Low Cost 100W HF Bands Dummy Load
General
The two resistors are 100ohm 50W thick film bolt down types and wired in parallel. See Farnell Electronic Components Ltd catalogue.
Coax socket is UHF type SO239, BNC, N etc. as preferred.
Heatsink is a finned type for use with TO-3 transistors like the 2N3055 with a centre gap in the fins to accommodate and protect the resistors and wiring.
Fan may be 12v DC or mains operated with the appropriate wiring and safety precautions but only needed if the dummy load is to be used continuously. Make sure that the overall structure is in free air and not covered. A small metal cover bolted to the heatsink may be required over but not touching the wiring to reduce radiation at very high frequencies.
Performance
Up to 30MHz the reflected power is negligable. However, at 144MHz, 12W forward power gives 3W reflected power which is thought to be due to the capacitance inherent in the construction of the thick film resistors.
Some alternative TO-220 style resistors will be tried when available.
The original prototype is shown below.
