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Index |
High impedance Baluns (published in
Electron #4, 2007) General This chapter is the fourth in a series of
articles on baluns for antenna applications. The first article is an
introduction with some background on balun
types. The second article is on wire baluns.
The third article is discussing baluns around
core materials. It is advisable to read the articles in the above order
especially since each next chapter is building on information already
explained earlier and referencing to this. Introduction Many types
of balun are designed for application in a low impedance environment e.g. for
matching to the input and output of (balanced, power) amplifiers, resonant
antenna's etc. System impedance will be between a few Ohm
to around 100 Ohm which requires the (current) balun to exhibit a sleeve
impedance of hundreds of Ohm that is not too difficult to realize over a wide
band-width. Low frequency behavior is determined by the sleeve reactance
which will quickly reach the required values when being constructed around a
ferrite core. High frequency drop-off is mainly determined by the parasitic
capacitance across the (sleeve) reactance, this in turn depending on the
number of turns and the proximity of the first to last winding. At
antenna systems outside resonance, system impedance may be considerably
higher than 100 Ohm, with comparably higher demands to the design and
construction of baluns. An interesting analysis of such situation may be
found in the article of Kevin Schmidt, W9CF, Putting a balun
and a tuner together. It is shown
that for high impedance antenna systems, balun (sleeve) impedance should be
above 1000 Ohm and preferably even higher. To ensure these high impedances over a wide
bandwidth is not trivial anymore. A balun to connect to a high impedance,
balanced feed-line is not a simple component. - It should force equal currents in the
balanced connections with as low a current as possible to ground. This is
always a problem at 'flux'- transformers that usually will not be constructed
in perfect symmetry (see e.g. trifilar transformer in previous chapter). Furthermore, any form of un-balance will make
the feed-line radiate / receive in an undesired way as your neighbors will
tell about your transmissions and you will discover yourself tapping into the
(usually vertically polarized) electro-smog in your environment. - It should exhibit high transfer efficiency at all
operating frequencies. With comparably low allowable internal power
dissipation (e.g. maximum 4 Watt in a Especially the first requirement differentiates
a balun for a symmetrical feed line from coaxial feed-line applications. Diagram Even to date we may find see proposals for antenna
tuners at with a voltage balun in the output circuit to take care of a balanced
output (-currents). To this extend the voltage balun is looking something
like figure 1.
For this voltage balun, the primary and
secondary winding usually have equal turns although step-up ratios may be
seen as well. In the 1 : 1 variation, the generator voltage 'U' will also
appear across the load and with proper winding techniques the secondary
winding may be balanced around the ground connection in order to also balance
the parasitic capacitance.
Construction A practical solution to this balancing
problem may be found in figure All turns have been spaced equally at the
inside of the coil-former, to minimize inter-winding capacity.
When designing this balun for a 50 Ohm
system, with a lower operating frequency of 2 MHz., at a Measurements As in previous chapters this transformer has
been measured for transmission and reflection. Because of the construction
details, a small unbalancing may be expected so the measurements have been
performed two times, each time with a different output terminal grounded. For
all measurements, the balun was terminated into 50 Ohm. Figure 3 presents the
transfer measurements.
In figure 3 it is shown that indeed some
transfer difference may be noticed . When measured
in counter phase (blue curve) transfer is improved at 20 MHz. by 0,3 dB and at 30 MHz. by 0,8 dB. This different transfer
behavior most probably is due to some capacitive unbalance in this voltage
transformer Apart from some unbalancing, transfer
characteristics are not really ideal either with transfer loss of 1,7 dB and 2,5 dB at 30 MHz. This will limit application in
a high(er) power system at this operating frequency.
Never-the-less, this 1 : 1 voltage transformer is doing
a better job than the trifilar 1 : 1 transformer we discussed earlier in the
chapter on baluns with cores. Reflection measurements We also performed some reflection
measurements, identical to earlier transformers. Results may be found in
figure 4.
The reflection graph (SWR) is showing some
interesting facts: - SWR never drops below 1,5 - Below 2 MHz. SWR is going up because of diminishing
impedance - Above 10 MHz SWR is going up sharply to
take impractical values above 20 MHz.
To investigate this disappointing behavior we
measured the real and imaginary impedance values that make-up the reflection
curve. In figure 4 we notice reactance (Xseries)
to not drop to insignificant values, which accounts for the not too low SWR
in the frequency range up to 10 MHz. Above
this frequency, the reactance rises further due to loss of inter-winding
coupling. This effect is showing up as a leakage induction, making (series) reactance
to go up again with frequency. For 4C65 materials, ferrimagnetic resonance
frequency (fr) is at 45 MHz., meaning permeability and loss to have the same value
(Q-factor is 1). This is showing in figure Around 65 MHz. rising of Xseries
breaks off to fall off sharply and become negative. This is the effect of the
parasitic parallel capacitance becoming the dominant impedance. We might consider applying higher
ferrimagnetic resonance materials, e.g. 4D2, with fr
at 150 MHz. Unfortunately the higher the ferrimagnetic resonance, the
lower the permeability (Snoecks law) so we would
need more turns to comply with reactance requirements at the low frequency
cut-off. More turns also would mean higher parasitic capacitance and this
would lower the high frequencies cut-off. Because of these effects this voltage balun
may not be improved too much beyond current, already not ideal performance.
To operate as a balun for a symmetrical
tuner, impedance to ground should be 1500 Ohm or higher. Going back to figure
3 one may notice the inter-winding reactance to be below this value already
at 10 MHz. and at 30 MHz. even below 350 Ohm. This again makes this voltage
balun less fit for the job. Current balun for a symmetrical tuner Let's look again to the earlier current transformer,
this time specifically for application in a symmetrical tuner. All earlier
requirements still apply and so we again arrive at the 4C65 current balun at
a
The transfer characteristic for this component may
be found in figure 6, this time with the graph for the sleeve impedance to
ground added.
In figure 6 we find this current balun already
better performing than the voltage balun at 2 MHz. with 0,2
dB of damping versus 0,25 dB. In the rest of the HF frequency range,
insertion loss is much lower for the current balun again with a damping of
0,1 dB at 30 MHz. as compared to 1,7 - 2,5 dB for the voltage balun. Also
beyond this frequency, the current balun is still in full service for quite
some time. Impedance to ground Using this component in a symmetrical tuner, the
impedance to ground is especially important. Although permeability of 4C65
ferrite is going down at higher frequencies, the vector summation of
permeability and materials-loss keeps on rising for a very long frequency
range so total sleeve impedance will keep going up. This is opposite the
impedance in the voltage transformer that is determined by the parasitic
inter-winding capacitance, generating a falling impedance graph. Starting at 10 MHz. the current balun is close to
the minimum impedance requirement (Z > 1500 Ohm). At lower frequencies
impedance is still too low. This is easily enhanced by adding ferrite to the
core. For each additional ferrite ring, impedance is added by the same
factor. With three toroides in a stack and the same ten turns of RG58, we
easily reach to over 1000 Ohm at 2 MHz. which is close enough for this lower
operational frequency. Stacking cores is the better method over
constructing three individual ten turn current baluns and putting these in
series. In this latter situation, parasitic capacitance of one sleeve coil will
inevitably resonate with the inductance of the next transformer, creating a low impedance instead. A last but useful feature of the current balun over
the voltage balun is the direct DC-path of the first type to ground; static
charge may not build up making this component safer to operate and more 'silent'
in dry seasons (no discharge noise). A frequently recurring discussion to put the
balun before of behind the tuner has
found a definitive answer in the article by Roy Lewallen:
'The 1 : 1 current balun' .
He proves that there is no difference in operation and the balun will have to
exhibit an equally high 'sleeve impedance' in both situations. Bob J. van Donselaar, |
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