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

[Ian White GM3SEK]

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.



-- 

73 from Ian GM3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek