At present jamming is considered one of the main threats to a GPS receiver’s user, both in dynamic and stationary conditions, and especially regarding telecommunication synchronization. Threats stemming from the use of cheap jammers, more and more commonly referred to as Personal Protecting Devices, available on the Internet, are especially dangerous. Despite the formal ban on using them, there is evidence that they are in common use. Therefore the problem which has hitherto been considered marginal needs to be urgently investigated. Reports dealing with investigations on jamming focus mostly on analyses of effects of various jammers on receivers. This article presents part of the results of experiments focused on the effects and it shows that the effects of jamming when the same device is used depend on both the receiver and the antenna it employs.