wrote:
I certainly would not call it "silly" which is exactly
how "old wives tales" start.

At first, Einstein thought quantum physics was silly
(spooky). Entangled particles violate the spirit of
Einstein's relativity, if not the letter (at least
not yet). :-)
--
73, Cecil http://www.qsl.net/w5dxp

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"Richard Harrison" wrote in message
...
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.

Generaly I can agree with that but it is NOT TRUE in all cases.

1 It can NEVER reflect all radiation from the rear
2 Coupling is reduced as the directors move away from the driven element
to the point where little current flows.!

A yagi does have an advantage in simplicity together with the ability
to "focus" the forward lobe but not at the expense of removing rearward
radiation.

This post is starting to be remoniscent of the old days where Guru's stated
that more
than one reflector was pointless and unproductive because they could only
think
Yagis to the ommission of all others . Need W8TI to complete the full circle
(On a prior post I brought up the corner reflector antenna but I had
temporarily forgotten
that the reflectors carried little current and thus could not be a good
contender
for high F/B)





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.

HUM.... isn't F/B values retarded when maximum gain occurs?
This suggests that max gain occurs when there is MORE energy to
work with at the rear does it not?
Regards
Art

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote in message
...
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.



No, No, No Richard! I do not


A grid-dish reflector is where the reflector assembly has voids in the
reflector panel
that R.F. cannot penetrate. A similar instance is the perforations in a
transmitter housing.
Maybe it was the term "dampen" that led you to think I had a "grid" type
reflector which is not the case.
My antenna consists of 8 - 1/2 wave elements of which one is fed. All others
are resonant reflectors
set on a parabolic curve as is the driven element, thus it does not
"focus".This can be
fabricated using standard straight dipole elements . Advantages are to be
had using elements
that are "shaped" to minimise "reactance" coupling and also the physical
turning radius.
It is this variation that I am building now to see if it becomes quieter
especially when static is around.
It is times like this that light telescopic fishing poles can be used to
advantage together with
aluminum tubular mast. You may remember that I sent a 60 foot boom yagi
picture to you
years ago with 13 elements ( 26 fishing poles) which was also light weight
and thus desirable
when testing or experimenting to find out where the knoweledge envelope can
be pushed.
Regards
Art





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

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

Cecil Moore wrote:
John Smith wrote:
I cannot find a configuration on the gamma which brings the swr under
that figure...

As Richard H. said, find the point where the feedpoint
resistance is 50 + jXL ohms. That must occur somewhere.
Then tune out the reactance with a series capacitor.

I've used EZNEC to model a gamma-fed 33' 20m dipole at
40' made out of 0.5" aluminum tubing. With a gamma element
three feet long 6" below the 33' element and a series
Xc=122 ohms, it indicates a feedpoint impedance of
49+j0.3 ohms. The EZNEC file can be downloaded by
clicking on the link below.
--
73, Cecil http://www.qsl.net/w5dxp/GAM20DIP.EZ


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Cecil:

Your work with EZNEC is greatly appreciated here.
Your configuration is for a horz ant and I am working with a
vertical--conversion is trivial.
"A picture is worth a thousand words..." has real significance here.
I am sure an analysis of your file will provide me with insight into the
workings of EZNEC and accelerate my learning curve!

THANKS!,
John

--
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!!!
"Cecil Moore" wrote in message
...
Cecil Moore wrote:
John Smith wrote:
I cannot find a configuration on the gamma which brings the swr under
that figure...

As Richard H. said, find the point where the feedpoint
resistance is 50 + jXL ohms. That must occur somewhere.
Then tune out the reactance with a series capacitor.

I've used EZNEC to model a gamma-fed 33' 20m dipole at
40' made out of 0.5" aluminum tubing. With a gamma element
three feet long 6" below the 33' element and a series
Xc=122 ohms, it indicates a feedpoint impedance of
49+j0.3 ohms. The EZNEC file can be downloaded by
clicking on the link below.
--
73, Cecil http://www.qsl.net/w5dxp/GAM20DIP.EZ


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John Smith wrote:
Your work with EZNEC is greatly appreciated here.
Your configuration is for a horz ant and I am working with a
vertical--conversion is trivial.
"A picture is worth a thousand words..." has real significance here.
I am sure an analysis of your file will provide me with insight into the
workings of EZNEC and accelerate my learning curve!

I knew I was confused about what you were trying to do.
If you just tilt the horizontal dipole to a vertical
position, it will still be center-fed. Are you trying
to gamma-match a 1/2WL vertical base-fed against a
radial system? If so, try this EZNEC file:
--
73, Cecil http://www.qsl.net/w5dxp/GAM20VHW.EZ

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Cecil Moore wrote:
I knew I was confused about what you were trying to do.

John, I just realized what is wrong with gamma feeding a
1/2WL vertical at the base. The currents in each side of
the gamma feed section are pretty much out-of-phase so
they don't radiate (much). Conclusion: Gamma feeding a
1/2WL antenna at the base results in a field strength
comparable to a 1/4WL vertical and the minuscule amount
of gain is not worth the effort. Antenna gurus probably
already knew that but it just dawned on my concrete brain.

In much the same way a Zepp or a J-Pole is not a 3/4WL
radiator even though they are 3/4WL long, a gamma match
on a base-fed 1/2WL vertical doesn't yield a 1/2WL
electrical radiator. For this reason, the length of a
gamma match should be considerably less than a wavelength.
A good rule of thumb is probably in the ballpark of less
than ~6% of a wavelength. Your idea seems to require about
16% of a wavelength.

Moral: Physical length and RF electrical length are not
always the same.

What you are trying to do apparently requires a J-Pole
or Zepp approach, i.e. a physical 3/4WL antenna.
--
73, Cecil http://www.qsl.net/w5dxp

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Cecil:

One element I do not understand in EZNEC is: say I end up with 225 ohms
inductive reactance in the gamma rod(just an example figure, use any you
please), how do I inform EZNEC I am inserting a 225 ohm capactive reactance,
at the feedpoint to offset it?
Or, I am all wet in considering this?

Regards,
John

--
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!!!
"John Smith" wrote in message
...
Cecil:

Your work with EZNEC is greatly appreciated here.
Your configuration is for a horz ant and I am working with a
vertical--conversion is trivial.
"A picture is worth a thousand words..." has real significance here.
I am sure an analysis of your file will provide me with insight into the
workings of EZNEC and accelerate my learning curve!

THANKS!,
John

--
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!!!
"Cecil Moore" wrote in message
...
Cecil Moore wrote:
John Smith wrote:
I cannot find a configuration on the gamma which brings the swr under
that figure...

As Richard H. said, find the point where the feedpoint
resistance is 50 + jXL ohms. That must occur somewhere.
Then tune out the reactance with a series capacitor.

I've used EZNEC to model a gamma-fed 33' 20m dipole at
40' made out of 0.5" aluminum tubing. With a gamma element
three feet long 6" below the 33' element and a series
Xc=122 ohms, it indicates a feedpoint impedance of
49+j0.3 ohms. The EZNEC file can be downloaded by
clicking on the link below.
--
73, Cecil http://www.qsl.net/w5dxp/GAM20DIP.EZ


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Cecil:
Yes Cecil, there is wisdom in your words...
However, as I pointed out, I am now focusing on "received signal", its'
quality and strength.
After having given it some thought, that is what is most important. In an
grave emergency I have the ability to bring multiple kilowatts on line (yes,
I have the generator to support it.) What I can't do is guarantee that for
the fellow at the other end of the contact. So, I am focusing my attention
towards his/her benefit--in the refinement of my ability to pull
sub-nano-watts from the ether...
EZNEC and all other calculations are great, I appreciate what can get me to
ballpark figures with matching, lengths, spacings, predictions, etc.
But, in the end, the humble s-meter on a known transceiver, compared against
know signals and similar conditions will be my final guide and the
determining factor.
To all which point out I "may only be"/am duplicating past work, while
making futile attempts, or expelling great amounts of energy in this
pursuit, I say: "It keeps me out of the bars and away from the wild women!"
(saves on my Jim Beam/Viagra bill too!)

Warmest regards,
John

--
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!!!
"Cecil Moore" wrote in message
...
Cecil Moore wrote:
I knew I was confused about what you were trying to do.

John, I just realized what is wrong with gamma feeding a
1/2WL vertical at the base. The currents in each side of
the gamma feed section are pretty much out-of-phase so
they don't radiate (much). Conclusion: Gamma feeding a
1/2WL antenna at the base results in a field strength
comparable to a 1/4WL vertical and the minuscule amount
of gain is not worth the effort. Antenna gurus probably
already knew that but it just dawned on my concrete brain.

In much the same way a Zepp or a J-Pole is not a 3/4WL
radiator even though they are 3/4WL long, a gamma match
on a base-fed 1/2WL vertical doesn't yield a 1/2WL
electrical radiator. For this reason, the length of a
gamma match should be considerably less than a wavelength.
A good rule of thumb is probably in the ballpark of less
than ~6% of a wavelength. Your idea seems to require about
16% of a wavelength.

Moral: Physical length and RF electrical length are not
always the same.

What you are trying to do apparently requires a J-Pole
or Zepp approach, i.e. a physical 3/4WL antenna.
--
73, Cecil http://www.qsl.net/w5dxp

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