[TowerTalk] Baluns and SWR

[Tom Rauch]

> All the baluns start out looking inductive at low frequencies, as
> indicated by the positive phase angles.  As the frequency is increased, a
> point is reached where the capacitance between the windings forms a
> parallel resonance with the coil's inductance.  Above this frequency, the
> winding reactance is reduced by this capacitance.  The interwinding
> capacitance increases with the number of turns and the diameter of the
> turns, so "more is not always better".

Actually the parallel impedance increases above the self-resonant 
frequency. The only reduction in impedance occurs at frequencies 
more than 1.4 times the resonant frequency.

If you have a choke of 500 ohms on 21 mHz and add enough stray 
capacitance to resonate the choke on 21 MHz, the impedance at 
28 MHz is the same or slightly higher!  The impedance at ALL 
frequencies below ~1.4 times the resonant frequency increases.

Of course the impedance at resonance, assuming a Q of 100 in the 
circuit, increases to 50,000 ohms! (Impedance at resonance is 
simply Q times reactance of one of the reactances. Q is found by 
adding the reciprocals of the inductor and capacitor Q's, and finding 
the reciprocal of that addition. So an inductor with a Q of 130 
parallelled with a capacitor Q of 500 results in a "tank system" Q of 
103 at resonance. These are typical values of coaxial cable tank 
circuits. )

Extra stray C in not always a bad thing. It can be useful even above 
the resonant frequency, but the upper limit is generally around 1.4 
times the resonant frequency.  
 
> impedance.  In the case of a 50 ohm balanced antenna, the balun's winding
> impedance is effectively shunted across one half the 50 ohm load
> impedance, or 25 ohms.  A reasonable critera for the balun's winding
> impedance for negligible common mode current in the shield is that it be
> at least 20 times this, or 500 ohms.  The measurements show, for example,
> that 6 turns 4-1/4 inches in diameter meet this criteria from 14 to 35
> MHz.

That is a good analysis, unless we are talking about systems like 
full size dipoles and feedlines leaving the dipole center at right 
angles. 

If we widen the scope to all antennas, the common mode 
impedance can be ANYTHING. Very few antennas look like a pair 
of 25 ohm resistors with a grounded junction.

That was an oversight in designing ferrite bead baluns. They are 
OK in some applications, but not all applications. 

One example is a center fed vertical, where the feedline drops 
along the antenna for some distance. In a case like this the 
common mode impedance can be very high, and is nothing like 25 
ohms. It takes many many thousand ohms of balun impedance, 
and the balun can not be effective when located at the feedpoint.

Asymmetrical antennas are another example. 

Small antennas (like small loops) often have very high common 
mode impedances, and do not even require a balun in most cases.

1/4 wl groundplanes (like those with four radials) DO require a 
balun, although the impedance required is very modest. Now look 
at 1/4 wl groundplanes, and see how many manufacturers 
decouple the feedline and the radials from the mast!

The balun impedance also depends greatly on the feedline's 
common mode impedance! Not just the antenna's common mode 
and differential mode impedances.

The entire issue is very complicated, and we tend to oversimplify it. 
But yours is a good example for center fed dipoles with feedlines 
leaving the center at right angles to the element when the feedline 
has a low common mode impedance.    

We should just remember the two 25 ohm resistor model is not a 
good example for all antennas, or even for most antenna types.

> resistive.  I didn't record the exact resonance points, but it can be seen
> from the tables that the four single-layer baluns are all above 15K ohms,
> while the ferrite bead balun read about 1.4K.  These baluns see half the
> load voltage, so at 1500 watts to a 50 ohm load, the power dissipated in
> the coaxial baluns will be less than 1.3 watts, and the ferrite bead balun
> will dissipate about 13.4 watts (neglecting possible core saturation and
> other non-linear effects). These losses are certainly negligible. 

My measurements of coaxial traps tend to agree with the 
impedances you measured for coaxial baluns. Coax makes a very 
poor inductor, but at least it is much better than 73-type ferrite 
beads!! 

The losses are negligible in all cases, but the heating certainly is 
not! 

Ferrite beads make poor baluns for high power because smaller 
beads have excessive temperature rise with much less than one 
watt per bead! We sometimes incorrectly assume the heating is 
saturation, but it is almost always due to the loss tangent of the 
material. It is a dissipative resistance problem, and beads make 
very poor heat spreaders.

Once the bead reaches curie temperature, it behaves like a lump of 
non-magnetic material. It certainly quits acting like a balun, 
sometimes forever from that point on!

At high power, or if the common mode voltage across the balun is 
high, you need a low loss-tangent core at the operating frequency 
or one with a very high curie temperature or a large area to 
dissipate heat.
73, Tom W8JI
W8JI@contesting.com 

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