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Diamond instructions for my CR8900 antenna state that proper grounding is required for the antenna to work. The battery negative is already grounded and has continuity to the coax outer so isn't the antenna thus already grounded without the need for a short grounding wire?

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A DC ground connection is not the same as an AC ground, especially at RF frequencies and in EM fields, and not the same as the ground plane or counterpoise that a quarter wave vertical antenna requires for decent efficiency and radiation pattern.

So mere DC continuity is not enough.

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The instructions could be worded better. The point that the instructions are trying to get across is that a proper ground plane is needed for the antenna to work as it was designed. The manufacturer doesn't want to be blamed for poor performance if someone tries to mount the antenna on the (non-metallic) roof of a house, or a fiberglass pickup truck topper, etc. On most bands that antenna is a quarter-wave vertical, and as such it needs to be mounted on a conductive surface perpendicular to the antenna (that's the definition of a ground plane), such the roof of a car. The larger the ground plane, and the closer the antenna is mounted to the center of the plane, the better the antenna will perform.

A grounding wire between the antenna and the ground plane is completely unnecessary, and might prevent the antenna from working as designed.

This is off-topic, but I'd like to discourage you from using a mag mount. I understand that you might not want to make any permanent alterations to the vehicle, but mag mounts are dangerous, because they can easily detach in a crash and turn your antenna and its mount into a dangerous missile. Alan K0BG says it better than I could on his web site, which is loaded with information about mobile operating.

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  • $\begingroup$ Funny enough, for this antenna, CR8900, Diamond say it must NOT be mounted in the centre of the roof but on the edge of the trunk. I queried this as my 4x4 doesn't have a trunk but Diamond didn't respond. It's a quad band antenna and I suspect Diamond have designed some witch craft to make it work, hence the need to be on the edge. I am wondering if this antenna isn't a serious compromise that's loosing me range. $\endgroup$
    – rcx935
    Feb 9, 2022 at 20:52
  • $\begingroup$ Sure enough, the manual says "Optimized for Trunk Lid Mounting" and "not recommended for magnet mounts or mounting in center of roof" (the latter is in upper case in the manual). I wonder why that is? The gain specifications they quote for 10m/6m/2m, 2.15 dBi, is suspiciously exactly the same as the theoretical gain of a dipole. Anyway their specified gain, 2.15 dBi or 5.5 dBi, is respectable, and VHF/UHF are line-of-sight, so if you actually get gain that is anything like that, then you should have no trouble communicating. $\endgroup$
    – rclocher3
    Feb 9, 2022 at 23:30
  • $\begingroup$ "Excessive VSWR may occur with the following antennas if used on a magnet mount: ..., CR8900A, ..." Their matching network does not want a ground plane. I bet the mounting bracket holds it up and over, and the first tuning coil is out of range if you add a large ground plane directly underneath. As you say, this is line of sight so it'll probably be ok, but it would be interesting to see how far from 1:5 it goes on the roof. You could probably mount it a little higher on the roof with a custom bracket to hold it away from the rest of the vehicle a little. $\endgroup$
    – user21417
    Feb 11, 2022 at 13:27
  • $\begingroup$ What you say makes sense @jdv. I'd think that the antenna still depends upon the vehicle being a ground plane, because any λ/4 vertical loses tens of decibels without one. But the designers could have designed the bracket to hold the antenna far away from the vehicle in an effort to trade efficiency for more consistent impedance, which would be easier to match. $\endgroup$
    – rclocher3
    Feb 23, 2022 at 0:36
  • $\begingroup$ As they say, any antenna wants to be a dipole, and if you don't give it a counterpoise, it'll find one anyway. I guess they didn't want this one to couple so closely with the vehicle I guess. $\endgroup$
    – user21417
    Feb 23, 2022 at 0:47

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