[Elecraft] OT: heat sinking resistors

[Ian White GM3SEK]

Don Wilhelm wrote:

>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


>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

The two of you may be applying different standards for VSWR performance.

If you wish the dummy load to be an accurate 50-ohm reference for 
measurement purposes, then 1-2pF of shunt capacitance can be quite 
serious. At 145MHz, 2pF of shunt capacitance would increase the VSWR to 
about 1.1; as a 50-ohm reference, that would be "quite poor".

On the other hand, if you only need somewhere to dump unwanted RF power, 
that same VSWR of 1.1 will be "just fine".

There are some examples on my 'In Practice' reference page:
http://www.ifwtech.co.uk/g3sek/in-prac#0908

Whatever your requirements, you can estimate the minimum VSWR quite 
accurately by assuming 1-1.5pF of shunt capacitance for each TO-220 
package. In practical layouts, the VSWR at VHF will almost always be 
dominated by the series inductance of your own connecting wires.



-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek