In most cases, a Supplemental Type Certificate is not required for the installation of an HF system. Major changes or alterations to an aircraft may include those which affect weight, balance, structural aspects, reliability, operational characteristics, airworthiness characteristics, etc. Some major alterations can be performed using the FAA Form 337 with approved data. This approved data can be in the form of engineering data with a FAA designee approval (i.e. DER, DAR). The DER or DAR then issues an FAA Form 8110 which is considered approved data adequate to proceed with the installation.
HF installations in the USA are installed and approved in the method detailed above. STCs are not required for almost all HF installations since the change to the existing aircraft Type Certificate (TC) is not so intensive as to require one.