|
Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
|
Thread Tools |
4th Jul 2020, 5:01 pm | #21 |
Hexode
Join Date: Aug 2010
Location: nr. Hannover, Germany
Posts: 372
|
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.
__________________
Eddie BVWS Member. Friend of the BVWTM |
4th Jul 2020, 5:51 pm | #22 |
Octode
Join Date: Feb 2009
Location: Bletchley, Buckinghamshire, UK.
Posts: 1,223
|
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. |
4th Jul 2020, 6:24 pm | #23 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,010
|
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. |
4th Jul 2020, 6:41 pm | #24 |
Dekatron
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
|
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.
__________________
Regards, Jeremy G0HZU |
4th Jul 2020, 8:22 pm | #25 |
Dekatron
Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 5,768
|
Re: UK Homebrew Dummy Load plans
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.
__________________
David. BVWS Member. G-QRP Club member 1339. |
4th Jul 2020, 8:25 pm | #26 | |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,903
|
Re: UK Homebrew Dummy Load plans
Quote:
Some don't say that the dissipation is asymmetric, you just have to be suspicious. Lucas-Weinshel made a thing about symmetry DAvid
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
|
5th Jul 2020, 12:04 pm | #27 |
Heptode
Join Date: May 2015
Location: Bradford, West Yorkshire, UK.
Posts: 512
|
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
__________________
worried about the electrons entering the circuit and the smoke leaving Andrew |
5th Jul 2020, 2:11 pm | #28 | |
Triode
Join Date: Aug 2019
Location: Longhoughton, Northumberland, UK.
Posts: 13
|
Re: UK Homebrew Dummy Load plans
Quote:
|
|
5th Jul 2020, 2:12 pm | #29 | |
Triode
Join Date: Aug 2019
Location: Longhoughton, Northumberland, UK.
Posts: 13
|
Re: UK Homebrew Dummy Load plans
Quote:
|
|
5th Jul 2020, 3:08 pm | #30 | |
Triode
Join Date: Aug 2019
Location: Longhoughton, Northumberland, UK.
Posts: 13
|
Re: UK Homebrew Dummy Load plans
Quote:
|
|
5th Jul 2020, 6:03 pm | #31 |
Hexode
Join Date: Jul 2014
Location: Magor, Monmouthshire, Wales, UK.
Posts: 436
|
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!
__________________
Adapt, Improvise, Oh Bother..... |