View Full Version : Gamma Match?
02-11-2004, 12:09 AM
I was wondering what exactly gamma match is? I was looking over some plans for a 2 element 10 meter beam and part of the construction called for a gamma match. The finished driven element looked to me like a j-pole turned on its side. Is that basically all it is? I've built a two element 2 meter beam once and basically all it consisted of is a dipole made of tubing and the director made of tubing.
I was just hoping someone could help clarify this a little for me.
02-11-2004, 12:20 AM
A gamma match taps a point on one side (only) of the center of the driven element and connects the feedline to that point through a (usually variable) capacitor. There's no particular advantage to this matching system (in fact, it's about my least favorite method of connecting a transmission line to a beam antenna), but it does allow the center of the driven element to be directly grounded to the antenna's boom, making driven element assembly a bit easier.
If you're following a construction article that discusses a gamma match but provides no details about how to make one, I'd pick a different article! A complete construction article includes all details, including the gamma match design, components and dimensions, and there are zillions of them available in dozens of antenna books.
02-11-2004, 01:02 AM
http://www.qrz.com/iB_html/non-cgi/emoticons/wow.gif9--></span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (k3who @ Feb. 10 2004,17http://www.qrz.com/iB_html/non-cgi/emoticons/wow.gif9)</td></tr><tr><td id="QUOTE">I was wondering what exactly gamma match is? I was looking over some plans for a 2 element 10 meter beam and part of the construction called for a gamma match. The finished driven element looked to me like a j-pole turned on its side. Is that basically all it is? I've built a two element 2 meter beam once and basically all it consisted of is a dipole made of tubing and the director made of tubing.
I was just hoping someone could help clarify this a little for me.
Are you referring to this article?
I built this antenna, and it turned out great. The gamma match described gets its capacitance from inserting the gamma rod through a section of acrylic tubing that has been inserted into a short length of larger diameter aluminum tubing. By feeding the signal through the "capacitor" created, the RF signal can get through without creating a DC short to ground, and it allows the fed element to remain in one piece, which makes for a more structurally sound antenna that is easier to assemble.
I wish the author had provided his sources for determining the capacitance of the "capacitor" he created by stating the formulas used and the dielectric constant of the acrylic tubing. But, it wasn't necessary for constructing this antenna, as long as you just follow his measurements.
Scaling the design for another frequency would be difficult, though, without those formulas and specifications. I think about 60pF to 70pF is called for in this case.
02-11-2004, 02:35 AM
If you download the yagimax program suite from this site, there is a program called "gamma" that will tell you all of the information that you need to know about making a gamma match.
You first run your design through the yagimax program and then write down the feed parameters that are given for whatever frequency you desire. You then decide what diameter of gamma rod that you wish to use (a good diameter is 1/2 that of the driven element) and how far away from the driven element that you want to place the rod (for 10 meters 3 or 4 inches is a good figure although you can use something different if you wish). You then "plug in" these plus the figure that you copied from yagimax and set the impedance of the coax that you want to use (usually 50 ohms, but you can use 75 ohms - TV type coax - or some other "off" value). In a few microseconds the gamma program will give you the length of the gamma rod and the value of the capacitor that needs to go in series between the center conductor of the feed line and the gamma rod.
Normally one uses a variable capacitor of some type for the gamma capacitor that can be varied around the figure given by the program. The program is very accurate, but you always need to have some method of varying things just in case your measurements are not exact (and they usually will be off slightly!).
There is also a variation on the gamma match called an "omega" match. When using the "omega" match you make the gamma rod 1/2 the length normally called for. Then, you insert a second variable capacitor from the end of the gamma rod to ground (the boom of the yagi) where the coaxial cable connects. The value of this capacitor should be slightly larger than the gamma capacitor. You then adjust both variable capacitors for best reflected power (or "SWR") at the desired frequency.
I have used both gamma and omega matches for antennas from 20 meters up through 6 meters. I usually use a folded dipole feed for 6 meters and above. When you use a 2:1 ratio between the diameters of the halves of the folded dipole the feed point impedance is very close to 200 ohms. You can then use a coaxial fixed 4:1 balun and feed the antenna with 50 ohm coax.
If you look at the antennas on my "short" tower on
you can see the folded dipole feeds.
02-11-2004, 08:48 PM
I read in your post that the gamma match was your least favorite of choices for connecting coax to the antenna...this has made me curious and so I thought I would ask what your most favorite choice of match that you prefer is. The only problem with gamma's that I have seen thus far, is the fact that they change the lobe shape ( I think I said that right). I prefer a more balanced match, which for that I use a T-Match. Also, the clemens match is a "neat" way of accomplishing what the gamma and t's do. http://www.qrz.com/iB_html/non-cgi/emoticons/smile.gif
02-11-2004, 09:08 PM
Theoretically a gamma match changes the pattern very slightly. In the "real world" it doesn't change it enough to really tell!
02-11-2004, 09:28 PM
Thank you for the information Glen http://www.qrz.com/iB_html/non-cgi/emoticons/smile.gif I thought it slightly messed with the shape of the side lobes of the antenna but I was not completely sure of that. Thanks for the clarification. http://www.qrz.com/iB_html/non-cgi/emoticons/smile.gif
02-11-2004, 09:48 PM
I doubt you could tell the difference in radiation pattern whether using a T-match, a gamma match, a delta match, a beta match, etc.
I'm not wild about the gamma match only because at VHF-UHF, it's difficult to make one that maintains very low loss for very long (period of time). Most (not all) gamma match construction involves using questionable dielectrics in an uncontrolled fashion, then using aluminum materials clamped to each other at the gamma tap point. Making a folded dipole out of copper tubing, so everything can be soldered together permanently, then using a well-sealed coaxial balun is almost sure to maintain performance over time in a better fashion. A well-engineered T-match has the same attributes.
02-11-2004, 11:02 PM
Thank you for the reply WB2WIK...I was just curious as to what your preference was. I have noticed that for antennas such as for eme the T-Match is used more commonly than a Gamma match is. I take it that this is because of the loss factor and the performance of the T-Match.
02-11-2004, 11:19 PM
I think the reason you'll see T-matches used more on e.m.e. arrays and such isn't so much because it's a superior matching device as much as it's the preference of the antenna makers who design and build the highest performance VHF-UHF antennas, in general.
M2, Directive Systems, K1FO, EME and other manufacturers of very high performance (read: big, and high gain) VHF-UHF antennas choose to use matching devices known for handling substantial power in all kinds of weather, over a long period of time. They can charge more for their antennas, because the users know they're better.
A very well-built gamma match is probably as good as anything else, but it's not so easy to make a very well-built one.
There's a famous experiment, an actual field test done on an antenna range, that occurred back in about 1976 at the Central States VHF Conference that I'll never forget. CSVHFS always held an antenna gain competition, to see who could build (or even just bring along a commercial version) the highest gain antenna for each VHF and UHF band, and they set up a range to measure results under very controlled conditions.
One fellow entered a standard Cushcraft 11 element yagi using the Cushcraft "Redi-Match," which is a gamma match. The antenna was brand new and shiny, and properly adjusted, and matched perfectly. It measured something like 10 dBi gain.
Wayne, N6NB, took that same antenna, removed the driven element and reflector, and bolted in their place a set of Quad elements (homebrew) made of #12 gauge copper household electric wire, with direct-feed (no matching device of any kind) to the driven element loop via an SO-239 UHF connector soldered to the wire.
Then, they measured that same antenna with this minor change, that took all of five minutes to assemble: It was now over 13 dB gain, using the same equipment. A 3dB improvement achievable in 5 minutes, almost entirely due to a more efficient driven element.
That's a lot of improvement for 5 minutes and about $2 worth of materials. This sent a strong message to nearly everyone who was there that the "Redi-Match" leaves a lot to be desired.
</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (K9STH @ Feb. 11 2004,17:08)</td></tr><tr><td id="QUOTE">Theoretically a gamma match changes the pattern very slightly. #In the "real world" it doesn't change it enough to really tell!
Glen, K9STH[/QUOTE]<span id='postcolor'>
The lower the ratio of a Yagi beam's element size and length to that of the gamma match, the more pronounced and measurable is the effect of this distortion. It is of little consequence at 50 MHz and below, but much more significant at UHF and above.
It is usually best to avoid gamma matches for amateur VHF/UHF single Yagis, above 6 meters. However, in certain "stacked" Yagi arrays, where antenna phasing becomes important (thus orienting feedpoints in opposition to one another), gamma feed "skewing" is usually negated.
Better-balanced feedpoint configurations include T, Delta, and Hairpin matches (to name a few), but the drawback to some balanced feed systems is the requirement for an isolated, sometimes divided, driven element. The chief advantages of using a gamma feed, are simplifying driven element design and eliminating the need to insulate the driven element from the boom, improving the Yagi's mechanical strength.
One way to reduce a gamma match assembly's parts count and complexity, is to use a piece of coaxial cable (with its jacket and shield braid removed) as the feed capacitor. The material surrounding the center conductor becomes the capacitor dielectric, and a piece of aluminum tubing serves as the static tube.
This tubing should be chosen in a size that allows for a close fit to the dielectric, while still easily slipped-over it. The adjustment to the gamma capacitor is then made by pruning the coaxial cable core that extends into the static tube, until the correct value is reached. This method eliminates the need for an exposed feedpoint connection, separate static tube, dielectric sleeve/spacers, gamma rod, and one mechanical support.
02-12-2004, 12:35 AM
CushCraft has used the "inside the gamma rod" type of gamma capacitor for decades including their HF, VHF, and UHF yagis. I have also used this when practical. However, sometimes there is a need for more capacitance than you can get with the "inside the gamma rod" capacitor. You can also make a capacitor out of coax. 50 ohm coax has a capacity of usually between 25 and 28 pf per foot. By using a program that gives the value of the capacitance needed to match the gamma rod you can create a capacitor by calculating the amount of coax needed and then adding a little for "trimming". Just "roll up" the coax capacitor and tape it to the boom. Works great!
When the length of the gamma rod is sufficient, I usually remove the shield from a length of coax that is just a little long (again allowing for "trimming") and connect the shield to the boom and insert the center conductor into the gamma rod. Then there isn't that much to weatherproof the connection.
As for the pattern being "skewed": Frankly, I haven't found that to be true on the gamma matched 2 meter, 1.25 meter, and 70 cm antennas that I have used. However, I have found it easier to get a good match by using the folded dipole with a 2:1 diameter ratio between the two halves of the driven element. This usually gets the feed point impedance pretty close to 200 ohms and then a 4:1 balun made from 50 ohm coax matches the feed to 50 ohms. By using a folded dipole you still can use the "plumbers delight" type of construction with the center of the larger side of the folded dipole connected directly to the boom.
Also, a folded dipole usually has a broader frequency response (lower reflected power or "SWR" for a wider range of frequencies).
I have a 27 element 432 MHz yagi that was manufactured by J-Beams Limited in the U.K. that has a 4 element reflector (these reflectors are mounted in the horizontal plane but are on a vertical mast at the rear of the antenna) and a folded dipole driven element. For a commercial antenna it works very well. However, the boom is long (requires a secondary set of boom to mast supports) and the pattern is very narrow. But, when you get it "aimed" correctly it does work!
02-14-2004, 03:56 AM
WOW!!! Thanks for all the replys folks. I'll admit, most of it was over my head but I get the gist of the whole thing. Esp the bit about the gamma match being used as a capacitor.
</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (KI4DOK @ Feb. 10 2004,21:02)</td></tr><tr><td id="QUOTE">The gamma match described gets its capacitance from inserting the gamma rod through a section of acrylic tubing that has been inserted into a short length of larger diameter aluminum tubing. # # By feeding the signal through the "capacitor" created, the RF signal can get through without creating a DC short to ground...[/QUOTE]<span id='postcolor'>
The purpose of the capacitor in a gamma-feed system, has nothing to do with avoiding a "short" to ground.
The capacitor's sole purpose, is to oppose unwanted inductive reactance presented by the gamma match. It is 1/2 of a series-tuned LC circuit between the coaxial cable's center conductor and the gamma's connection point at the driven element.
Electrically speaking, the center of a resonant dipole is at zero voltage potential. Therefore, it is of no consequence to RF, and is not "seen" by RF as a "short". The entire issue of whether or not to separate the halves of a dipole, or to leave it intact through its center, AND whether or not to insulate it from the boom of a Yagi, is dictated by the type of feedline used and the matching system necessary to couple the feedline to the antenna.
Yagi antennas are not the only ones that lend themselves well to the gamma match; delta-loop antennas can also utilize "plumber's delight" construction and be fed easily with a gamma match.