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Author Topic: air vs oil dummy load ?  (Read 4914 times)
BOOTYMONSTER
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Posts: 95




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« on: August 24, 2018, 10:21:10 AM »

what's your thoughts or experiences on using a air vs a oil cooled dummy load for 1k pep ? looking at the MFJ 264 air vs the ameritron ADL-1500 oil included . both are $75 at DXengineering . also open to suggestions on something with similar power capability for similar money .
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KE6EE
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« Reply #1 on: August 24, 2018, 10:58:02 AM »

Read the specs.

Oil dummy gives you about 10 minutes at about 1 KW.

Air dummy gives you about 10 seconds at about 1 KW.
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N8GD
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« Reply #2 on: August 24, 2018, 11:09:13 AM »

The differences are in how long they can handle rated power and in cleanliness.

The MFJ-264 is rated for 1500 W @ 10 seconds, 100 W @ 10 minutes.

The ADL-1500 is rated for 1500 W @ 5 minutes, under 200 W @ continuous use.

The difference I see between dry and oil type loads is the "messy" factor.  The oil filled units all leak to some degree.  You have to be careful when transporting them, and they are best placed on a solid, non-absorbing floor (think concrete, not carpet) when in use or stored.  I have both types, and the dry loads are on a shelf above my operating positions (where the coaxes come in to the shack).  The oil filled units are more for testing and stay on the concrete floor, out of the way and in a safe area where they can't get knocked over.

If you need loads that can handle a lot of power for a longer time, get the oil filled type.  If cleanliness is a factor (at the expense of longer power handling capability), get the dry/air type.
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KM1H
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« Reply #3 on: August 24, 2018, 11:32:16 AM »

The oil loads also take longer to cool down to the point of running rated power again.

There are often several sizes large commercial loads by Bird and others that can run continuous at 500 to 3000W and at a fraction of new cost.

Mine is a RCA 3000W made by Bird and rated to 1300 MHz found at a hamfest over 30 years ago. No leakage ever from that type.

Carl
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K6JH
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« Reply #4 on: August 24, 2018, 02:37:25 PM »

I've got an old oil filled Bird Termaline that was old 30 years ago when I obtained it. But I've always wondered whether the oil in it contains PCBs. Anyone have any idea? Use it and play dumb? ;-)
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73
Jim K6JH
KE6EE
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« Reply #5 on: August 24, 2018, 02:56:13 PM »

Anyone have any idea? Use it and play dumb? ;-)

PCBs widely used back when.

Use it but be smart. Don't try to change the oil, etc.
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G0HZU
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« Reply #6 on: August 24, 2018, 04:53:51 PM »

The other option is to buy a big used aircooled load or better still a 30dB or 40dB attenuator from one of the big brands. eg something aircooled made by Bird. An well made attenuator is going to have lower VSWR over a wider bandwidth and should be more useful if you want to do any monitoring on a scope or spectrum analyser. But you would have to pay a bit more for a used attenuator. But worth looking on ebay I think?
« Last Edit: August 24, 2018, 04:56:11 PM by G0HZU » Logged
K6JH
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« Reply #7 on: August 24, 2018, 05:44:54 PM »

Anyone have any idea? Use it and play dumb? ;-)

PCBs widely used back when.

Use it but be smart. Don't try to change the oil, etc.

It doesn't leak. (Yet! I don't currently have an amp able to challenge it!)

It does slosh around a little if you shake it, so there's some air space in it - but I believe that is by design to allow for heat expansion. (Termalines have a rubber diaphragm in the domed section on the rear to allow for expansion. Who knows what condition the rubber is in?) I'm reluctant to open for inspection, for obvious reasons.

There are still pole pigs around with PCB's in them. The power company leaves them in place until they fail, then dispose of them properly (hopefully not dumping out all the oil!). Which is the handling method I've chosen to use.
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73
Jim K6JH
K8AXW
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« Reply #8 on: August 24, 2018, 05:48:09 PM »

I use an oil filled dummy load.  (Cantenna)  I keep it in a plastic bucket which eliminates any problem with a spill or leak.....or tipping over.

It also creates a longer cool down time but if I anticipate a long use I take it out of the bucket and set it on a small piece of plywood.

I replaced Heath's rubber/spring vent with a short piece of 1/4" copper tubing soldered to the lid.  While I was at it I installed a lab dial type thermometer to monitor the oil temp.
« Last Edit: August 24, 2018, 05:53:29 PM by K8AXW » Logged

A Pessimist is Never Disappointed!
K6BRN
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« Reply #9 on: August 24, 2018, 06:13:31 PM »

Just get a decent used Bird 8201 and be done with it.  They are common on line and at ham fests and are rock solid reliable.  Way better than the vast majority of amateur equipment.  Will handle 500 watts so tinuous, 1 KW intermittently.

https://www.ebay.com/itm/BIRD-termaline-coaxial-resistor-model-8201-500-Watts-50-ohms-DUMMY-LOAD/273402026329?epid=1100592431&hash=item3fa807d959:g:Q38AAOSwE0JbbzpR

Need more power capability?  Get the 8251  which adds an expansion tank.


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N7ZDR
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« Reply #10 on: August 24, 2018, 08:41:14 PM »

Or you can just make a salt water load--- My 5 gallon home depot bucket load will handle 5KW all day. The hardest part of the build was snapping on the lid--- Less then 10 bucks!

Cheers,
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WA7PRC
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« Reply #11 on: August 24, 2018, 11:57:02 PM »

My homebrew air-cooled DL can handle 1500W at 100% duty cycle... until I run out of air:

It uses four 50Ω/400W RF power resistors (Anaren pn RFP-400-50R) in series-parallel.
A 100 cfm tubeaxial (aka Muffin™) fan moves air thru two heatsinks (140 in2 each). The construction (including the sheetmetal housing) contributes to forcing most of the air thru the heatsink fins. An LM35DT "Precision Centigrade Temperature Sensor" IC keeps track of heatsink temperature.

Mine used scrapped parts from my then-employer but, I see suitable parts available for cheep on that watery auction site.

Bryan WA7PRC
http://www.tinyurl.com/wa7prc-dummy
« Last Edit: August 25, 2018, 12:00:38 AM by WA7PRC » Logged
W9IQ
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Posts: 3558




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« Reply #12 on: August 25, 2018, 04:25:29 AM »

Or you can just make a salt water load--- My 5 gallon home depot bucket load will handle 5KW all day. The hardest part of the build was snapping on the lid--- Less then 10 bucks!

Cheers,

That seems like a rather "enthusiastic" claim.

Let's do some rough calculations. You want to keep it below the boiling point. Saturated salt water boils at 228°F.

     Temperature rise = 228°F - 72°F = 156°F

     Weight of water = 5 gallons * 8.34 pounds / gallon = 42 pounds

A BTU is the amount of heat needed to raise one pound of water one degree Fahrenheit.

     BTU required = 42 lbs * 156°F = 6552 BTU

One kilowatt is equivalent to 3412 BTU per hour.

     5 kw * 3412 BTU/hr = 17060 BTU/hr

Time to reach the boiling point:

     6552 BTU / 17060 BTU/hr = 0.38 hours or about 23 minutes

So while this doesn't account for the water to plastic to air thermal interfaces and a few other minor factors, it is clear that 5 kw into your dummy load would make the water boil in less than 30 minutes. As a "sanity" check, consider that the large burner on an electric stove top generates about 3 kW of heat. It seems reasonable that it could get a 5 gallon pot of salt water boiling in about 30 minutes.

So it seems that your claim doesn't hold water...

-Glenn W9IQ

« Last Edit: August 25, 2018, 04:28:29 AM by W9IQ » Logged

- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
BOOTYMONSTER
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Posts: 95




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« Reply #13 on: August 25, 2018, 04:48:47 AM »

thanks folks . i'm not knowledgeable enough or confident enough to build one and the birds even on eek-bay are out of my budget . didn't think about the oil leaking or evaporating ....... i'll go with air since i'll only be about half of the 1500 watts capability .

thanks again Smiley
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VR2AX
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Posts: 1016




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« Reply #14 on: August 25, 2018, 05:21:28 AM »

I use a Deltec CT-50N it has a Kiwi brand but I don’t know the actual origin. It’s good intermittently for a bit more but air cooling depends on temperature difference and air flow. Oil is absorptive- it is also a fuel so one has to be careful.
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