UK Homebrew Dummy Load plans

[Your_Sinclair]

Hi all,

I would like to build a homebrew Dummy Load, of 50 Ohms and approximately 100W-300W.

A search online has shown there are a lot of ideas, but a lot of them are based on designs by people based in the USA and utilising components obtained from American suppliers.

I cannot find any past posts that match such a requirement on this site.

Are there any UK designs available, with components that can be obtained with relative ease from UK based stockists?
Ideally, I would like to build a Homebrew Dummy Load along the lines of an MFJ-260C.

Many thanks in advance.

[crackle]

I believe Dummy loads need to be made using non inductive resistances, e.g. carbon types.
Maybe there is a use for all those old dog bone resistors I changed.

Note; the MFJ-260C is 300 watts for 30 seconds.
Mike

[Julesomega]

Use modern non-inductive insulated case resistors like the Arcol 50Ω, 50 and 100W from the usual suppliers

What frequency range do you want?

[mickm3for]

Hi lots of 50 ohm ceramic non inductive resistors on ebay built one using them 200w cont. 1.2swr at 440mhz 1to1 at 144mhz and below keep all conection lead lengths short Mick

[Sinewave]

I use the Arcol resistors. Have a bank of them depending on what I need to do, I can configure the load accordingly. I mounted them on a nice chunk of aluminium.

[mickm3for]

link to resistors https://www.ebay.co.uk/itm/RF-Termin...50ohms-DC-3GHz

[Your_Sinclair]

Originally Posted by merlinmaxwell

What frequency range do you want?

1.8 to 30Mhz, ideally.

Thanks to all for all the replies and suggestions, most appreciated.
I'll have a look at the resistors suggested.

[Bazz4CQJ]

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

[Terry_VK5TM]

Originally Posted by Bazz4CQJ

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....

B

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

[Bazz4CQJ]

Best tell the folks at MFJ too https://www.radioworld.co.uk/generat...uct_id=1603242.

B

[Radio Wrangler]

Oil immersed dummy loads have been used for about half a century. Heathkit's "Cantenna" was on the go long before MFJ. QST and the ARRL handbook have carried designs including shaped ground structures around large stick resistors to provide the correct capacitance to operate the resistor as part of a matched transmission line, allowing a six inch resistor to dissipate hundreds of watts and produce a reasonable impedance up to UHF.

Then there are all the oil filled finned jobs sold by the Bird company.

They have a track record of being OK so long as you don't get them too hot. In the extreme they would become a fire bomb, but you have to be careful to keep long term power within bounds set by their surface area, and higher power for time set by their thermal mass.

If in doubt, put it outside on a length of cable. Microwave people sometimes just use a long length of cable with nothing on the end.

And there must be millions of Dimplex oil filled electric heaters in the UK, running a kilowatt or two.

You need to think about venting. The ones in gallon paint cans can pop their lids. Birds have little pressure relief valves.

If you're building or buying a dummy load, consider instead a power attenuator. Having a reduced power output port allows you to check your transmitter with an oscilloscope, a power meter or a spectrum analyser - or a wavemeter for good vintage trad testing.

David

[Bazz4CQJ]

I think K4EAA (whom I think passed away quite recently) was pretty much aiming his design (which I built) at Tx's running ~100W out. He ran a business specialising in looking after the old Kenwood Hybrids. I suspect that a pair of 6146's are unlikely to ignite anything much.

B

[David G4EBT]

Back in the 80s there were several designs for dummy loads in magazines such as Practical Wireless which generally used carbon film 2 Watt resistors in parallel, typically, twenty 1K Ohm for a 50 Ohm dummy load. Invariably they'd be housed in an (empty!) treacle tin. I made one by John Thornton Lawrence who - rather than use mineral oil as a coolant, recommended dry silver sand, which is what I used. I used the dummy load for more than twenty years with a Trio TS830 transceiver till I pulled the plug on amateur radio in 2003.

There was a similar design which featured in an informative two-part article by the much respected late F.C. Judd, G2BCX in Practical Wireless in January and February 1983 which would handle up to 200 Watts continuous up to 150 MHz. (Never quite sure in what circumstances anyone would want to use a dummy load for more than say ten seconds at a time for tuning up off air).

The two PW magazines can be found at the links below at the World Radio History website (Formerly known as 'American Radio History'):

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

https://worldradiohistory.com/UK/Pra...01983%2002.pdf

The February PW has lots of other articles of interest - an 'Antenna Special', and part 1 of a two-part article on a regenerative receiver for Long, Medium and Short Waves, using six easily wound plug-in coils - one of many designs over several years by the talented 'R.F. Haigh' who seemed to disappear without trace. (Was it a pseudonym?).

As to 'mineral' oil it's more widely available in the UK as 'liquid paraffin' - a more refined form used for medicinal use, (for humans and horses etc), for treating food chopping boards, and scented, as baby oil.

https://en.wikipedia.org/wiki/Mineral_oil

https://en.wikipedia.org/wiki/Liquid_paraffin_(drug)

Hope that's of interest.

[Terry_VK5TM]

Originally Posted by David G4EBT

As to 'mineral' oil it's more widely available in the UK as 'liquid paraffin' - a more refined form used for medicinal use, (for humans and horses etc), for treating food chopping boards, and scented, as baby oil.

Now that's interesting, to me, paraffin was what you used to stick in a heater and lit with a match.


Different country, different definition, hence the confusion

[Ambientnoise]

Yes, it’s an odd colloquialism. Paraffin is called that and is indeed burned in heaters and is a liquid, but liquid paraffin is fed to people and animals !

[David G4EBT]

Originally Posted by Ambientnoise

Yes, it’s an odd colloquialism. Paraffin is called that and is indeed burned in heaters and is a liquid, but liquid paraffin is fed to people and animals !

For reasons best not gone into!

[Cobaltblue]

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

Given the very low resistance of a 50 ohm load cooling with good old tap water will do very well, boiled tap water even better, less mineral content and no outgassing.

[Radio Wrangler]

The Americans did a bit of research into available oils for dummy loads.

Car engine oil is a complete no-no. There are nasty additives.
Medicinal oil is OK
Proper transformer oil (no PCBs type) came out best
'Turbine oil' was almost as good as the transformer oil. When you refuel your jet, ask the driver of the fuel truck, they carry cans of this stuff for topping up your engines. Not cheap, available all over the place just not in easily accessible places!

David