There are a lot of tradeoffs in yagi antenna design, so first, some
1) how much gain is needed? 5dB? 10dB? 20dB?
2) what are the mechanical limitations for the desired application,
such as wind loading, boom length, portability? Does it need to be
rotatable, or can it be fixed in one direction?
3) what are the construction parameters - do you have a machine
shop available? a lathe? hacksaw? just a pocket knife? What
materials do you feel comfortable working with - copper pipe?
welding steel? threading aluminum? PVC pipe? wood?
4) if the low SWR is centered at 147.0 MHz, what is the allowable
SWR at the band edges?
Most yagis up to 7 elements or so probably have enough bandwidth
to cover 144 - 148 MHz, so whether the minimum SWR is at 146
or 147 is really irrevelant. Longer antennas will have more gain,
but typically narrower bandwidth (though there are some tradeoffs
to be made here.) Another thing to remember is that the gain drops
off more quickly on the high frequency side than the low frequency
side (because all those directors start looking like reflectors at the
higher frequencies.) The longer antennas become more complex
mechanically, needing a larger diameter boom and/or stays to keep
the boom from sagging. Also, higher gain means a sharper pattern,
so aiming the antenna must be more precise to take advantage of
the extra gain.
The SWR is not necessarily the best indicator of the bandwidth of
a yagi, however. The gain, side-lobe level, and front-to-back
ratio will all change with frequency. Actually, the feed point
impedance for many yagis is NOT 50 ohms: the yagi is designed
for best performance (as a trade-off among several parameters)
then some sort of impedance matching is applied to the driven
element to make it match the feedline.
A good resource on many of these design issues (including plots
of the gain vs. frequency, etc.) is W4RNL's site www.cebik.com
All that said, here is some advice from my own experience.
1) Antenna height is often more important that gain, particularly on
VHF. That is, a smaller antenna on a higher mast may work better
than a higher gain antenna on a shorter mast.
2) If you are trying to reach a distant repeater, they you want
vertical polarization, of course. But if you want to maintain simplex
communication with a specific site over an obstructed path, then
horizontal polarization (at both ends) may be better.
3) On an obstructed path, the strongest signal may not always be
in the direction of the other station. Check around for reflections
from mountians or other sources of indirect paths.
4) Instead of using a single long yagi, consider using a pair of
shorter ones mounted side-by-side. The mechanics and
construction are often simpler, and if they are mounted on the ends
of a common cross-bar then the vertical mast has less effect on
the pattern. The SWR bandwidth is generally wider when you use
several smaller yagis than one larger one.
5) unless you have already run some tests, you probably don't
really know how much gain is required in your situation. Perhaps
the best solution is to try out some inexpensive antennas to see
what works for you, then build one in a more robust version.
Yagis are actually quite simple to build. I've built a number of them,
the latest took me 20 minutes or so. The key to simple construction
is to use PVC pipe for the boom and #8 aluminum ground wire (from
Radio Shack, or large diameter aluminum electric fence wire)
for the elements. Sure, this won't hold up in a
tornado, but you can try several designs for a very small investment
in parts. Construction is simple: cut a piece of thin-wall PVC pipe
a bit longer than the specified boom length. Mark the element
positions on the boom and drill holes in the pipe with a 1/8" drill.
Straighten the wire enough to measure one element, cut it to
length, and stick it through the hole in the boom. Secure with tape,
glue, or a rubber band so it doesn't slide through.
Now, the only thing remaining is to attach the feedline. After trying
several approaches, this is what I have had the best luck with.
First, make a 4 : 1 coaxial balun. (This consists of an electrical
quarter wavelength of coax with one end connected to the feedline
and both ends of the shield connected together.) On the two ends
of the balun center conductor, attach about 12" (30cm) of #18
solid, insulated wire. Mark the middle of the driven element and
measure out about 2" (5cm). Lay the midpoint of the insulated wires
over this point on each side of the boom, and wrap the insulated
wire around the driven element in a snug, compact coil until you
run out of wire. Attach the balun to the boom temporarily, point the
antenna straight up (preferably outdoors) and check the SWR.
Adjust the match by sliding the coils along the driven element,
keeping them about equally spaced either side of center. You can
also do some matching by changing the shape of the wires from the
driven element to the balun somewhere between a triangle and a
I built a pair of 5-element yagis using this technique and they
worked quite well. (I don't have the dimensions handy, but can
probably find them if needed.) In that case, after tuning each yagi
individually to 50 ohms, I connected each through a 3/4 wave
section of 75 ohm cable to a T connector, which gave a good
match for the pair of them. The crossbar was also PVC, using
standard PVC "T" fittings at each end and for the boom. I did
find that I needed a short length of dowell (or broomstick) in the
crossbar to keep it from sagging under the weight of the two yagis.
Another approach is a Quagi. The ARRL Handbook and Antenna
Book have carried a number of designs by N6NB (formerly K6YNB).
These are simple to build in various lengths.
Also remember that the dimensions for a yagi are dependent on the
diameter of the elements and how they are connected to the boom.
So if you find a design which uses 1/2" tubing, or copper pipe
soldered to the boom, you can't just build it with the same
dimensions out of #8 wire in an insulated boom and expect to get
the same performance.
That should give you something to chew on. If you post back with
more specific information I'll try to help you find a design which will
meet your needs.
Good luck! - Dale WB6BYU