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  #1  
Old 02-03-2008, 08:37 AM
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Thumbs up homebrew 70cm oscar yagi

First I like to thank the following.
N4CZT Doug, For getting me involed in Amateur satellite.

These guys helped me make my mind up on what I wanted to do.
K9STH Glen
KA4DPO John
WA9SVD Larry
KL7AJ Eric

W9GB Gregory, For helping me with some vital information


I been searching for months now trying to find a good design for a circular polarized 70cm yagi. All I could find is bits and pieces.
I took those bits and put them all together and this is the results.
This is a 10 X 10, total 20 element yagi.

Will edit this post as it moves along.
These first four pics shows how the boom is drilled.
I made a simple jig to hold the boom on the milling table.
Mark boom on two locations (vertical and horizontal). You and lay the boom in a piece of long angle iron to get it straight.

Align the bit with the mark and your hole will be at 90 degrees each time.

My boom is 1 inch x 72 inches.
All elements are 3/16 except driven. It will be 3/8"
Elements are insulated from boom. To reduce noise.
Driven element will have "gamma match"

Element length:
Ref- 12 15/16
drv- 12 3/4
D1- 11 15/16
D2- 11 13/16
D3- 11 5/8
D4- 11 9/16
D5- 11 1/2
D6- 11 7/16
D7- 11 3/8
D8- This one in still in question. 11 5/16 would be the next down step. 11 15/16 would blunt the pattern alot to take the sharpness off. 11 1/2 may be the better choice.

Spacing:
Ref= 0
Drv= 0-5 3/4
D1= 0-8 1/2
D2= 0-13 1/8
D3= 0-18 3/8
D4= 0-25 1/8
D5= 0-32 5/8
D6= 0-40 3/4
D7= 0-49 1/2
D8= 0-58 1/2

Horizontal plane will mount first.
Vertical plane will be shifted forward by 1/4 wave length.

Phasing harness will be two 75 ohm cables 1/4 wave length each.
Most antennas are on the same plane. You have to install a 1/4 wave length jumper to get circular polarization. You feed it 90 degrees out of phase.
Since the vertical elements at shifted forward 1/4 wave length (90 degrees electrical in space), the phasing harness is equal on each side, should give identical transformation of antenna impedance.
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Old 02-03-2008, 08:44 AM
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After the boom is marked and drilled it time to think about mounting the elements.

The elements are insulated from the boom.
I could had ordered the insulators but I am thinking, this is home brew. Lets use and alternative.

I went to the hardware store and bought some number 10 wall anchors. These are the blue ones that you insert in a hole in sheet rock.

I cut the ends off and was left with a piece about 3/8 long.
These had 3 runners along the site that keep them from twisting in the wall. I cut these smooth at the mounting edge, left the rest on.

If you do this right, it will hold the element firmly with out anything else. If not you will have to find some push nuts.
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Old 02-03-2008, 06:07 PM
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.
Now for the driven element.
Remember this one is 3/8 in diameter.
What I did was get some old tubes the end mill bits come in. See you local machine shop. You want the caps also.

Take the element and mark center. 1/4 inch on either side of center wrap electrical tape around it a few turns. Take the tube and cut the bottom off. Mark center of tube. If you have enough tape around element, the tube should slide over element and fit very snug.

The hole in the boom for the driven element should be just over sized from 3/8. Use a reamer to element with insulated tube to fit. It will be tight.

Once element is in and centered cut the red cap from tube so you have 2 pieces. Slide these over element to fit snug against boom.
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Old 02-03-2008, 06:25 PM
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Now for the gamma match.
What I used was a piece of element from a TV antenna.
This is 3/8 of an inch. Cut 2 pieces 3 3/4 long. Now grab a piece of RG-8 about 8 inches long and remove the outer cover. Remove braid.
Mark end about 3/8. Then from that mark measure 2 3/4.
Cut material so the 3/8 of the center conductor is exposed.
Slide the coax into the 3 3/4 tube. You now have a 70 cm gamma.

Shorting bar.
You now need an aluminum strip about 1/2 inch wide by 3 1/2 inch long.
Cant find any? I used an old aluminum yard stick.
You will have to fabricate this so that it will go around both the element and the gamma tube. Plus enough to put two bolts in. The spacing from element center top gamma center should be 1 3/4 inch.

You will also need a piece to hold the connector. I used a S)-239 for the time being but this should be a N-type connector.
I will change these out later after testing.

Sorry about the last pic of the shorting bar. I must had sneezed when taking that one.
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Old 02-03-2008, 06:31 PM
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Now the end results. First pic shows the gamma completed.
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Old 02-03-2008, 06:35 PM
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The completed antenna.
3 days to plan things out.
Half day to prepair materials.
Half day to assemble.

Years of testing
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Old 02-14-2008, 10:36 AM
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Next I will show how the build the 2 meter yagi.
Coming soon
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Old 02-14-2008, 09:54 PM
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Woah! Ease up there big boy! Dude, you have to give us a little meat-n-taters on it's performance. Man that is one beauty of an antenna and a fine assembly job to boot, but, for us 30MHz and below doofuses, tell us how she performs, how you track the bird and what your on the air experiences have been!

Even us old toadies like to learn.
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Old 02-15-2008, 06:28 AM
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The computer model shows 16 db of gain front to back.
I use a radio shack rotor for azimuth and on top of it I have an Alliance U100 turn sideways. This allows me to adjust elevation.

With the bit of a blunt pattern due to the last 2 directors being longer that they should be, it allows me to just be close and not dead on the bird.
I have made about 35 contacts with this antenna so far.

On the ssb birds you need a little more "umpt" so to say. This one is a winner and a keeper. The easy sats (FM) this is not needed. They are low in orbit.

What I like about the antenna is the simple feeding. Gamma matches are the easy match to build. Plus the 2 equal 75 ohm stub harness is no biggy to make.
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Old 02-15-2008, 10:29 PM
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Fantastic! I'll bet you're having a blast with that one. That is one fine / good looking antenna!
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Old 02-20-2008, 09:22 AM
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Thanks Tom.
I works better than planned.

Now if I can ever get the time to talk on the radio.
So many projects
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Old 03-12-2008, 01:42 AM
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It is quit nice. I would like to build one like that oneday. Good Job Buddy!

73

Chris
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