maybe their 'smaller' means smaller absolute values.. so that would give
numbers like -15db, -10db, etc... which would indeed be worse.

"Dave" wrote in message
...
Ron J wrote:

Hello, I have Balanis' Antenna Theory 3rd Edition. I read on page 31 at
the very last sentence that "Side lobe levels of -20 dB or smaller are
usually not desirable in most applications."

Why? On the same paragraph it states that "minor lobes represent
radiation in undesired directions, and they should be minimized." If a
side lobe is a minor lobe, then why is it undesirable to have side lobe
levels less than -20 dB? Thanks!


-20 dB from a transmit point of view means that 99% of your radiated power
is going in the forward [desired] direction. That's GOOD!

From a receiving point of view, -20 dB means that an undesired signal from
the side or back of the antenna is approximately 3 1/2 S units [-20 dB]
from it's maximum value. It is easily conceivable that you may be trying
to copy a weak barely detectable signal from your desired direction and be
unable to copy that signal because of a stronger signal that happens to be
in a side lobe and on or very close to your frequency. That's NOT too GOOD
:-(

For most practical Ham antennas in the DX or contest mode a -20 dB
sidelobe is adequate. You can spend a lot of $$$$ buying antennas with
better side lobe specifications. You are the only one who can decide if
the benefit to your operations is worth the expense.

For VHF/UHF EME work you really want the best side lobe and F/B ratio
[front to back] performance you can buy, primarily to minimize undesired
signals from the non preferred direction.