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.
--
73, Cecil http://www.qsl.net/w5dxp

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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

J:

If the formulas, charts, etc. don't exist, then I am extremely lucky. Most
of the time (OK! Maybe ALL the time) my efforts just end up duplicating what
past expermenters have already done. I make notes as I go, if I don't find
what I am doing in some other obscure work--or as knowledge another has, I
will be tempted to place it in a form where others might use it.
What I mean is, I will not be disappointed--and very happy if what you
suspect is true.
"Top Down" (hey, is that guy in a convertible?) is great, however, bottom up
has its uses too... OOP(s) (a pun--Object Orientated Programing--for
non-coders), I hope some of my old professors aren't around here! (Darn!
that Richard, now he has me doing it) grin

Regards

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


"J. Mc Laughlin" wrote in message
...
Dear John Smith (no call, no location)

Your questions contain assumptions. You assume that formulas amiable
of
being programmed into a TI calculator exist for what it is you wish to do.
I doubt that they exist.

To borrow a term from your job, top-down is needed. Start with the
ARRL's Antenna Book. When you have digested its teachings, read either
Kraus' 3rd edition or Balanis' 2nd edition. Start at the beginning and
follow the road. It will be an interesting journey.
Regards, Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
Home:
"John Smith" wrote in message
...
Yes, I am quite sure you are correct.

And, perhaps no one knows, or no one does it the old fashioned way and
has
the formulas and steps at their disposal, I may have to seek other
sources
than here.

However, I am a software engineer, once I understand the underlying math
and
methods I can write such an application myself.

When the final drafts of these programs are released, antenna design, I
am
sure you will just draw your antenna on virtual graph paper with ruled
lines, such as many of the draw programs have. And, be able to add any
inductance or capacitance just by selecting the point with the mouse, and
then entering values--the application will do everything else for you,
including making suggestions for improvement, matching, special
conditions,
etc, etc--much like CAD (Computer Assisted Drafting) applications.

However, whether I use software, or simply divide 468/Fmhz, I still get a
half-wave.

It is fine if one wants to rely on software--I make my living on those
who
do, I LOVE those people!--I just don't wish to invoke that method, later
when my understanding is absolute (well, greater than it is now) I may,
or
write software specific to my needs...


Warmest regards



"Cecil Moore" wrote in message
...
John Smith wrote:
I am sure EZNEC is an excellent application. But with such
applications,
you spend your time learning the application, rather than the
underlying
principals;

I have learned many, many principles from using EZNEC which is
a really great learning tool. Look at it as reverse engineering.
You wonder why EZNEC reports such results and then you go discover
why. Modeling with ELNEC and EZNEC has taught me as much about
antennas as any other single source including Dr. Balanis' graduate
level
college antenna course at ASU. One doesn't learn anything by avoiding
learning EZNEC. Dr. Balanis was amazed that an Intel
digital engineer already knew so much about antennas and I owe
that, in large part, to ELNEC and EZNEC.
--
73, Cecil http://www.qsl.net/w5dxp

Richard H.:

I had only used a "universal stub" and "L-Network" to match a 1/2 wave end
fed before now. The gamma seems interesting.
Just for my curriosity, I will do a full-wave with a T-Match in the next few
days (seems a 1 wave vertical is best suited for high angle radiation
pattern.) I will keep your analysis in mind, especially at that
time--thanks!

Regards

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


"Richard Harrison" wrote in message
...
John Smith wrote:
"In other words, what set of formulas would give you length, conductor
spacing, gamma capacitor value----?"

It looks experimental to me.

My ARRL Handbook says that the T match has an advantage in that a pair
of transmission line attachment points equidistant from the center of a
resonant wire has a resistance between them. Therefore it is possible to
choose points which match the line Zo. Problem is the physical distance
between the attachment points probably does not match the line spacing.
The handbook suggests a T match made like a folded dipole with
single-wire extensions at its ends. This looks as if it would require
experimentation.. It seems suited for parallel-wire lines.

The gamma match seems to be similar but an ubbalanced scheme where the
grounded line conductor is attached to the center of the radiator and
the ungrounded conductor is connected to the wire at a distance from the
center of the radiator which results in a low SWR on the transmission
line. A variation treats the gamma match as if it were a shunt tower
feed arangement.

The gamma match becomes a feed loop containing a ground path. The feed
loop is sized to that required for a resistive component which matches
the Zo of the transmission line. The loop inductively couples the
antenna with the transmission line. The loop`s reactance is always
inductive and can be tuned out with a series connected variable
capacitor.

It seems a good cut-and-try project to me. Maybe perfect for Art Unwin.

Best regards, Richard Harrison, KB5WZI

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

Well "John Smith," wherever and whoever you are, you are receiving valuable
advice from several quarters.
Our students are required to use top-down programming in computer
engineering and in computer science classes. If your professors did not
convince you of its importance, far be it for me to try.

Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
Home:

John Smith wrote:
Just for my curriosity, I will do a full-wave with a T-Match ...
(seems a 1 wave vertical is best suited for high angle radiation
pattern.)

An *end-fed* one wavelength vertical fed against ground has
a high angle radiation pattern, e.g. 36 deg TOA. A one wavelength
vertical with a T-Match is a center-fed antenna. Such an antenna
has a *low angle* radiation pattern, e.g. 11 deg TOA.
--
73, Cecil http://www.qsl.net/w5dxp

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