The most common obstructions are electricity pylons or TV aerials. Mogul skis are used in moguls and sometimes in aerials. Eston Nab is also home to Bauer Teesside and aerials and transmitters. Broadcasting aerials of longwave broadcasting stations apply it often. Snowboard tricks are aerials or maneuvers performed on snowboards for fun, or in competitions. Lassila achieved a Bronze medal in the Freestyle Skiing Women's Aerials event. Ladder companies are generally equipped with Seagrave aerials. The installation of directional aerials also encouraged more AM stations. Two aerials at ground stations were rotated so that their beams converged over the target. For all transmissions indoor, loft-mounted, and external aerials are available. Various periscope-mounted aerials have been developed to allow communication without surfacing. The last three topics deal with radio propagation, aerials, and frequency measurement. The electrical lengthening allows the construction of shorter aerials. "Flip tricks" are a subset of aerials which are all based on the ollie. The dance routines are very fast and athletic and include many aerials.