Dummy Load...

[Rick K5IAR]

I'm looking for an economical way to build a dummy load capable of safely handling 1000 watts carrier for a minute or longer without having to wait an hour or more for it to cool off.  If I were to use the proper number of series/parallel non-inductive resistors at say, 100 watts each, what kind of support system would be best to insure at least a one minute key down time?  Would immersing them in oil or attaching them to heat dissipating metal with a blower be the better choice?

Thanks for any and all advice..

Rick/K5IAR

[W7IXZ]

Rick, you might try oven elements!  When working in the broadcast field for years, that was a quick and dirty way to put together a high power dummy load.  I built one for a 50 KW station that ran 50 KW full modulation with no problems.  It had some reactance but a simple "L" network took care of that.  That was for one fixed frequency however and they would be more reactive at the ham bands.  However, one could connect it to an antenna tuner.  It is better than the old light bulb trick we used to do back in the early 50"s.

Larry W7IXZ

[KE6DF]

Wouldn't the old light bulb trick still work?

Four 300 watt bulbs, two groups of two in series would have about 50 ohms resistance.

Better buy them now, because in a few years  you won't be able to get 300 watt incandescent bulbs any more.

Not sure about the reactance, however. Might be a problem because filaments are little coils of wire??

[John K5PRO]

Rick
I built a load a few years ago, made from two paralleled bulk carbon resistor rods, something I found surplus. They were 100 ohms, and solid carbon stuff, not tubular like Carborundums. It think they were marked US RESISTOR, a company I cannot find anywhere. I put them into an alum. chassis with a forced fan on the front. They held up to a 1 kW broadcast transmitter fine, for short tests (10 minutes with full modulation of tones). Problem was the tempco of the resistors, they would drift in resistance as they heated up to near 90 deg C when I cooked them. They would always return back to cold value when turned off though. What this did was to make my transmitter kick off VSWR after a while, otherwise it tested fine.

If you're interested in this thing, I can send it to you (minus the fan which i used for something else). It should work fine for 1 minute tests as they don't have a chance to really get hot enough to drift. You pay for the postage.  I eventually got a used 7 kW rated Ohmspun frame resistor bank, from a broadcast station that didn't need it, and removed all the burnt elements to reduce it to about a 2 kW unit now. It is still 50 ohms, and flat across the BC band and up to 160, I tuned it with a series mica capacitor.

[Rick K5IAR]

Sounds pretty cool, John.  Thanks for the offer, but I am really looking for something that will allow me to leave the hammer down for a while.  I use the Catenna for short bursts, but it too gets hot real fast at high wattage.

Thanks again for the offer.

Rick

[WA1GFZ]

I hear a dipole works well

[John K5PRO]

Are you only looking to get to 2 MHz or entire HF range? The broadcast tx loads tend toward inductance above a few MHz. Get carborundum resistors and a fan.

[Opcom]

nothing to do but build a bank of non inductive resistors rated for more power so they don't get so hot and either put a fan on them or arrange them for convection. Might not be very cheap! surplus sales of nebraska has a decent selection, but pricey (12 pcs. 600 ohm 200W at $35 each). The oven elements are starting to look good..

[Rick K5IAR]

It is looking more and more like there is no cheap way out if I want to go with resistors and a fan.  I'll look around for a surplus load and/or resistors.  I went through this once before, but I wanted a ready to roll unit.  Maybe I'll luck out and run across a good combination of high wattage non inductive resistors that aren't pirced out of range.  ..it could happen. 

Rick

[N3DRB The Derb]

I bought my 500 watt Bird Termaline DL offa ebay for 80 bucks. put a fan on it and it will handle a KW with ease. Think it's a 84A.

The deals are out there, but you gotta be able to put in some mental sweat equity. In this case, it wasn't a pretty gray color - but that yucky dark gray-blue they used on some models years ago. So the bid went cheap, because everyone wants a light gray current color model.

[Ed-VA3ES]

I bought my 500 watt Bird Termaline DL offa ebay for 80 bucks. put a fan on it and it will handle a KW with ease. Think it's a 84A.

The deals are out there, but you gotta be able to put in some mental sweat equity. In this case, it wasn't a pretty gray color - but that yucky dark gray-blue they used on some models years ago. So the bid went cheap, because everyone wants a light gray current color model.

I have one of each. They're cheap up here at festers.   I also picked up a dark gray-blue 2KW model, also cheap.    I think that's  the way to go.

[N3DRB The Derb]

yeah, I checked, mine is actually a 82B. Nobody wants those with that color, they want the current color, the light gray. I also have a model 81B 80 watt that is an even worse color blue green ( I call it barf green ) but it works great for exciters.

Whatever is in demand, just go the other way.

[Ian VK3KRI]

The oven elements are starting to look good..

Australian 240V 1.25KW hotplate elements are close to 50 ohms DC. Works OK on 160m but the resistance seems to increase with frequency.  I suspect the skin effect causes less of cross section of the element to be used as the frequency goes up, thus higher R.  One day I'll try cutting one down in length to see if making a set of monoband dummy loads will work. The price is certainly right...
                                                                                 Ian VK3KRI

[Ed-VA3ES]

The oven elements are starting to look good..

  Australian 240V 1.25KW hotplate elements are close to 50 ohms DC. Works OK on 160m but the resistance seems to increase with frequency.  I suspect the skin effect causes less of cross section of the element to be used as the frequency goes up, thus higher R.  One day I'll try cutting one down in length to see if making a set of monoband dummy loads will work. The price is certainly right...   Ian VK3KRI   

The typical North American stove runs off of 240VAC.     Hmmm....

[John K5PRO]

Rick
I just remembered that I got some Carborundum resistors from a friend that are 100 or 150 ohms thereabouts. I was going to use them for a load for my 1 kW rig, but ended up with some Bird loads that I use plus the Ohmspun BC load. I think I had enough to do a kW but I will check at home tonight. They would need to be done series/parallel to make close to 50 ohms, I think. Hold on, will let you know.
John

[flintstone mop]

I'll be a witness for the DERB on the Bird Dummies. I have one and use two Rotron type fans on it and it handles 1kw with modulation for a half hour. It was always nice for testing the Gates BC1-H I USED to own or the K1KW "big rig" I USED to own, that's stored in a Ham shack somewhere in Pa. and probably will never be heard on the air again, sorry Chuck...............
Fred

[Carl WA1KPD]

Whatever is in demand, just go the other way.

Yeah Derb!
You be da Warren Buffet of ham radio!

http://en.wikipedia.org/wiki/Contrarian
In finance, a contrarian is one who attempts to profit by investing in a manner that differs from the conventional wisdom, when the consensus opinion appears to be wrong.

A contrarian believes that certain crowd behavior among investors can lead to exploitable mispricings in securities markets. For example, widespread pessimism about a stock can drive a price so low that it overstates the company's risks, and understates its prospects for returning to profitability. Identifying and purchasing such distressed stocks, and selling them after the company recovers, can lead to above-average gains. Conversely, widespread optimism can result in unjustifiably high valuations that will eventually lead to drops, when those high expectations don't pan out. Avoiding investments in over-hyped investments reduces the risk of such drops. These general principles can apply whether the investment in question is an individual stock, an industry sector, or an entire market or any other asset class.

[John K5PRO]

Rick,
I have four Kanthal Globar type 889 12 x 1 inch tubular carborundum resistors, 180 ohms each +/-20%. Each is rated for 275 watts average at 40 deg C. They can withstand 350 deg C. Paralleled you would have 45 ohms, at 1100 watts continuous, and a fan would be needed. So VSWR would be 1.11. If you found one 5 ohm of high power (110 Watt) to put in series, then it would be a perfect match. Let me know if you can use them.

John
K5PRO

[Rick K5IAR]

Thanks John.. I'm checking into something right now, but if it falls apart, I may take a look at that.

Rick/K5IAR