Sherry Rees, All regulations pertaining to Radio transmissions, including Amateur Radio, is regulated by the F.C.C. (Federal Communication Commission) and in conjunction with international agreements applies to all U.S. licensed radio operators, their equipment and their transmissions. The F.C.C. recognizes Amateur Radio Operators, i.e. HAMS, as part of the national emergency communication network and in that spirit encourages HAMS to police themselves. There are some regulations regarding retail sale equipment and frequency use, power, tower heights, etc but for the most part HAMS regulate themselves. That being said areas of greater congestion may present unique, non-regulatory, problems for HAM radio operators. These problems are most often resolved by responsible HAMS themselves and may involve overcoming problems with radio transmission interfering with broadcast radio, TV transmissions, visible antenna systems, etc. These problems are rare but when they do occur it’s up to the HAM to resolve them. In short, there are no special federal limitation for congested areas but as a HAM you will responsible for correcting any problems that occur because of your equipment. You also may want to check with the city building department or H.O.A. before erecting large antenna arrays.