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1-10 of 89 messages
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RE: Homebrew Dummy Load Project
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by K1CJS on June 18, 2009
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Only one drawback with this arrangement--you've got to keep on checking your match to heat your shack! ;-)
Seriously though this is a nice thought for reuse of those old radiators.
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RE: Homebrew Dummy Load Project
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by WX1F on June 18, 2009
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Your article lacks any mention of resistor values, mounting methods or parts sources other than your local landfill. Not very helpful to newbies who haven't built up a deep junk box.
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RE: Homebrew Dummy Load Project
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by K5END on June 18, 2009
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My water heater draws roughly 22 amps at 240 volts.
Considering the error of my clamp on meter (not so much designed for current this low) I'd say the element was around 10 ohms, give or take a few.
But that curve in the folded element may have some reactance.
Best Ham gear investment I made so far: MFJ-259B
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RE: Homebrew Dummy Load Project
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by K5END on June 18, 2009
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Meant to add, good article. Very resourceful.
I used something like this for tube guitar amps, sharing the load with the speaker.
I can drive the amp to get the tone I want, but this way the neighbors don't call the cops when I like to practice late at night.
:)
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RE: Homebrew Dummy Load Project
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by N5TGL on June 18, 2009
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"But that curve in the folded element may have some reactance. "
The article doesn't state that you should use the factory installed element as a load. Maybe you missed this part of the article?
"Cut the heating element off close to the bottom but leave enough of a stub on it to install the desired dummy load resistors on it."
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RE: Homebrew Dummy Load Project
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by K0BG on June 18, 2009
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Bobbing along with the jetsam and flotsam on the Internet, is a almost identical article. In that one, a salvaged Heathkit Cantenna resistor was used. It's a lot of work, for not much gain in my opinion, and here's why.
The resistor in this case, is the limiting factor. No matter how much oil surrounds the resistor, there is only so much surface area to dissipate the heat. Engineers call this Energy Density, and it's usually rated in Joules. At some given level of RF, the maximum power level will be reached, regardless of the amount of oil surrounding the resistor, and the resistor will fail. The increase in oil volume will allow a lower power input to be sustained for a longer period, but how many amateurs need to transmit a 100 watt carrier for say an hour or more?
There is another issue, and that's insertion loss. For example, the Heathkit Cantenna was well designed, in that the resistor was surrounded by an aluminum, coaxial sleeve. This kept the SWR relatively low into the lower VHF region. If you carefully assembled it, the SWR at 440 MHz, was still under 2:1. If you look at the current dummy load offerings from MFJ, most of them aren't rated past 30 MHz, and those that are have very low max power ratings. Low Energy Densities, in other words. Only Alpha Radio Products makes a decently rated one (1,500 watts continuous at 30 MHz max), all for just $2,000!
In all fairness, if you have a burned out heater, and you don't want to replace the element, I suspect making a dummy load out of it is a worthy project. However, if you're going to use it upwards of 30 MHz, you'd best do a little research first.
Alan, KØBG
www.k0bg.com
PS: You can buy the necessary resistor, and other parts directly from MFJ, if you're so inclined to repeat Charlie's project.
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RE: Homebrew Dummy Load Project
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by W6RMK on June 18, 2009
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Caddock non-inductive resistors are available from Mouser (and others), and would be very suitable for this kind of project. They're inexpensive and available in various power ratings and resistances, and pretty cheap.
Data on thermal resistance immersed in oil is hard to come by, but you could probably guestimate and go from there.
What you might want to do is find a plumbing fitting with the same thread as the heater element (it's a standard pipe thread, usually), and drill it for a suitable hermetic coax connector. Why hermetic? Because you don't want oil on your floor, and the connector has to be at the bottom (you don't want your resistor surrounded by an air bubble, right?).
For what it's worth, silicone sealer does not seal oil tight. You get a slow seepage.
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RE: Homebrew Dummy Load Project
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by K0BG on June 18, 2009
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Jim, non-inductive is a bit of a misnomer, as all objects used at any reasonable radio frequency will have some inductance; even a straight piece of wire! Knowing that, it's easy to design a coaxial holder which essentially forms a transmission line. This is what I alluded to above with respect to the Heathkit Cantenna.
While a simple carbon resistor soldered on the end of an SO239 will act as a dummy load at low frequencies, as the frequency increases, so does the reactance. You have to cancel it out in some fashion.
If you go to the Alpha site (http://www.alpharadioproducts.com/2100.asp) you'll see a photo of their compensating network. Their overall design limits the dummy load to HF.
An HF-only dummy load is fine, but I use mine clear into the SHF range (1,296 MHz). My thought is (opinion really), if you're going to have one, don't limit yourself to just HF.
Alan, KØBG
www.k0bg.com
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