Road Squawker brings up a point about SWR over-emphasis. So I read back over the hypothesis and realize I left something out. This operator wants to run at least 500 watts on this system. Does the addition of high power into the hypothesis change the need to emphasize he best match to the antenna system or not?
A hypothetical center loaded antenna that is mounted on a stake hole mount on the front right of the pickup truck bed. The best SWR obtainable here is 1.3:1 on ch 1, 1.6:1 on ch 19, and 1.9:1 on ch 40
There will be a 500 watt kicker in the mobile.
Ideally, when possible, the coil on a mobile antenna should be set as near to the center of the shaft lengths as mechanical and aesthetic considerations will allow. When the coil of the antenna is mounted below the roof height of a vehicle the placement of the coil is effectively reduced to much less than approximately half the combined shafts height. MOST of the radiating power occurs from the bottom of the coil upward. The longer the upward part of a loaded antenna the closer it resembles a non-loaded 1/4 wave whip, and the better your TX/RX. So the idea of getting a longer whip could help when the mounting position is less than optimum, or below the topmost height of the vehicle, but may not necessarily correct an SWR due to the lowered coil height in relation to higher part of the vehicle body, or reflect from the vehicle body. Although the placement in the center of the bed rail behind a pickup cab (or on a toolbox) does not raise the antenna up completely above the cab, it does center it on the truck body. This can help with the reflect, and even out the radiation pattern some. The ground effects from the earth beneath the truck is the true groundplane, the truck is simply the mount, and functions as a part of your antenna system; that's why good grounding to the vehicle is needed. The vehicle body influences performance, and it is in your best interest to USE it to your maximum advantage. If not, it will serve to your disadvantage. We often place antennas where we can, but trade offs in performance are always in play. I suggest finding a way to raise your antenna up high if you cannot put it on the cab itself.
The funny thing about mobile antennas like this is that more than SWR changes when we make alterations to the design. The way an antenna is built decides its performance parameters, but every install changes those things, and SWR only tells us the reflect is controlled. SWR is an important measurement, particularly if you are running high power, and having a low SWR helps to prevent damages to equipment. That said, SWR doesn't tell the whole story of an antenna's performance. It actually tells less about the performance of the antenna then we think. It speaks to the match of the antenna system and the transceiver. What I'm trying to get to is antennas are usually designed with the optimum performance in mind, and as a result it's expected they are to be mounted in the optimum location to take advantage of the design. Changing the SWR may even make your antenna perform less well if you aren't changing the antenna setup in ways that make it work best. It can be assumed your SWR is high because of where it is located on the vehicle. An SWR solution could be either getting a longer riser to raise the coil up, or a longer whip. You will by these methods reconfigure the shape/design of the antenna. So, leaving the antenna where it is, yet bringing down the SWR may only make us feel better about things, but the antenna is still not in the location for best performance. In fact, it is now actually a different antenna. Likely, it will not hurt the equipment; reflect is better controlled (maybe). But he antenna was designed to be installed differently.
Anything else? If no one finds anything that needs correcting, or saying better in all this I'll have to assume it is correct throughout.
