[HTC EVO 3D] Sad Panda:
I am not really sure what you mean, [the antenna is at the bottom]. The antenna runs from the top of the device on the lower part of the motherboard down to the bottom, but it isn't like the antenna is actually at the bottom only.

It doesn't matter where the antenna is at in a device. There was a comment that was made that does have a slight amount of validity. I don't know if you guys have any knowledge of physics specifically electromagnetics, but I would like to do a little briefing for everyone, including those that have no knowledge of electromagnetism. I will keep this explanation as simple as I can without leaving out the most important details needed to have a good idea of the fundamentals of antennas, receiving and transmitting "signal". I will leave out the complex details like how it works on a subatomic level, because in this case it just won't be necessary to know that amount of detail to understand exactly what is happening on the macro level (ie our phones).

Transmitting and receiving "signal"

So, when you apply electricity to a conductive substance; copper wire, certain metals, etc the electric current produces electromagnetic radiation (radio waves). When electromagnetic radiation (radio waves) are applied to conductive substances it produces electric current in that substance. So, a cell tower uses electricity to produce the radio wave, the radio wave reaches the antenna of our cell phone, that produces electricity in our cell phone's antenna [which is just a thin copper wire inside the phone], the current that was produced by this process is amplified by the phone's hardware, and that electricity is sent to the speaker, or input/output system of the phone to process the electricity into data, and used to display webpages,download pictures, or whatever else you were intending to do at that time. So that is an oversimplified account of what is happening with the signal.

Antennas

Now to understand a little more about what may cause a particular phone, or cell tower, or the communication between the two to function at a standard lower than what we were expecting or hoping for we must now know a little about antennas. Antennas transmit, and phones receive via the science I gave above about electromagnetism.

Antennas transmit electromagnetic radiation (radio waves) in a direction that is perpendicular to the substance that the electricity was ran through. For a cell phone to receive and transform that electromagnetic radiation into an electric current the antenna has to be in a position that is perpendicular to the incoming radiation (radio waves). The figure below shows electric field (blue), magnetic field (red) and the black in the center will represent a cell tower transmitter, or antenna inside our phones.

Most cell towers nowadays have multiple transmitting antennas that will transmit signal in 360 degrees, and in multiple directions on an XY axis.

SO, in conclusion because of the way cell towers are PHYSICALLY designed, the will transmit and receive electromagnetic radiation (radio waves) that are going, and coming in from all three spacial dimensions. So if you have your phone vertical, horizontal, angled, upside down, etc the phone will transmit and receive signal the same way efficiently.

However: Because that is not the only thing that determines how much "signal" you have we can't rely on that to predict the quality of signal for a particular device, and network. The above explanation only illustrates proof that the antennas' location within the phone, and the way you hold your phone should have no bearing on signal quality. Some things that will affect your "signal" though:

Location from cell tower(s)

Frequency

Amount of traffic

(Important!) The amount of electromagnetic radiation from other technologies that could interfere with a cell towers' transmission

Transmitters electrical power

Receivers electrical power

Geographic factors like mountains, bridges, valleys, buildings, and if you are inside what the building is constructed of.

This bullet represents the plethora of other factors that can make your signal go catywampus.

Anyway, thanks for letting me ramble. Hope I helped someone, and someone learned something they didn't know. Otherwise ignore me.

Citing sources for images, and a few ideas incase I may have created something too similar to someone else' work.

http://en.wikipedia.org/wiki/Cell_site#Radio_power_and_health

http://en.wikipedia.org/wiki/Cellular_network#Directional_antennas

http://en.wikipedia.org/wiki/Antenna_(radio)