Are your dipoles 1/2 wavelength long, or some other length?
Are you using a specific length of twinlead to match them, or a random length?
What is the SWR on your coax if you don't use a tuner?
All of these affect the answer.
Some of the basic principles you need to understand the various facets of what
is happening:
1) Baluns are designed to operate at a specific impedance level over a given
frequency range. When used outside that range they will still do something, but
might not work as you would expect. Most antennas do NOT match the impedance
of the balun, especially when feed through a length of ladder line.
2) Feedlines act as an impedance transformer when they are not perfectly matched.
This is usually demonstrated using resistive loads and quarter wave feedlines: for
example, with a 100 ohm load connected through 1/4 of 50 ohm coax it will actually
look like (50 * 50 / 100 = ) 25 ohms at the rig end. Using a half wavelength of coax
brings it back up to 100 ohms, and so it goes down the feedline. In between these
points the resistance will take on an intermediate value, along with some reactance
so that, in this case, the SWR will be (100 / 50 = ) 2 : 1 at every point along the
coax, even though the impedance is changing.
3) All feedlines have loss. One possible definition of an antenna "working well" is
that most of the power generated by the transmitter is radiated by the antenna,
and to do that we need to keep the losses low in the feedline. Generally losses
increase with SWR (though there are some exceptions for very short lengths.) Many
hams can have high feedline losses and not notice it.
So now lets put these together and see if we can give you some examples.
a) a full wave dipole (2 half waves in phase) has a high feedpoint impedance, let's
say 4000 ohms. (In practice it depends on the diameter of the wire, etc.) If we
connect 1/2 wavelength of ladder line between this and the balun, the balun will
also see 4000 ohms. A 4 : 1 balun would, in theory, step this down to 1000 ohms,
but in practice it will depend on balun construction. Connecting RG-58 to the
1000 ohm load gives us an SWR of 20 : 1, and if you use 50' of it on 20m then
about 2/3 of your transmitter power goes to heating the coax, and 1/3 gets
radiated.
I make liberal use of VK1OD's transmission line loss calculator for such problems:
http://vk1od.net/calc/tl/tllc.phpb) Same situation, but we'll use 1/4 wavelength of ladder line. Now there will
be a transformation of impedance and the balun will see (450 * 450 / 4000 = )
50.6 ohms! We'd now choose a 1 : 1 balun and have a very low SWR on the coax.
now only 17% of your power is dissipated in the RG-58.
See how a small change in feedline length makes a big difference in efficiency?
c) What if we used a half wave dipole instead. Let's say that the impedance is
50 ohms (it will depend on height above ground, among other things.) With a
half wave of ladder line we'll again see 50 ohms at the balun, so would use a
1 : 1 balun and have a low SWR on the coax. But with a quarter wavelength
line we're back with ( 450 * 450 / 50 = ) 4050 ohms at the balun, and even
a 4 : 1 balun will still give us a 20 : 1 SWR on the coax, with high losses.
d) Now you can get creative. If you choose an antenna with a feedpoint
impedance that gives a 9 : 1 SWR on 450 line, at some point the impedance
along that line will be ( 450 / 9 = ) 50 ohms. Similarly if you find a point
where the SWR is 2.25 : 1, then there will be points where the impedance is
( 450 / 2.25 = ) 200 ohms, and a 4 : 1 balun should give you a good match.
But in either case, changing the length of either the antenna or the feedline
will result in higher SWR.
e) Another common approach is to use, say, an 80m dipole fed with ladder
line as an all-band antenna with a tuner in the shack. In this case the
antenna impedance varies widely from band to band, as does the length of
the ladder line in wavelengths. (The physical length stays the same, but the
electrical length varies because the wavelength changes with frequency.)
So we end up with a relatively random impedance at the balun. You can use
either a 4 : 1 or 1 : 1 here - the 4 : 1 voltage balun was commonly used,
and while a modern 1 : 1 current balun is often a better choice, the difference
will depend on the actual impedance you have to match on each band.
That presents a relatively random impedance to the coax, so it may operate
with a high SWR on some bands, but reasonable on others. My advice in
such cases is to keep the coax as short as possible (inches are better than
feet) to keep efficiency high.
One or more of these might apply to your specific antennas, but at least
it will give you a start on the underlying principles. If you can give some
specific examples we can work them through.
