[Elecraft] OT: Now Balun effectiveness

[Darrell Bellerive]

Don,

Thanks very much for your detailed replies! I would really like to
understand this much better than I do.

Choke baluns seem to be the favoured solution, but I can't help but
wonder about the losses in the coax cable due to the impedance mismatch.
I can see where a choke balun of ferrite beads or from a coil of coax
will keep the RF off the shield of coax feedline when used with a
resonant dipole, but in multiband operation, I would expect the vast
differences in reactance and impedance would create vast differences in
losses. Particularly with long runs of coax.

As you have stated a doublet with open wire feeders can be built to have
a finite and manageable impedance range over two or three bands, but
getting a manageable impedance range on all 9 bands from 160 to 10
metres is almost impossible to achieve.

Therefore I would believe that a non-resonant doublet connected at the
feed point to a remote balanced-balanced tuner either directly or via
ladder line would have less loss across all 9 bands from 160 to 10
metres than would a coax fed doublet with a choke balun or current
balun.

Certainly, from a practical standpoint using the KAT2 or KAT100 and a
coax feedline to a choke balun or current balun fed doublet would be
easier to design and construct than coax feedline and control cable to a
remote tuner, however, I can't help but wonder what one looses in
efficiency for the sake of convenience. Does the decrease in loss
justify the complexity of the remote tuner? How much loss are we really
dealing with? 

And finally yes, it is at saturation of a toroidal balun that harmonics
are generated. The source of my understanding about harmonic generation
is from an article by Richard L. Measures, AG6K where he states:
"More turns means more ampere-turns of magnetic flux in the balun's
core, and high magnetic flux densities can cause the ferrite-core to
saturate. This distorts the RF waveform and creates harmonics. These
harmonics extend well into the UHF TV band."

Darrell VE7CLA K2 #1973



On Tue, 2004-03-23 at 18:14, Don Wilhelm wrote:
> 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
> 

>