Avanti Sigma4: An alternative view point

[HomerBB]

I am trying to upload some photos so you can see the materials of which the antenna at the hub and lower is made. They are non-conductive PVC. The actual bottom of the main vertical is at the top of the coil. Even with the coil gone on my QuasiV4k, the vertical is still that high above the hub. That is roughly 4" above the attachment points for the bottom of the gamma, and 3" above the bottoms of the 4 radials.

My gamma is attached to the outside of the 4" hub. The radials are attached to the outside of the 2" portion of the hub. This puts radials at their closest about 1/2" from the exterior side of the main aerial, and the gamma about 1 1/2" from the main vertical.. I could be wrong, but I see my job is to move these connections up the main aerial of the antenna as the original is, and to try to copy the spacings more closely as you suggested were influential. What I need to know is whether the bottom of the radials are grounded to the main aerial, and whether the bottom of the gamma is isolated from it. The diagrams are not particular about that as they are for assembling rather than explanation.

These are photos from when I was redoing the 5/8 after an ice storm.The coil is gone from the antenna now. The bottom of the gamma is attached to the center of the coax connector, and the four radials are fed by the coax shield being attached to the outside of the PVC hub.

 

[Marconi]

Here are some close-ups of the bottom of my Sigma 4. Hopefully you can see that everything is at ground potential. Only the lower end of the gamma is attached to the center pin of the SO239 and thus is insulated from everything else.









You will also notice that my gamma is small and therefore probably has a small range for capacitance. I have a gamma off of a Larry 150 and it is twice the size and that probably allows it to tune from 10 thru 12 meters.

 

[HomerBB]

Mucho thanks.

The photos are very good and self explanatory. We'll see how it goes when I get the thing back on the ground.
Gonna have to figure out my connections. As you can see, everything I use is always hardware store variety. Ideas are already buzzing in my head.

 

[Marconi]

I am trying to upload some photos so you can see the materials of which the antenna at the hub and lower is made. They are non-conductive PVC. The actual bottom of the main vertical is at the top of the coil. Even with the coil gone on my QuasiV4k, the vertical is still that high above the hub. That is roughly 4" above the attachment points for the bottom of the gamma, and 3" above the bottoms of the 4 radials.

My gamma is attached to the outside of the 4" hub. The radials are attached to the outside of the 2" portion of the hub. This puts radials at their closest about 1/2" from the exterior side of the main aerial, and the gamma about 1 1/2" from the main vertical.. I could be wrong, but I see my job is to move these connections up the main aerial of the antenna as the original is, and to try to copy the spacings more closely as you suggested were influential. What I need to know is whether the bottom of the radials are grounded to the main aerial, and whether the bottom of the gamma is isolated from it. The diagrams are not particular about that as they are for assembling rather than explanation.

These are photos from when I was redoing the 5/8 after an ice storm.The coil is gone from the antenna now. The bottom of the gamma is attached to the center of the coax connector, and the four radials are fed by the coax shield being attached to the outside of the PVC hub.

I see now what I thought was the radiator is actually PVC and the actual radiator is several inches above the hub where the radials are attached. You described you situation correctly.

I guess the old saying applies, "...pictures are worth a thousand words."

 

[bob85]

homerbb,
if you want your homebrew to work like a vector you will likely have to use tubing that is close to the same diameters as the vector/sigma antennas as a starting point,
you can see from eddies pics how it needs to be configured, note the vector radials are slightly closer to the monopole at the base than they are in the original sigma4,

with regular antennas, groundplanes ect almost any conductor of the correct electrical length will work ok, the sigma style is not like any regular antenna, imho relative tube diameters, lengths, spacing and radial sleeve to monopole length ratio are important, you will only discover what works best for you with any particular combination of conductor diameters by experimentation with distant stable signals ( no skip )

using 4 rather than 3 radials works better in our tests here, as you sweep the radials up towards the monopole you get the effect of both radials and monopole becomming electrically shorter for the same physical length, thus it can be seen that you cannot use maths that work for groundplanes to guesstimate any particular monopole or radial electrical length when using the sigma design,
the gamma allows a good match with a wide range of radial and monopole lengths
there is no need to have a resonant 3/4wave nor in our tests does that give the best performance,

good luck homer, i apreciate your willingness to experiment

 

[HomerBB]

Thanks, Bob

I have come to that conclusion by both the sad results of the homebrew so far, and carefully reading the thread as you fellows discussed the findings (or lack thereof) in your experiences and modeling.

The antenna was a homebrew 5/8, but is now longer - 3/4 according to the measuring tape. Also, I need to redo other things than just the placement of the feedpoint and radial sleeve conjunction. I am using four of the radials, but they will need to be solid skinned aluminum instead of wire supported by fiberglass rods as they now are. Etc . . .

The results I've gotten have been less scientifically arguable than modeling, which has been the central piece of this thread, but quite eye-opening for me. In all respects it has shown less TX and RX than the other sleeved monopole - something it should not do. All local is less on the receive meter, and any stable distant station I am accustomed to hearing is barely audible to not audible. With a mere switch of the coax switch I was able to converse with a stable base station on the 1/2 wave sleeve monopole about 40 miles out (sorry best I can do here) that required me to have someone half that distance between us relay when on the Qv4k*. Even the nearer station was down 2 S-units and considerable audio on the Qv4k compared to the sleeved monopole. As for local mobile traffic, just as bad. Some mobiles are completely inaudible on the Qv4k that I hear quite well on the sleeved antenna. As for skip - same thing. All of this with a great SWR, however, although that is currently centered in the band below CB 40.
So far, the Qv4k is a wash.

All-in-all, I figure if I'm going to have the odd looking thing above my property it should at least pull its intended load.


*Qv4k = QuasiV4k (homebrew Vector 4000)

 

[CDX-007]

Thanks, Bob

I have come to that conclusion by both the sad results of the homebrew so far, and carefully reading the thread as you fellows discussed the findings (or lack thereof) in your experiences and modeling.

The antenna was a homebrew 5/8, but is now longer - 3/4 according to the measuring tape. Also, I need to redo other things than just the placement of the feedpoint and radial sleeve conjunction. I am using four of the radials, but they will need to be solid skinned aluminum instead of wire supported by fiberglass rods as they now are. Etc . . .

The results I've gotten have been less scientifically arguable than modeling, which has been the central piece of this thread, but quite eye-opening for me. In all respects it has shown less TX and RX than the other sleeved monopole - something it should not do. All local is less on the receive meter, and any stable distant station I am accustomed to hearing is barely audible to not audible. With a mere switch of the coax switch I was able to converse with a stable base station on the 1/2 wave sleeve monopole about 40 miles out (sorry best I can do here) that required me to have someone half that distance between us relay when on the Qv4k*. Even the nearer station was down 2 S-units and considerable audio on the Qv4k compared to the sleeved monopole. As for local mobile traffic, just as bad. Some mobiles are completely inaudible on the Qv4k that I hear quite well on the sleeved antenna. As for skip - same thing. All of this with a great SWR, however, although that is currently centered in the band below CB 40.
So far, the Qv4k is a wash.

All-in-all, I figure if I'm going to have the odd looking thing above my property it should at least pull its intended load.


*Qv4k = QuasiV4k (homebrew Vector 4000)

Why don't you try the exact 11m .82 - measurements are found here oops, never mind, I just read that he's retracted those.

I'd love to read how a .64 compares in your real world test against your successful earlier 5/8.

 

[HomerBB]

Why don't you try the exact 11m .82 - measurements are found here oops, never mind, I just read that he's retracted those.

I did use a posted formula for the length. I know I said tape measure, but I just wanted to be clear that I had made a physical change to the length of it from the 5/8

I'd love to read how a .64 compares in your real world test against your successful earlier 5/8.

Perhaps in due time.

 

[HomerBB]

For Marconi

I need to know whether the center radiator is isolated from the mounting mast or not. It isn't clear to me in the photo. Is there a plastic sleeve at the mount point? I have pointed to where I am talking about in the attached photo.

Thanks,
Homer

 

[Marconi]

I need to know whether the center radiator is isolated from the mounting mast or not. It isn't clear to me in the photo. Is there a plastic sleeve at the mount point? I have pointed to where I am talking about in the attached photo.

Thanks,
Homer

The Vector/Sigma 4 is all metal. Only one insulator that supports the radials.

The radiator you see here is one piece of aluminun tubing 88" long with 16.5" at the bottom for mounting. The feed point and the gamma provide the necessary insulation.

 

[HomerBB]

I think I understand your answer, but let me be sure.

1.) The radials are not in direct contact with the main vertical? Yes or No?

2.) The supporting mast the antenna is mounted on is not in direct contact with the main vertical? Yes or No?

From the photo both of those points appear to be in direct contact with the main vertical radial. I can not see anything between the radial sleeve clamp and the main radial, nor can I see anything between the mast going into the bottom of the main vertical radial and the vertical itself.
Thanks again.

 

[Marconi]

I think I understand your answer, but let me be sure.

1.) The radials are not in direct contact with the main vertical? Yes or No?

2.) The supporting mast the antenna is mounted on is not in direct contact with the main vertical? Yes or No?

From the photo both of those points appear to be in direct contact with the main vertical radial. I can not see anything between the radial sleeve clamp and the main radial, nor can I see anything between the mast going into the bottom of the main vertical radial and the vertical itself.
Thanks again.

1. NO! the radials are in direct contact with the main vertical element. Except for the gamma showing an open circuity (continuity), the whole antenna shows a short at every point.

2. NO! Same answer as above, but this point may be insulated if desired.

You are correct, except for the insulation at the feed point and the gamma, the whole antenna will show continuity between every other point on this antenna. It is constructed just like a gamma fed multi-element yagi beam antenna.

 

[HomerBB]

Ok. That's what I thought. You were clear earlier. I just didn't want to be wrong on my end.
Thanks

 

[HomerBB]

At this point the only thing I've accomplished is to become aware that the antenna I had tuned to low SWR much easier than this one will . (I ran out of time today due to work schedule and rain.) I'm finding it a challenge to grasp the mechanics of an antenna that grounds its sleeve to the driven radial. . . .

 

[CTStallion]

I wonder WHY everybody keeps calling this antenna a J-POLE? I thought the ASTROPLANE was a J-POLE and the Sigma IV is merely a gamma-matched 3/4 (7/8) wavelength monopole with the inverted cone to decouple the coax to prevent the shield (braid) from radiating???