On 21 jun, 10:20, Helmut Wabnig [email protected] --- -.dotat 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?

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

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?

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.

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

w.

Hello Helmut,

There is influence, whether or not the elements are electrically
connected. When (thickness of the boom) (element length), the
influence is negligible.

When the above isn't met, you may experience a noticeable shift in
design frequency (it will go up). when connecting the elements to the
boom, the influence will be more. So to compensate for this, the
elements should be somewhat longer. Note that it also depends on where
the metallic contacts are. An element through a boom will experience
more shift then an element having just a single-point electrical
connection to a boom.

When you have bad connections (for example due to corrosion or a loose
nut), movement (induced by wind) will change the electrical
characteristics in a time varying way. This may result in cracking
noise, even on strong signals. Clean strong signals may introduce
noise at other frequencies due to the time varying change in antenna
properties.

This may be a reason for manufactures to use non-contact mounting of
beam elements. I saw several UHF yagis (with relative thick square
booms w.r.t. element length) with loose elements making (bad or no)
contact with the square boom. So if you decide to make electrical
contact, make sure it is long lasting.

Here you may find useful info http://www.qsl.net/yu1aw/Misc/vhf_ant.htm

With kind regards,


Wim
PA3DJS
www.tetech.nl