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
--
FAQ on WWW: http://www.contesting.com/FAQ/topband
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|