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Will a mobile antenna --either mag mount or NMO mount-- couple to surrounding objects such as tall trees, taller trucks, buses, or even people walking by?

If that is the case, would you get slightly better transmission signal if you parked in a crowded parking lot? As you get a better ground plane?

I have an SG7500 NMO mount on the roof of my car, a little bit to the left side. Radio is a TMV71A. Last night, after I stepped out while talking on the repeater, the NET controller then told me that my signal just degraded a lot. The signal returned to normal after I got back in. It was not noisy outside, and so I believe it is a bonding issue.

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You've got plenty of ground plane on your roof and there is probably nothing wrong with your antenna installation.

It's better thought of as a matter of propagation. Radio waves are reflected, refracted, diffracted, or absorbed, to various amounts, by everything in the environment. Objects nearby will change the radiation pattern of your antenna, but whether that change is for the better or for the worse depends on which direction the other station is.

When using FM, there's a fairly sharp transition from enough signal to be clearly heard to a very noisy signal, so a small change in how well your signal reaches the repeater can make a big audible difference. You've probably heard the "picket fencing" effect when your radio or someone else's is in motion and quickly flutters into and out of intelligibility. This is the same thing you experienced, except from a different cause, and it didn't cycle since the object (you) didn't keep moving.

Don't bother trying to fix your system — there will always be better and worse physical arrangements, and you'll just be changing which ones there are. Instead, be prepared to try parking differently if you need to make a contact while stationary.

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