What are "front" and "back"? If the maximum forward lobe is +10 dBi at
an elevation angle of 23 degrees and the best null is -30 dBi at an
azimuth angle 160 degrees from the peak forward lobe, and at an
elevation angle of 47 degrees, are you saying that by your definition
the front/back ratio is 40 dB? If so, I guess that's interesting but I
can't imagine what it might be good for.

Who's "we"?

Roy Lewallen, W7EL

Jerry Martes wrote:
Roy

When I was working with antennas, we considered the antenna's F/B ratio
used the max of the front compared to the max of the back. But, I get the
impression that the rules are different now.

Jerry

Roy

I make no claim to being qualified to discuss antennas with you when we
are in disagreement. I worked as an antenna design engineer for 15 years
till 1968 when I was layed off from TRW. I was never a high level theorist
but managed to hold a decent position with designing hardware. I did work
with some highly qualified engineers from whom I thought I obtained alot of
knowledge about antennas. Thats why I was bold enough to say "we". I
still maintain a casual relationship with George Oltman who you might know
from his association with antenna groups with IEEE.
As for the F/B, I considered that to identify Front to Back of the
antenna's radiation pattern. I would consider it appropriate to identify
the radiation toward the "Front" as the max radiation to the front. Then,
it seems that the numerical level used for the "F/B ratio" should be max to
the Back.
I make no argument that this definition I use is *the* way F/B is. But,
dont we define "side lobe level" as the ratio of the main beam Max to the
side lobe Max? Be aware, I dont write to correct your thinking. I did
consider the F/B to be flawed when the main beam Max is compared with a rear
radiation Min.

I'll consider myself corrected and stop posting.

Jerry

"Roy Lewallen" wrote in message
...
What are "front" and "back"? If the maximum forward lobe is +10 dBi at an
elevation angle of 23 degrees and the best null is -30 dBi at an azimuth
angle 160 degrees from the peak forward lobe, and at an elevation angle of
47 degrees, are you saying that by your definition the front/back ratio is
40 dB? If so, I guess that's interesting but I can't imagine what it might
be good for.

Who's "we"?

Roy Lewallen, W7EL

Jerry Martes wrote:
Roy

When I was working with antennas, we considered the antenna's F/B ratio
used the max of the front compared to the max of the back. But, I get
the impression that the rules are different now.

Jerry

Don't quit posting Jerry, you are an asset to the group.
It is just that courtesy is not a requirement in this group
so some posts tend to be a bit sharp and personal
Look forward to hearing from you again
Regards
Art






"Jerry Martes" wrote in message
news:xNr4e.3912$%b1.1814@trnddc08...
Roy

I make no claim to being qualified to discuss antennas with you when we
are in disagreement. I worked as an antenna design engineer for 15
years till 1968 when I was layed off from TRW. I was never a high level
theorist but managed to hold a decent position with designing hardware. I
did work with some highly qualified engineers from whom I thought I
obtained alot of knowledge about antennas. Thats why I was bold enough
to say "we". I still maintain a casual relationship with George Oltman
who you might know from his association with antenna groups with IEEE.
As for the F/B, I considered that to identify Front to Back of the
antenna's radiation pattern. I would consider it appropriate to identify
the radiation toward the "Front" as the max radiation to the front. Then,
it seems that the numerical level used for the "F/B ratio" should be max
to the Back.
I make no argument that this definition I use is *the* way F/B is. But,
dont we define "side lobe level" as the ratio of the main beam Max to the
side lobe Max? Be aware, I dont write to correct your thinking. I did
consider the F/B to be flawed when the main beam Max is compared with a
rear radiation Min.

I'll consider myself corrected and stop posting.

Jerry

"Roy Lewallen" wrote in message
...
What are "front" and "back"? If the maximum forward lobe is +10 dBi at an
elevation angle of 23 degrees and the best null is -30 dBi at an azimuth
angle 160 degrees from the peak forward lobe, and at an elevation angle
of 47 degrees, are you saying that by your definition the front/back
ratio is 40 dB? If so, I guess that's interesting but I can't imagine
what it might be good for.

Who's "we"?

Roy Lewallen, W7EL

Jerry Martes wrote:
Roy

When I was working with antennas, we considered the antenna's F/B
ratio used the max of the front compared to the max of the back. But,
I get the impression that the rules are different now.

Jerry

Please don't consider yourself unqualified. I don't think anyone posting
on this newsgroup should, and with your background you certainly shouldn't.

My question about who "we" meant was to establish a context for the
definition you used, which you've supplied -- thanks.

The definition you use isn't a bad one, although it might not be the
most useful, provided that you restrict the analysis to free space and
are speaking only of a single plane of the 3D pattern. This is commonly
done in discussing Yagi arrays, for example. Perhaps your experience was
largely in Yagi, log periodic, or other planar arrays which lend
themselves to this simplification. The meaning of "back" is open to some
interpretation, though. Sometimes it means the precise direction that's
exactly 180 degrees from the main forward lobe. Sometimes, though, it
refers to a range of angles, even as great as the whole rear semicircle.
Let me give an example. Suppose an antenna nominally has a deep null
directly to the rear of the front lobe. But a slight asymmetry in the
antenna moves the lobe a few degrees to the side. This could easily
degrade a strictly defined ("rear" meaning exactly to the rear of the
peak of the front lobe) front/back ratio by 10 or 20 dB. It's hard to
conceive the application where it would really change the usefulness of
the antenna. But a very slightly asymmetrical antenna would look much
worse on paper. In your experience, would you consider this to be a poor
f/b ratio, or would you give the definition some slack and allow "rear"
to vary a few degrees? If you'd give it some slack, then the next
question is how much -- could the null be skewed 5 degrees? 10? more?

In my limited experience, when the second convention is used (allowing
the whole rear semicircle to count as "rear"), the "rear" figure often
comes from the largest lobe in the "rear" region. So the gain in the
precise direction opposite the front lobe doesn't matter, if there are
lobes in other directions in the rear semicircle. This definition would
be useful for amateur beam applications, because it tells you the
minimum amount of attenuation you'll get from signals coming from any
direction within the rear 180 degrees of the pattern. Who cares that you
have a 50 dB null in one very narrow direction, if a few degrees away
the response is 40 or 50 dB greater. The definition of front/back ratio
seems flexible, sometimes used to make the measure more meaningful or
useful, but sometimes, I'm sure, to obscure the quality of the pattern.

In the example I mentioned in my earlier posting, though, of the complex
pattern of an antenna over ground, the definition can get muddy indeed.
So it's often necessary to carefully define the term and state exactly
what you mean if you really want to communicate meaningful information
when you quote a "front/back" ratio.

Roy Lewallen, W7EL

Jerry Martes wrote:
Roy

I make no claim to being qualified to discuss antennas with you when we
are in disagreement. I worked as an antenna design engineer for 15 years
till 1968 when I was layed off from TRW. I was never a high level theorist
but managed to hold a decent position with designing hardware. I did work
with some highly qualified engineers from whom I thought I obtained alot of
knowledge about antennas. Thats why I was bold enough to say "we". I
still maintain a casual relationship with George Oltman who you might know
from his association with antenna groups with IEEE.
As for the F/B, I considered that to identify Front to Back of the
antenna's radiation pattern. I would consider it appropriate to identify
the radiation toward the "Front" as the max radiation to the front. Then,
it seems that the numerical level used for the "F/B ratio" should be max to
the Back.
I make no argument that this definition I use is *the* way F/B is. But,
dont we define "side lobe level" as the ratio of the main beam Max to the
side lobe Max? Be aware, I dont write to correct your thinking. I did
consider the F/B to be flawed when the main beam Max is compared with a rear
radiation Min.

I'll consider myself corrected and stop posting.

Jerry

Roy, W7EL wrote:
"Who cares that you have a 50 dB null in one particular very narrow
direction, if a few degrees away the response is 40 or 50 dB greater."

This is an example of Roy`s earlier post which noted differences between
amateur requirements and commercial requirements. Roy is right. There
are real differences.

Commercial licensees operate on assigned frequencies and enjoy some
protection from interference on their assignments. A broadcaster may as
a condition of his license be required to have a null in one or more
azimuths in his pattern to protect another broadcaster who was there
first. In this broadcaster`s case, he is interested in the narrow null
and may very well expect and hope his signal a few dgrees away is 40 or
50 dB greater.

Best regards, Richard Harrison, KB5WZI