How to determine the viability of a cell phone amplifier with an iPhone

in order to calculate whether a cellular boosting system will help in a particular location  it might be useful to perform a site survey. The site survey involves taking several accurate readings of signal strength in and around your coach, Normally, this is determined by looking at how many bars you have on your phone. However, the following provides  a far more accurate method:

Access the field test mode by dialing *3001#12345#* + “Call”

The numeric value for signal strength is now in the upper left hand corner of the screen where the signal strength was previously displayed in bars. To exit and return your iPhone to normal status, hit the Home button. The mode is available on iPhone's running iOS 4.1 and all later versions.

If you want your iPhone to always display numerical signal strength instead of signal bars,  perform the following process:

1.       Once in Field-test mode  hold down the power button until you see “Slide to Power Off”, then release it.

2.       hold the Home button until you’re returned to your main app screen. You’ll now see your numerical signal strength while you use your phone, and you’ll be able to tap the signal numbers to switch to signal bars, and vice versa.

3.       To exit this permanent field-test mode, simply reboot the phone or re-load Field Test Mode and exit it via the Home button.

The numeric value is known as RSSI, which stands for Received Signal Strength Indication. It will generally be double or triple digits, and it will be negative. The closer the number is to zero, the better the reception, -80 is a stronger signal than -100. The unit of measurement of RSSI is the decibel (dB), it is a measure of the power of a signal. The decibel scale is logarithmic, not linear. An increase of 3 dB corresponds a signal that is twice as strong while a 10 dB increase corresponds to a 10 times increase in signal strength. Therefore, an RSSI value of -90 is actually ten times stronger than an RSSI of -100.  think about the volume of a radio, the numeric RSSI value is really telling you exactly how “loudly” your phone is receiving the signal from your service provider’s cellular network.

 Anything numerically less than  -80 is good, and is equivalent to five  bars. Anything numerically greater than -110 is bad, and would be considered one or two bars.
 Once you get the hang of reading the numbers, you’ll find it’s much more accurate, and it becomes easier to predict when you may drop a call or start to get a bad signal or connection, which creates the weird artifacts and sounds on phone calls, often before it starts to cut out or even drop completely. Problems  typically starts happening around -110 or so, before dropping the connection or call completely when it hits -120 to -130.

 The signal strength seems to vary frequently, Why?

Carriers pass off signal between towers based on a number of criteria including traffic volume, signal quality as well as legal reasons related to coverage areas.  Base ID (Serving Cell Info) will change even when standing in the same place, or be different depending on the time of day.  

The tower that handles your traffic at any given time may not be the closest one.