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Old 12th Jun 2021, 8:03 am   #1
G4_Pete
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Default PW Jan 1983 50 Ohm Dummy load

This PW article:

https://worldradiohistory.com/UK/Pra...01983%2001.pdf

Describes a sand filled dummy load, in the article the resistors are mounted vertically and a diagram is given to specifically immerse the resistors to a half way depth.

I still have mine built in 1983 and was building another recently but it puzzled me then, and still does now, as to why the article specified only to cover half the resistor body.

Any Ideas?
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Old 12th Jun 2021, 8:43 am   #2
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Default Re: PW Jan 1983 50 Ohm Dummy load

I built a similar one but I used a mineral oil instead, although a small hole at the top is useful as a vent. I can only think that the sand is used as a heat absorber/retarder.

There are a range of commercial "paint tin" loads all of which require oil as a coolant.
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Old 12th Jun 2021, 10:46 am   #3
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Default Re: PW Jan 1983 50 Ohm Dummy load

I suspect the author was playing games with dielectric constant and trying to improve the impedance at higher frequencies.

If you think about the resistor at different levels down its length, the effective impedance changes. At the bottom it is a short to ground, at the top it is 50 Ohms to ground and is connected to whatever is driving it looks like.

A cunning designer of wide band loads... ones that will do VHF as well as HF, puts a contoured-diameter tube around the resistor and this contributes the right amount of C to ground to turn the big cylindrical resistor into a lossy transmission line whose Z suits each level along it.

I seem to remember an example in an American publication. Oil filled (They used either Regent or Aeroshell turbine oil) the grounded tube was a simple cone outwards and a simple cone back inwards and it made a useful difference at 2m and 70cm.

Sand provides a bit of thermal mass, but it's a good heat insulator.

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Old 12th Jun 2021, 4:23 pm   #4
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Default Re: PW Jan 1983 50 Ohm Dummy load

I guess the test for that would be to try it on an impedance analyser with different sand depths but it is really messy getting the sand in and out plus if anyone has built one of these they will know how difficult it is to persuade the resistor assembly to sink in the sand so I think I will just leave it as is. Plus it wont be going above the 80 meter band.

I recall taking my original one to a club construction evening in the 1080's I pulled it apart to show the internals and was still trying to shake the tin to get the resistors back in as people were leaving not to mention sand all over the place!

I appreciate the sand is a short term sink and there are far better ways with specific RF resistors available now but this was a low value resistor specified in the manufactures transmitter set up and once I had set off down the construction path I was reluctant to turn back or consider other technical set up options.
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Old 12th Jun 2021, 4:50 pm   #5
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Default Re: PW Jan 1983 50 Ohm Dummy load

I built something similar based on a design by the American guy (Ken ??) who passed away last year who had run a business for many years servicing Kenwood (Trio) Hybrid gear. He had used Liquid Paraffin (the sort of stuff you give to horses and other animals to help avoid digestive blockages) complete filled the can, and had had it in service for a long time (100W to 30MHz).

Probably cheaper that Aeroshell by some margin!

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Old 12th Jun 2021, 5:41 pm   #6
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Default Re: PW Jan 1983 50 Ohm Dummy load

The dummy-load-resistor with conical shroud was a frequent item in 60s/70s ARRL handbooks, and I think it made an appearance in one RSGB VHF/UHF Handbook too.

Oil-immersed dummy loads - as typified by the Heathkit "Cantenna" - were always popular too: I made a smaller version using a series-parallel-connected network of 470-Ohm 2-Watt carbon resistors in a large screw-top glass jar (which once held pickled onions) filled with Castrol GTX. Good for 100 Watts.

I remember something though about certain oils reacting in an unhelpful way with the surface of the big Morganite-type carbon-on-ceramic resistors (which did not have any enamel covering the carbon-coating).

Work-wise I was able to buy a nice big 300-Watt rated-up-to-1GHz air cooled job from Aspen Electronics [which spent more time in my shack than at the office QTH].
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Old 12th Jun 2021, 7:57 pm   #7
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Default Re: PW Jan 1983 50 Ohm Dummy load

Automotive oils have a whole cocktail of additives in them which change their electrical properties and also might react with metal surfaces. They start off by being quite basic, and when used in car engines, head towards becoming acidic, but I doubt that is encountered in 100W dummy loads. Aircraft oils are made of different stuff and need fewer additives. I think the animal feed liquid paraffin is pure(ish).

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Old 13th Jun 2021, 7:06 am   #8
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Default Re: PW Jan 1983 50 Ohm Dummy load

If choosing an aircraft oil, go for oil for turbine, not piston engines, the latter are no better than automotive engine oils as they need to be chemically active to deal with combustion products. The additive packages in oils for piston engines are rather nasty. Avoid skin contact even with fresh, new oil.

Transformer oil is a good choice, but that opens the PCBs can of worms, so let's dodge that one. Transformer oil which you know is PCB free is good.

Medicinal/feed stuff can have some water content. but you should be able to drive that off by heating it for a while.

Aviation stuff has to be water-free or you are in trouble.

More people have access to pharmacists than to aeroshell. The tanker drivers topping-off your private jet, do stock it on their truck and add it to the bill

David
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Old 13th Jun 2021, 4:26 pm   #9
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Default Re: PW Jan 1983 50 Ohm Dummy load

I recall buying Aeroshell (oil for turbine aero engines) in Quart cans (2 pints for the younger people) and it was fairly pricey; made car oil seem damn cheap.

I got the liquid paraffin on-line for not much money. The fact that Ken the Kenwood man (K4EAA) had used it over a long period of time, and described it in fine detail on his website https://k4eaa.com/dummy.html, made it good enough for me.

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Old 13th Jun 2021, 5:36 pm   #10
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Default Re: PW Jan 1983 50 Ohm Dummy load

Probably US quarts, too so you even get short-changed on the quantity, making it even dearer. The medicinal stuff seems a good deal.

David
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Old 13th Jun 2021, 9:02 pm   #11
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Default Re: PW Jan 1983 50 Ohm Dummy load

Would gear oil be better than engine oil? I understand that, because gear oils do not have to cope with exhaust gases that inevitably leak past the piston rings, they do not contain engine oils' detergents and other additives.
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Old 13th Jun 2021, 10:58 pm   #12
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Default Re: PW Jan 1983 50 Ohm Dummy load

I'm pretty certain that gear box oils also contain cocktails of additives. The role of the oil in modern synchro boxes is very critical, not only providing lubrication, but also, spinning up the transfer shaft. In fact, some makes now insist on referring to "gear box oil" specifically as "transmission fluid" even in manual boxes. Even in the distant past, the old "EP" gear oils were the foulest-smelling fluids you'd be likely to find in a garage.

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Old 14th Jun 2021, 11:53 am   #13
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Default Re: PW Jan 1983 50 Ohm Dummy load

I have always used cooking oil in my tate & Lyle syrup container dummy loads, have I been doing it wrong all these years ?
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Old 14th Jun 2021, 2:20 pm   #14
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Default Re: PW Jan 1983 50 Ohm Dummy load

Don't touch EP (Extreme Pressure) gear oils. The additives include sulphur, phosphorus and chlorine compounds. There is a reason behind their stink. They are a very specialist lubricant for point-contact geartooth shapes, like hypoids, and to survive this even nasty additives were accepted. Not as dangerous as PCBs but rather unpleasant in their own way.

David
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Old 14th Jun 2021, 3:41 pm   #15
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Default Re: PW Jan 1983 50 Ohm Dummy load

I was going to suggest silicone oil and then was prompted by Andrews "cooking oil" post, that must be the best solution.
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Old 14th Jun 2021, 4:00 pm   #16
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Default Re: PW Jan 1983 50 Ohm Dummy load

If you're using cooking oil, be aware that some versions do have a tendency to oxidise and go distinctly rancid after a few years.

[Hmmm... would you use sunflower-oil, corn-oil or Extra-Expensive hand-pressed Extra-Virgin olive-oil I wonder? ]
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Old 14th Jun 2021, 5:28 pm   #17
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Default Re: PW Jan 1983 50 Ohm Dummy load

Silicone oil would probably be good, but what does that stuff cost?

I would have thought that K4EAA's idea of using liquid paraffin met the need; easy to obtain in small quantities, fairly cheap and used successfully by a very respected amateur for some period of time. Why "re-invent the wheel"?

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Old 18th Jun 2021, 9:19 pm   #18
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Default Re: PW Jan 1983 50 Ohm Dummy load

Quote:
Originally Posted by G6Tanuki View Post
If you're using cooking oil, be aware that some versions do have a tendency to oxidise and go distinctly rancid after a few years.

[Hmmm... would you use sunflower-oil, corn-oil or Extra-Expensive hand-pressed Extra-Virgin olive-oil I wonder? ]
Extra virgin olive oil doesn't handle high temperatures very well. Rapeseed oil (Canola) is much better. And a lot cheaper!
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Old 19th Jun 2021, 1:40 pm   #19
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Default Re: PW Jan 1983 50 Ohm Dummy load

When I was first licenced as a young lad back in 1982, I went to collect my nice new shiny TS530S from Leeds Amateur Radio. Tom the owner showed me how to build a dummy load in a Tate and Lyle golden syrup tin.

It consisted of 20 1kohm resistors arranged in a cylinder, with a so239 in the tin lid. He then told me to fill it with cooking oil (cant remember the brand). It's still in use today, with no sign of the oil smelling.

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Old 19th Jun 2021, 3:38 pm   #20
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Default Re: PW Jan 1983 50 Ohm Dummy load

An original Heathkit "Cantenna" [joke] RF LOAD [Tanuki post 6*] has emerged here in Sussex. It appears to handle up to 1k according to the de-rating curve printed on the side, using an element immersed in Transformer and Mineral Oil. It's fairly original, basically a 5 Litre Paint Can, itself painted black with the distinctive white lettering and a small box with a BNC? RF in socket on top. Unfortunately there is some rust on the bottom lip and underneath but it's not breached [there's liquid inside] and otherwise, pretty much "as was". This is the result of a cellar flood up North in the eighties but it's probably very restorable. I'll pass it on for free if anyone wants it for that "vintage" look-perhaps with a different interior set up? [I've mentioned it here rather than "Parts Offered" Mods as this seems to be the Dummy Load Interest Group] .

Dave W

I've attempted to get the lid off. It looks ok but it's probably rusted in to some degree. I've left it for now as I don't want to deform the lid or the can!

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