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Starting a new yagi today

RickC.

RickC.

Hopeless antenna junkie
Mar 30, 2009
400
8
28
Alabama
The plan is for 2 elements on 20m, 3 for 10. Wood/fiberglass/wire. Sort of a cross between the VE7CA and a C3. Common feed, hairpin match.

Wish me luck (and no rain in the evenings after work), I'll post photos along the way if anybody's interested. Hope to have it finished by the end of the weekend.



Rick
 

OK, this doesn't look very "antenna-y" so far, but I was finally able to get back to it today.

Here's the boom with the element brackets attached and curing. Boom is 12 feet of straight grained pine, 1 1/2" square, element mounts are 3/8" exterior ply cased in 3" fiberglass tape. Mounts are 3 x 36" for the driven element and 20m reflector, 3 x 24" for the 10m reflector and director. Spacing:

20M reflector- 8'
10m reflector- 4 1/2'
10m director- 4'


boom.jpg


The element mounts are attached to the boom with screws and PL Premium- the screws are there mostly to hold the ply in place while the PL cures, and the undersides where the ply meets the boom will be filleted and glassed. Nothing fancy, but conserving weight is the key here since the 20m elements will be full size and this yagi is going on a 40' pole.

elementmount.jpg


More later.

Rick
 
A little more progress today.

The boom-to-mast plate I used for my last three beams is made from exterior ply laminated with fiberglass (very tough and lot cheaper than a piece of aluminum that size) so I made a new one-- 5 x 12":

plate.jpg


Plate marked for drilling:

marked.jpg



Plate drilled for U bolts and primed, mount for driven element primed, plexiglass feedpoint insulator with stainless steel bolts:

primed.jpg


If the rain holds off, I'll have an antenna in a couple of days. The Palomar Engineers balun kit came in yesterday.

It would have been better to mark and drill the element mounts on the drill press before mounting them on the boom, but I got in a hurry and didn't think about it until later. Figures... Well, I'll dig out the hand drill.


More when I have more.


Rick
 
Now this is very exciting I know, but it's all I had time for today (spent most of the day giving an old friend a tour of the track at Talladega):

painted.jpg


Like watching paint dry...

I'm posting these pics for this reason: When this antenna is finished, it will be built totally from stuff bought at:

Walmart
Lowes
Home Depot
Bass Pro Shops
Advance Auto Parts

[EDIT: Not entirely, I'd forgotten about the Palomar Balun-- but I went with that to avoid a coaxial coil balun, though those have worked just fine when I've used them.]

No earth-shaking design innovations, and no one will mistake me for the new Hiram Percy Maxim. BUT-- maybe it will start would-be antenna builders thinking about possibilites. If you've looked back through QST from past decades, hams used what they had on hand where possible and the stuff worked. If construction similar to this were used on CB, the cost and weight would be even lower-- not to mention the simpler task of building a single band antenna.

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

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

2. Whether the element spacing I've chosen will give me a good compromise feedpoint impedance on both bands.

3. If the poor f/b ratio I experienced on VE7CA's design on 20m will be improved with this spacing (he used 6.99' because that's what would fit in his car).

4. Whether I'll need to give up on the 3 elements for 10 meters when the moment of truth arrives in tuning, and just saw off the forward 4' of the boom and have 2 elements on each band.

So, we'll see.


Rick
 
Last edited:
Captain Kilowatt said:
Two questions.

1. What are the elements going to be made of?

2. Where did you come up with the element spacing? They appear to be about half of what is normally used
Click to expand...

1. 14 AWG wire and crappie poles

2. The element spacing is something I mulled over for a few weeks. For 20m 8 ½ feet is what I had originally wanted and in fact cut the first boom 8 ½ ‘. From the VE7CA antenna I built a while back I knew his 4.1’ for the 10 meter reflector worked very well at 33’ above ground. I was surprised to find nearly identical performance with the 2 element 10m yagi to the 2 el quad it replaced— f/b was the only noticeable difference, and that was not a deal breaker for 10m.

So as a compromise to fit the 12’ boom I chose 8’ for 20, which is just a shade under 1/8 wave spacing. I say compromise because had I used 8 ½’ I’d only have 3 ½’ feet left for the 10m director since I’m using a common feed and hairpin for the driven elements on both bands.

10m was a bit of a quandry, since I had plenty of boom space to play with IF I wanted to use separate driven elements and feedlines. In this case I was looking at modeling of feedpoint impedances with different boom lengths and element spacings, some that I did myself and some from reading. Cebik lists a short boom 10m yagi with only a 7 ½’ boom, and the Cushcraft TEN3 has an 8.0’ boom. So 8 ½ feet is not out of line for a 10m 3 element yagi. Not optimum perhaps, but any multiband design is a compromise, even the A3S has a boom of only 14’ for 3 elements on 10/15/20, and it’s a very popular antenna.

So again, we’ll see. My aim here is not to copy any commercial design but to see if I can come up with something to fit my needs and that would be fun to build.

Good questions, and thanks!

Rick
 
Last edited:
I forgot to mention that the modeling shows the feedpoint impedance of this array to be 33 ohms at 20m, 28 ohms on 10m. It'll be interesting to see how close that is with a common feedpoint and the DE's separated 3", and how the match works out with the interactance between bands. With 2 elements on each band the VE7CA yagi tuned pretty quickly.

What I think I'll see is a good match across most of 20m, and probably more reduced bandwidth on 10m as opposed to a monoband antenna. Now if this rain will move on out I can get this finished....
 
RickC. said:
1. 14 AWG wire and crappie poles
2. The element spacing is something I mulled over for a few weeks. For 20m 8 ½ feet is what I had originally wanted and in fact cut the first boom 8 ½ ‘. From the VE7CA antenna I built a while back I knew his 4.1’ for the 10 meter reflector worked very well at 33’ above ground. I was surprised to find nearly identical performance with the 2 element 10m yagi to the 2 el quad it replaced— f/b was the only noticeable difference, and that was not a deal breaker for 10m.

So as a compromise to fit the 12’ boom I chose 8’ for 20, which is just a shade under 1/8 wave spacing. I say compromise because had I used 8 ½’ I’d only have 3 ½’ feet left for the 10m director since I’m using a common feed and hairpin for the driven elements on both bands.

10m was a bit of a quandry, since I had plenty of boom space to play with IF I wanted to use separate driven elements and feedlines. In this case I was looking at modeling of feedpoint impedances with different boom lengths and element spacings, some that I did myself and some from reading. Cebik lists a short boom 10m yagi with only a 7 ½’ boom, and the Cushcraft TEN3 has an 8.0’ boom. So 8 ½ feet is not out of line for a 10m 3 element yagi. Not optimum perhaps, but any multiband design is a compromise, even the A3S has a boom of only 14’ for 3 elements on 10/15/20, and it’s a very popular antenna.

So again, we’ll see. My aim here is not to copy any commercial design but to see if I can come up with something to fit my needs and that would be fun to build.

Good questions, and thanks!

Rick
Click to expand...

When you say "crappie" poles do you mean junk or fishing as in cane pole?
 
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
 
Last edited:

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