i was looking for a high power dummyload for my Gi7B 144 MHz PA project, but most of the dummys are too expensive for me or nt useable with power >100w.
Here is the 20 Euro solution:
124 * 6,2 k 3 Watt Resistors (13 Euro at Oppermann Electronic )
PL Socket
10 cm diameter Coppertube, 50 cm long
4mm Copperwire
4 hours of time and a good soldering iron....
the resistors are solderd on two copperplate from a central copperwire, two layers of resistors on each side. All resistorwires are kept very short. The cover with the N-Socket is soldered on top of the two copperplates. Don´t use PL-Sockets, there are NOT oil proof after some time oil is flowing out of the socket. Use Teflon N-Sockets (Suhner), these Sockets are 100% oil proof and you don´t have to take care of the socket while soldering the cap on the tube. The resistors are connected to the N-Socket with 50Ohm Coaxialcable (RG58, Aircell7...)
The whole thing is now put in a 10 cm diameter coppertube and filled with silicon oil. Silicon oil has a
dielectric constant about 3,5 -4. Together with the distance of the inner wire and the two copperplates impendance of the square coax is about 50 Ohm.
The Siliconoil is easy to get by ebay, look for CANON S20 Siliconoil for Plotters or Xeroxmachines. Any other Silionoil should work too.
Although all wires are kept short there is a inductive reactance on 144 MHz which can be matched with a 7-10 pF capacitor.
Don´t use an aircapacitor in the oil !!! the aircapacitor on the pictures is only for testing !!!
Without oil you can use this dummyload with 130*3 = 390 Watt, with oil about 1 kW. You can test the dummyload easily by connecting 230V AC directly to it (DON´T TOUCH THE COPPERTUBE WHILE DOING THIS !!!)
Power is about 1kW. After 1min the tube is about 70°C, for longer testing put the dummyload into a bucket of water.
Don´t fill the whole tube with oil: if the oil heats up it need a bit more space.
SWR/Frequency (meassured with commercial SWR-Meter Diamond SX 200 and with homebrew directional coupler 144-435 MHz):
Ready for use:
