[Elecraft] OT: Now Balun effectiveness

[Don Wilhelm]

Darrell,

Baluns differ in design, but the ferrite bead baluns are normally quite
low-loss (others are too).  Charles Green has made several balun
measurements with mismatched loading and has not found any major problem
that I an aware of.  Sorry, but I cannot recall where his results were
published.  If he is still monitoring this reflector, perhaps we will hear
from him.

The main problem with using a balun on a non-resonant antenna/feedline
system is that the balun can be rendered ineffective when the feedpoint
impedance is high.
For good balun operation, a common rule of thumb is that the reactance
across the balun (from input ot output) be 10 times the impedance at the
output.  If the impedance at the feedpoint is very high - say 3000 ohms at a
voltage feed point - then the input to output impedance of the balun should
be 30,000 ohms!!!  That would take MANY beads - so many that it is not
practical.

The better solution is to use a length of balanced feedline that produces a
reasonable feedpoint impedance (somewhere between 20 and 600 ohms).  Finding
a single length that will keep the feed impedance within that range for all
bands of interest is the real challenge - two bands is easy, three is a bit
harder, but 4 or more can produce hair pulling and other acts of
despairation <G>.

As an added point, I fail to see how a balun (or any other passive
component) can generate harmonics unless the core is driven into hard
saturation (a result of using a too small core).  It takes a lot of power to
saturate the core in a properly designed balun.

73,
Don W3FPR

----- Original Message ----- 
>
> Anyone have any references to the efficiency of baluns with respect to
> mismatched impedances? For instance the non-resonant doublet used on 40
> through 10 metres.
>
> I have always believed that baluns were only useful at the design
> impedance and that losses would dramatically increase as the impedance
> changed from the design value. Not to mention harmonic generation.
>