On 1/24/2012 8:28 AM, Harry Brown wrote:
Ok, sounds good and thanks. You can send it to
(remove nospam).
Thanks,
Harry
One note.. When designing a loaded dipole or yagi, you can
consider it pretty much the same as designing a loaded vertical,
except you have two back to back.
The same rules apply as far as current distribution, etc.
The only difference is the vertical is half an antenna
fed against ground, and the yagi is two verticals back to
back, which form a complete antenna independent of ground.
So.. if you look at the various forms of loaded verticals,
you can apply the same designs to loaded yagi's.
For instance, up to a point, the farther up the vertical
the loading coil is, the better the current distribution
will be. The tradeoff point where the coil losses outweigh
the improved current distribution, which can be modeled, but
is probably around 3/4 the length of the radiator in most
cases. Maybe slightly more.. I checked this one time, but
forgot the best exact location that I came up with..
Been a long time since I did that.
If the loading coils are at the feed point, that
would have the least desirable current distribution with
a vertical, and this will apply also to the elements of a
loaded yagi.
The farther up/out the loading coil is from the feedpoint,
the more coil turns you will need to tune.
If the world was perfect, and coils had no loss, all
the way at the end would be the best place. :/
In the real world, you have to use the best compromise.
Which averages about 3/4 the way up/out from the feed
in most cases.
You could use the usual antenna modeling program to
calculate the performance, but you could also use
a vertical design program, such as vertload, and
also determine the best place for the loading coils.
You could then use the modeling program to design the
best spacing for what you want to optimize. IE: most
gain, or best f/b, etc.
Placing the coils at 1/2 out from the feedpoint is
a fairly good location as far as coil loss, vs current
distribution. But if you wanted every last drop of gain,
you could go out a bit farther. I probably wouldn't go
any farther than 3/4 out though. You can also use capacitive
loading hats on the ends of the elements. That will further
improve the current distribution, and also slightly reduce
the number of coil turns you need.