GSM 3G VSWR and antenna testing question
I will quote out of my M2M Iot Cookbook:


  Changing the position of the feeding point d has an influence on both
  - the resonance frequencies and the antenna matching. An antenna is matched if the reflection coefficient is less than -6 dB. Before I go
  ahead to explain the further graphics I will analyze the upper graphic
  more detailed. If the parameter S1,1 is 0 dB then all power generated
  by the radio module will be reflected to the module. For some radio
  modules this is a dangerous situation. The module could break and even
  if they do no break the rejected radio wave could force
  electromagnetic radiation inside the cellular module and outside at
  other parts on your PCB as well. Remember that the target was to
  generate up to 2 Watt TX power on GSM 850/900 and to radiate it to the
  air. The gap between the - 6 dB on the lower end of the curve and the
  higher end of the curve is called bandwidth of the antenna. As we can
  see the curve is crossing the -6 dB border in four times. The antenna
  is resonant in two bands. The two resonances are already covering the
  GSM 850, the GSM 900 band, the GSM 1800 MHz band and the GSM 1900 MHz
  band. GSM 850 and GSM 900 are very close together and overlap in the
  frequency range and with GSM 1800 and GSM 1900 it is the same. Please
  note that the European UMTS 2100 is overlapping with the GSM 1900
  band. If you have a closer look to some quad band GSM antennas then
  they often already cover the UMTS 2100 MHz too. Anyhow, the antenna
  under test in this chapter offers not enough bandwidth to cover UMTS
  2100. The biggest bandwidth we get at the position d of the feeding point X =3. A one to one copy of the origin documentation is maybe not
  the best choose. It makes maybe sense to move the feeding point of the
  antenna a little bit to the right. More about the optimization will
  follow later in this chapter. Be aware that a reasonable reflection
  coefficient only shows you how many power will not rejected from the
  antenna. If the radio wave will not be reflected the wave has two
  options. Option one is that the antenna will convert the power in
  thermal energy. Just replace your embedded antenna to a 50 Ohm
  resistor and detect the result. No energy will be rejected. The
  reflection coefficient will look perfect. If you measure the emitted
  TX power, then it will be zero. A good antenna will show a reasonable
  reflection coefficient at its data sheet and will tell the antenna
  efficiency as well. The antenna efficiency is the value of the
  generated energy in comparison to the emitted energy. The antenna
  efficiency is an important parameter and is a part of this simulation
  as well. Even more, the simulation includes the plastic enclosure too.


Just have a look on of the four curces. All curves are passing the -6 dB border in four times. - 6 dB is the same like a VSWR of 3. A VSWR is ok for an embedded antenna. For an external antenna the target is - 10 dB or a VSWR of 2. As told before the VSWR is an indicator. The important parameter is the antenna effiency. 

See also Nothing will interfere your embedded antenna?