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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On Sun, 3 Apr 2005 20:06:45 -0700, "John Smith"
wrote:

Lenght of coax from rf source to ant seems critical, when coax is shortened
or lengthened, gamma must be adjusted (but is always around 3 ft)

Hi Brett,

This is a classic symptom of feedline interaction with the feedpoint
Impedance. Basically, the exterior of the cable is acting as an
uncontrolled tuning element that is bridging your attempt to tune the
antenna. To correct this one indication, you need to choke the
feedline. This is accomplished by one of several methods.

We frequently recommend what is called a "Current BalUn" which is a
series of ferrite beads strung onto the coax near the feed point.
Another method is to simply coil the coax with half a dozen turns of
about 6 inches diameter. Either method will decouple the feedline
from the feedpoint. There's more to be said, but that can be
discussed at another time, or you can research that in the archives
using the keywords in quotes above. This is successful with Low-Z
loads because the choke is Hi-Z in comparison - your drive point
choice presents a problem in that regard.

That problem is that a halfwave is extremely sensitive to environment,
especially in the face of unchoked lines. This is a fact of life in
that for a Hi-Z load, nearly everything nearby looks like a short
circuit unless you can hoist the antenna out of the way. The choking
action will have to be extremely good to overcome this (or you will
have to insure that the external coax length is also halfwave in
length - this is one of those CB antenna/coax issues that rarely gets
discussed and becomes lore instead of learning). Hi-Z loads are
usually matched with Hi-Z circuits through voltage matching, the Gamma
match is more a Low-Z device working as a current transformer. Its
standard application, nearer the middle of the halfwave antenna, would
be more suitable.

To achieve this, isolate the bottom end of the halfwave (mount it on
an insulator, the details which follow will be challenging). Make
sure you can snake the coax up inside the antenna (this presumes it is
tubular of sufficient diameter to pass cable up inside). Break out
the cable halfway up (care must be given that this does not
substantially weaken the whole of the antenna). Build the gamma match
(external of course) here and drive it against the radiator body with
one lead of the coax going to the gamma structure, the other side
going to the break out hole.

All in all, a SWR of 1.7 is not shabby for a first pass approximation
for a physical solution. If you decide to stick with your current
implementation, you might try making the gamma tube larger than the
diameter of the radiator (capacitance stays the same). We are
departing from the regime of nutshell math.

By this (classical design), the work revealed in full sized, folded
dipoles (and monopoles) teaches that the size ratio of the two
elements (in this case approximated by the radiator and the gamma
element), and their proximity, yield a step-up or step-down
relationship in drive point Z (depending on which, larger or smaller,
is being driven). This is like conventional transformer winding
ratios, except in RF it is embodied in diameters and separation with
complex results (not as simple as counting turns and being done with
it). This is further complicated by the structure not being a classic
folded design - I am merely extending the metaphor, returning this to
nutshell theory.

This means it is up to you to close the gap through experimentation,
observation, and correlation. There may be a limit in what you can
achieve, but you seem close enough to vary a few parameters to see if
there is any progress.

73's
Richard Clark, KB7QHC

On Sun, 03 Apr 2005 21:47:02 -0700, Richard Clark
wrote:

By this (classical design), the work revealed in full sized, folded
dipoles (and monopoles) teaches that the size ratio of the two
elements (in this case approximated by the radiator and the gamma
element), and their proximity, yield a step-up or step-down
relationship in drive point Z (depending on which, larger or smaller,
is being driven). This is like conventional transformer winding
ratios, except in RF it is embodied in diameters and separation with
complex results (not as simple as counting turns and being done with
it). This is further complicated by the structure not being a classic
folded design - I am merely extending the metaphor, returning this to
nutshell theory.

Hi All,

For others following my strained metaphors, consult Johnson and
Jasik's "Antenna Engineering Handbook," "Impedance Transformation as a
Function of the Ratio of Conductor Sizes" (pg. 4-17 of the second
edition).

This topic has always been confined to full sized folded quarterwave
monopoles or halfwave dipoles and I have not observed any equal (that
is, comprehensive) treatment given in terms of T or Gamma matches. Of
course, there may be every reason why such has never been done.

73's
Richard Clark, KB7QHC

Yes, I understand about the transformer action of differing conductor sizes
at rf freqs.
I interned for a tranformer manufacturer many moons ago.
I will experment with differing diameters of the gamma rod. That is one new
variable I can introduce....

Regards

--
Hay, if'n ya'll cun't konstructivly partecipete in this har disscusion, haw
aboot speel-checkin it fer me?


"Richard Clark" wrote in message
...
On Sun, 03 Apr 2005 21:47:02 -0700, Richard Clark
wrote:

By this (classical design), the work revealed in full sized, folded
dipoles (and monopoles) teaches that the size ratio of the two
elements (in this case approximated by the radiator and the gamma
element), and their proximity, yield a step-up or step-down
relationship in drive point Z (depending on which, larger or smaller,
is being driven). This is like conventional transformer winding
ratios, except in RF it is embodied in diameters and separation with
complex results (not as simple as counting turns and being done with
it). This is further complicated by the structure not being a classic
folded design - I am merely extending the metaphor, returning this to
nutshell theory.

Hi All,

For others following my strained metaphors, consult Johnson and
Jasik's "Antenna Engineering Handbook," "Impedance Transformation as a
Function of the Ratio of Conductor Sizes" (pg. 4-17 of the second
edition).

This topic has always been confined to full sized folded quarterwave
monopoles or halfwave dipoles and I have not observed any equal (that
is, comprehensive) treatment given in terms of T or Gamma matches. Of
course, there may be every reason why such has never been done.

73's
Richard Clark, KB7QHC

Richard:

Thanks, I think you have at least one problem I created in check. I forgot
to place ferrite beads over the coax, at the antenna, I guess I was thinking
the gamma was some sort of increased isolation. It is now so apparent--now
that you mentioned it...

Regards

--
Hay, if'n ya'll cun't konstructivly partecipete in this har disscusion, haw
aboot speel-checkin it fer me?


"Richard Clark" wrote in message
...
On Sun, 3 Apr 2005 20:06:45 -0700, "John Smith"
wrote:

Lenght of coax from rf source to ant seems critical, when coax is
shortened
or lengthened, gamma must be adjusted (but is always around 3 ft)

Hi Brett,

This is a classic symptom of feedline interaction with the feedpoint
Impedance. Basically, the exterior of the cable is acting as an
uncontrolled tuning element that is bridging your attempt to tune the
antenna. To correct this one indication, you need to choke the
feedline. This is accomplished by one of several methods.

We frequently recommend what is called a "Current BalUn" which is a
series of ferrite beads strung onto the coax near the feed point.
Another method is to simply coil the coax with half a dozen turns of
about 6 inches diameter. Either method will decouple the feedline
from the feedpoint. There's more to be said, but that can be
discussed at another time, or you can research that in the archives
using the keywords in quotes above. This is successful with Low-Z
loads because the choke is Hi-Z in comparison - your drive point
choice presents a problem in that regard.

That problem is that a halfwave is extremely sensitive to environment,
especially in the face of unchoked lines. This is a fact of life in
that for a Hi-Z load, nearly everything nearby looks like a short
circuit unless you can hoist the antenna out of the way. The choking
action will have to be extremely good to overcome this (or you will
have to insure that the external coax length is also halfwave in
length - this is one of those CB antenna/coax issues that rarely gets
discussed and becomes lore instead of learning). Hi-Z loads are
usually matched with Hi-Z circuits through voltage matching, the Gamma
match is more a Low-Z device working as a current transformer. Its
standard application, nearer the middle of the halfwave antenna, would
be more suitable.

To achieve this, isolate the bottom end of the halfwave (mount it on
an insulator, the details which follow will be challenging). Make
sure you can snake the coax up inside the antenna (this presumes it is
tubular of sufficient diameter to pass cable up inside). Break out
the cable halfway up (care must be given that this does not
substantially weaken the whole of the antenna). Build the gamma match
(external of course) here and drive it against the radiator body with
one lead of the coax going to the gamma structure, the other side
going to the break out hole.

All in all, a SWR of 1.7 is not shabby for a first pass approximation
for a physical solution. If you decide to stick with your current
implementation, you might try making the gamma tube larger than the
diameter of the radiator (capacitance stays the same). We are
departing from the regime of nutshell math.

By this (classical design), the work revealed in full sized, folded
dipoles (and monopoles) teaches that the size ratio of the two
elements (in this case approximated by the radiator and the gamma
element), and their proximity, yield a step-up or step-down
relationship in drive point Z (depending on which, larger or smaller,
is being driven). This is like conventional transformer winding
ratios, except in RF it is embodied in diameters and separation with
complex results (not as simple as counting turns and being done with
it). This is further complicated by the structure not being a classic
folded design - I am merely extending the metaphor, returning this to
nutshell theory.

This means it is up to you to close the gap through experimentation,
observation, and correlation. There may be a limit in what you can
achieve, but you seem close enough to vary a few parameters to see if
there is any progress.

73's
Richard Clark, KB7QHC

John Smith wrote:
Anyone work with the gamma-match to the point of becomming a guru?
I am looking for "Everything you wanted to know about a gamma-match--in a
nutshell."

Start with the ARRL Antenna Book. My 19th Edition, (C) 2000, discusses gamma
matches in chapter 26: Coupling the Line to the Antenna.

In other words, what set of formulas would give you length, conductor
spacing, gamma capacitor value, ratio of gamma rod to driven element
diameter, and a starting measurement for the shorting bar between
conductors-- for a given frequency?

The best resource I've found for this is lecture notes by Dr. Thomas Montoya
at:

http://montoya.sdsmt.edu/ee492/fall2...2_fall2004.htm

Glen, kc0iyt

Glen:

THAT is a very interesting URL, thank you for the time in bringing it to my
attention; it is apparent that my internet searchs are missing important
documents!

Dr. Montoya is "my kinda guy."

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!!!
"Glen Overby" wrote in message
...
John Smith wrote:
Anyone work with the gamma-match to the point of becomming a guru?
I am looking for "Everything you wanted to know about a gamma-match--in a
nutshell."

Start with the ARRL Antenna Book. My 19th Edition, (C) 2000, discusses
gamma
matches in chapter 26: Coupling the Line to the Antenna.

In other words, what set of formulas would give you length, conductor
spacing, gamma capacitor value, ratio of gamma rod to driven element
diameter, and a starting measurement for the shorting bar between
conductors-- for a given frequency?

The best resource I've found for this is lecture notes by Dr. Thomas
Montoya
at:

http://montoya.sdsmt.edu/ee492/fall2...2_fall2004.htm

Glen, kc0iyt

After so many waffling contributions to the newsgroup from you
'experts' - where is the Gamma-match design formula? There are only
2 or 3 dimensions involved. It should be simple and straightforward
enough!

Or is this newsgroup just a farce?
----
Reg.

On Mon, 4 Apr 2005 15:56:39 +0000 (UTC), "Reg Edwards"
wrote:
Or is this newsgroup just a farce?

Hi Reggie,

Surely at your age, and experience writing here, you MUST know the
answer to that by now.

73's
Richard Clark, KB7QHC