A power combiner (10, 10', 150, 200) having four ports (16, 18, 20, 22), the combiner comprising: first and second transmission lines (24, 26, 152, 154, 202, 204) arranged side-by-side, each of the first and second transmission lines having a first end coupled to a respective one of four ports of the combiner and having a second end coupled to a respective one of four ports of the combiner; an intermediate conductor arrangement (28, 156, 158, 206, 208) disposed around the first and second transmission lines between the ports of the combiner and electrically insulated from the first and second transmission lines; an outer conductor arrangement (12, 14, 162, 164, 216) electrically insulated from the intermediate conductor arrangement at least partially by air; and heat transfer pathways extending along at least a portion of the length of the intermediate conductor arrangement, the heat transfer pathways being formed from a dielectric material (42, 44, 160, 210, 212) having good thermal conductivity disposed between the first transmission line and the intermediate conductor arrangement and between the second transmission line and the intermediate conductor arrangement, the dielectric material having a dielectric constant higher than that of air; the intermediate conductor arrangement having capacitive loads (50, 52, 54, 56, 166-172, 174-180, 222-228) to compensate for the imbalance in the odd and even mode propagation constants caused by the difference in the dielectric constants of a) the heat transfer pathways and b) the dielectric constant of the at least partial air insulation between the intermediate conductor arrangement and the outer conductor arrangement. first and second transmission lines (24, 26, 152, 154, 202, 204) arranged side-by-side, each of the first and second transmission lines having a first end coupled to a respective one of four ports of the combiner and having a second end coupled to a respective one of four ports of the combiner; an intermediate conductor arrangement (28, 156, 158, 206, 208) disposed around the first and second transmission lines between the ports of the combiner and electrically insulated from the first and second transmission lines; an outer conductor arrangement (12, 14, 162, 164, 216) electrically insulated from the intermediate conductor arrangement at least partially by air; and heat transfer pathways extending along at least a portion of the length of the intermediate conductor arrangement, the heat transfer pathways being formed from a dielectric material (42, 44, 160, 210, 212) having good thermal conductivity disposed between the first transmission line and the intermediate conductor arrangement and between the second transmission line and the intermediate conductor arrangement, the dielectric material having a dielectric constant higher than that of air; the intermediate conductor arrangement having capacitive loads (50, 52, 54, 56, 166-172, 174-180, 222-228) to compensate for the imbalance in the odd and even mode propagation constants caused by the difference in the dielectric constants of a) the heat transfer pathways and b) the dielectric constant of the at least partial air insulation between the intermediate conductor arrangement and the outer conductor arrangement.