Hello,
New guy here. I am wanting to build dummy load that is 50ohm and at least a capacity of 6 to 7kw. I have access to large quantity of termination resistors that are 50ohm and 800w rating also heatsinks.
Could someone advise me on how to build one and provide me with schematic ?
I’m just starting hobby CB/Ham radio and electronics. I thought this might be a good entry level project to build.
Thanks !
Mitch

 

Why do you need 6 to 7kW? Something like that will need to be oil-cooled and heavy. Our Bird 5kW for industrial RF 3 kW amp testing was 119 lbs (54 kg).

https://birdrf.com/hubfs/learning-center/manual-files/920-8920S_REV J.pdf

 

I’m just starting hobby CB/Ham radio and electronics. I

You don't need more than 1.5kW for (legal) Ham radio.
Where do you expect to need 6-7KW?

 

If you have the room, you could make a length of balanced transmission line with a short at the end, and use stainless steel wire as your dummy load.
Here is a wire resistance table for example.
https://www.remingtonindustries.com...A0tl4AWU6uuS15kPC343W741nms_UA5eW5n80DDcpr9Yx
Radio Australia in Shepparton had the Rohmbic termination resistor made like that.
But as mentioned, why the high power rating?
What are you actually trying to do?

 

Your resistors have to be non-inductive for RF dummy loads. That means they must be some form of composition, not wire wound types. Large non inductive resistors are something like carbon composition, special film/ceramic or similar "solid" conductive material.
Cooling is the next issue. Heat sinks for 7kw would be huge and would require fan cooling possibly. Oil immersed loads are more common on the 2-5kW range. Higher ranges usually have cooling fans as well as oil immersion.... The transformer oil helps dissipate the heat equally to all surfaces (in theory...)

 

Check the ARRL Handbook on how to make a dummy load.



Also here:
https://www.dxzone.com/9-ideas-to-make-your-own-dummy-load/

 

I sill have my Heathkit Cantenna beneath my bench. Rember though that the 50Ω is the Impedance, not the resistance of the load. The gallon sized Cantenna filled with mineral oil and the dummy load was rated for 1kW so 7kW is quite a stretch. The largest stand alone bare dummy loads are typically rated only 100Ω so to imagine that element increased to 7kW would be huge. The oil immersion allows for smaller elements to expend energy by heating a much denser material than air but even an oil immersion one would be nearly 10 gallons in size. I have attached the Heath Kit manual for the HN-31 Cantenna. Note that as the oil is heated the device is rapidly derated by 80% so short-term usage is required. You do not want an oil fire starting under your bench... I keep mine on a pie tin due to oil leakage as its internal mineral oil heats, expands, and leaks out of the vent that keeps the can from exploding as the oil expands due to heating. Allowing the oil to reach its boiling point is highly dangerous and to be strenuously avoided. Operationally, I reduce my transmit power to around 10-20W for transmitter tuning purposes and never do so at full power for many reasons.

 

Even with transformer oil, this load is good for 9 minutes at 1kw. The 1-gallon can is only a 200-watt load. I remember getting these very hot.
A 10 gallon metal drum probably would get you 10x the power. You will need 10 resistors. I think you will need many fans blowing on the can. The fans will really help bring up the long-term power rating.

 

Askeral transformer oil is preferred as it is both non-combustible and has better heat response which is why it was designed and preferred for commercial transformer usage. However, in the 70s and prior, it contained PCBs which is a carcinogen and both more expensive and near impossible to buy in small quantities. Great if you have a source for the modern non-PCB variety but for home/hobby usage mineral oil is much more easily and cheaply found. We spent quite a bit of money taking test samples of transformer oil, having them analyzed for PCBs, and replacing all contaminated transformers on the plant site back in the day. On large substation XFMRs it was drain, flush and decontaminate, then refill with non-contaminated oil.

oemNqV5XB5a1Q1rV7XXqB2RLR.pdf

 

50ohm and at least a capacity of 6 to 7kw.

Years ago I made a 10kw 100mhz transmitter. I used 1 5/8 inch air coax. We had problems with it sparking over. I moved up to 3 1/8 inch piping. I maintained some big transmitters using 6" pipe.
7kw at 10mhz will work with 7/8 inch piping.
Getting the right connector is important.

 

Since the TS already has 50 ohm termination resistors available it makes no sense to suggest buying something else. There is a simple scheme to achieve a power rating in excess of amature radio requirements, with not much effort or expense. It will, however, require adding connectors to sections of coaxial cable, possibly, depending on what sort of connectors are on the 50 ohm, 800 watt resistors.
The simple plan is to make two series strings of two 50 ohm resistors, resulting in two 100 ohm strings. Then connect the two 100 ohm strings in parallel, resulting in a 50 ohm array with a power handling capacity of 4X800 watts. that is 3200 watts rating, plenty adequate for any legal Ham RADIO USE.

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