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KD0BRD
10-23-2007, 05:23 PM
I am a very new ham, and have built a 5 element 2 meter yagi. However, I have no idea how to make a matching network. I think I would like to go with a hairpin (beta) match because it's the simplest in terms of materials needed.

Could someone explain to me in (very) simple terms what I would need to do the match my yagi to 50ohm RG-8 coax?

Thanks!
Isaac, KD0BRD

KA4DPO
10-23-2007, 06:14 PM
A hairpin matching arrangement will work but usually requires a 1:4 Balun.

A gama match is simple to construct and is a little more broad banded than a hairpin. You can make a gamma match using brass tubing from the hobby store and the insulated center conductor of some RG-8 coax. By sliding the coax in and out of the tube it acts like a variable capacitor while the whole assembly works like an L network.

K8CQ
10-23-2007, 06:17 PM
Isaac,

If you can get a copy of the ARRL Antenna Handbook, you will find all the info you need (e.g., in Chapt 26, page 26-11ff, of the 19th edition) about hairpin matches, how to construct, design details, etc.

My own experience would prefer making a gamma match work with minimum effort and mechanical fooling around. And as I said in my response to your earlier question on this subject, for 2m, the construction is fairly straightforward.

I didn't mention it in the earlier response, but you might also consider making your driven element a quad loop rather than a straight element. Feeding it couldn't be simpler - straight from the coax, no baluns, no matching network, etc. And I can tell you that such an antenna will perform admirably. Such an antenna is called a quagi, and there has been much posted and published on this type of antenna. I have built more than one. For 2m, the quad loop can be made from #12 electrical wire (I got mine as scrap from a building site). I simply bent the wire into a square loop, epoxied it to PVC pipe to hold it, and mounted it to the boom with one reflector loop and 9 straight driven elements. That antenna was at about 70 feet for over 15 years. When I took it down, I gave it away. I understand it is still in use 13 years later.

Good luck. 73, Jeff K8CQ

KD0BRD
10-23-2007, 08:21 PM
Quote[/b] (KA4DPO @ Oct. 22 2007,12:14)]A hairpin matching arrangement will work but usually requires a 1:4 Balun.

A gama match is simple to construct and is a little more broad banded than a hairpin. You can make a gamma match using brass tubing from the hobby store and the insulated center conductor of some RG-8 coax. By sliding the coax in and out of the tube it acts like a variable capacitor while the whole assembly works like an L network.
OK, this sounds like it would be pretty easy. Do I attach the brass rod to the driven element? How long does the rod and coax need to be?

Thanks for the responses.

Isaac

KC7YPJ
10-23-2007, 09:29 PM
simplest method of matching would be making the driven element a dipole

KD0BRD
10-23-2007, 10:11 PM
Isn't the driven element already a dipole?

Isaac

KA0GKT
10-23-2007, 10:24 PM
Quote[/b] (KC7YPJ @ Oct. 23 2007,14:29)]simplest method of matching would be making the driven element a dipole
The driven element of a Yagi IS a dipole, the problem is that in a parasitic array like a Yagi, the feedpoint impedance, most times, is very low along the order of a few Ohms and doesn't match the 50-Ohm impedance of common coaxial transmission line.

The biggest problem with a hairpin or beta match is that the elemsnt must be split at the center and therefore isn't useful with plumbers' delight methods of construction. A "T" match along with a 4:1 balun is an easy to implement method, and is covered in great detail in the ARRL antenna book. In my 14th edition, it is described on page 5-14, and again on page 11-11 where dimesions for a 70 cm yagi are illustrated along with details for the "T" match.

Gamma matches are also fairly easy to build, but IM (not so) HO, the "T" match can be much easier to implement for the first-time antenna builder.


73 DE KAGKT/7

--Steve

KA4DPO
10-23-2007, 10:34 PM
Quote[/b] (KD0BRD @ Oct. 23 2007,15:21)]
Quote[/b] (KA4DPO @ Oct. 22 2007,12:14)]A hairpin matching arrangement will work but usually requires a 1:4 Balun.

A gama match is simple to construct and is a little more broad banded than a hairpin. #You can make a gamma match using brass tubing from the hobby store and the insulated center conductor of some RG-8 coax. #By sliding the coax in and out of the tube it acts like a variable capacitor while the whole assembly works like an L network.
OK, this sounds like it would be pretty easy. #Do I attach the brass rod to the driven element? #How long does the rod and coax need to be?

Thanks for the responses.

Isaac
Isaac, The rod needs to be about 7 inches long but you will need a sliding connection to the driven element.

The way it works is, the the sliding capacitor tunes out the inductive reactance while the slider on the rod is adjusted for the best match. Just slide the insulated coax into the rod about two or three inches to start and adjust the slider between the rod and driven element for the lowest SWR at center frequncy. Then adjust the capacitor and repeat the process until you get the best match.

WB2WIK
10-23-2007, 10:41 PM
Hairpin match for 2m is pictured and discussed here, among many places I found using Google in about two seconds:

http://home.att.net/~jleggio/projects/rdf/tape_bm.htm

WB2WIK/6

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