I used a 'dry' dummy load for a long time. The good thing is that they're rather cheap and compact. The down side is that they are sensitive for overloading and the dissipating resistor is rather small thus resulting in 'hot spots' on the contact surface between the chip resistor and the heat sink. Even a pure copper heat sink is rather 'slow' to conduct enough heat from the hot spot. A better solution is an oil filled dummy load. The non inductive thick film resistor has a rather large surface and is submerged in oil. The heat of the oil is conducted to the aluminium housing and radiates the heat via the rather large surface to the surrounding environment. I thought a 500 Watts dummy load should be more than enough so I started looking for a 500 Watts version...
I started looking for a Bird 8201 (the 500 Watts model) dummy load and it turned out that they are rather rare. I found only one ridiculous overpriced one in The Netherlands so I started looking abroad. I found some Bird 8201's on eBay in various conditions and with various types of connectors. The connectors can be changed though. Most of the dummy loads are not in mint condition and I like a good looking one. So I decided to pay more for a very good looking device as advertised below. The connector is the N-type, so perfect for me. So I payd a fair amount of money for the Bird 8201 including shipping and started waiting for delivery...
After a wile, a big box arrived! Yey!
Unfortunately the contents was very disappointing... There was very little packaging material so the dunny load was rather loose in the box. The dummy load was so loose that it had bounced around in the box during transport. The rather thick aluminium mounting was bent, the two contacts of the temperature switch were bent and one corner of the device was punched trough the box leaving a floor mark in the corner of the dummy load. It's very disappointing to spent a fair amount of money for a device in perfect nick and gets damaged by the neglect during packaging. In hindsight it was maybe more wise to buy an average looking one which was cheaper...
I contacted the seller and he expained that the device was packed by the eBay shipping service and he offered to rent the device back. But 'I know what I have' now and I don't want a hassle over this. It's a pity that the seller doesn't take pride and spends effort in packing customers equipment. A perfectly good device was degraded to an average device due to the neglect during packaging. Lesson learned: Just another seller to avoid... Note: I received recently another package that was packed by the eBay shipping service and was also a very bad packing job. So it's wise to request the seller not to use the eBay packing service to avoid damage. Two out of two packages form the eBay packaging service resulted in damaged equipment for me...
After 'crying with on eye' over the cosmetic damage I started the technical inspection. The main thing to check is the resistance of the dummy load. The resistance should be 50 Ohms and I measured 50,21 Ohms. That's close enough and well within limits. This means that the device isn't overloaded and the resistor is not broken. Since the device was 'dropped from an air plane' by the shipping damage, a broken resistor would not be a surprise to me.
The dummy load is filled with refined mineral oil. The oil can degrade over time and an inspection is therefore wise. After removing the rear cap, the membrane is visible. Oil expands very much when heated, so the membrane acts as a buffer for expansion of the oil. There's an oil film on the membrane, but there's no leak. I cleaned the membrane and removed the membrane to check the oil condition.
The oil level is fine and the condition is not too bad. Since I've got a can of new Shell Diala transformer oil, I'll replace the oil anyway. Bird mentioned that never oil is used containing PCB's. PCB's are a health hazard so beware for PCB containing (transformer) oils. Under normal operating conditions the oil can last for years, but after a (a couple of) decade(s) is wise to change the oil so the device is in good condition. Use always the correct oil type to keep the dielectric constant the same. Shell Diala B oil is quite common for Bird dummy loads.
The Bird model 8201 is a 50 Ohms RF load which can dissipate 500 Watts of power. No external power is needed. The resistor is a cylindrical type 'thick film' ceramic tube resistor which is submerged in oil for heat exchanging to the aluminium shell. The resistor is mounted in a tapered housing to provide good linearity in impedance for a large frequency range up to 2,5 GHz. There is a carrying handle for easy handling during transport. The device is rugged and is well known for it's professional usage.
|Bird model 8201 specifications
|500 Watt, Oil-Cooled Terminations
|500 W continuous
|Frequency Range and VSWR
|DC to 1 GHz @ 1.1 max.
|Frequency Range and VSWR
|1 GHz to 2.5 GHz @ 1.25 max.
|3,42 liters (0.9 gallon) Refined Mineral Oil
|Gray powder coat
|427 x 216 x 151 mm (L x H x W)
Bird made a smart move using QC (Quick Change) connectors. Therefore it's rather easy to change the connector type to the desired type. An N-connector is 'my choice' but a BNC or TNC for example are also possible. N-type has a low loss for higher frequencies and for other types I use an adapter since the losses are not that relevant for me. Beware not to loose the screws since the screws are not metric like in Europe is common. ;-)
(Over)load and durability
The Bird 8201 is designed to withstand 500 Watts of power continuous. But it can be 'overloaded' for a short time according to the original Bird specifications below. The average power of the load should never exceed the maximum power of 500 Watts so the duty factor has to be reduced according to the power.
25.000 Watts for 5 ms
57.000 Watts for 1 ms
105.000 Watts for 100 us
150.000 Watts for 10 us
200.000 Watts for 1 us
There's a 8230 model that's almost the same as the 8201, except that there's a water tube in the 'oil bath' that acts as a heat exchanger. Water can transfer a lot of heat and therefore the same 500 Watt resistor is used and the dummy load can handle 2.500 Watts!
So if you want to test a 1.000 Watts transmitter, that should be possible but don't exceed the duty cycle of 50%. Testing for a short amount of time should be no problem.
These numbers are really different than for chip resistors. Chip resistors can't handle that much of 'overload'.
The MTBF (mean time before failure) for the Bird model 8890 is 640,300 hours! The MTBF is an estimate for the life of the dummy load. The actual time to failure is based on how the item is used and the environment it is installed in. It's likely that the MTBF is also applicable for the 8201 load. This is recalculated a lifespan of 73 years continuous use! That's quality.
The support is Bird is great! I had a couple of questions and they're answered very quick. Even documentation of products are after the end of the service life available. Thumbs up for Bird!