Image

Building a Gamma Match for a Yagi

Help with installing an Antenna, or just choosing the right antenna to go with your radio, or your mobile.

Moderator: JR Moderator

User avatar
KAAA 8324
4 PILL USER
4 PILL USER
Posts: 49
Joined: Tuesday, 10 February 2009, 8:34 AM
Radio: 2950DX
Name: Greg
Location: Around Seattle

Building a Gamma Match for a Yagi

Post by KAAA 8324 » Tuesday, 10 February 2009, 16:40 PM

I'd like to build a 4 Element Yagi. I have a machine shop so I can make what I need to and I DO like to make things.
I got to looking around for some plans for a Yagi and ran across this page


but it doesn't go into the Gamma Match part. So I looked around some more and started to put the pieces together.
I drew up this drawing so I could ask questions.

The red is the driven element
The white square on the left is the beam and the bracket for the coax to hook up to
The yellow is an insulting arm
The White arm at the right is a non insulting arm between the driven element and the slider gizmo
The green is the tube that the blue slider gizmo slides into and it is insulate with the gray sleeve
The birght pink line is the center of the coax hooking to the gamma gizmo

Now for the questions......
Does it matter how far the gamma tubes are from the driven element?
Does it matter what the diameter of the gamma tubes are?
What should the length of the gamma tubes be?
About how much should the gamma tubes be inserted into each other?
Is there a way to get close to the proper setting with a VOM?

That's all I can think of for now but if you can think of something I should know please tell me

I tried a couple times to post a link to the picture I made but I am too new to post on this forum. If you know about Gamma Matches I can send you a link to the picture. I have it posted on one of my domains.
Image

edit by HI-TECH for pic


User avatar
drdx
Donor
Donor
Posts: 5966
Joined: Wednesday, 25 April 2007, 11:32 AM

Post by drdx » Saturday, 14 February 2009, 8:44 AM

I"ve got a beam antenna handbook, by ORR. It has a bunch of good gamma info. I've gotta run, just posting to mark it, but I can either give you some rough numbers later. It is mainly on one page but if you PM me I can fax it to you. Basically .04 to .05 wavelength(20-22 inches), on the length of it, spacing would be about 4 inches. You can use a piece of rubber and shield removed coax for the sleeve I believe(like rg213 or something), and you'll need a capacitor on there about 6 times the wavelength in meters (11 in this case) so about 77pf, or as close as you can find. I think you can use probably 3/8 or 1/2 tubing and you'll fab straps to attach it to the element. I have other element length and spacing math I can throw your way as well, but that's all over the web and dimensions on yagi's that short (less than a full wavelength) are not that critical overall as long as you are within decent parameters.

-drdx
Yes it's me, Dollar-98, drdx, the original all *Censored*, shot cawla on workin this no-fade technology.

-drdx



User avatar
KAAA 8324
4 PILL USER
4 PILL USER
Posts: 49
Joined: Tuesday, 10 February 2009, 8:34 AM
Radio: 2950DX
Name: Greg
Location: Around Seattle

Post by KAAA 8324 » Saturday, 14 February 2009, 11:18 AM

I just checked with the local library and they have a copy of The Beam Antenna Handbook by Orr, so I'm off to check it out.
Thanks for the tips

I found a used copy of The Truth About CB Antennas by Orr which I scarfed up ($5 plus shipping) I've had a few copies of this book and loaned them out, never to be seen again. I won't loan this one

User avatar
KAAA 8324
4 PILL USER
4 PILL USER
Posts: 49
Joined: Tuesday, 10 February 2009, 8:34 AM
Radio: 2950DX
Name: Greg
Location: Around Seattle

Re: Building a Gamma Match for a Yagi

Post by KAAA 8324 » Sunday, 15 February 2009, 14:15 PM

Been reading Orr's Beam Antenna Handbook and found an interesting section about building a Gamma Match on page 178

I was able to extrapolate some numbers for 11 meters.
A= 5.5689"
B= 26.4"
C= 7.04"

User avatar
drdx
Donor
Donor
Posts: 5966
Joined: Wednesday, 25 April 2007, 11:32 AM

Re: Building a Gamma Match for a Yagi

Post by drdx » Sunday, 15 February 2009, 14:46 PM

I'll look again, but those numbers are a little different than what i had, but I was in a hurry. I think your numbers are a little larger than mine, but better too long than too short.

-drdx
Yes it's me, Dollar-98, drdx, the original all *Censored*, shot cawla on workin this no-fade technology.

-drdx




User avatar
KAAA 8324
4 PILL USER
4 PILL USER
Posts: 49
Joined: Tuesday, 10 February 2009, 8:34 AM
Radio: 2950DX
Name: Greg
Location: Around Seattle

Re: Building a Gamma Match for a Yagi

Post by KAAA 8324 » Sunday, 15 February 2009, 15:58 PM

I tried to post a scanned picture of that gamma device, but I think my computer is goofy. The board is not showing up like it used to and the attached jpeg didn't make it (at least that I can see)

User avatar
KAAA 8324
4 PILL USER
4 PILL USER
Posts: 49
Joined: Tuesday, 10 February 2009, 8:34 AM
Radio: 2950DX
Name: Greg
Location: Around Seattle

Re: Building a Gamma Match for a Yagi

Post by KAAA 8324 » Sunday, 15 February 2009, 17:36 PM

Lets try this again
(I hope I get it right this time)

Image

User avatar
Dirty Harry
6 PILL USER
6 PILL USER
Posts: 54
Joined: Tuesday, 18 March 2008, 7:38 AM

Re: Building a Gamma Match for a Yagi

Post by Dirty Harry » Sunday, 15 February 2009, 23:42 PM

There's a handy program to calculate all the required dimensions for gamma matches on this site:

http://www.ve3sqb.com

I've used it a couple of time's & it's been very close (with only very minor tweaks needed)

An antenna analyser (like the MFJ 259B) would be useful if you can borrow one to see exact reactance & resistance.

Good luck & let us know how you get on. 8)

User avatar
KAAA 8324
4 PILL USER
4 PILL USER
Posts: 49
Joined: Tuesday, 10 February 2009, 8:34 AM
Radio: 2950DX
Name: Greg
Location: Around Seattle

Re: Building a Gamma Match for a Yagi

Post by KAAA 8324 » Monday, 16 February 2009, 6:09 AM

Thanks Dirty Harry
That's pretty close to what I was coming up with. I was quite worried about blowing up a radio, keying up with some terribly mismatched antenna.

I wonder if you could hook a VOM meter to your coax (hooked to your antenna) and tell if it is tuned or not? The radio wants to see 50 ohms right? So if you checked you coax and it was 25 ????

Maybe my reasoning is off

User avatar
Dirty Harry
6 PILL USER
6 PILL USER
Posts: 54
Joined: Tuesday, 18 March 2008, 7:38 AM

Re: Building a Gamma Match for a Yagi

Post by Dirty Harry » Monday, 16 February 2009, 7:55 AM

I'm no expert but it's a little more complicated than that because you're trying to measure several things which will provide a sum impedance (Z) rather than simple resistance (R)

The impedance (Z) of an element is its value of pure resistance at the feed point plus any reactance (capacitive or inductive) that is present at that feed point - according to the boffins. :icon_e_geek:

As you have no way of measuring the reactance (X) resistance (R) inductance (L) then you can't calculate or measure the impedance.

Have you modelled your antenna with EZNEC or something similar???

I have the measurements for my four element design if you're interested plus gamma details.

User avatar
drdx
Donor
Donor
Posts: 5966
Joined: Wednesday, 25 April 2007, 11:32 AM

Re: Building a Gamma Match for a Yagi

Post by drdx » Monday, 16 February 2009, 8:24 AM

An antennal analyzer, like an MFJ259 would help with that. If you're doing it on the cheap, use a junker radio to iron it out. Even then, the duration you'd be transmitting into it would be short, not like you'd use it for weeks and blow something up.

-drdx
Yes it's me, Dollar-98, drdx, the original all *Censored*, shot cawla on workin this no-fade technology.

-drdx



User avatar
KAAA 8324
4 PILL USER
4 PILL USER
Posts: 49
Joined: Tuesday, 10 February 2009, 8:34 AM
Radio: 2950DX
Name: Greg
Location: Around Seattle

Re: Building a Gamma Match for a Yagi

Post by KAAA 8324 » Monday, 16 February 2009, 8:56 AM

Have you modelled your antenna with EZNEC or something similar???


I think now is a good time to start a new thread about Yagi building


There's no more quote button


Post Reply Previous topicNext topic

Who is online

Users browsing this forum: Russ and 2 guests