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View Full Version : Direct Connect to Element -VS- Using a Gamma-Match



mcjim555
11-05-2005, 05:53 PM
Hey there Guys,I have been tinkering with and building antennas for about 6 or 7 years now. I've built both Base and Mobile (mostly base) antennas, from simple Dipoles and Verticles to One five element horizontal Yagi. I have never used a Gamma-Match, I've always just used a split element direct connect on the Yagi and Dipole, because I wasn't sure about tuning the antenna and a little nervous that I wouldn't get it tuned properly. I don't know if it's that I've become more curious or maybe I'm starting to gain some Testicular Fortitude about this subject. So which is better or more efficiant and how hard is it to get a gamma-match set up properly. Thanks Guys and 73. Jim http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif http://www.qrz.com/iB_html/non-cgi/emoticons/rock.gif

KC0NJA
11-05-2005, 06:38 PM
I build direct cut antennas myself.. As far as gamma-match goes I've found the direct cut to be much better.. placing gamma-matchs would reduce the gain on my antenna.. Made antennas for 2 meters and 6 meters Hope this helpes KC0NJA JIM

KE7ENC
11-05-2005, 06:45 PM
My 6 meter beam was built to frequency. I did not need a match to make it work per se. I did make several coax loops (balun) to lower the -j to 7 ohms. I have not built a big frequency range yagi yet. Now a gamma or T match may have doen a better job, I don't know, but what I've done has worked for me.

K9STH
11-05-2005, 08:01 PM
NJA:

A properly constructed gamma match (or "T" match, or omega match, or hairpin match, etc.) is not going to have any effect on the "gain" of your antenna. As for the effective radiated power use of those types of matches over an insulated "split feed" they might even help very slightly.

The gain of the antenna has to do with the physical dimensions (as well as the front-to-back ratio) and has nothing to do with how the driven element is fed.

For VHF I often use a folded dipole arrangement with a 1:2 ratio on the diameter of the halves of the driven element. This gives an impedance pretty near 200 ohms instead of the "normal" 300 ohms for a folded dipole. Then I use a 4:1 coaxial balun to match to 50 ohm coaxial cable. This works very well and a folded dipole has a somewhat broader bandwith than the more normal single type of element.

Glen, K9STH

WB2WIK
11-06-2005, 04:01 AM
A reason to use a gamma match, or any type adjustable matching device, is that when building beam antennas (like Yagis), the resonant driven element won't be 50 Ohm impedance when the parasitic elements are optimized for maximum gain.

In fact, the driven element impedance is usually quite a bit lower than 50 Ohms. So, if you direct-feed your driven element, you cannot possibly have a good match -- unless you make adjustments to the driven element length specifically to adjust for a match, in which case you're making it the wrong length to achieve other goals.

Another reason to *not* "direct feed" a driven element is there will always be some skewing due to using an unbalanced feedline (coax) to a balanced load (dipole). This actually *reduces* antenna gain because without a good balun, you're not exciting the parasitic elements equilaterally for optimum gain.

People who build and sell commercial yagis don't use matching devices such as gamma matches, beta matches, t-matches and so forth for their health: They use them because they make the antennas work better.

A gamma match is simple to build and simple to adjust, and has pretty broad matching range; so if your driven element really has a feedpoint Z of 32 Ohms and not 50 Ohms, the gamma can adjust for that and create a "perfect" match for your feedline, without creating other system problems. An advantage of the gamma match, and one reason why it's so popular, is that the driven element needn't be isolated from the antenna boom when using one. This makes yagi construction simpler, and less critical.

WB2WIK/6

mcjim555
11-06-2005, 02:55 PM
Hey thanks for the replies Guys, a couple of things though.
one: can a gamma-match be used on a quad antenna such as a two element quad beam, (that's my next project).
two: a gamma-match is basically a type of capacitor, is that correct ? and if so how would you figure out how much capacitance a given gamma-match has ?
Sorry if I'm being difficult in my questions, I just trying to understand the theory behind all of this.
Thanks again Guys and 73 Jim http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif

WB2WIK
11-06-2005, 03:57 PM
No reason to use a gamma match on a quad loop.

A quad loop impedance varies with length and spacing to its parasitic elements, and on VHF it's pretty easy to achieve about 50 Ohms without any matching device at all. The natural impedance of a resonant loop is higher than 50 Ohms (usually about 100 Ohms), in which case it may be fed through a 1/4-wave matching section of 75 Ohm coax to achieve a match. This is easy and efficient. No reason for a gamma match.

I'd suggest you follow the well established designs for quads and just copy one of those. L.B. Cebik's website has some, and of course they're also in the Antenna Handbook and many other recognized references. I doubt you'll see gamma matches there.

For that matter, you won't see gamma matches used on simple dipoles (that aren't beams), either, because there's just no reason to use one there when a simple current choke can be made very broadbanded to accomplish the same purpose.

WB2WIK/6

KA4DPO
11-06-2005, 04:35 PM
Quote[/b] (WB2WIK @ Nov. 06 2005,08:57)]No reason to use a gamma match on a quad loop.

A quad loop impedance varies with length and spacing to its parasitic elements, and on VHF it's pretty easy to achieve about 50 Ohms without any matching device at all. #The natural impedance of a resonant loop is higher than 50 Ohms (usually about 100 Ohms), in which case it may be fed through a 1/4-wave matching section of 75 Ohm coax to achieve a match. #This is easy and efficient. #No reason for a gamma match.

I'd suggest you follow the well established designs for quads and just copy one of those. #L.B. Cebik's website has some, and of course they're also in the Antenna Handbook and many other recognized references. #I doubt you'll see gamma matches there.

For that matter, you won't see gamma matches used on simple dipoles (that aren't beams), either, because there's just no reason to use one there when a simple current choke can be made very broadbanded to accomplish the same purpose.

WB2WIK/6
Actually, you can use a gamma match on a dipole to match a high impedance feed line same as a Y match.

I have built a bunch of quads for two meters. A one wavelength loop is around 110 ohms but placing parasitic elements closer than .25 wavelength will lower the feed point impedance to around 50 ohms depending on spacing and proximity of other objects.

I highly recommend a 1:1 choke balun since a loop is a balanced antenna. It will work without a balun but the pattern ( front to back and gain ) will be messed up. Also, quad antennas have a very narrow angle but deep side null which can be usefull for nulling unwante signals.

KL7AJ
11-07-2005, 04:52 PM
The real advantage of a gamma match is that you don't have to cut the driven element...it makes for a much stronger antenna...especially those of the more generous in proportion.

Eric

KE7ENC
11-07-2005, 05:04 PM
Not to rile anyone but isn't "Direct Connect" a form of match as it is in reality a "Y" match but probably placed haphazardly over planned and optimized. If the goal is i=50 and j=0 if the "Y" match accomplishes this isn't it then matched? Granted getting i=50 and j=0 is fairly tough but once attained no further matching is neccesary, right??? Basic questions from a newbie.

WB2WIK
11-07-2005, 05:33 PM
Quote[/b] (KE7ENC @ Nov. 07 2005,10:04)]Not to rile anyone but isn't "Direct Connect" a form of match as it is in reality a "Y" match but probably placed haphazardly over planned and optimized. If the goal is i=50 and j=0 if the "Y" match accomplishes this isn't it then matched? Granted getting i=50 and j=0 is fairly tough but once attained no further matching is neccesary, right??? Basic questions from a newbie.
Not really.

A "Y" match, or Delta match, is a linear transformer that isn't a balun at all, so it's normally used with twin lead or ladder-line, not coax. It's an inappropriate application here.

A "T" match which uses a balun along with the sliding, adjustable "T" taps, is a very appropriate application here. But then, so is a simple Gamma match (for connecting coax to the driven element of a Yagi).

A gamma is very silly for a quad because it's more complex and heavier than anyone needs to accomplish the task.

The 50 +/- j0 objective, while downright noble, isn't important, either. It cannot be achieved over any bandwidth and represents a single-frequency match point and thus isn't very ham radio friendly. Actual performance isn't predicated on meeting this objective.

WB2WIK/6

KA0GKT
11-07-2005, 10:16 PM
Quote[/b] (KE7ENC @ Nov. 07 2005,10:04)]Not to rile anyone but isn't "Direct Connect" a form of match as it is in reality a "Y" match but probably placed haphazardly over planned and optimized. If the goal is i=50 and j=0 if the "Y" match accomplishes this isn't it then matched? Granted getting i=50 and j=0 is fairly tough but once attained no further matching is neccesary, right??? Basic questions from a newbie.
They're all related...sort of.

The "Y" match which you describe is actually a "Delta" match. #The delta match is used to match a high impedance feed (like 600-Ohm ladder Line) to the low impedance antenna.

The "T" Match is a decendent of the delta match. #If you feed a dipole at the center by splitting the element, the impedance is approximately 75-ohms. #If you feed an antenna as a folded dipole with equal sized parallel elements, the impedance is approximately 300-Ohms or a 4X step up. #as you move the place where the fed element connects to the driven element symetrically, step-up ratios of anywhere from 1X to 4X may be realized.

Just as a folded dipole can be designed with unequal size elements, so can a "T" match. #In the instance of a folded dipole, Z ratios of 2:1 to 15:1 are easily realized. #By varying the sizes of the split element and the driven element, plus the spacing between the split and driven elements, it is possible to match 300-Ohm, 400-Ohm or 600-Ohm ladder line to just about any antenna, even those with a normal split driven element impedance of less than 20-Ohms.

The Gamma Match is basically half of a "T" match with a variable capacitor to help get a match. #The advantages to the Gamma Match are:

1) It takes an unbalanced feed and matches it to a balanced antenna.

2) The antenna may be assembled using plumbers delight methods and is very strong.

3) #Insulators for split feed antennas often break-down electrically or mechanically.

4) Gamma matches are far easier to adjust than Delta (or "Y") matches.

Yes, you are right, once a match is made, assuming nothing changes, no further adjustments need be made.

The delta match is difficult to put together since, generally it is done with open wire line of one sort or another. #The distance between the splayed ends of the ladder line and the vertical distance from the matched antenna to the point where the feed lines cease to be parallel tend to be determined by experimentation. Too many random factors exist in the design of the typical delta match to be easily quantified.

Formulas exist for the design of "T" matches and Gamma Matches which will get the antenna experimenter into the ballpark dimension-wise, hense the prevalence of these types of matches in commercially built antennas.


73 DE KAGKT/7

--Steve

mcjim555
11-08-2005, 03:23 AM
Maybe "Direct Connect" was the wrong term for me to use, but I didn't know what else to call it. I appologize if I confused anyone, I figured that since when you put a dipole or split element antenna together you can "Connect" the Feedline "Directly" to the element as compared to using a gamma-match where the Feedline goes to a capacitor before the energy is radiated through the element. Just call me confused, hi hi. Thanks for all of the replies guys, I think that I'm beginning to get the picture now. 73 Jim http://www.qrz.com/iB_html/non-cgi/emoticons/rock.gif http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif

VE2NSM
11-08-2005, 02:34 PM
I was never lucky with my attemps to build gamma matches. On all the beams I built, several 10 and 6 meters, I used beta matches with the driven element slighly shortened to achieve an impedance of 200ohms. The hairpin cancels the capacitive reactance (due to the short DE) and a simple 1/2 wave coax is used as a balun transformer.

Of course, it's not a "plumber's delight" construction but I learned to work around it and the element integrity is not a problem at all.

K0RGR
11-08-2005, 02:59 PM
The center of a dipole element always has the lowest impedance. When you put it in a yagi configuration, that impedance will be much lower than 50 ohms. So, you really want to move the feedpoint away from the center, out toward one end of the element, where the impedance is higher. That's exactly what the gamma match does.

Oddly enough, if you build an all-metal delta loop antenna, which is another form of full-wave loop antenna like a quad, a gamma match is a convenient way to feed it, though it can be interesting finding the 50 ohm point.

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