[hackpgh-ham] Re: Antennas: Delta match versus Gamma match, Mega Super Bonus and Confusing Thing at the End
- From: Matt Stultz <matt@xxxxxxxxxxxxxxxxxx>
- To: hackpgh-ham@xxxxxxxxxxxxx
- Date: Mon, 6 Dec 2010 16:23:17 -0500
Not to mention well bearded!
On Mon, Dec 6, 2010 at 4:06 PM, Matthew Beckler <matthew@xxxxxxxxxxxx>wrote:
> Again, thank you Ed for the excellent explanation. If only my professors
> were so timely, un-forgetful, and helpful as your have been.
>
> --
> Matthew
>
> On 12/05/2010 11:16 AM, Ed Paradis wrote:
> > 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.
> >
> > A gamma match looks like this:
> > http://www.hamuniverse.com/n5wvxgammamatch1.GIF
> > ( from this article: http://www.hamuniverse.com/n5wvx2m7eleyagi.html )
> >
> > 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.
> >
> > Ed
> > KE5AYE
> >
>
>
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