Topband: A reminder about core selection

[Tom Rauch]

I can't help but this hardly gets mentioned. 

Remember the ui of a core is expressed at dc, and changes with 
frequency. 

Always look at the impedance slope of the core at radio 
frequencies, and pick a core material that has the highest 
impedance at the lowest operating frequency desired!

Highest impedance often occurs where the loss tangent is sloping 
upwards sharply, and the reactance hasn't fallen off yet.

For example, a 10000 ui core will sometimes have less impedance 
than a 2500 ui core at 2 MHz. 

It's best to check or look up the exact core selected, rather than 
selecting only by dc parameters. Good goal is to set the no-load 
winding impedance at the lowest frequency at about five times the 
RF impedance across that winding, although less can often be 
used.

Reflection transformers and high impedance transformers like for 
EWE's and Flags (etc) are the most critical.


73, Tom W8JI
w8ji@contesting.com



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