On 6/21/2011 1:20 AM, Helmut Wabnig wrote:
What influence has a metallic boom on a Yagi antenna?
Normally the design is made without taking the boom into
consideration.

http://yfrog.com/janextgen14p

The voltage curves on the parasitic elements have their maximum
in the center. When using a non-conductive beam the situation is not
affected.

With a metallic boom and with the parasitic elements fastened
to the metal would the oscillation pattern be changed?

Yes.. not a lot, but important on a high gain antenna (one that is
"super directive".. i.e. has gain N, where N is number of elements)
because the amplitude may not change much, but the phase might, and
small phase errors ruin the nulls in the pattern, which degrades the
overall directivity.

A modeling program like NEC can easily show the difference.


http://img28.imageshack.us/img28/3096/boommounted.jpg


That's the distribution of the voltage, but your picture doesn't show
phase and magnitude, which is what's important.

There are a whole variety of techniques to account for the boom in a
model, ranging from actually modeling the boom (tricky with some
modeling codes) to empirically determined correction factors.




Do I have to insulate the parasitic elements from the metallic boom
or is it advisable and allowed to fix them with metal fasteners?
Does the connection have to be made electrically good,
or does it not matter at all?

It does matter. What you really don't want is something that changes.
Either always connected or always insulated is what you want.





In fact there are both designs used, but probably without further
consideration. Some manufacturers use plastic holding clamps
for mounting the elements, for no other reason than saving time
during assembly. Other manufacturers just drive a self cutting
thread screw through everything.

The YAGICAD software does talk about mounting elements
on the boom, and NEC is too difficult for me to understand.


basically, a directive antenna gets its directivity from having
different phase and magnitude of current in each of the elements. In a
Yagi-Uda, only one element is driven, so the current in the other
elements comes from mutual coupling among the elements.

That coupling is affected by the spacing, length, and diameter(s) of the
elements, so having the boom connected is sort of like having an element
that is somewhat "fat" in the middle. It's slightly different.

On a 3 element wideband Yagi, the "boom correction" is going to be
pretty small. But on a 432 MHz moonbounce array with 20 elements, and
where you want very low side/back lobes to keep the noise temperature
down, it gets more important.

back before computers got fast and cheap, there were a lot of analytical
(or semi-analytical) approaches to designing Yagi antennas. You'd have
an approximation of mutual coupling (from, say, King) and form a matrix
equation. You'd figure out what element currents you wanted for your
pattern, and iterate your element lengths and spacings until it all
worked out. (or, you use the matrix equation to determine the element
currents, given the excitation, and then calculate the pattern from
that, using simple phased array techniques).

The problem is that simple approximations like King's typically assume
constant diameter elements. So, by dint of a bunch of empirical work,
people found "correction factors" that you could apply to an idealized
model (no boom) that would make it work ok on a boom.








Thank you in advance for your answers, and a sixpack
will be kept ready to compensate your efforts at your next visit.

w.