Antennas Delta and Gamma Matching
-- Ed Paradis, email on 5 Dec 2010:
Matt B raised a great question about antenna building at a recent shop night.
"What is the difference between a Gamma Match and a Delta Match?"
If you ever see designs for Yagi or "Beam" antennas, you'll see reference to these two types of "matches". The purpose of both of these is to match the impedance of the antenna to the coax feeding the antenna. Adjusting the match is one way to tune an antenna to better radiate the available energy at the frequency you're transmitting at. They have no significant purpose for receiving.
In that image, you see the two halves of the driven element going vertically with the boom in the center. In the Yagi antennas we built in the shop, the driven element was the copper wire that was bent into a U shape and the boom was the wood supporting the whole thing. You'll see in that image a smaller piece parallel to one side side of the driven element with a "clamp" on one end connecting it to the element and the other side is being fed by the center of the coax.
In these designs, your driven element doesn't have a big U shape, instead it looks just like all the other elements on the antennas we built. But to efficiently get the signal from the coax to the antenna, the energy is fed in a different place on the element. At radio frequencies, the shape of your parts matters, so this is solving the electrical problem of an impedance mismatch with a geometrical solution of connecting the coax in a different place along the element.
So you'll see gamma matches are sort of asymmetric: The little piece parallel to the driven element is only on one side. The clamp is conductive and you would tune the match by moving that clamp further or nearer the boom.
A delta match, performs a similar function. It also matches the impedance of the coax to the antenna.
Here is an image of a delta match. It is the two triangular pieces that look like braces going from the boom to two of the antenna elements. http://www.qsl.net/pa3hbb/dm39.gif ( from http://www.qsl.net/pa3hbb/DeltaM1.htm#_Toc442949240 )
In this image, you can see that the coax connects to the ends of the diagonal pieces and also uses a loop of coax at a transformer to produce a "balanced" signal. http://www.qsl.net/pa3hbb/dm38.jpg ( same site as above )
Coax is "unbalanced" because the RF energy it carries is one sided. There are two conductors, the center wire and the outer cylindrical shield, but energy travels via the center conductor and not the outer metal shielding. Antennas, on the other hand, are inherently balanced. You always have to have two halves that oppose each other. Think of the two halves of a dipole antenna. At any moment, one half will have a positive voltage and the other half will have a negative voltage. You'll see antennas that look like they only have one half, but that the other half exists. It might be the ground, your metal car body, or in bad situations, your feed coax, radio and your hand!
So the other sides of the diagonal pieces of the delta match connect somewhere to the driven element of the antenna, like a gamma match, but on both sides of the boom. Here is an image of "half a gamma match". http://www.qsl.net/pa3hbb/dm33.gif ( same site as above )
Notice how the side on the boom is insulated and the side connecting to the element is riveted so that it conducts energy.
So there you have it! Gamma matches and Delta matches are very similar and perform pretty much the same tasks. A Gamma match is super easy to build and the Delta match is a bit more complicated.
A Gamma match is unbalanced and a Delta match is balanced.
--- Mega Super Bonus Stage --- So if you built or ended up with one of the 70cm Yagi antennas we built, you might be wondering why there is no Delta or Gamma match. Or you might be wondering what the weird U-shaped thing is all about. Or you might be wondering how people build Yagi antennas out of tape measures with no match what-so-ever.
The bent piece is doing the same sort of thing that a Gamma match is doing, but instead of taking the energy out on a short feed piece, it brings the antenna all the way back around. Again, distance matters at Radio Frequency, so feeding the energy "all the way around" a half wavelength distance is significant. I can't find a good diagram of what is going on, so if you'd like me to draw you one, ask me next time you see me in the shop. This was fine at 70cm where the length of the antenna elements are short. If you were making an antenna that had 10 or 20 foot elements, that would be a significant mechanical challenge compared to a Gamma or Delta match.
The reason you can get away with no match on some antennas is that they are resonant at a wide enough range of frequencies that they do not require a match. Or the match might be "close enough" that you aren't concerned about a mismatch. The frequency you're using the antenna at might make a match so strange (tiny or huge) that it does more harm than good.
Both a $2000 state of the art camera and a $5 disposable camera will take a picture. Understanding the limitations and options will lead to great photo with either. Antennas are similar.
--- Confusing Thing at the End --- There is another type of match, called the Beta or Hairpin Match. These are different than the U-shaped thing we used on our 70cm Yagi antennas. Both are hairpin-shaped, but in the following image you can see that the hairpin match is connected across the feed point. http://www.vk1od.net/antenna/misc/BetaMatch.htm
Hairpin matches are balanced like Delta matches.
Interestingly, here is someone using a hairpin match on a tape measure antenna: http://mcars.us/test_000038.htm
Notice how in the second image, there is a piece of wire soldered vertically between the two wires traveling horizontally. Changing the position of that wire to the left or right will tune the antenna.
I hope this helps people understand a little more about the different parts of the antennas you'll see around. If you would like to know more, there are plenty of other bits to talk about; just ask or check out the ARRL Antenna book in the workshop.