Hi Jerry,
I wouldn't use mineral oil in a HP dummy load. The Cantenna has limited
capacity. Using mineral oil will further reduce it. Actual transformer oil is
as close as your power utility's transformer shop. Mine was glad to give me a
gallon. The trick was getting ONLY a gallon (their pump has a high flow rate).
Otherwise, MFJ sells it. When you're done, you still have a dummy that can
handle a kilowatt for not very long.
That's why I built an air-cooled dummy that uses four Anaren RFP50-400R (50 ohm
400W resistors) in series-parallel, mounted to two heatsinks. The heatsinks
have about 140 square inches of area each, and I push about 100 cfm through
them. During testing, using an RF source of 650W at 40.68 MHz (ISM), the
heatsinks got no warmer than about 55°C (at 23°C ambient). This is a
temperature rise of 32°C, or 0.049°C/Watt. Extrapolating to 1500W, the
temperature would be 73.8°C rise + 23°C ambient = 96.8°C (just within the
resistors ratings. I haven't made that measurement because my SB-220 won't
produce 1500W. At 1000W, the load runs cool as a cucumber. It's made from
scrapped parts from my employer at the time. Total cost: zero! Photos here:
http://tinyurl.com/3bkembs
vy 73,
Bryan WA7PRC
> From: Jerry Kaidor
> Subject: [Amps] Dummy Load Blues
>
> Hello,
>
> To mess with amps, a good dummy load is a
> must. I went to my storage,
> and pulled out a Cantenna of unknown provenance that I had
> purchased about
> 30 years ago at a flea market. Not knowing what was in
> it, I assume it's
> poisonous PCB transformer oil. I had kept the dummy
> load in a plastic bucket
> with a hole for the coax, sealed with coax seal. Now I
> figured I'd
> dispose of the old oil, clean it out, and put in fresh
> mineral oil. What
> a mess!
>
> OK, the old oil is gone; it'll be headed
> to the local hazardous waste
> dropoff. The guts are all washed out in hot water and
> detergent. The
> heathkit can is toast - rusty and leaking. The silver
> straps around each end of the resistor are broken. So
> I can reconstitute
> this thing with new silver straps ( luckily I have some
> silver sheet
> laying around ). And the can - can be replaced with an
> ordinary one from
> the paint factory. Only trouble with that is - 1
> gallon paint cans are
> now *plastic*, not metal. I guess it'll still work,
> just the heat
> transfer won't be quite as good.
>
> I also scored a 250W load cheap on
> Ebay. Seller said it came out of a
> cell phone installation. I popped it open - the
> resistor is the size
> of a postage stamp! I was messing with one of my
> SB2-LA's, used that
> load - figured a momentary toot from the amp wouldn't hurt
> it. I was
> wrong. Apparently these little resistors are not only
> heat limited,
> but *voltage* limited. I hit transmit, there was a
> small *pop* and a
> puff of smoke. Hole burned right through it.
>
> There are similar load resistors on sale
> on Ebay - 800W for IIRC $40.
> But that's just the resistor, you have to bond it to a
> heatsink. I
> have
> the heatsink from the defunct 250W load - I could machine
> off a big enough
> flat spot for the 800W resistor.... failing that, I have a
> really big heat
> sink in my junk.
>
>
> - Jerry Kaidor, KF6VB
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