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Author Topic: Homebrew 1KW Wet Dummy Load  (Read 5908 times)
KC8BNR
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Posts: 10




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« on: April 18, 2005, 06:55:44 AM »

Hello and a quick question on homebrewing a dummy load.
I would like to build a 1KW wet dummy load using a 1 gallon paint can like the Heath Cantenna.
The use for this will be to tune up the amp before transmitting.
Any suggestions on resistors to use that would be readily available like from Radio Shack.
Any suggestions would be greatly appriciated.
Thanks and 73's
Brian KC8BNR
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WB2WIK
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Posts: 20543




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« Reply #1 on: April 18, 2005, 08:59:29 AM »

Nothing Radio Shack sells is even remotely proper for building a 1 kW wet dummy load like a Cantenna.

You want to find a 50 Ohm Globar resistor, just like Heathkit used.  They can still be found, although "new," they are expensive today.  Surplus, better deal.

The Globar looks like a big hollow cylinder of graphite with metallized bands around its two ends to make electrical contact.  It is ideally suited for this application, and almost nothing else is.

This type resistor is usually rated only about 100W or 200W "in air," but when immersed in a good cooling oil (which is also a good dielectric oil), ratings are greatly increased.

To use "Radio Shack" resistors, which have a maximum 3W rating, you'd need hundreds of them to create a suitable load -- very unfeasible.  It would be far cheaper to just find a used Heath Cantenna and buy it.

WB2WIK/6
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W7DJM
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« Reply #2 on: April 18, 2005, 11:59:34 AM »

The other thing about "building your own"  is

it can be hard to find a good resistor, and,

It ain't all that obvious.  True high power loads have things like "chimneys"  (sleeves) around the resistor, to do two things--optimize the impedance of the load, and to cause a convection current in the oil to enhance cooling.

You must be careful (as I found out with an old Waters load) that you don't get the load in a "wrong" position and expose part of the resistor out of the oil.

For truely high power use, the Heathkits aren't all that great.  It doesn't take very long with a high power amplifier, to heat them up enough to change the resistance (temporarily)  which of course "changes" the amplifier tuning.     'Yo bea settin' thea a wondan' whuts wrong wit dat amp'.

All that's "wrong" may be the resistor going down in R.
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HA5RXZ
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Posts: 380




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« Reply #3 on: April 19, 2005, 07:12:38 AM »

http://www.surplussales.com/Resistors/Res-NonInd.html

No connection, just a satisfied customer.

HA5RXZ
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WB2WIK
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Posts: 20543




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« Reply #4 on: April 19, 2005, 10:10:08 AM »

>http://www.surplussales.com/Resistors/Res-NonInd.html

No connection, just a satisfied customer.

HA5RXZ<

Most of the resistors shown there as "non inductive" are absolutely wrong for RF work.  They're non-inductive at audio frequencies.  

However, the "Type C" resistor shown on that web page is the correct type for an RF dummy load, so good tip!

If you look at the Surplus Sales website a bit further, you'll find an item called P/N (RNI)00050-50C, for $56.00, which is just about exactly right.  It's a 50 Ohm Globar resistor appropriate for dummy load work.

Of course, I can buy a Heath Cantenna (used) already containing one for less than that!

But it's good to know that Surplus Sales does seem to have some inventory of real carbon Globar resistors.  The "non-inductive wirewound" types shown in the other photographs don't work at RF.

WB2WIK/6
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K0IZ
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Posts: 737




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« Reply #5 on: April 19, 2005, 11:05:49 AM »

Might I suggest an alternative to building:  Check on eBay for some Bird brand dummy loads (do a search on "Bird dummy".  They are available in various wattages, rated continuous duty.  Based on my experience you can safely run 2 or 3 times their rated power for periods up to several minutes.  I have a 500 watt bird that I have run 3000 watts into for about 1 minute with only small temperature rise.

The Bird units are super on SWR, typically flat to 1GHZ or more.  MUCH BETTER than any loads being sold, and much better than what you can make yourself.

I have seen various Bird units for sale with ratings of 10Watts, 20, 50, 100, 500, and 1KW.
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NS6Y_
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« Reply #6 on: April 29, 2005, 09:51:47 PM »

Yep there are some big Bird Termalines out there, and the magic keyword is Termaline, I was just drooling over one in a local place today.
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KB4EMF
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Posts: 387




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« Reply #7 on: June 15, 2005, 07:27:33 PM »

The rule of thumb is, when you oil cool the resister(s), you get 10X the power rating.  That said, you could use whole bunch of radioshack resisters and come up with 100watts rating at 50 ohms, but that will require 33 of 3watt resisters, at minimum.  It won't be cheap at radio shack....

Since those resisters weren't meant to be used this way, I would use more than 33, like 50.  Now, you are talking MORE money.  Plus, many of the higher powered radioshack resisters are wirewound, which would be useless for RF.

Your best bet is to go to junk shops and find metal film resisters at 5 watts or higher rating and parallel abunchofthem.  (I've done this)

Even better than best bet is to purchase a real resisters made for dummyloads.  They show up at hamfests every now and then.

....and.....

I'm guessing you WANT to make one for yourself, not purchase it.  By the time you spend this much time and effort, you will probablly be spending more money than you can just go to your local ham outlet and buy one already made out right.

Yes, you can find all sorts of commercial grade dummyload on e-bay.  Those are great.

By the way, I'm not discouraging home brewing.  I do it knowing I can probablly buy it cheaper.
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DW1SCO
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Posts: 14




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« Reply #8 on: June 19, 2005, 04:15:49 PM »

here is my documentation of my homebrew wet HF dummy load, it is made from a big milk can and it never heats up when keyed up 30minutes  100watts CW. its a  nice project and when given a dedication and real hard work, it will surpass those commercially available substitutes. haha Happy homebrewing Smiley

http://www.geocities.com/noynoy_2000/homebrew/test_equipment/qro_dummy_HF
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WA1RNE
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Posts: 823




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« Reply #9 on: June 19, 2005, 06:08:58 PM »

 
 There is another way:


 You can use Vishay-Dale NH-100 non-inductive resistors in various parallel and series combinations to achieve 50 ohms and the necessary power dissipation.

 NH series resistors require a proper heatsink sized according to the power being dissipated and the characteristics of the heatsink.

 This method has its advantages, such as no oil required and and with the right heatsink can be forced air cooled.


 I have used these same resistors for T-pads to reduce drive levels to amplifiers and transverters and they work great.

 Here's the link to the datasheet:  http://www.vishay.com/docs/30201/rhnh.pdf
 

 73,  Chris
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NO6L
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Posts: 179




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« Reply #10 on: September 06, 2008, 10:42:45 AM »

I know this thread is very old, but MFJ now sells "Globars" in a few values with one 50 ohm one at 200W. They are not expensive, either. You should also install it inside a 2" metal tube vertically to aid in heat transfer and oil circulation. You can use mineral oil from drug stores or if you want to build a couple of these use Texaco turbine oil. It's the same stuff with corrosion and rust inhibiting additives and is cheaper in larger quantities.

If you really want to increase the power handling, stack two one gallon cans, submerge the resistor half way down a longer tube and feed it with coax
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KC7NOA
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Posts: 58




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« Reply #11 on: October 23, 2009, 11:31:26 AM »

I have a Para Dynamics PDC-DL50, ITs a 100W dry dummy load. I have used these before in oil and now that i have need for a 1KW load again ill submerge the last one of these that i have in oil...

KC7NOA
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KG4RUL
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« Reply #12 on: October 23, 2009, 12:03:59 PM »

$70.00 for an MFJ oil filled dummy load.
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W6RMK
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Posts: 649




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« Reply #13 on: October 25, 2009, 05:01:46 PM »

A previous post mentioned
"real carbon Globar resistors."

Those aren't carbon, they're Carborundum (a trade name for silicon carbide), made in electric arc furnaces by, oddly enough, the Carborundum corporation, with electricity coming from Niagara Falls (one of the first industries to use hydro power).  It's really hard (Mohs 9) so it's used as a commercial abrasive.

SiC has a very high temperature limit, so it's used for thermal elements.  It's also used for dummy loads and absorber (as for anechoic chambers) in vacuum chambers, because it doesn't outgas.

They also make ceramic non-inductive resistors with a thin film of SiC.  The core is a ceramic tube, with a thin layer of the resistive material, often with silver plated ends.  These are used as dummy loads for RF, in pulse discharge apparatus, etc.

The names have changed over the years, Carborundum became Cesewid, then Sandvik, now a division of Saint Gobain abrasives.
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