Baluns and SWR

When choosing a 4:1 balun you should consider the surroundings in which the antenna will be installed.  If there are many trees, large structures, metal towers etc. these items could cause a significant imbalance at your antenna feedpoint.  A consequence of this is RF current will flow back down your feedline and can cause abnormal SWR readings and also readings that constantly fluctuate.

If you feel there is a chance this may occur or is occuring, you should consider installing the Dual Core 4:1 Balun model 4115.  This model has two separate cores, one for each conductor, and has significantly more windings on each core than the standard 4:1 model 4113.  This provides a very high choking impedance factor and will eliminate the difficult common mode currents caused by the RF. 

An alternate solution would be to install a 1:1 Isolation/choke balun such as the model 1113 or 1115 in your coax feedline to also supress and eliminate the common mode currents.