i'll try to answer as best i can within a brief context. if you want me to go into more detail, i'll need more time (than i have right now) and i'll need to brush up on my electromagnetic physics.

first -- let's clarify that the strength of the signal of your local wifi and that of a satellite are significantly different (both at the source and at the receiver). (then again, your XM receiver doesn't need to return transmit to the source.)
so, you've got these extremely powerful satellites beaming down electromagnetic (radio) waves. but the signal deteriorates exponentially (power of 2) over distance especially through the ionosphere. by the time it gets to us, it has already lost the majority of it's initial strength, and additional terrestrial distances don't have a huge impact (since it's relatively minor compared to the orbit to earth distance changes).
whereas your WIFI is much lower power, but also much closer. it's signal strength also decreases at a rate of the square of the distance. but since you're initially close to the receiver, (relatively) short distances can make a significant impact, since you're early exponential losses are so high.
(i hope this makes sense - because it's relative to the reasoning i'll continue to use. )


RE: metal vs vinyl top
different materials have different atomic structures (duh ). those structures absorb energy in different ways depending on their own molecular resonance frequencies (kinda). that's what makes some materials transparent, some translucent, and others neither. thickness also plays into the equation as well as the wave is subject to the absorption "rules" of the other material. since metals are generally denser and conductive, they are more prone to absorb radio/electromagnetic energy than non-conductives.
(if you want more detail here, i'll have to do some research)

trees?
well, keep in mind that trees contain a significant quantity of water, as do heavy storm clouds. i can't be sure (as i don't have XM and don't have time to check), but i would suspect that heavy rain clouds may also affect your signal quality. (i know it does for satellite TV)
since water is again fairly dense and conductive, it absorbs radar and radio waves very aggressively (which is why submarines use sonar and not radar).

regarding satellite being "line of sight" --
this would seem to be an easy rule of thumb to make sure you can get a signal. clearly, it isn't required, as many can attest to since they are able to get reception in places where there's no "true" visible line of sight.


another thing to keep in mind, is that w/ a satellite, you've got a very low power signal that's already come a long way. reflections are probably meaningless. with local WIFI, you're still close to the source, and reflections can be significant.

i hope this helps... i've kinda slapped it out quick here at work.


interesting references:
http://illumin.usc.edu/article.php?articleID=32&page=1
http://en.wikibooks.org/wiki/Astronomy/Mol..._and_Absorption
http://en.wikipedia.org/wiki/Absorption_of...netic_radiation
http://en.wikipedia.org/wiki/Electro...c_spectroscopy
http://en.wikipedia.org/wiki/Radio

 

Thanks for the information. I am going to try two things...

1) inside of the secret compartment where the antenna is totaly unseen from outside.

If #1 doesn't work satisfactorily, I will stay with...

2) on the car just forward of the trunk lid.

I am actualy not too concerned with the current location of it near the trunk lid. The antenna is actualy not that bad looking IMO and blends in with the black paint OK. I would obviously like the invisible solution better, but we will just have to see with real world tests. My plate is really full right now with trying to balance work and a getting everything ready for the dragon, so it may have to wait until afterward.

 

Makes me glad I got Sirius. The antenna has been in my secret compartment for 6 months with no problems.

 

A BIG part of reception has to do with where you live. NY/NJ has a lot of repeaters, thus, making reception a lot easier. When you are out in the middle of nowhere, that's where the true test is.

 

I don't think one service or the other will matter much. It may well be that the XM will work fine in my compartment. I am just skeptical until I try it out.

 

I've had mine in the secret compartment after moving it from the front dash for a few weeks now. I, too, didn't believe it would work there, but it's been great - just as good as on the front dash. I've mostly had my top up, too.

 

I install both xm and sirius here at work. And out of both of them the xm ant. are def better. We have very few customers come in complaining or replacing them. Were as with the siruis ones we replace around 2 a day because they keep going bad. Sirius tells us they the reason they go bad is cause they are using a fiber optic cable which gets cracked or broken after time. But when we install them we try and put them above the windsheild and run the wire down the weather stripping. If not there on the trunk, but sometimes the constant opening and closeing of the trunk can cut threw the wire if its not run properly.


-mark-

 

why not put in on the center of your dash right by the windshield between the dash vents?

have your old one there now and it works GREAT

 

Steve, what about when you are facing due north? Does it cut out? Do you guys have repeaters in Franklin?

 

i don't think there are any repeaters here in Franklin and i have no problems with drops it works great and it's out of sight