Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: Gamma match vs. phasing line  (Read 1140 times)
KE6WNH
Member

Posts: 126




Ignore
« on: July 13, 2008, 06:10:15 AM »

Just a thought... with all the yagis I copy (70cm, 6m, 2m, soon to copy 1.25m), I omit the complicated phasing line in favor of a gamma match, simply because it's less complicated to machine, with fewer parts to corrode. BTW the standard coax connector thread = 5/8x24 NEF.
Logged
WB6BYU
Member

Posts: 13253




Ignore
« Reply #1 on: July 13, 2008, 07:17:51 AM »

"Complicated phasing line" Huh??

Do you mean a half-wave coax balun?  Or a beta match stub? Clemmens
Match?  I certainly wouldn't consider the first two to be complicated.

The only "phasing line" applications that I can think of with common
VHF/UHF beam antennas would be the line joining the two elements on
a phased array such as a HB9CV, or the one between two yagis phased
for circular polarization.  Neither of those can be replaced with a gamma
match (and, indeed, the antennas may also use a gamma match along
with the phasing stub.)


There are many different matching methods for the impedances
commonly encountered with yagi antennas.  The choices among them
are more mechanical or practical than anything else.  Those of us who
don't have machining equipment will probably choose something easier
to implement.  I've had good results with a delta match, but more
recently have been using the half-folded driven element method from
WA5VJB because it is easy to build and provides mechanical strength
by not splitting the driven element.  Both of these are easy to implement
with stiff wire antennas on PVC booms (cheap and easy to make.)  For
antennas that can't have anything attached on the elements (for example,
tape measure yagis where the elements have to be able to bend when
they hit branches) I use a beta match (though DK7ZB's 28 ohm method
would be a good alternative.)

Gamma matches have been problematic for me.  I have a simple
implementation with a single insulated wire forming the gamma rod
and then wrapped around the element to form the capacitor, and I can
get a good match.  But I have more problems with pattern skewing
when I use a gamma match than with the other balanced designs.  (For
radio direction-finding, having a reliable pattern is very important.)
For many years I have built gamma-matched quads and here are many
of them around here as a result of club projects.  Recently when I tried
modelling it with EZNEC I got strange results, so I set one up and
measured it.  Sure enough, the polarization was rotated about 50
degrees from what I expected!   Converting to a T match by adding a
gamma arm to the other side of the feedpoint solved the problem.
Logged
N8EKT
Member

Posts: 371




Ignore
« Reply #2 on: July 13, 2008, 08:48:38 AM »

Why bother?

Quad driven element is 50 ohms and fed directly with coax.

No matching loss no machine work and wide bandwidth.
Logged
WB2WIK
Member

Posts: 20595




Ignore
« Reply #3 on: July 13, 2008, 03:11:11 PM »

RE: Gamma match vs. phasing line  Reply  
by WB6BYU on July 13, 2008  Mail this to a friend!  
Gamma matches have been problematic for me. I have a simple
implementation with a single insulated wire forming the gamma rod
and then wrapped around the element to form the capacitor, and I can
get a good match. But I have more problems with pattern skewing
when I use a gamma match than with the other balanced designs. (For
radio direction-finding, having a reliable pattern is very important.)
For many years I have built gamma-matched quads and here are many
of them around here as a result of club projects. Recently when I tried
modelling it with EZNEC I got strange results, so I set one up and
measured it. Sure enough, the polarization was rotated about 50
degrees from what I expected! Converting to a T match by adding a
gamma arm to the other side of the feedpoint solved the problem.<

::I've had exactly this same problem with gamma matched yagi's, and this is exactly why I don't use them any longer.  For a halo/squalo or something, probably not an issue at all.  But for a yagi, I want the cleanest pattern I can achieve, and never get that with a gamma match.

I use "T" matches on all my yagi antennas.  Easy to build, easy to adjust, very stable, and very balanced.  And no finnicky capacitor that can drift with age.

WB2WIK/6  
Logged
WB6BYU
Member

Posts: 13253




Ignore
« Reply #4 on: July 15, 2008, 04:19:37 PM »

N8EKT wrote:
>> Quad driven element is 50 ohms and fed directly with coax.


My three and four element quads are in the 50 to 75 ohm
range, but the 2-element ones are around 140 ohms.

Sometimes there is a mechanical advantage to not breaking
the driven element, in which case a gamma, T or delta match
may be useful even if the impedance wouldn't otherwise
require it.
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!