It depends on how the two antennas are wired. In general, if you're feeding two antennas with the same signal, you're not going to get "omni" coverage, you're just going to shift the directionality to a new angle. For example, a single loopstick has a figure-8 pattern, with nulls in two directions. Two loopsticks wired in parallel will still have a figure-8 pattern, but the nulls will point in different directions.
The same thing applies on the receive side. If you wire two antennas to a single receiver, you're just going to shift the directionality. To get true omni coverage, you need two separate receiver front-ends.
If you can phase-shift the signal to/from one of the antennas by 90° relative to the other, you can create a "circular" polarization pattern that will help mitigate this problem. Now the nulls will be in the plane of the PCB.
Question 1: Does the gain of an antenna always increase with the
decrease in directivity?
Real antenna gain is nearly always referred to the theoretical isotropic antenna. The isotropic antenna emits power in all directions equally therefore it projects power onto the surface of a sphere where the antenna is at the centre of the sphere.
At distance r (radius of sphere), the power from an isotropic antenna is passing thru a spherical area of \$4 \pi r^2\$ square metres.
Normal antennas (such as dipoles) do not transmit this power in all directions therefore they are said to have a gain in certain directions compared to the isotropic antenna and, indeed there is more power per sq metre at a comparable distance, but this is beginning to become "directional". Therefore the higher the directionality of an antenna, the more power it concentrates in one direction (reciprocal for receiving antennas too) and the higher the gain.
Question 2 : What type of GSM antenna is recommended for an
application that can run in such low GSM coverage areas?
What is the likely incident power received and what is the minimum power needed by the receiver. A good figure for required power by the receiver is based on the signal data rate: -
Received power is -154dBm + 10\$log_{10}\$(data rate) - from this you can calculate the headroom, add maybe 20 dB for fade margin (could be lower if you accept a longer delay and you are moving).
Question 3 : What sort of an effect would the animals giant body have
in the reception of GSM signals? If yes, what are the remedies I can
do to reduce the impact of the same.
I have no idea.
Best Answer
You can't wire two antennas together because each antenna presents an impedance to the feed cable and TV that needs to be kept under control. Without controlled impedance matching you could end up with zero signal being received.
Therefore you can't just join up two antennas unless you really know what you are doing and of course there will be some losses that are inevitable. Those losses (even when you have done the best you can to match impedances and splice things together properly) might be enough to get really crappy reception from either antenna.
So it is best to use two antenna feed wires and have a selector box at the TV.
Regards having the two antennas close, this can cause other problems in that an antenna likes to have space around itself or the field patterns get disturbed. Again, a guy knowing what he is doing might be able to get the two antennas within a metre of each other but someone who doesn't know what they are doing is probably best to keep them as far apart as they can be and pointing in different directions.
Any radio antenna has a capture aperture (basically an area in square feet or sq metres) that is like the lens on a telescope - get in the way of that lens and you are reducing the signal. That aperture is usually a fair bit bigger than the mechanical dimensions of the physical antenna so this underpins what I said earlier - point them in different directions and keep them apart.