"Richard Harrison" wrote in message
...
Art, KB9MZ wrote:
"O.K. Reg, I`ll take you up on the 50 dB F/B."

It requires good balance for such cancellation.

Yes, it will not come by accident but Reg has said that 50 db F/B is
"silly"!
Why would he say that without back up evidence? He also did not offer
a max F/B that was obtainable in theory. I have seen computor results
that offer 50 db F/B based on NEC, Can I trust gain if F/B cannot
be trusted? Obviously 50 db is hard to get but is it beyond the realms
of possibility?
The F/B that you offered is a bit unfair but then it is an antenna and
Reg did not qualify his statement.
I personaly modelled a antenna that comprised of a driven element
with the remaining elements acting as reflectors only. And we all know
that extra reflectors are a waste of time for H.F.( grin ) but they do give
very high F/B figures.
I wonder what a corner reflector antenna would provide

Regards
Art KB9MZ......XG


Kraus gives the gain for a 100-meter dish near Bonn, Germany on page 676
of the 3rd edition of "Antennas". Gain is a function of frequency and
varies from about 48 dB at 300 MHz to about 98 dB at 150 GHz. I don`t
know if shielding has been added to make this a "high-performance" dish
but I would wager that this enormous radio ear and mouth has an
excellent front-to-back ratio. Probably exceeds forward gain at some
azimuths and elevation angles.

Arnold B. Bailey has a lot to say anout a "connected" (driven) element
and a parasitic reflector, starting about on 447 of "TV and Other
Receiving Antennas", Bailey says:
"The optimum spacing for highest gain of a parasitic reflector is S=0.15
wavelength plus or minus 0.025. Here the relleector is operated at Q=+1
(longer than resonant) and the gain in direction 1 is approximately 5.5
dB. Reasonable compromise for a less critical system is to use a spacing
of 0.2 wavelength and a parasitic element longer than resonant (at
Q=+1). This case gives a gain in direction 1 of 5 dB."

On page 440 Bailey says: The front-to-back ratio ---is 17.5 dB in this
case, and the Y/X ratio only about 9 dB, where +X represents the optimum
direction.---" A picture is worth 1000 words.

Best regards, Richard Harrison, KB5WZI

Just for the sake of curriosity: what if a RASER "Radio (frequency)
Amplification by Stimulated Emission of Radiation" device existed?
50 db would become acceptable and common place overnight, huh? MASER's
exist, why now long wavelength? Sure the antenna at HF would be the size of
the pentagon, but Bill Gates wouldn't blink an eye! grin

Regards

--
I would like to point out, I do appreciate the "Been there--done that!"
posts. Indeed, now your observations, comments and discourse should be
filled with wisdom--I am listening!!!
" wrote in message
news:P2o4e.22497$NW5.1099@attbi_s02...

"Richard Harrison" wrote in message
...
Art, KB9MZ wrote:
"O.K. Reg, I`ll take you up on the 50 dB F/B."

It requires good balance for such cancellation.

Yes, it will not come by accident but Reg has said that 50 db F/B is
"silly"!
Why would he say that without back up evidence? He also did not offer
a max F/B that was obtainable in theory. I have seen computor results
that offer 50 db F/B based on NEC, Can I trust gain if F/B cannot
be trusted? Obviously 50 db is hard to get but is it beyond the realms
of possibility?
The F/B that you offered is a bit unfair but then it is an antenna and
Reg did not qualify his statement.
I personaly modelled a antenna that comprised of a driven element
with the remaining elements acting as reflectors only. And we all know
that extra reflectors are a waste of time for H.F.( grin ) but they do
give
very high F/B figures.
I wonder what a corner reflector antenna would provide

Regards
Art KB9MZ......XG


Kraus gives the gain for a 100-meter dish near Bonn, Germany on page 676
of the 3rd edition of "Antennas". Gain is a function of frequency and
varies from about 48 dB at 300 MHz to about 98 dB at 150 GHz. I don`t
know if shielding has been added to make this a "high-performance" dish
but I would wager that this enormous radio ear and mouth has an
excellent front-to-back ratio. Probably exceeds forward gain at some
azimuths and elevation angles.

Arnold B. Bailey has a lot to say anout a "connected" (driven) element
and a parasitic reflector, starting about on 447 of "TV and Other
Receiving Antennas", Bailey says:
"The optimum spacing for highest gain of a parasitic reflector is S=0.15
wavelength plus or minus 0.025. Here the relleector is operated at Q=+1
(longer than resonant) and the gain in direction 1 is approximately 5.5
dB. Reasonable compromise for a less critical system is to use a spacing
of 0.2 wavelength and a parasitic element longer than resonant (at
Q=+1). This case gives a gain in direction 1 of 5 dB."

On page 440 Bailey says: The front-to-back ratio ---is 17.5 dB in this
case, and the Y/X ratio only about 9 dB, where +X represents the optimum
direction.---" A picture is worth 1000 words.

Best regards, Richard Harrison, KB5WZI

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 Lewallen" wrote in message
...
wrote:
. . .
I have seen computor results
that offer 50 db F/B based on NEC, Can I trust gain if F/B cannot
be trusted?

Absolutely! While you might get some very deep nulls at some particular
points in space, and fairly deep nulls in some particular
azimuth/elevation angle combinations, they're not likely to be exactly as
deep or in the directions the program reports. Gain, on the other hand,
can be strikingly accurate in many cases.

Put together any model you want with an extreme F/B ratio. Then fiddle the
model just slightly -- change the frequency, element length or diameter,
etc. Look at how much the gain changes, and how much the F/B changes.
Modify it more, and look again.

You'll see that the F/B is *much* more critical than gain. You can goof up
the model -- or real antenna -- a lot more without any appreciable change
in gain than you can before seeing major changes in F/B.

The reason is simple. To get a deep null and therefore good F/B ratio, you
have to add the fields from all parts of the antenna together to get zero
within a tiny, tiny fraction of a percent. If any one of the fields
changes just a tiny amount, they no longer sum precisely to zero. But
small change like that won't noticeably affect the gain. No model is good
enough to precisely predict extremely deep nulls -- there's always too
much difference between the model and reality.

I don't recall what Reg recently said, but I've gotten 50 dB and greater
F/B ratios from an array by adjusting the phasing network while listening
to a receiver placed in the null direction. But the null is that deep only
in that direction, at that height above ground. It's also noticeably
shallower a little ways away even in the same direction, because I've
compensated for re-radiation from nearby objects, too. Even coax shield
leakage becomes a very noticeable factor. So while I can tweak an array to
get a very deep null, there's no way I can expect that to hold when
anything changes, even just a little. I'd even expect it to change from
day to day as the ground moisture changes and the sap rises in the trees.

Obviously 50 db is hard to get but is it beyond the realms
of possibility?

For what, one particular azimuth/elevation combination at one single
frequency? You might be able to do it. But it would be only of academic
interest at best.

. . .

Roy Lewallen, W7EL

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

I agree Roy, I did say it wasn't easy and I suspect you are correct if the
null is "deep"
But if the general" Front to rear"is exceptionaly good then
there is no reason for the F/B not to fairly accurate.
I certainly would not call it "silly" which is exactly
how "old wives tales" start.
The particular case I was referring to was where multiple
reflectors were used to dampen rearward radiation to a minimum.

Regards
Art




"Roy Lewallen" wrote in message
...
wrote:
. . .
I have seen computor results
that offer 50 db F/B based on NEC, Can I trust gain if F/B cannot
be trusted?

Absolutely! While you might get some very deep nulls at some particular
points in space, and fairly deep nulls in some particular
azimuth/elevation angle combinations, they're not likely to be exactly as
deep or in the directions the program reports. Gain, on the other hand,
can be strikingly accurate in many cases.

Put together any model you want with an extreme F/B ratio. Then fiddle the
model just slightly -- change the frequency, element length or diameter,
etc. Look at how much the gain changes, and how much the F/B changes.
Modify it more, and look again.

You'll see that the F/B is *much* more critical than gain. You can goof up
the model -- or real antenna -- a lot more without any appreciable change
in gain than you can before seeing major changes in F/B.

The reason is simple. To get a deep null and therefore good F/B ratio, you
have to add the fields from all parts of the antenna together to get zero
within a tiny, tiny fraction of a percent. If any one of the fields
changes just a tiny amount, they no longer sum precisely to zero. But
small change like that won't noticeably affect the gain. No model is good
enough to precisely predict extremely deep nulls -- there's always too
much difference between the model and reality.

I don't recall what Reg recently said, but I've gotten 50 dB and greater
F/B ratios from an array by adjusting the phasing network while listening
to a receiver placed in the null direction. But the null is that deep only
in that direction, at that height above ground. It's also noticeably
shallower a little ways away even in the same direction, because I've
compensated for re-radiation from nearby objects, too. Even coax shield
leakage becomes a very noticeable factor. So while I can tweak an array to
get a very deep null, there's no way I can expect that to hold when
anything changes, even just a little. I'd even expect it to change from
day to day as the ground moisture changes and the sap rises in the trees.

Obviously 50 db is hard to get but is it beyond the realms
of possibility?

For what, one particular azimuth/elevation combination at one single
frequency? You might be able to do it. But it would be only of academic
interest at best.

. . .

Roy Lewallen, W7EL

Jerry Martes wrote:
"When I was working with antwennas, we considered the antenna`s F/B
ratio used the max of the front to the max of the back. But I get the
impression that the erules are different now."

Front-to-back ratio is defined as the ratio of power gain between front
and rear of a directional antenna.

A.W.P. King writes on page 209 of "Transision Lines, Antennas, and Wave
Guides"*:

"In practice, it is usually important to maximize the ratio of forward
to backward field or the ratio of backward to forward field. If the
ratio of forward to backward field is maximized, the parasite is called
a reflector; if the ratio of backward to forward field is maximized, the
parasite is called a director."

I don`t think the above has changed since 1945.

Best regards, Rihard Harrison, KB5WZI

Richard
That rule is not all encompasing tho it may well suffice for yagi's and the
like
Regards
Art
"Richard Harrison" wrote in message
...
Jerry Martes wrote:
"When I was working with antwennas, we considered the antenna`s F/B
ratio used the max of the front to the max of the back. But I get the
impression that the erules are different now."

Front-to-back ratio is defined as the ratio of power gain between front
and rear of a directional antenna.

A.W.P. King writes on page 209 of "Transision Lines, Antennas, and Wave
Guides"*:

"In practice, it is usually important to maximize the ratio of forward
to backward field or the ratio of backward to forward field. If the
ratio of forward to backward field is maximized, the parasite is called
a reflector; if the ratio of backward to forward field is maximized, the
parasite is called a director."

I don`t think the above has changed since 1945.

Best regards, Rihard Harrison, KB5WZI

Art Unwin wrote:
"The particular case I was referring to was where multiple reflectors
were used to dampen radiation to a maximum."

It works. Taken to an extreme, you have a grid-dish reflector.

Additional reflectors in the plane of the other Yagi-Uda elements don`t
make best use of materials aloft. Placed ahead of the driven element,
additional elements are in a strong field where they may work as
directors to sharpen the forward gain. Placed behind the reflector which
has already reversed most of the energy in its direction, a parasitic
element is less effective as rhere is less energy to work with.

Best regards, Richard Harrison, KB5WZI