[Waverley ARS] Dummy Load calculations

[George Georgevits]

Hi Henrik & Adam,

Saw this thread and thought it worth mentioning that it might not be quite
so straight forward. You will need to use non-inductive resistors,
especially if you want to use it for VHF or higher. Likewise, you will need
to pay attention to layout at the higher frequencies. You need to keep the
leads short and arrange them in a symmetrical, coaxial pattern. I haven't
seen the AR article (and if you have it in pdf I wouldn't mind a copy if its
no trouble) but I bet they specified carbon composition resistors, as these
are non-inductive. They are also not that easy to get.

When you go to 1kW you will need a LOT of 2W resistors.

The other way to go is to use wire wound resistors, but these have to be a
special type, wound specifically to be non-inductive. Needless to say, they
are much more expensive and harder to get than the normal wire-wound
resistors. Also, I doubt they would be good enough for VHF.

Often, such a load is mounted in a can of oil, to help spread the heat
quickly. I have seen a paint tin used for this purpose, with a coax
connector mounted on the lid, and the resistors hanging underneath, and when
you close the lid they become immersed in the oil.

You also need to be a little careful with the voltage across such a load.
Remember Power = 1,000 watts = V^2/R, where R=50 ohms. Thus V = 223 volts
rms, which is DANGEROUS!

And lastly, you may find that if you only want to use it for intermittent
applications, you can get away with, say, 500W worth of resistors,
especially if they are mounted in oil. Just make sure you don't apply the
power for too long.

Cheers,
George Georgevits
VK2KGG

> -----Original Message-----
> From: members-bounces at us.cactii.net
> [mailto:members-bounces at us.cactii.net]On Behalf Of Henrik Stenstrom
> Sent: Monday, 23 April 2007 8:08 AM
> To: Adam Carmichael; members at vk2bv.org
> Subject: Re: [Waverley ARS] Dummy Load calculations
>
>
> Hi Adam,
>
> Amateur Radio Magazine Jan/Feb 2007 has a good construction article for
> a 200W dummy load with power meter using a complement of 2W resistors.
> It would be an easy job to sub higher wattage resistors in this design.
>
> I may be able to PDF the article, let me know.
>
> Henrik Stenstrom VK2FHHS
>
>