Tech-Talk
Part 5
Welcome to Part 5 of our discussion.
In Part 4, we talked about practical installation of ladder
line. You'll find that section (and all of the other parts) right below
this one.
This month, we're going to take a first look at baluns --
what they do, why they are (sometimes) needed, and perhaps most
importantly, how to pronounce "balun" so you don't sound like a lid.
Very simply, baluns match a balanced antenna -- like any
typical dipole or loop -- to an unbalanced feedline, most often coax
cable. Hence the name: BAL (as in balanced) + un (as in unbalanced) =
BAL-un. Not "BAY-lin". "Not buh-LOON". BAL (rhymes with Hal and Sal)
un (pronounced like the "en" in fallen. Now you know.
Okay, what do we mean by a balanced antenna?
In an ideal
dipole, both legs are exactly equal with respect to ground. The current
and voltage on each is exactly the same, although 180 degrees out of
phase. As we discussed last month, that symmetry carries down along a
balanced feedline -- like ladder line. But since our antenna legs are
not close to each other and parallel, as they are in ladder line, the
fields no longer cancel. In fact, they radiate out into space. And
Voila! -- we have a signal on the air.
Coax cable, by contrast, is unbalanced. The shield is
typically connected to our station and RF grounds, while the center
conductor floats. And here's the question that a large percentage of
hams, both newbies and old hands, will answer incorrectly --
How many conductors are there in a coaxial cable?
If you said two, read on. If you said three, go to the head
of the class. That's right, there are three conductors in a coax
cable. The center conductor, the inside of the shield, and the outside
of the shield. Because of the skin effect in an AC circuit -- and an RF
signal is simply one type of an AC signal -- the current divides
between the inside and outside of the shield. And now we've "lost our
balance". We no longer have equal and opposite fields canceling each
other out. The result? The outside of the shield begins to act like an
antenna, and radiates a signal. One problem with this is that it will
distort, to some degree at least, the pattern of our antenna. Now our
megabuck BandBlaster Yagi is not concentrating our signal -- and power
-- where we intend it to. But the bigger problem, as you've probably
guessed, is RFI/EMI. That unwanted signal is radiating all along the
coax, including in the shack and throughout the house. And as the
number of electronic gadgets in a typical home multiplies, so does the
potential for headaches. Trust me on this -- when your XYL can't watch
her football game, or the OM misses the latest episode of his favorite
soap opera, things can go downhill rapidly. I don't suggest that every
RFI problem can be cured with a balun, but adding one is the best first
step in almost all cases.
So what does a balun do? To oversimplify a bit, a balun
chokes off the flow of current on the outside of the shield, forcing it
all to the inside. We've now "regained our balance" -- equal and
opposite fields are generated inside the coax, cancel each other out,
and nearly all of our power reaches the antenna.
Baluns can also perform another function at the same time --
they can be used to improve the impedance match between a source and a
load. We could more properly call them "balun and X:Y transformer",
where X:Y is the ratio. But common use is to call them X:Y baluns.
Just keep in mind that they are performing two distinct functions.
Most common are the 1:1 and 4:1 types. The 1:1 is what
we've been talking about so far. The 4:1 will improve the match (lower
the SWR) on high impedance antennas. A folded dipole, for example, has
an impedance of about 300 ohms. We could feed it with 300 ohm ladder
line for an almost perfect match. But sooner or later we need to
transition to 50 ohm coax to connect to our transceiver. Recall from
previous installments that coax losses can increase greatly with high
SWR. Using a 4:1 balun steps up our 50 ohm coax to a 200 ohm load
impedance, and a perfectly acceptable 1.5:1 SWR on the coax.
Next time, we'll look at some practical baluns that you can
make for just about nothing, and talk about advantages and disadvantages
of some of the different types. Until then,
73 for now,
John Bee, N1GNV
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