Help with a yagi

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temchik

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Hi All,

I just built my first antenna - a 2m 3 element yagi, followed ARRL specs. Elements are made of 1/4 soft copper pipe.

uploadfromtaptalk1410350210880.jpg

It seems to work fine, not a huge gain over a JPole I had, but better on one distant repeater, full quieting.

A couple of issues though

- SWR is around 2.5

- couldn't find if it mattered where exactly to connect the wires - the very edge or not. Right now the are simply connected to the whole 1/2 inch pieces

- soldering to copper proved to be impossible, I am not very good at soldering

- the schematics didn't address element width at all, was not sure if I need to measure distances from the center or not.

Any insight will be appreciated

Thanks!
 
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W3DMV

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Nice looking job. If you look closely at the article, he has adjustable
sections on both ends of the driven element. These are used to adjust the
length of the driven element for best match.

Take some SWR readings. If the SWR is better at the high end of the band,
you need to add some length to the driven element. Equal pieces at both ends
of the element. If the SWR is better at the low end of the band, then the element
is too long and you need to shorten the element.
I usually try for the middle of the band and leave both ends fall where they may.

.. Good luck
 

temchik

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I did try different frequencies, it's pretty much flat at around 2.5.........
 

temchik

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Could this mean the match isn't working properly? I suspect the connection and the length of those stubs are incorrect, but it doesn't specifically state anything about those
 

temchik

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Ok. I did a quick test and found that I am way outside the band... the lowest SWR of about 1.2 is at 140.000. So... do I just trim the driven element a little to raise the center freq? Or I actually need to trip all elements?
 

temchik

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Here's what I did:

I moved the driven elements in 1/2 inch more extending the matching section and shortening its total working length to 36"

The SWR is 1.5 across the whole 144-148 band, I don't see the 1.2 anymore possibly because I also changed the matching sections. This is good enough for me, now if I can only figure out a way to solder the wires onto copper pipes.... The solder simply rolls off the copper, I'm using Rosin core 40/60, possibly the wrong type for the job. Maybe I'll just clamp them with a garden hose clamp and then pour hot solder in?
 

jonwienke

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You need more heat. The pipe has to be hot enough to melt solder, and you aren't going to achieve that with a soldering iron. Use a propane torch.

Also, for proper matching, the driven element should be one solid pipe, not two separate pieces. The coax shield should connect to the exact center of the driven element, and the coax center conductor should connect to a point on the driven maybe 1/8 of the way from the center of the driven element to the tip. You'll need to experiment to find the best connection point.

The length and spacing of the elements determines the resonant frequency, and the distance of the coax center connection point from the coax shield connection point will adjust the impedance of the connection between the coax and antenna. Both need to be adjusted properly for low SWR.

Also, the way you have your feedline routed is not optimal--it is acting like an additional excessively long element interfering with the antenna design, and you will have issues with RF emitting from the coax. Route the coax along the horizontal boom, and loop it down behind the longest element so there is some clearance between the coax and the element, and the coax meets up with the vertical PVC a ways below the bottom tip of your driven element.
 

mm

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What are the outside diameters of the elements that you used?

That ARRL/QST article is old and it doesn't really mention the diameter of the rods used just something about welding rod or such.

If you can measure the diameter of all elements then I can model the entire antenna in 4NEC or EZNEC and give you optimized lengths depending on the element diameter you supply.

In fact I think I have that same antenna in my EZNEC files, with optimized lengths, using .25 inch diameter aluminum elements.
 

temchik

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I am using 1/4 soft copper gas/water pipe, I believe it's almost exactly 1/4 diameter.
 

temchik

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Jim, next time :) I already built this one and I am going to use it for now as it is, works wonders for that repeater I wanted to get to...
 

jonwienke

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I think (not an expert though), you are describing a different matching design
The split design you have is non-adjustable and could vary between 50 and 100 ohms, depending on height above ground and other factors. If feeding with 50 ohm coax, you're pretty much guaranteed an impedance mismatch between the coax and antenna, and thus non-optimal SWR. The matching setup I describe will allow you to get an exact between coax and antenna, and will also prevent static buildup on your antenna, especially if you connect the coax center wire to the driven element below the center of the antenna instead of above.

As far as the cable RF goes - will coiling up the cable around the vertical pvc work?
Yes, coiling the coax around the vertical PVC will create an inductor coil that will block RF from traveling down the coax. But it's better to avoid RF then trying to brute-force it into submission after the fact.
 

jonwienke

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I am using 1/4 soft copper gas/water pipe, I believe it's almost exactly 1/4 diameter.
1/2" hard copper is a better choice. 1/4" soft copper will probably bend in a high wind.
 

temchik

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I see. Can you point me towards a link to schematics with measurements? I am a newbie, I saw this one referenced many times and went for it

Thanks!
 
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