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!

On 8 Apr 2006 10:28:28 -0700, "Ron J" wrote:

If a
side lobe is a minor lobe, then why is it undesirable to have side lobe
levels less than -20 dB? Thanks!

Hi Ron,

If it were one side lobe, that would be great. If it is several dozen
minor side lobes, then that is entirely another matter. The antenna
design is devoting too much power in this aggregate.

73's
Richard Clark, KB7QHC

Richard Clark wrote:

On 8 Apr 2006 10:28:28 -0700, "Ron J" wrote:


If a
side lobe is a minor lobe, then why is it undesirable to have side lobe
levels less than -20 dB? Thanks!


Hi Ron,

If it were one side lobe, that would be great. If it is several dozen
minor side lobes, then that is entirely another matter. The antenna
design is devoting too much power in this aggregate.

73's
Richard Clark, KB7QHC

As someone who does a lot of V/UHF antenna design, I'd say that -20 is
fine, and less than -20 is generally better if it doesn't screw up the
whole pattern, which it seldom does. The aggregate of a lot of -20 is
an average of less than -20 across the included angles. For EME, which
is pickier than HF, it's fine. I'm not sure what Richard could be
talking about.

Now if you want a 6 meter contest antenna for use in the midwest, you do
not want very low sidelobes, but that's a different story.

Tom
K0TAR

On Sat, 08 Apr 2006 20:37:31 -0500, Tom Ring
wrote:

I'm not sure what Richard could be talking about.

Hi Tom,

Compare a yagi to a rhomboid.

73's
Richard Clark, KB7QHC

On 8 Apr 2006 10:28:28 -0700, "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!

Another possible explanation is that if the main lobe is pointed in a
direction with low ambient noise (eg quiet sky), the last thing you
want to capture is significant power from hot earth (or other high
temperature sources) via minor lobes.

Owen
--

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.

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.

Richard Clark wrote:

On Sat, 08 Apr 2006 20:37:31 -0500, Tom Ring
wrote:


I'm not sure what Richard could be talking about.


Hi Tom,

Compare a yagi to a rhomboid.

73's
Richard Clark, KB7QHC

Point taken. They are tough to rotate, though. Also if you stack yagis
properly, things get much better.

tom
K0TAR

Richard Clark wrote:

On Sat, 08 Apr 2006 20:37:31 -0500, Tom Ring
wrote:


I'm not sure what Richard could be talking about.


Hi Tom,

Compare a yagi to a rhomboid.

73's
Richard Clark, KB7QHC

And, as I said, for some, perhaps many, contesting situations, low side
lobes are a good idea. On the east and west coasts, it's often good to
have as much F/B and F/S as possible because of the population density
and direction of it, while in the midwest the situation is quite
different, with few stations scattered all over the map, and it's
desirable to have sidelobes and rear lobes around -15 or so.

tom
K0TAR

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

Somebody finally got the problem.

Side lobes smaller than -20 dB are better. Smaller than -20 dB is -25
dB, -30 dB.

If it was stated as "-20 dB or less", that could/migh be interpreted as -15
dB, -10db. It is a language problem.

Now... What was the OP really asking? and what was the author really trying
to say? Probably what most assumed, but that isn't what he wrote.


73, Steve, K9DCI