On Jan 21, 10:58 am, Richard Clark wrote:
On Mon, 21 Jan 2008 10:50:29 -0800 (PST), K7ITM wrote:
http://www.caddock.com/Online_catalo...e/Current.html
MP9100
Dont lure good people to smacks shops! Just DIY!

Unlikely to be useful above the LW to AM band.

On the contrary, I've measured some of the Caddock resistors similar
to those and they're quite useful to 2 meters and beyond. We were
using a 100-ohm 25 watt one in a Wilkinson combiner at 450MHz
successfully. YMMV, and as I wrote in this thread before, it does
help to have a reliable analyzer to test them with. Much more useful,
IMO, than a lot of the drivel that goes on here would be info on ways
to bootstrap yourself into some decent measurements. It can be done
with very inexpensive home-brew equipment, but it takes some thought
and careful construction.

Hi Tom,

Did you put the MP9100 against a heat sink? The large surface area,
thin construction for heat flow, bound on both sides by metal
clamping, a high dielectric constant, all point to a considerable
sized capacitor (I visualized a 0.01µFd ceramic cap).

73's
Richard Clark, KB7QHC

Surely you don't think they use high-K ceramic for their substrate.
Assuming alumina, A8 grade if you wish, the thermal conductivity is
sufficient, I believe, to allow a 2mm thick piece 13mm square to serve
the MP9100 at better than the data sheet thermal performance.
Relative permittivity of that alumina is about 10 at most. Even if
the 13mm square were silvered both sides, it represents a whopping
7.5pF. Assuming that's distributed evenly across the resistive
element, it gives you better than 20dB return loss out to beyond
800MHz. For a ham dummy load, that's generally fine. I'll bet this
simple back-of-the-envelope calculation yields a worse result than
you'd see in practice with an MP9100. Please note that I didn't say I
either had, or have used, an MP9100. It was a 25-watt part in a
TO-220 style package from Caddock, and I believe it was a precursor of
the series with exposed ceramic backs. It was about 20 years ago; I'd
have a hard time finding the data at this point. Please feel free to
get one of the MP9100s and measure it yourself. I'm confident the
capacitance won't be anything like 0.01uF.

Cheers,
Tom