One question that frequently comes up in antenna design is how sensitive an antenna will be. While a fair question, one trouble that arises is the fact that there is no antenna parameter called “sensitivity.” Furthermore, the word sensitivity means different things to different people.
While there are many different things that can be meant by sensitivity, here is one possible answer:
Signal Strength
One common modern definition of sensitivity is used by individuals in connection with wireless devices and the bars representing signal strength that we see on cell phones. Of course many other portable devices also come with similar indicators.
In connection with an antenna, one might very well ask, “How many bars will I see with this antenna versus another?” Unfortunately, bars on a cell phone are a remarkably abstract and an imprecise way of measuring signal strength. Yet, the question still makes a valid point: namely, how well does one antenna versus another receive signal?
Antenna Efficiency
How well an antenna receives (and transmits, for that matter) signal is a function of antenna gain. While antenna gain is a function of directivity, most mobile devices do not use directive antennas; if they did the coverage would tend to be sporadic, even if very good in some directions.
The main factor playing into the gain of an antenna for many mobile applications, then, is efficiency. The efficiency of an antenna ends up being directly correlated to size.
At any given frequency, an antenna is the most efficient when it is a certain size. As the ideal antenna size seems to always end up being larger than the space available for the antenna, the antennas are usually shrunk in some fashion. However, shrinking the antenna will result in less efficiency — read, the antenna will not be as sensitive. This can cause real problems with coverage in some cases.
Testing
If, when using the term sensitivity, one is referring to how well an antenna will receive signals (perhaps as indicated by the bars of a cell phone), it often ends up that the question really being asked is how efficient the antenna is. So to determine this, the antenna will need to be tested against a standard of known gain, and the gain numbers compared.
The more directive an antenna is, the higher the gain, but for many mobile applications this is not desirable. To determine the efficiency, it is important to determine the radiation pattern of the antenna in question to see how evenly it distributes power. This tells us how much directivity plays into the gain of the antenna.
A less sophisticated test is to position your hand next to an antenna while testing its VSWR; if the antenna is radiating, the VSWR numbers should change.
In the end, though, nothing short of a real-world test will determine conclusively if the antenna is sensitive enough to meet the customer’s needs.
ARSI 