[Elecraft] OT: heat sinking resistors

[Phil & Debbie Salas]

If they are mounted against a ground-plane (which is often the case for 
heat-sinking), I've found that the 100-watt resistors are not too good above 
50 MHz.  The 30 watt resistors are better, but the 15 watt resistors are 
best thru 2-meters.  I built a series of 50 ohm terminations where the 
resistors were bolted to a brass plate, and the brass plate was attached to 
a N-connector.  The resistor leads were about 0.2" long (one pin attached to 
the connector center pin, the other soldered to ground).  Measured with an 
AIM4170 up to 180 MHz.

Phil - AD5X
----- Original Message ----- 
From: "Don Wilhelm" <w3fpr at embarqmail.com>
To: "Phil & Debbie Salas" <dpsalas at tx.rr.com>
Cc: <elecraft at mailman.qth.net>
Sent: Friday, May 21, 2010 5:50 PM
Subject: Re: [Elecraft] OT: heat sinking resistors


> Phil,
>
> Is this recent data? Has something changed?  I have had plots of the 
> Caddock 50 ohm 100 watt resistors taken to 250 MHz and the return loss 
> indicated that they made very good attenuators and dummy loads at that 
> frequency - assuming they were mounted with zero length leads directly to 
> a BNC or SO-239 connector.  My data is 2 years old.
>
> BTW - the facts I have quoted are good only for the 50 ohm Caddock 
> resistors - I have found that other values were "as stated" non-inductive, 
> but they certainly were capacitive.  A capacitive dummy load is just as 
> bad as an inductive load.
>
> 73,
> Don W3FPR
>
> Phil & Debbie Salas wrote:
>> "I just received a few Ohmite thick film power resistors for an 
>> attenuator I'm making, rated 20 watts. They are 15mm x 10mm x 3mm. 
>> Obviously they have to be heat-sunk (heat-sinked?) if they are going to 
>> dissipate that much power."
>>
>> FYI - Both Ohmite and Caddock make versions of these resistors that have 
>> screw holes for mounting.  The lower power parts (15-20 watts) are pretty 
>> good thru 2-meters.  However, the higher power versions (especially the 
>> 100-watt versions) can start to look bad at 50 MHz due to the large 
>> package capacitance-to-ground.
>>
>> Phil - AD5X
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