View Full Version : Ballparking a gamma match
12-09-2003, 03:13 PM
Most new hams just want a place to start when building a gamma match. Many inquirees ask what are the dimensions for a gamma match? I have included some ball park figures and should anyone disagree, please add to the thread.
A 20 meter Yagi: gamma tube about 4 feet long. Spaced about 5 to 6 inches from driven element.
A 15 meter Yagi: gamma tube about 3 feet long. Spaced about 4 to 5 inches from driven element.
A 10 meter Yagi: gamma tube about 2 feet long. Spaced about 3 to 4 inches from driven element.
A 6 meter Yagi: gamma tube about 1 to 1.5 Feet long. spaced about 2 to 2.5 inches from driven element.
A 2 meter Yagi: gamma tube about 5 to 6 inches long. Spaced about 2 inches from driven element.
Coaxial cable RG8 with the outside jacket and shield removed, makes a good conductor well insulated to be inserted into the gamma tube. Remove some wire in the center on the end to avoid arcing. Seal well. Use seamless 1/2 inch or 3/8 inch aluminum tubing. Keep metal the same type as driven element. If 1/2 in tubing is used and you cannot get enough capacitance from the RG8, then you may have to use and 1/4 in.rod with insulating something to keep it centered. the shorting clamp slides on the end of the gamma tube for tuning. Also the RG8 cable will be moved in or out for lowest SWR. Variable air capacitors may be needed for the 20m, 15m, and 10m gamma tubes.
350 pf, 250pf and 150pf mounted in series with the RG8 will help fine tune the Capacitance of the gamma tube. Keep air variables in closed plastic waterproof boxes.If you have access to an MFJ antenna analyzer, this would be a helpful tool to let you know when you are approaching a good match. Antennas can be tuned on the ground with the reflector on the ground and the beam aimed upward. I have made all of these antennas over the years except the 20 meter beam and started out not knowing exactly the dimensions, but never encountered any hangups on the gamma matches ever. Without writing a book on the matter, you will just have to experiment with the dimensions. Lastly the non conductor used to support the gamma tube near the boom, should be made of a strong plastic or plexiglass. Often, you can look at a picture of an antenna using a gamma match and calculate its dimensions. Since I didn't include all details of construction, the feedback may have some of those. In short the gamma tube is about 20 to 25% of the 1/4 wave for that frequency.
Good luck. Paul KōYNE
12-09-2003, 03:21 PM
Correction , the variable capacitors 350pf ,250pf, and 150pf should match up with 20m, 15m, and 10 meters respectfully. Paul
12-09-2003, 04:51 PM
A very easy way of finding the starting point on Gamma match dimensions is to use the program called "Gamma" that is part of the Yagimax program suite that is available for free on this site.
Run your antenna design through Yagimax and it will give you the parameters of the feed point. Then run these figures through the Gamma program and it will give you the length of the gamma rod and the capacitance necessary to match your coax. You tell the program what the diameter of the gamma rod is, the diameter of the driven element, and how far from the driven element that the gamma rod is spaced. Also you tell the program what impedance of coax that you are using. In less than a second the program will give you the details.
You can make a capacitor from coax by running the center conductor inside the gamma rod. If the capacitance is greater than what you can accomplish by this method, then you can also make an external capacitor from a piece of coax and "roll" it up. Or, you can use a conventional variable capacitor that you house in a protective plastic box.
A variation on the gamma match is the Omega match. In this configuration the gamma rod is made 1/2 as long as normal. There is a second variable capacitor from the gamma rod to ground which has to be tuned as well as the "normal" gamma capacitor. The length of the gamma rod is not varied but you tune both variable capacitors until you get minimum SWR (or reflected power). I have used the Omega match on several yagis that I have built over the years. It works fine and is a little easier to adjust than having to adjust the length of the gamma rod and the variable capacitor. The drawback is that it requires a 2nd variable capacitor.
12-09-2003, 09:37 PM
I've got a pair of phased (vert polarization) 2m yagi's with single gamma matches. Is it worth my time to add 2 more so that I have a double gamma on each yagi. This would require splitting power 4 ways, 2 gamma's per yagi, 2 yagi's so it'd be a bunch of work. Just throwing this out there to see what I get,
12-10-2003, 11:32 PM
The "T" match is nothing like a "double gamma," and I cannot imagine why anyone would need a "double gamma" match, or what benefit it would provide.
The advantage of the "T" match is that it doesn't require any capacitor, at all. It feeds both "sides" (from center) of a dipole driven element using nothing but hardware (solid conductors, no reactive devices) and a balun and is thus less tricky to tune than a gamma, and if materials are good conductors, it's a lossless system not prone to changes with the weather.
To make it really foolproof, some VHF-UHF antenna manufacturers used T-matches with silver plated brass conductors, so once the interconnections were attached, everything could be flow soldered together to make a very permanent and weatherproof assembly. They also used UT141 rigid coaxitube (Teflon dielectric, solid silver plated copper outer conductor and silver plated copperclad steel inner conductor) for the balun, so even that was extremely weatherproof (moisture can't intrude extruded Teflon coax). Those designs lasted decades, not years, and most of them are probably still out there working perfectly.
After using all sorts of beams over the years, I try to shy away from gamma matches unless they're within reach and serviceable. For antennas up so high I hope to never touch them again, I'll use something else, like the T match.
My favorite is the hairpin match, requires insulating the driven element from the boom but no big deal as the heavy duty plastic edging from landscape stores will fit around the driven element easily ( on HF that is) then just a short jumper wire across the elements and attached to the boom and your off. I can't remember now which software program I used to get ballpark figures but I thing hairpin matches can be ballparked from the yagimax software.
12-12-2003, 07:57 AM
AAARRGGG! # Gamma Match!
# # http://www.qrz.com/iB_html/non-cgi/emoticons/sad.gif
#I'm about ready to pull my hair out! (And I got a lot of it too.)
#I've used all the gamma match software I can get a hold of, and I've created Yagi beams with the proper software for that as well. #I've even ďmanuallyĒ calculated everything to the "Unth-Degree", spending weeks on a particular design to get it just right for my purposes. . . #I've even tuned the length of the coax as well, to 50 ohms. #Using a MFJ Antenna Analyzer, and after hours of tweaking, the best I ever achieved was 1.75:1! #Thatís just Not good enough.
#Everything is measured, and re-measured, and then checked again upon installation. #Everything is "Right on the Nut!" and it just doesnít work worth a darn!! #I see descriptions of items from some folk that contradict others on the same topic. #I see some measure from center to center and others from edge to edge without clarification. #I see folk call the "Gamma Match Tube" #the "ROD", and not care one way or another if its mistaken or not. #I've even copied manufactured antennas right down to the smallest item which again resulted in miserable performance.
#I'm beginning to believe its some sort of sick curse Iím under . . #I know darn good and well that itís some very fundamental and basic thing thatís kicking me. #Something thatís so obvious to everybody else that it's simply overlooked, that is causing me these headaches. #Even checking my designs against the smith chart shows me that my design is of a very good quality and calculation, but does it actually work? #Oh HECK no!
#Is there an Elmer out there that can take me off-line, and back to the beginning to help me figure out what the heck I'm screwing up? #Itís just GOT to be something really simple, cuzí Iíve beaten all the hard stuff to rubble. . .
# # # # # # # http://www.qrz.com/iB_html/non-cgi/emoticons/confused.gif
12-12-2003, 05:12 PM
I have built antennas that the gamma match worked the very first time and I have built antennas that no matter what I did the gamma match wouldn't work!
Thus, for antennas that I use on the 50 MHz and higher bands I have gone primarily to a folded dipole driven element using a 2:1 diamater ratio in the material used for building the element. That gives just about a 200 ohm feedpoint (not 300 ohm like when the entire folded dipole is made from the same diameter tubing). Then I use a 1/2 wave (or 3/2 wave) coaxial balun to convert the 200 ohms balanced to 50 ohms unbalanced. Works great!
If you want to see a photo of my VHF beams go to http://home.comcast.net/~k9sth and the link is near the top of the list of links.