There is no such thing as a "standard yagi", and there is
no "normal" answer to any of your questions.
For yagis, and any antennas for that matter, you need to
think of the 4 basic pieces of the antenna:
1) the electrical design (the length and spacing of the
elements/wires/radiators.) For yagis, this includes the
diameter of the elements, and also requires a correction
for using a metal boom (depending on how the elements are
attached.) This determines the gain, F/B ratio, etc.
2) The design frequency. You can take the same design and
scale it to other frequencies, and it really is still the
same antenna, even though the dimensions have changed.
3) The feed method. You can use any of a number of
different feed methods on the same antenna. Some may
require that the driven element be split, others don't.
4) The actual mechanical construction of the antenna.
This includes details such as the boom material, how
the elements are attached to the boom, the element material
(tubing? solid rod? wire? copper? aluminium?) This
usually has only a small effect on performance (if you
use lossy materials), but makes a big difference in how
long the antenna will survive the weather.
Although there are some interactions among these four
characteristics (scaling an antenna to a lower frequency
changes the mechanical considerations, for example), to a
large extent a builder can take liberties in one area
without changing the others.
My usual method for building a yagi is to start by choosing
the properties I want: generally this means reasonable
gain, impedance of 20 ohms or higher, and all rear lobes
of the pattern down at least 20dB. In some cases, the
physical length is the limiting factor. Then I look
through various designs to find one that looks good.
The next step is to decide how to build it. My favorite
method is to stick #8 aluminum ground wire (from Radio
Shack) through holes in thin-wall PVC pipe, but for
DF use in the woods I use tape measure material for the
elements instead. (It bends when it hits a branch, then
snaps back into place.) For more permanent installations
I might use heavy aluminum tubing for the boom and
3/8" rod for the elements (with or without insulating
spacers between the elements and the boom.) This all
depends on the intended application and how much time and
money I am willing to put into the antenna.
Then comes the question of how to feed it. For mechanical
rigidity in short elements, having a continuous element
is an advantage. I have a couple of UHF yagis that I
made with a split driven element (440 and 730 MHz), and
even a good pile of hot-melt glue at the feedpoint
doesn't take all of the wobble out of the driven elements.
On the other hand, where there is adequate mechanical
structure to support the elements, splitting the driven
element and using a beta match is probably the easiest
approach. (Also, the W4RNL "OWA" antennas have direct
50 ohm feedpoints, so don't need a hairpin for matching.)
So my tape measure antennas tend to have split feedpoints,
since the elements are firmly attached to the PVC pipe
framework. Wire yagis are usually not split, and use
a delta or "T" match, or the "half-folded driven element"
approach shown in the "Cheap UHF Yagis" web site listed
below. This latter method is quite simple and has become
my favorite for many uses.
Using an insulated boom simplifies the calculation of
element lengths. If you use a metal boom, then you have
to correct the element lengths by some amount that
depends on just how the element is mounted with respect
to the boom.
Here are some links for more information:
www.cebik.com/radio.html W4RNL's antenna site - one of
the best antenna resources on the web. Start with his
article, "I Want to Build a 3-Element Yagi". I uses the
designs he lists here on several bands, even though they
are originally given for 10m.
http://www.clarc.org/Articles/uhf.htm Cheap, easy-to-build
yagis using a "half-folded" driven element. My current
favorite construction technique for many uses.
http://www.ifwtech.co.uk/g3sek/ Ian G3SEK has links
to a lot of information on building yagis.
http://fermi.la.asu.edu/w9cf/ W9CF has a yagi modelling
applet that you can run to check the pattern, gain, and
impedance of different yagi designs.
