[Elecraft] Dummy Loads and SWR

[G3VVT at aol.com]

In a message dated 08/06/05 17:50:09 GMT Daylight Time, sbralr at cox.net  
writes:

Just a  thought on dummy loads, a number of years ago I noticed my Heath
Cantenna  was giving me a slightly higher SWR reading instead of 1 to 1.
When I  measured it with a VTVM I found it was 40 ohms, not the original 50
ohms  when I built it 25 or so years ago. 


Reply:---------------------------------------------------
 
There was a similar query in Radcom quite a few years back, I think in the  
Technical Topics column about the value of resistors changing in oil filled RF  
dummy loads.
 
The answer to the problem came back from no less a person than Reg Varney,  
G5RV who was I understand involved in the chemistry profession at one time.  
Certain types of oils apparently degrade the carbon material that the resistors  
are constructed from and over a time alter the resistance value. Vegetable  
oils were particularly bad in this respect it turned out. As these  vegetable 
oils are available cheaply in all grocery stores, this would be a  popular 
choice for the cash strapped ham, to their eventual downfall. There are  oils 
available that do not exhibit or minimise this effect, though probably are  much 
more expensive to purchase and more difficult to source. Some transformer  oils 
such as PCBs have to be avoided as they are reputed to be  carcinogenic.
 
Bob, G3VVT