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Old 4th Jul 2020, 5:01 pm   #21
eddie_ce
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Default Re: UK Homebrew Dummy Load plans

Years ago, I made a dummy load out of 20 carbon resistors, each 1k 3w in parallel. These were soldered vertically and equally spaced around the diameter between two copper discs approx. 2,5" in diameter, The SO239 connected to each disc.

The whole thing was immersed in a treacle or similar tin filled with transformer oil, a canister of which I blagged from a mate working for the electricity board. It worked perfectly with a good VSWR at least up to 2m. Although not designed for 100% duty cycle it was more than sufficient for tuning my TS820S.
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Old 4th Jul 2020, 5:51 pm   #22
m0cemdave
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Default Re: UK Homebrew Dummy Load plans

More recent Bird loads use a Silicone oil. Rather expensive in small quantities, I think it's one of the (guaranteed non-toxic) types used on food processing machinery.
It has to be the correct viscosity. I used to have the spec somewhere, but seem to have lost it.
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Old 4th Jul 2020, 6:24 pm   #23
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Default Re: UK Homebrew Dummy Load plans

These days, I'd use modern 'slab' resistors coupled to a fan-cooled heatsink.

A couple of decades ago "KDI" was the brand to look for - though beware that a lot of those offered on the world's-favourite-tat-bazaar are either fake or are recovered-from-failed-equipment scrap.

KDI were bought out by "Aeroflex" which in turn became "Inmet".

https://www.apitech.com/brands/inmet...l/inmet-brand/

who could be worth a call if you want a good dummy-load.
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Old 4th Jul 2020, 6:41 pm   #24
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Default Re: UK Homebrew Dummy Load plans

The other way to do it would be with graded 50R attenuator sections. This would distribute the heat better and the coolant oil wouldn't be needed. Just a big long heatsink with the flanged attenuators mounted along it in series. It might be possible to buy 1dB, 2dB...10dB flanged attenuators at a reasonable price on ebay. I have a Bird 40dB attenuator that uses this principle and I think it is rated to 1.5kW up to 2GHz without needing any cooling oil. It does mean that you have to be careful to always feed the high RF power in at the correct end or the attenuator would be damaged very quickly.
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Old 4th Jul 2020, 8:22 pm   #25
David G4EBT
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Default Re: UK Homebrew Dummy Load plans

Quote:
Originally Posted by Cobaltblue View Post
I have a couple of large tubular carbon resistors you can have for the cost of postage if you want to experiment

They are hollow 250mm long 25MM outside diameter

They are shown as 66 Ohms

They can be posted Via Hermes for £2.90

Cheers

Mike T
They're conductive throughout their length of course so if the termination clips at either end are adjusted, each of the resistors could be set to 50 Ohms and make two perfectly adequate dummy loads if just used for ten seconds or so at a time when tuning up off air. It's a rather more polite than using the mike as some 'Billy no mates' do (or did when I was on air) bellowing 'Ahhhhhhhlow' into the mike and while tuned up, then with everything tweaked, calling CQ and expecting someone to answer them. I don't think so chum.
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Old 4th Jul 2020, 8:25 pm   #26
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Default Re: UK Homebrew Dummy Load plans

Quote:
Originally Posted by G0HZU_JMR View Post
It does mean that you have to be careful to always feed the high RF power in at the correct end or the attenuator would be damaged very quickly.
A common problem with commercial RF power attenuators.

Some don't say that the dissipation is asymmetric, you just have to be suspicious. Lucas-Weinshel made a thing about symmetry

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Old 5th Jul 2020, 12:04 pm   #27
Andrew B
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Default Re: UK Homebrew Dummy Load plans

I made a load from 20 1K 3W resistors arranged in a ring, this got put into a "Tate & Lyle" syrup tin (other types available) and 3/4 filled with sunflower cooking oil, good to 180 degrees C
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Old 5th Jul 2020, 2:11 pm   #28
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Default Re: UK Homebrew Dummy Load plans

Quote:
Originally Posted by Terry_VK5TM View Post
Mineral oil, paraffin of any type is the last thing you want in an enclosed tin with a heat source

https://k4eaa.com/dummy.html
No worries, as I rent my house, I will exercise extreme caution.
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Old 5th Jul 2020, 2:12 pm   #29
Your_Sinclair
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Default Re: UK Homebrew Dummy Load plans

Quote:
Originally Posted by Bazz4CQJ View Post
Some of the high frequency termination resistors have a very low duty cycle and get very hot very quickly.

There's an interesting article "Building a Dummy Load" by Ken Kemski, K4EAA on the net, in which he uses 20 off, 1k, 3W metal film resistors, which he immerses in liquid paraffin (as used with horse and ponies) in a 1 litre paint tin.

B
Sounds interesting and I am planning on reading the article later on tonight.
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Old 5th Jul 2020, 3:08 pm   #30
Your_Sinclair
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Default Re: UK Homebrew Dummy Load plans

Quote:
Originally Posted by David G4EBT View Post
They're conductive throughout their length of course so if the termination clips at either end are adjusted, each of the resistors could be set to 50 Ohms and make two perfectly adequate dummy loads if just used for ten seconds or so at a time when tuning up off air. It's a rather more polite than using the mike as some 'Billy no mates' do (or did when I was on air) bellowing 'Ahhhhhhhlow' into the mike and while tuned up, then with everything tweaked, calling CQ and expecting someone to answer them. I don't think so chum.
Agreed, I don't want to start making enemies before I even start my initial CQ.
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Old 5th Jul 2020, 6:03 pm   #31
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Default Re: UK Homebrew Dummy Load plans

Here is one I made earlier, (about 20 years ago), good to 2M blow a fan over it when QRO!
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