[Elecraft] OT: Now Balun effectiveness

[George, W5YR]

Just a comment about balun material generating harmonics, Don:

Bead baluns are subject only to the common-mode current flowing on the outer
braid of the coax or the "parallel" current on a balanced line. The
differential-mode current does not affect the beads directly, unlike the
older voltage baluns in which the windings carried differential-mode
current. In those, the core material had to stand up to the full load
current.

While bead baluns can become hot at maximum legal power (or beyond!), they
are far less likely to saturate and have any effect on the differential mode
operation of the line.

In the case of a balun placed at a point of high differential-mode impedance
on the line, the core balun is much to be preferred over the bead balun for
the reason you cite: it takes a lot of beads to deal with several thousand
ohms of load impedance. With the core balun, the effectiveness improves as
the sq2uare of the number of turns while with a bead balun each bead adds
only its own incremental effect. But even the core balun itself in a current
balun configuration deals only with the common-mode current, not the load
current, and is relatively immune to saturation.

Incidentally, I transition my ladderlines to coax with  five-bead baluns on
RG-8X. The measured loss of these baluns is about 0.1 dB on 20 meters using
the method developed by Frank Witt.


Good summary of balun operation - thanks for posting it.

73, George W5YR
Fairview, TX
w5yr@att.net
http://www.w5yr.com


----- Original Message ----- 
From: "Don Wilhelm" <w3fpr@isp.com>
To: "Darrell Bellerive" <ve7cla@shaw.ca>; "Elecraft Email List"
<elecraft@mailman.qth.net>
Sent: Tuesday, March 23, 2004 8:14 PM
Subject: Re: [Elecraft] OT: Now Balun effectiveness


> 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