid AG6YO Jul 24 '14 at 20:38
  • $\begingroup$ If the antenna is mounted in such a way that it penetrates the roof, and you are mounting it such that the roof is where the radials were, does that mean these "radials" are in the middle of the antenna? $\endgroup$ – Phil Frost - W8II Jul 24 '14 at 21:17
  • $\begingroup$ Sorry, I meant to add images and the current antenna I'm looking at. See above for a little more information. I'm planning on extending the antenna through a hole in the roof. The antenna mount will be below the roof. The hole will be smaller than the radials, so they won't go through the hole, but I can connect them to the roof electrically at the hole so the roof becomes an extension of the radials. $\endgroup$ – Adam Davis Jul 24 '14 at 22:34
  • $\begingroup$ Can you tell if this antenna is a dipole or a monopole? If it's a dipole, then there must be some way to get the feedpoint to the middle. If it's a monopole, then the long part above the radials is just a simple tube. $\endgroup$ – Phil Frost - W8II Jul 30 '14 at 19:57
  • $\begingroup$ @PhilFrost In their documentation it is described as "6/8 wave C-Load(2m) - 5/8 wave three element C-Load(70cm)" - though I admit I'm not sure what that means. I'm guessing it's a capacitatively loaded monopole. $\endgroup$ – Adam Davis Jul 30 '14 at 20:28
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You can remove them and instead attach the shield of the transmission line to the actual roof! It will make a great ground-plane, probably much better than the radials that are currently attached.

Alternatively, connecting a copper mesh will work too.

You will definitely want something though, as you'll be giving up some power and efficiency if you just leave it off completely.

Example: next time you have a 2 meter HT with a rubber-duck in your hand, grab a 19-inch (doesn't need to be exact) piece of wire and connect it to the shield/outside of the antenna connector with an alligator clip or similar... basically, you've made it a dipole. You can then test just how much the signals improve/degrade with & without that single wire.

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Those radials look too small to be a ground plane for 2m. They may be a capacity hat to decouple the coax feedline from the antenna. That would be true if the antenna inside the radome is a vertical dipole rather than a ground plane vertical.

I would leave those on the antenna and keep them a reasonable distance from the metal roof, as much as a wavelength if you can manage it. Luckily, that is only six feet on 2m.

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  • $\begingroup$ They are mechanically and electrically connected to ground (shield). If it is a capacitance hat, how will mounting it so the hat is at the same height as the roof affect the antenna? $\endgroup$ – Adam Davis Jul 30 '14 at 17:53

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