Is this homebrew worth it?

[Uplink]

How do you figure out spacing for 75 ohm?



Even if the coax is 75 ohm?

It's been a while since I built it, but I believe I used 24" spacing between driven elements it what I eventually came to after some field tuning. I used the 1/8" steel welding rods from Home Depot which were 4 feet, so no splicing needed. Herztian may have the exact dimensions you can start with.

 

[cassidy1190]

Thanks, Ill give that a try. So definitely go with the thin welding rods? I have so much 1/2" copper tubing on my hands, would it be worth a try?

 

[davidmc36]

I have so much 1/2" copper tubing on my hands, would it be worth a try?

Maybe not for this particular antenna but there are lots of J-Poles I have seen made from 1/2 in copper. Depends on the application/design.

 

[cassidy1190]

Just curious if this would work. Say I build the antenna to specs, not chaging the design for my 75 ohm RG-6. Could I use a 4:1 balun at the feedpoint as a transmatch?

 

[nanZor]

If you were to make this antenna, and then use a 4:1 transformer/balun, it would basically transform 50 ohms down to 12.5 ohms - something far worse than a small 50-75 ohm mismatch.

So a direct-connection to your 75 ohm cable is better. Are you perhaps thinking of a "current balun" that only suppresses common-mode shield current? If so, here a direct-connection is used, but perhaps a few snap-on chokes near the feedpoint is all you need - no impedance transformation necessary.

You could try using your 1/2 inch tubing, BUT I can almost guarantee that it won't perform as expected unless you change spacing, change element lengths etc that are designed for a larger conductor than what is shown in the article. Instead of a high-gain unidirectional yagi, you end up in the real world with a low-gain bidirectional antenna or something odd since it wasn't designed up front for 1/2" tubing.

I don't know what your mounting / environment is, but if it isn't too bad, you could even use straight #12 copper wire if that is more convenient, say up in an attic or other shielded area.

A 4:1 transformer here will do more harm than good. And it isn't in the article either.

 

[kb2vxa]

If you look at the driven element of a commercially produced Yagi you'll see there is a much simpler way to do it. Like the parasitic elements it is one continuous length connected to the boom at the center (ground) and fed slightly to one side through a series gamma match.

What it consists of electrically is an inductor and capacitor in series forming a loop between the feed point along the element and the center where it's grounded and the coax shield connected. Inside the box that mounts the RF fitting may be a fixed capacitor or the center conductor connected directly in which case the capacitor is formed by a smaller metal tube inside a larger one separated by a plastic tube. In the latter case both the inductor and capacitor may be variable giving a finer adjustment. In any case an antenna analyzer is a good bet for tuning, that is matching impedance, tune for lowest SWR at the center operating frequency and you're good to go.

Sorry I'm lousy at drawing pictures and worse when it comes to cluttering a post with multiple scans so your best bet is Google "antenna gamma match" and you'll come up with plenty of home brew designs complete with photographs.

 

[cassidy1190]

I don't know what your mounting / environment is, but if it isn't too bad, you could even use straight #12 copper wire if that is more convenient, say up in an attic or other shielded area.

I am in fact going to put it in the attic (grounding sounds like a nightmare to me). I thought you could only get copper wire in spools, Ive never seen it actually straight before.

And that gama match sounds and looks very interesting. Ill definitely try to expiriment with that once I get going with this thing.

 

[davidmc36]

I thought you could only get copper wire in spools, Ive never seen it actually straight before.

What is often done is buy a length of household wiring. You get three pieces of #12 in a piece of 12/2 wire. It is super easy to peel and soft as butter to straighten out.

 

[nanZor]

If you are going to keep the antenna pointed in one direction in the attic, you don't even need a boom!

I've done this before by just hanging the elements from the rafters and tying them securely with cord to the wooden crossbeams on the ceiling below at my feet. Keep tension on the elements and measure carefully.

 

[cassidy1190]

All great info, as always! Thank you to all. One more question though..how important is the straightness of the elements? I changed my medium to 1/4 copper tubing (fridge coil type) which comes coiled in a box. I was able to straigten it out pretty well, but Its not factory perfection. Will a small hairline kink here or there mess up the design at all? I am assuming that as long as the lengths are spot on I should be in the clear.