Accurate Antenna Measurements

Accurate Antenna Measurements and the NetTek YBA250:Reflection measurements are usually made as a comparison of
the unknown (device being measured) to a known standard. This
standard is assumed to be perfect. The process of comparing is
called “calibrating” the measurement equipment.
Any non-precision cable that is placed in between the YBA250
(where the comparison to the standard was made) and the antenna
system to be measured can dramatically affect the reflection or VSWR
measurement. The higher the frequency of measurement, the worse
the degradation will likely be. The measurements made here are done
for a sweep of 100 MHz to 2000 MHz. The comparisons are mostly
made at the US PCS band at 1900 MHz.
Two distinct types of errors are introduced by a jumper cable.
The first type includes two separate errors: the reflection of the cable
itself and loss of the cable. The reflection of the cable usually causes
a device being tested to seem to have a worse VSWR or return loss
(RL) than it actually has. The loss of a cable will cause the device
being tested to seem to have a better VSWR or return loss than it
actually has. Any cable loss simply adds directly to the measured
return loss, but a reflection due to the cable itself can have a huge
effect, and may actually dominate the measurement result.
Interestingly, the two errors have different effects. Loss in the cable
has more effect on measurements of a device with small return loss
(poor VSWR) and any reflections in the jumper have a much larger
effect on measurements of high return loss (good VSWR).
The second type of error is the change in reflection, phase, or loss
of the cable as it is flexed or bent. This causes readings to change
as the jumper is bent (particularly for measurements of antennas with good VSWR).