Combiners are a vital system component for improving the quality of ATC radio communications. They are used to allow several radio channels to operate at the same site using a common antenna thus saving installation space. Each channel consists of two cavities connected in series. Their high power rating make them ideal for working with AM transmitters with high peak power. The combiner can be adjusted to obtain the optimum combination of selectivity and insertion loss. The combiners can be supplied with or without external isolators.