[Elecraft] K3 - CQWW SSB Observations - R U kidding?

[David Gilbert]

As I said, stay below the rated power and they should work fine.  When 
they go, they don't go gradually, though.  They really are simply fuses 
on a heat sink.

As far as their RF characteristics is concerned, no argument from me.

73,
Dave   AB7E

 


Ian White GM3SEK wrote:
> David Gilbert wrote:
>   
>> juergen piezo wrote:
>>     
>>> A 2nd option could also be a dummy load using one of those very cheap 
>>> RF film resistors. This would have to be mounted onto the case for a 
>>> heatsink.
>>>
>>> John
>>>       
>> Metal film resistors are notoriously fragile.  They will typically meet
>> their published dissipation specs just fine, but they go from being OK
>> to being an open in a flash (literally) when overstressed.  I personally
>> would never use one in any application where the power they were
>> expected to handle wasn't rigid controlled.  Check with any industrial
>> control manufacturer and you will hear the same story.
>>
>> 73,
>> Dave   AB7E
>>     
>
> This isn't an industrial control application where huge surges may be 
> encountered.
>
> TO220 film resistors make excellent RF dummy loads, up to at least 
> 50MHz. Many of these resistors are also used in RF power attenuators and 
> as  "passive grid" loads in tetrode amplifiers. Power handling depends 
> on the resistor rating, but is limited mostly by the performance of the 
> heatsink (which is mandatory, because the power dissipation of the bare 
> T220 package is only a few watts).
>
> A 50-ohm dummy load will normally use two 100-ohm resistors in parallel. 
> VSWR at higher frequencies is limited mostly by layout and construction 
> technique. For example, see
> http://tinyurl.com/inpractice/#0908
>
> Turning to conventional wire-ended metal film resistors, again my 
> experience is entirely the opposite. I have deliberately overloaded 
> samples until they were glowing bright red and the paint has completely 
> burned off... and still they did NOT fail. Furthermore, when they cooled 
> down their resistance was still within a few percent of the correct 
> value.
>
> Also, it's largely a myth that tubular wire-ended metal film resistors 
> are "too inductive for RF". If you calculate or measure the inductance, 
> you'll find it's generally no more than a few tens of nanohenries, and 
> the effective inductance can be reduced even more by paralleling several 
> higher-value resistors. With care, they are even usable at 144MHz.
>
>
>
>