Eruvin 11a – The Height of a Tzuras HaPesach
The Avnei Nezer, Orach Chaim §291 proves that a tzuras ha'pesach need not be recognizable. He proves this from the Gemara's ruling here that there is no maximum height above which a tzuras ha'pesach is invalid, even though the Gemara (in parallel discussions both at the beginning of Messeches Eruvin and at the beginning of Messeches Sukkah) states that the eye does not discern that which is above twenty amos.
However, Pri Megadim, Orach Chaim 363:19 states that when the tzuras ha'pesach incorporates a gud aseik [the halachic principle of “gud asek mechitzta” (literally: extend the walls up) creates imaginary lines directly up from the top of a lechi to an overhead cable —these imaginary lines may be drawn from any point on the top of a rod or a barrel, and if any of these imaginary lines hit the cable overhead, we may view the barrel or rod as the lechi for the tzuras ha'pesach ] there may not be a gap of twenty amos between the top of the barrel or other object used as a lechi and the overhead cable. Nevertheless, it is not clear whether this chumra is universally accepted l’halacha, as Mishnah Berurah 362:62, in discussing the parameters of gud aseik, omits Pri Megadim's ruling.