Gamma match really necessary?

[W4LKR]

I'm a beginner building a portable dual-band Yagi for 2m/70cm to work the satellites with a handheld radio, similar to the one Arrow sells. I've read several articles and how-to guides, some of which say to install a gamma match and others do not. I'm getting the impression the gamma match is helpful but not required and that an antenna that lacks a gamma match will still be a huge improvement over the rubber duck on my handheld.

What I have not found is a good article explaining how to determine the dimensions for a gamma match and install it to your antenna.

Should I bother with a gamma match or not?

 

[KC9Q]

You will likely need some form of matching for your yagi, and the gamma match is probably the easiest method for matching. A matching network is needed as the native impedance of a yagi is somewhere between 12 to 200 ohms, depending on the number of elements, and element spacing. You can search the internet to find dimensions for a gamma for 2 Meters and UHF. There are other methods such as a hairpin match, a T-match, a delta-match, etc. If you have a copy, or can get one, of the ARRL Antenna Book, there is information in there about all the different matches available.

The only yagi-type antenna that is 50 ohms (or very close to) is the two-element Moxon. That antenna can be fed directly with 50 ohm coax.

Hope that helps.

73,
Mike

 

[W4LKR]

Thanks Mike. The 2m is a 3-element and the 70cm is six-element. I'm using the patterns laid out in this article.

I'll if I can order a copy of that book from Amazon. I had hoped there might be a good online article but I haven't one that's on a beginner level yet.

 

[W4LKR]

After a lot of digging and studying diagrams I now understand the gamma match.

Last question - what device do I need to tune the gamma match? Directional watt meter? Can you recommend one that will be adequate to tune a wide range of antennas so I can keep using it as I grow into HF?

 

[W4LKR]

Thanks again to Moleculo who explained via PM that a half-folded dipole can be used for the driven element and provide some matching in lieu of a gamma match. That explains why all the patterns shown in the PDF article I linked (and all of the dula-band yagis I've seen online with no gamma, beta, or other kind of match whatsoever) are all using folded dipoles for the driven element. Duh.

Surely it is not that simple though. There must be some considerations that affect the matching behavior of the folded dipole, or a way to get the most out of it?? For example, this write-up indicates that the feedpoint impedence of the half-folded driven element is about 150 ohms in free space, but the additional parasitic elements bring it down to the desired 50 ohms.

How do I apply this to my dual-band yagi project? Is there a formula I can use to determine how many parasitic elements I would need for each band in order to approach 50 ohms on the driven element? What other variables affect the final impedence of the half-folded driven element?

I'll just focus on the 2m array for now. Currently my plan is to build it as follows:

2m yagi, 3 elements:

Reflector - 41.0" long

DE - 38.5" effective length after being half-folded (actual length 57.75") - spaced 8.5" from reflector

D1 - 37.0" long - spaced 20.0" from reflector

 

[Moleculo]

How do I apply this to my dual-band yagi project? Is there a formula I can use to determine how many parasitic elements I would need for each band in order to approach 50 ohms on the driven element?

I don't know the answer to that, but maybe someone else here does?

Last question - what device do I need to tune the gamma match? Directional watt meter? Can you recommend one that will be adequate to tune a wide range of antennas so I can keep using it as I grow into HF?

An analyzer of some type is your best bet. They're a couple hundred bucks, but worth every penny if you like rolling your own antennas. The MFJ 269 covers HF through 500Mhz, so you will get a lot of use out of it.

 

[KC9Q]

If you can swing it, get an MFJ-269. This unit covers 1.7 to 170 MHz continuous, plus 415 to 470 MHz in the UHF band.

The impedance of a full half wave dipole or a half folded dipole is dependent on the element diameter, the spacing between the elements, plus the dielectric (k) of the material between the elements. Note that air has a k=1, any material (insulation will bring the k down below 1). Generally the smaller the spacing between wire, the lower the impedance, and the greater the spacing, the higher the impedance. Notice the differences in parallel transmission line: 300 ohm line is about 1/2" spacing; 450 ohms is a bout 1" spacing, and 600 ohm line is about 2.5" spacing.

And as you have noticed: placing elements alongside a dipole will affect the load impedance. I still suggest getting the ARRL Antenna Book: It will explain a lot.

Mike