Comments on: Caddock Dummy Load Performance http://www.nt7s.com/blog/2009/01/caddock-dummy-load-performance/ Emanations from Amateur Radio Station NT7S Thu, 02 Feb 2012 14:01:01 +0000 hourly 1 http://wordpress.org/?v=3.3.1 By: k1trb http://www.nt7s.com/blog/2009/01/caddock-dummy-load-performance/comment-page-1/#comment-50 k1trb Sun, 24 May 2009 03:05:49 +0000 http://www.nt7s.com/blog/?p=245#comment-50 Thanks for the quick reply. You know the size of the resistor from the ones you use. That is the contact area. The heatsink is about 2" by 4" with fins about 1". I don't think that's terribly important because the heatsink stays pretty cool. I.e. the heat is not getting out of the resistor. 70w should be instantly noticeable with such a small contact area. More than one resistor makes it difficult to get it right at VHF and UHF since suddenly there is much more inductance and capacitance. My resistor connects directly to a BNC female fitting. Very short leads. I was just hoping someone would find workable solutions since these are amazing resistors. thanks tom . k1trb Thanks for the quick reply. You know the size of the resistor from
the ones you use. That is the contact area. The heatsink is
about 2" by 4" with fins about 1". I don't think that's terribly
important because the heatsink stays pretty cool. I.e. the
heat is not getting out of the resistor. 70w should be
instantly noticeable with such a small contact area.

More than one resistor makes it difficult to get it right
at VHF and UHF since suddenly there is much more inductance and
capacitance. My resistor connects directly to a BNC female
fitting. Very short leads.

I was just hoping someone would find workable solutions since these
are amazing resistors.

thanks
tom
.
k1trb

]]> By: NT7S http://www.nt7s.com/blog/2009/01/caddock-dummy-load-performance/comment-page-1/#comment-49 NT7S Fri, 22 May 2009 18:14:03 +0000 http://www.nt7s.com/blog/?p=245#comment-49 Thanks for that information, Tom. That's a very interesting response from the resistor. How big of a surface area was your copper clad/heatsink combo? I've never seen a write-up of anyone using the 100W resistor in ham radio use. I'm wondering if it would work better to parallel two 100 ohm 100 watt resistors, to give you a bit more headroom in heat dissipation? 73 de NT7S Thanks for that information, Tom. That's a very interesting response from the resistor. How big of a surface area was your copper clad/heatsink combo? I've never seen a write-up of anyone using the 100W resistor in ham radio use. I'm wondering if it would work better to parallel two 100 ohm 100 watt resistors, to give you a bit more headroom in heat dissipation?

73 de NT7S

]]> By: k1trb http://www.nt7s.com/blog/2009/01/caddock-dummy-load-performance/comment-page-1/#comment-48 k1trb Fri, 22 May 2009 17:57:45 +0000 http://www.nt7s.com/blog/?p=245#comment-48 I discovered the MP9100 50ohm 100w resistor and built a dummy load. I put the resistor on sheet copper thence on a big heatsink. It was held down with a big fender washer. I used silicon heatsink grease. My N2PK VNA showed this to be a great dummy load on HF and 2m. First, I put it on my 2m rig (70w). Something funny. I measured DC resistance. 56ohms. I loaded up again. Again measured: 58ohms DC. Second, I contacted Caddock and a very helpful rep talked with me. He concluded I had a faulty resistor. Third, I thought a lot on that. I don't think so. At low wattage the resistor is fine. At above 50w it must burn away a little of the film because the resistance keeps rising. I think my problem is I can't get the heat out of the resistor. Pressed onto copper thence onto aluminum doesn't seem good enough. Are people able to get the heat away from these resistors? If so, it would be great to know how, since they make good dummy loads right up to UHF. thanks tom . k1trb I discovered the MP9100 50ohm 100w resistor and built a dummy load.
I put the resistor on sheet copper thence on a big heatsink.
It was held down with a big fender washer. I used silicon
heatsink grease.
My N2PK VNA showed this to be a great dummy load on HF and 2m.

First, I put it on my 2m rig (70w). Something funny.
I measured DC resistance. 56ohms. I loaded up again.
Again measured: 58ohms DC.
Second, I contacted Caddock and a very helpful rep
talked with me. He concluded I had a faulty resistor.
Third, I thought a lot on that. I don't think so.
At low wattage the resistor is fine. At above
50w it must burn away a little of the film because
the resistance keeps rising.
I think my problem is I can't get the heat out of
the resistor. Pressed onto copper thence onto
aluminum doesn't seem good enough.
Are people able to get the heat away from these resistors?
If so, it would be great to know how, since they make
good dummy loads right up to UHF.

thanks
tom
.
k1trb

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