Ground plane question

[Turbo-T]

I think I've heard it said but wanted to confirm....

Doesn't more metal under the antenna equal a better (larger) ground plane?

In other words....if I took....a pick up truck....and a Blazer or a Bronco....and put the same antenna on both of them and even put it in the dead center of the roof....wouldn't the one on the Blazer/Bronco get out further than the one on the roof of the pick up?

Thanks.

['Doc]

"More metal 'under' an antenna is better than more metal 'beside' an antenna.", is a very true GENERAL statement. Just like saying the more boat directly under you is better than beside you. Doesn't mean you can't walk from one end/side of that boat to the other end/side of it, only that being in the center of that boat can be 'better' for a number of reasons. Practicality enters into both of those statements to some degree. If there's a reason for you to be on the side of that boat, then your getting your 'bottom' as wet as the boat's 'bottom' may not be a really bad thing, sort of. Being in the middle means it's usually easier to keep things 'balanced', so to speak, to compensate for you. (That's about as far as I'm going with that analogy! Boats make me think in terms of bikini's, and that leads to "The Good, the Bad, and the Uuugly!", and I just don't feel like going there.)

To a large extent, that 'under' refers the whole vehicle, not just the particular part that the antenna is attached to. Sort of a center of 'mass' thingy. Depending on the 'shape' of that 'mass', good antenna placement can vary to some undetermined-by-me degree. Sort of like shining a light on a mirror that isn't exactly 'flat'. What spot on that mirror will give you the best amount of light where you want it to go, when that light reflects off that 'non-flat' mirror? Some times that 'spot' won't be in the exact center of the mirror, which relates to the 'practical' aspects of it, and there are humongus variables in that! (Dat's as far as I'm going with that analogy too!)
There is no always 'best' antenna mounting position that is always the 'best' in every situation, sorry 'bout that, but it's true. After figuring in the practical aspects, that 'best' location typically amounts to the highest and most 'central' spot that you can manage.

Mercy... my first cup of coffee this morning! You have my permission to 'ponder' as I do my sipp'in. Don't bother me for a while, measured in gallons of coffee...
- 'Doc

[Turbo-T]

Doc, you lost me there....

[Red Warrior]

Let me see if I can expand Docs explanation.

It is the metal mass beneath the antenna not necessarily the amount of "flat" surface under the antenna. A ground plane becomes more effective with the "longer" metal under the antenna. Visualize the vehicle replacing the ground radials of a base station antenna. In other words the metal of the vehicle acts like radials in a ground plane. The longer the radials, the more effective they are. So the body, frame, exhaust and other metal parts become a part of the ground radial. That is why bonding all the metal on a vehicle is so important.

[Tinman590]

Here is the way I would take it, If the top of the blazer was 10x10 sheet metal. Then the rest of the blazer was plastic you would not have a good ground plane. The idea of a good ground plane is to utilize all the metal as "one". I hope this helps.

['Doc]

And now you know why I don't think that 'mass' analogy is a very good one.
There's two things about 'groundplanes' you have to remember. One, that groundplane's only importance is that it is replacing the other half of a typical vertical antenna. So, it has to provide the matching characteristics of the vertical part, the reactances and resistance which make up impedance. The second thing is that larger than necessary is easier to compensate for than smaller than necessary. (Also remember than all of this size thingy if frequency dependent, lower frequencies require a larger groundplane than higher frequencies.) A 'Grey Hound' bus would be much easier to work with than a 'Mini Cooper'. (By the way, if it makes the Mini Cooper look like a wind up toy, that's in your favor, and who cares what 'they' think anyway? They're just jealous... me too.)
-Tinman590-, that 100 square feet of sheet metal isn't gonna be the best, but it can certainly be made usable!
- 'Doc

[Turbo-T]

Here is the way I would take it, If the top of the blazer was 10x10 sheet metal. Then the rest of the blazer was plastic you would not have a good ground plane. The idea of a good ground plane is to utilize all the metal as "one". I hope this helps.

I believe Blazers have all metal roofs?

Now when you say "utilize all the metal as "one", are you referring to the entire vehicle top, bumper to bumper?

For some reason since a pick up has a small roof and a Blazer has a larger roof, I was thinking the larger roof would constitute as a larger ground plane.

Unless what you're trying to say is that it doesn't matter....because if the antenna were in the dead center of the roof on let's say a single cab pick up, the antenna would also use the surface of the hood and the bed floor surface as the ground plane....???

[WL74]

So the vehicle itself acts as the ground plane? That would make since; so you would want a good radial plane for the bounce. Wouldn't that mean that a vehicle shaped like a saucer would have a really good ground plane?

['Doc]

Right! A saucer shaped vehicle would certainly be more omnidirectional than a long narrow one. Little bit harder to park, but who cares?
The whole mess, top, hood, trunk, etc, make up that 'groundplane' (hate that word). Along with the dirt, concrete, whatever that's under that vehicle (capacitive coupling). So, there's always going to be a changing groundplane unless you never move. Probably will never be 'perfect', so just make it as good as possible. Don't get too 'wrapped up in it'.
- 'Doc

[Tinman590]

Here is the way I would take it, If the top of the blazer was 10x10 sheet metal. Then the rest of the blazer was plastic you would not have a good ground plane. The idea of a good ground plane is to utilize all the metal as "one". I hope this helps.

I believe Blazers have all metal roofs?

Now when you say "utilize all the metal as "one", are you referring to the entire vehicle top, bumper to bumper?

For some reason since a pick up has a small roof and a Blazer has a larger roof, I was thinking the larger roof would constitute as a larger ground plane.

Unless what you're trying to say is that it doesn't matter....because if the antenna were in the dead center of the roof on let's say a single cab pick up, the antenna would also use the surface of the hood and the bed floor surface as the ground plane....???

all the way down to the muffler, I worded my first post kinda far out I guess. Reason because my 2007 toyota tacoma is the hardest vehicle ive ever tried to get a good "groundplane" I hate that word too for some reason lol. Anyway everything is plastic down from the top pretty much but when I did grounding straps I got it all working together. My theory.