# Gain vs. Directivity and effect of animal body to Antennas

My design works in a foresty area inside a collar of an elephant. These areas have at least -110dBm GSM strength (accurate ref. map linked) but, usually higher as I live in a small country where GSM coverage is easily provided.

Following is an excerpt from the datasheet of my GSM receiver

I am currently using a GSM antenna to receive GSM signals in harsh environment. But it may not be appropriate or the best suited. I'm wondering if I would have better reception if I chose an antenna with lower gain in my design.

Question 1: Does the gain of an antenna always increase with the decrease in directivity?

Question 2 : What type of GSM antenna is recommended for an application that can run in such low GSM coverage areas? What are the factors I should keep in mind to optimise this design to have higher possibility of connectivity?

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.

• Signal strength is usually measured in dBm never dBi. dBi is gain. What signl strength does your receiver need (ignoring the antenna) - this determines how much lower gain you can go (less directivity). – Andy aka Oct 8 '14 at 11:44
• If oriented properly, I'm inclined to say that a directional antenna has higher gain, not lower. Also, do you actually have a PCB antenna as it says on the tag? – clabacchio Oct 8 '14 at 11:52
• Dear All, I have updated the question accordingly to answer the questions raised through comments. once again, thanks for your valuable knowledge shared. – Denis Oct 8 '14 at 12:02
• A bit anecdotal but I've placed 915 MHz RF modules on racehorses (in the saddle cloth) and during various trials it didn't seem to affect range versus someone just holding it in the air. – PeterJ Oct 8 '14 at 12:11

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.

• Thank you @andy aka your your reply. I have updated my question to mention that my receiver has -109dBm receive sensitivity. Also I updated the current GSM antenna I am using. I do not understand what you mean in the equation by data rate. could you please tell me how I could find that data rate? – Denis Oct 8 '14 at 12:09
• If you have the sensitivity of the receiver you don't need to use the equation but -107dBm converts to a data rate of 50kbps. I don't know what the GSM data rate is but -107dBm implies it is 50kbps. – Andy aka Oct 8 '14 at 12:13
• Referring to your earlier comment -"What signal strength does your receiver need - this determines how much lower gain you can go.." As I have a -107dBm receiver and a 3dBi antenna, What is the least gain of the antenna in dBi I can chose to fix to this module to have highest directivity in the forest? – Denis Oct 8 '14 at 12:18
• With a 3dBi antenna you can reasonably receive -110dBm with half decent data purity but you have no margin at all and you may easily find that this prevents reception in areas such as up several tens of metres squared. Also the antenna will need to be generally orientated in the right direction. – Andy aka Oct 8 '14 at 14:01