Starting a new yagi today

[Captain Kilowatt]

Well, let's hope it's not crappy! The last antenna I built along these lines served me well for the year or so it was up (that's in my avatar)-- I took it down only because I wanted to mess with Moxons.

Captain-- I'm scratching my head over your comments about half the usual number... That's element spacing, not length!


Rick

Yes, I know the spacing is about half what is normally used. When I said about it may end up being a crappy crappie idea I was thinking about wind and ice loading but after seeing you questioning my comments I saw that you are in 'Bama so not much to worry about ice loads there. I am so conditioned to automatically thinking about both snow and ice as well as the winds from the wild 'noreasters we get here on the east coast that I forget that some people don't have the ice even if they do have the winds. It's amazing what even 1/4 inch of ice will do to the windloading of an antenna or tower.

 

[RickC.]

Right about the ice-- we don't get ice storms here often and if we do, that antenna will be the least of my worries! Ice loading is bad news sure enough, and while I have been fortunate enough to avoid losing an antenna at home due to ice, some of my sites at work over the years have not fared so well. We have other fun things to deal with though-- after Katrina, wind loading specs all along the Gulf Coast were increased to 120 mph-- and it doesn't take much imagination to know what happened with antennas, feedlines, leases, structurals, and all that. No fun, lemme tellya.

Now, about the spacing-- you're saying that a 3 element 10m yagi would normally have a 17 foot boom and that a 2 el 20m yagi's would be 16 feet? Even if I wanted a direct feed with the 20 meter elements the spacing would be only around 10 feet. Even an A3S has only a 14' boom, so I'm not making sense of the "half the spacing" thing.

Or am I misunderstanding what you're saying?


Rick

 

[RickC.]

Slight change in plans-- I was not able to come up with a way of spacing the 10 and 20m driven elements 3" that suits me. Either it's mechanically not sound or it looks like crap. Had the same issue last time and ended up just adding a totally separate DE for 10. It worked fine since the two were spaced about 5" and I saw very little interaction between the two.

What I'm going to try instead this time is just taping the 14 AWG directly to the 20' pole and using the diameter of the pole to seperate the two. That comes to nearly 2" at the feedpoint and .985" at 8'. Not what I had in mind, but still greater overall spacing than if 450 ohm open line were used. That may make it harder to match on both bands as well as more restricted bandwidth-- but maybe not, we'll see.

No more pics at the moment-- black painted wood with holes drilled for the EMT clamps to mount the poles are not very exciting to see.

More later.

Rick

 

[Captain Kilowatt]

Right about the ice-- we don't get ice storms here often and if we do, that antenna will be the least of my worries! Ice loading is bad news sure enough, and while I have been fortunate enough to avoid losing an antenna at home due to ice, some of my sites at work over the years have not fared so well. We have other fun things to deal with though-- after Katrina, wind loading specs all along the Gulf Coast were increased to 120 mph-- and it doesn't take much imagination to know what happened with antennas, feedlines, leases, structurals, and all that. No fun, lemme tellya.

Now, about the spacing-- you're saying that a 3 element 10m yagi would normally have a 17 foot boom and that a 2 el 20m yagi's would be 16 feet? Even if I wanted a direct feed with the 20 meter elements the spacing would be only around 10 feet. Even an A3S has only a 14' boom, so I'm not making sense of the "half the spacing" thing.

Or am I misunderstanding what you're saying?


Rick

You are not the one misunderstanding something. I am. :headbang I went back and re-read your posts and now I realise that you were talking TWO elements on 20m not three. Forget what I was talking about in regards to the element spacing. I looked at the date I posted that and it was at the end of a night shift run with a 54 hour run with only 4 hours sleep. :blink: Your spacing looks reasonable.

Man I have got to stop doing shit like that and not sleeping after working nights. Sorry about that.

 

[RickC.]

HA! No sweat, don't give it another thought. Been there/done that, many times.. I went back and read my post on the spacings to make sure I hadn't listed something really strange, though.

Anyway, it's supposed to rain on and off for the next 4 days so maybe I can get this antenna finished and first tests done by Sunday afternoon.

I'm still not happy with the separation between the two driven elements, but I won't know what I have until I get it all connected and see whether that's going to be a problem. It'll work like this, I just don't want to end up with say, 125 KHz on 20 and 300 or so on 10. Or I may yet have a genius idea, but those have been rare in recent years!


Rick

 

[RickC.]

Ugh.

Maybe tomorrow.

 

[RickC.]

OK, finally some progress after several days of rain.

Getting ready to build the 10 meter reflector:

10m reflector in place:

Element mounting detail, ½” EMT conduit strap and #8 hardware— bolts, flat & lock washers. #8 works fine with elements this short and light:

To save weight and keep the parasitic elements from sagging, I opted for 17 gauge electric fence wire. I’d forgotten I’d used that in the VE7CA antenna I built until I went to steal the EMT brackets from it this morning. In fact I used that for the 10m driven element on that antenna and it worked fine.

Building the 10m director:

Progress at day’s end (or when I had to stop and get the charcoal going on the patio)—10m director and reflector in place. Left to go: Driven element, 20m reflector, balun, and hairpin match. I have family obligations tomorrow so it may be Monday before I can get back to it.

Bear in mind this is a wire yagi and poles are just the supporting structure. I exchanged emails with an Italian ham who had built the VE7CA tribander as I did with fishing poles, and his observations were the same as mine—that it was a good lightweight antenna but the f/b on 20 was not all that great.

More when I have more.

Rick

 

[HomerBB]

It's looking good.

 

[RickC.]

Thanks Homer.

Startin' to look like an antenna now anyway, instead of a pile of sticks! Looks like I may finally get it on the air Memorial Day weekend.


Rick

 

[mackmobile43]

Many crappie were spared due to the construction of this antenna.

 

[RickC.]

Ah, I wondered how this was going to go. The 20m reflector goes on:

Even when I used angle aluminum with the 20 foot poles on previous antennas I had some droop, but the glassed plywood gives a bit more than I had counted on. Not a deal breaker though, I'll have a solution for it after supper.

That 20' pole at the top of the photo is 6 years old, by the way. It's been part of several antennas: 5/8 wave 11m antenna, 1/2 wave 10m vertical, 20m j-pole, 20m Moxon, 30m ground plane, and a couple other I can't recall at the moment. It needs more paint, but it's still hanging in there after 6 years of constant heat, cold, and UV exposure.

(And Mack, almost as many crappie are being spared as when I've actually fished for 'em...)

Rick

 

[RickC.]

Got a little bit more done today, but I’m on the homestretch now.

20’ crappie poles are lightweight, but they do sag quite a bit when supported horizontally from one end (10-14 foot ones don’t, which would be a big advantage to someone building a 10, 11, 12, or 15 meter beam with them), so to counteract as much of that as I could, I added a 1/8” cord support, using ½” wood dowel as a standoff:

The nylon cord is attached to the poles about 10’ from the mounting plate (photo of that later) using tie-wraps—bowline on one end, through the eyelet in the center, trucker’s hitch on the other. That arrangement worked well with the VE7CA yagi I built and was not much the worse for the wear after 2 ½ years after I took it down.

With the 20m reflector finished, the dual band driven element could finally be addressed. This is the center insulator mounted on a PVC standoff to give the feedpoint the same height above the boom as the center of the poles:

Finally the 10 and 20 meter driven elements were attached. The solder lugs were both crimped and soldered. You can see the spacing there between the 10 and 20 meter wires, we’ll see how that works out when it’s time for tuning this array. Because the 1/8” anti-sag cord was an afterthought, I mounted the dowel standoff slightly forward of the element mounting plate. This makes the driven element pull just a bit to the front when it’s under tension but it’s not too bad. Again, if a 10 or 11 meter beam were built using crappie poles, that would not be necessary.

Well, that’s the report for now. Still to come: balun, feedline, getting it in the air, and tuning. No big deal, right?

Rick

 

[RickC.]

DadGUM this rain!!

Maybe I can get it finished by Monday.

I did have to back track on the reflector-- to keep the long poles from sagging too much the cord has to put a considerable amount of tension on the poles- which caused a couple of the joints on either side to telescope back into themselves about an inch-- they had been tightly wrapped with duct tape, which worked fine in other antenna applications.

So, I took all the tape off the joint, got all the adhesive off with mineral spirits, roughed the mating surfaces up just a bit with sandpaper, and epoxied those joints.

Given this issue, I thinking maybe 15 meters is as large as you'd want to go with a full-size yagi using this type of construction. My last 20m experiment along these lines used 16' poles and a loaded, shortened reflector element-- and I didn't have this problem. There are other options such as using angle aluminum for the element bracket but from what I can see it would not make much difference.

I almost built a G4ZU Bird beam with the poles instead, which would have been a lighter, but BIG antenna with roughly the same forward gain.

 

[packrat]

The questions I won't be able to answer until it's built are:

1. Total weight (I'm shooting for under 20 lbs)


Comment. Dont use the glass tape. No benefit... adds weight and cost.



Other than that, great project. Cant wait to see it perform.

 

[RickC.]

Didn't add to the cost since I had a bunch left over from the last boat project-- and allowed me to get away with using exterior ply for the element mounts and mast plate. It lacks only the balun and mast plate now and still feels pretty light. But thanks-- and yeah, if I didn' t already have some sitting around I would not have used that for the element mounts. Honestly for 10m you don't need it. The 10m reflector on the VE7CA I had up was just exterior ply sealed with resin and couple of coats of flat black spray paint. It was plenty stiff enough for the lightweight poles and wire and was only starting to delaminate a bit at the edges after nearly 3 1/2 years in the elements, though the crappy paint was peeling:

These are the same two poles and EMT clamps used in this new antenna for the 10m reflector, by the way. These element mounts in this old antenna were not screwed to the boom- it's fillets and fiberglass tape, much lighter than a couple of U bolts.



I am considering using angle aluminum for the 20m reflector mount though. Whoever's doing a rain dance can stop now.


Rick