On 6/21/2011 9:52 AM, John Smith wrote:
On 6/21/2011 2:17 AM, Owen Duffy wrote:
On Tuesday, 21 June 2011 18:20:25 UTC+10, Helmut Wabnig wrote:
What influence has a metallic boom on a Yagi antenna?
Normally the design is made without taking the boom into
consideration.

No necessarily, but it is common to design for a non-conductive boom
and then make a correction to element lengths for the type of boom in
use.

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.

That is not true.

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

Depending on how the boom is or is not bonded to the element, and the
geometry (sizes, position etc), the conductive boom may reduce the
magnetic flux nearest to the element, effectively reducing its
inductance in the middle area, so tuning it higher.

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?

If the 'connection' has infinite R (ie no connection) or zero R, there
is no power converted to heat, otherwise you invite conversion of RF
energy to heat.


In fact there are both designs used, but probably without further
consideration. Some manufacturers use plastic holding clamps

Not by competent designers, they consider the effects of the boom.

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.

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


The foregoing may challenge your thinking on the subject.

Owen


An interesting experiment might be to cast the upper elements above a
metal slab of 1/4 wave depth which extends a full wave out in all
direction of the compass, in eznec/nec/mmana, and serves the purpose of
the lower elements connected to a huge boom.

I'm having a tough time visualizing what you're describing. Half
elements sticking up from a "perfectly conductive plane"?

A comparison between a standard yagi and such a design should show a
stark comparison of the importance/function of the lower/ground
elements, between the two. Or, perhaps just the extension of the boom in
the frontal and trailing directions would provide usable data.

IN general, the boom extending front or back doesn't make a big
difference. If I had to "rule of thumb" it, I'd say that the
"influence" of the boom is several element radii.



In any case, I suspect the influence of the boom, in such a greatly
exaggerated example, would be exposed in more defining light ...

What could be done to the upper elements, reflector(s) and director(s),
to provide such a comparison(s) is a matter which provokes thought ...

Frankly, I don't even know if any antenna modeling software would allow
such an "abuse."



Step right up and fire up that copy of HFSS or ADS

in most modern codes you can model *anything* given sufficient time to
build the model and run it. If you have lots of big chunks of "stuff" a
method of moments code like NEC might not be the best choice. One of
the FDTD or similar codes might be better.