An 18' gate is a long one. Is the gate itself actually deforming? Is the post dead nuts plumb - any out-of-plumb is going to be really apparent at 18'. Is the ground under the gate level?

If you want to go the support cable route an eyelet ring should suffice. If I had to, I would use a welded ring rather than just a bent one. Obviously the taller you go the better the angle.

You could also install a gate caster.

 

If the post and hinges are solid, you can run the diagonal tension rod with the turnbuckle from near the top hinge to the bottom of the gate near the far end. I'd use at least a 1/4" all-thread rod and/or 3/16" steel cable.

 

Maybe weld or machine bolt a flat metal bar or pipe diagonally on the gate, as in my sketch?

 

I will assume, 18' gate has at least 1 vertical support brace.
Wrap a chain around one of the horizontal steel, far side of the vertical support. Use a clevis to connect, making a loop of sorts. Kinda like Greg's picture.
Steel cable from clevis to a stout turn buckle.
Prop up end of gate w/a piece of wood, a rock, etc.

How you attach it to your post kinda all depends.
I've screwed a 2x6 to the far side of the post that was 2' taller than the post.
Drilled a hole for a thru eye bolt, nuts and washers on both sides of 2x.
Use another clevis to connect turnbuckle to eye bolt.

I've strapped a piece of 1" black pipe to far side of post.
Drilled hole, clevis, as above.

Gate is not all that heavy, relatively speaking, its the leverage.

HTH...Good luck...Don.

 

That is a wide gate. Would adding a wheel at the far end work? That would support that end if the ground is compatible.

 

I would start by inspecting the post and hinges. Those tube cattle gates usually sag because the hinges get loose or the pins start to bend.
Is the gate currently mounted with J bolts?

 

Don't think I have ever seen a farm gate that was close enough to the ground to keep the critters in that did not sag and drag. I doubt that a tension bar is going to pull it up.

Mark's wheel is the best suggestion. They sell them at the farm supply stores.

 

close enough to the ground to keep the critters in that did not sag and drag.

Why would I want my gate to drag the ground if I had llamas?
Mini-donkeys? Mini-horse? Regular size horse? Cattle or cows?

I'll give ya, goats were a problem to keep inside an enclosure.
They would shimmy under a gate that was not close, or if the fencing needed work.

YMMV...Don.

 

What OP proposes is an option; although not that common in areas I'm familiar with. It is simply an external diagonal brace as opposed to an 'internal' one that braces the diagonal corner of the gate itself.

If the hinge post is plumb and doesn't flex when the gate is swung, another option is two shorter gate panels joined by a chain.

I think the widest gate we had was 14'. We didn't really have any great problems with it but I purposely installed an 8' gate to move the critters and left the big one for just tractor access.

 

From the OP, " Metal tube gate that spans about 18 ft across cattle guard "

It just hit me. Why does anyone need a swinging gate, when they have a buried "cattle guard"? Has the cattle guard been allowed to clog solid with debris to the point, the livestock can just walk over it maybe?

 

Why does anyone need a swinging gate, when they have a buried "cattle guard"?

The cattle guard keeps them in if the gate is open.
Gate is there to keep trespassers out.

FWIW...Don.