Avanti Sigma4: An alternative view point

[HomerBB]

Well, I've tried three different gamma matches that have all been used successfully on other antennas and still can't get under a 3.0:1 SWR with this thing. So . . . back to the drawing board.

The main radial is 27' 8"
the four sleeve radials are 104" mounted 16.5" from the bottom of the center radial.

I guess I'll have to lengthen the radials 4" and shorten the main radial to ______?

Lost.

Checked coax - good, changed coax anyway, checked all connections - good. All materials are aluminum and stainless steel. Tried it isolated from mounting mast, tried it not isolated from the mounting mast.

Hmmm...

maybe I'll just make a dipole

 

[Marconi]

That's a bummer Homer. How about a picture of the base connections and the gamma dimensions. I know the new Vector has longer radials, but the 104" for the radials sound a bit too long for me. Are you still using wire elements for the radials?

In case it might help, my Sigma 4 has 3 radials that are about 91.5" and the hoop is .5" from the ends. The gamma tap point is 33" from the feed point mount that you see in my previous pictures.

Bob, might be right since you're building your own. The diameters, and tubing lengths that make up the proper taper of this antenna may be far more important than we think. I know there are probably different ways to accomplish this, because those factors on my antenna are different from the New Vector antenna, but it is something to consider in building your own.

 

[Shockwave]

Many gamma matches are too short to work on the Sigma. The rod length of the gamma for this antenna is longer because the tap point on the main radiator is higher. How long is your current gamma? I will measure the stock one for you.

 

[Marconi]

Many gamma matches are too short to work on the Sigma. The rod length of the gamma for this antenna is longer because the tap point on the main radiator is higher. How long is your current gamma? I will measure the stock one for you.

That is a good point, SW. My gamma is 9" x 1/2" with a 36" x 1/4" rod. The LW150 gamma is 14.5" x 5/8" with a 34" x 3/8" rod.

SW, how high is the tap point on the New Vector from the bottom of the feed point to the tap point. My old Sigma 4 is about 33".

 

[HomerBB]

That's a bummer Homer. How about a picture of the base connections and the gamma dimensions. I know the new Vector has longer radials, but the 104" for the radials sound a bit too long for me. Are you still using wire elements for the radials?

Maybe I can get a photo tomorrow - 09/03/10. The length of the radials are what they were before i took apart the 5/8 antenna. It's a dimension I have no solid number for, but saw some discussion of them being as long as 108". . .
No, I am not using only the wire radials. Although the wire is still present, it is beneath a aluminum duct tape wrap of the fiberglass poles of which the radials are made.

In case it might help, my Sigma 4 has 3 radials that are about 91.5" and the hoop is .5" from the ends. The gamma tap point is 33" from the feed point mount that you see in my previous pictures.

My hoop is within an inch of the end of the radials. The tap point on one of the gammas was at 33", but the outer tube was too long, I think. Also, the diameters of the tubes were heavier than what your's apparently are. That one was a quick throw together and not well made.

Bob, might be right since you're building your own. The diameters, and tubing lengths that make up the proper taper of this antenna may be far more important than we think. I know there are probably different ways to accomplish this, because those factors on my antenna are different from the New Vector antenna, but it is something to consider in building your own.

The lowest end of my main radial is about 1.5" diameter. The upper most end is 0.5" diameter. Naturally, being of found materials as a homebrew redo, I have less control over the exact duplication of the original materials. I am wondering if the bigger diameter might call for a shorter length. All I've got is simple math - 36.08' * .75 = 27.06' (27' 3/4"). I am long here by 8" (27' 8"). any input here?

 

[HomerBB]

Many gamma matches are too short to work on the Sigma. The rod length of the gamma for this antenna is longer because the tap point on the main radiator is higher. How long is your current gamma? I will measure the stock one for you.

The gamma matches I've got are more like the typical lengths used for 11 meter Yagis. In the 24" - 27" range. I have purchased another piece of 1/4" x 36" aluminum rod and will try to rebuild a suitable gamma tomorrow. a measurement of the stock gamma would be a God-send. Thanks.

 

[HomerBB]

That is a good point, SW. My gamma is 9" x 1/2" with a 36" x 1/4" rod. The LW150 gamma is 14.5" x 5/8" with a 34" x 3/8" rod.

SW, how high is the tap point on the New Vector from the bottom of the feed point to the tap point. My old Sigma 4 is about 33".

Now I've got two opposing measurements. I will look forward to the stock measurement for the V4k from SW. All of this info is great!
I'll have to try each of the measurements till I find a keeper I suppose.

Thanks, Marconi. Maybe together we can get this thing built.

 

[CDX-007]

I'd like to try Bob85s version, are the measurements available or posted somewhere?

For what frequency is it tuned for center?

 

[Marconi]

Homer, I just had a thought about the gamma on the New Vector. It is adjustable based on frequency and the settings are noted in their 2 page manual below.

View attachment 3299

SW was right, the gamma setting for L1 tap point are longer between the feed point and the tap point, it shows 36 5/8" @ 27.185 than my Sigma 4.

The rod part of the gamma measures about 25" from the cap that covers the gamma to the end of the rod, and the instructions show a setting range for this measurement L2 = 630 to 640 mm or 24.8" to 25.2".

I do not know the actual dimensions for the New Vector gamma, but in the instructions it looks exactly like the LW150 gamma that I gave dimensions for earlier.

 

[Marconi]

Another note Homer. If your radials are made of small PVC that has aluminum tape around them, then they are likely much larger in diameter than the real radials, so if you made them as long as the thinner radials they are really too long, which may be making the antenna very low in resonance. Have you tried to move around in frequency, go down to 26 mhz to see if you might show a better match with the setup you currently have?

I'm not sure, but I have a feeling that the Sigma 4's resonance may be affected as much by adjusting the radial length as when adjusting the overall length of the radiator.

 

[HomerBB]

@Marconi
I see. I've got that file on my hard drive, and feel like a dunce for not remembering those settings were there.
Thanks again.

@CDX-007

I don't know what the dimensions are for Bob85's version, but here is a set of dimensions I forgot I had downloaded:
Vector4000 Dimensions

 

[HomerBB]

Another note Homer. If your radials are made of small PVC that has aluminum tape around them, then they are likely much larger in diameter than the real radials, so if you made them as long as the thinner radials they are really too long, which may be making the antenna very low in resonance. Have you tried to move around in frequency, go down to 26 mhz to see if you might show a better match with the setup you currently have?

I'm not sure, but I have a feeling that the Sigma 4's resonance may be affected as much by adjusting the radial length as when adjusting the overall length of the radiator.

The radials are constructed of 3/8" fiberglass tent poles wrapped in aluminum tape.

I check SWR in this range 28.755 - 25.165

 

[CDX-007]

@CDX-007

I don't know what the dimensions are for Bob85's version, but here is a set of dimensions I forgot I had downloaded:
Vector4000 Dimensions

Hi Homer,

All I get is this:
The connection has timed out
The server at s1205.photobucket.com is taking too long to respond.

So I guess I'll try tomorrow.

Thanks!

 

[AudioShockwav]

Lets see if this works

73
Jeff

 

[Marconi]

The radials are constructed of 3/8" fiberglass tent poles wrapped in aluminum tape.

I check SWR in this range 28.755 - 25.165

Well for some reason I was thinking the PVC was about 3/4" inch, and you covered enough frequency to know the 3:1 SWR was not a tuning problem. I was wrong.

BTW, those dimensions were just some guess-timates I made. I'm waiting on a guy from another forum to see if he'll give me some more of his dimensions. He just bought a New Vector.

So far, however, he has told me the overlap is 2.5" inches, and at first he told me it was 31' feet long. I questioned that, then he said he didn't really measure it, so I'm not sure what I'll get if anything reliable from him.