When testing an antenna, it is often necessary to add ferrite around the cable feeding the antenna. This ferrite tends to attenuate RF currents running along the surface of the cable, preventing them from radiating.
Why is it important to add ferrite?
If we do not use ferrite around the RF cable, RF currents can run along the surface of the cable and radiate. This can seriously foul the measurements. If the cable is radiating, it has become part of the antenna.
In this scenario, measured data will include the test cable. The upshot is that the data may very well be meaningless, as the test cable is a temporary addition used only for testing.
The problem with small antennas
Testing very small antennas can be challenging.
Small antennas are very susceptible to radiating cables. Antennas that are too small for a given frequency generally do not radiate well. In an effort to compact antennas to fit small devices, some performance is lost. For many antennas, then, increasing the size would increase efficiency.
If the test cable attached to the antenna is radiating, the antenna will, in fact, “appear” to be larger. This means that the antenna when tested may work significantly better than the same antenna in normal use.
Obviously in this case the test data is, quite frankly, misleading, as it makes the antenna appear to work better than it does.
Adding ferrite around the cable will give real, meaningful results.
Cautions about ferrite
One trouble with ferrite is that for very small antennas, the size of the ferrite can overwhelm the antenna, causing other problems related to having large metallic objects next to an antenna.
Positioning the ferrite strategically away from the antenna will help. The idea is to get the ferrite out of the main radiating sectors of the antenna. If the antenna uses a ground plane, the back of the ground plane can be used to “hide” the ferrite from the antenna.
In some cases, unfortunately, there really is no great way to add ferrite without affecting the data.
ARSI 
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