Avanti Sigma4: An alternative view point

[HomerBB]

Homer, the Sigma has a very wide bandwidth and what you are seeing is normal. The bandwidth can be increased even more with different tuning methods if this is your goal. By tuning the radiator to favor one end of the band and the gamma to favor the other, you can extend bandwidth further at the slightest expense of gain.

homer,
i agree with shockwave, thats not an unusual bandwidth, i have seen them wider than that when measured at the radio end of a fairly long feedline,
i used my vector hybrid on 11mtrs and 10mtrs with good results on both bands and no tuner needed to keep the radio happy.

I am intrigued by the possibility of increasing the bandwidth as you fellows have indicated is possible, however my journey to this point has been so challenging I am also reluctant to touch the antenna anymore. If using it on 10 meters after I get a license is my goal do you think I'd have anything to gain with further adjustment from where it is? My current radio for testing SWR does not go any higher than 28.755 so I am asking for your best guess.

Thanks,
Homer

 

[CDX-007]

Sorry to semi-hijack this thread

- but I just have to share this.

I put the Imax back up this morning in a hurry because I was having breakfast with a friend, so I haven't yet measured the exact lengths of mast & guy wire.
OK, with one 23 foot guy wire attached to the top Imax U-bolt, and about 8 feet of mast down to 3 @ 9 foot guy wires going to insulators, a total of about 34 feet of Rohn telescoping mast atop an 11 foot high, grounded galvanized vent pipe, with ~89 feet of RG-213...

- The Imax with 100 watts shows 2:1 at 23.150 & 29.550.

...or, 6.4 MHz wide at 2:1 swr.

I'm using three meters, an Heathkit 2040, Drake W7 and the internal meter, all of which were within .1 of the others.

It's got to be something in the various lengths of guy wire and mast / coax length that's causing some sort of an active resonance tuning circuit, but I've never before seen anything this wide and I'd love it if someone else tried this same combination of variables then posted their findings, especially if they attained similar results.

How about <1.5:1 from 24.650 to 29.050MHz!!!

MHz ...... SWR

23.0 ...... 2.5:1
23.150 .. 2:1
23.5 ...... 1.75:1
24.0 ...... 1.7:1
24.5 ...... 1.6:1
25.0 ...... 1.3:1
25.5 ...... 1.25:1
26.0 ...... 1.5:1 (back up a bit)
26.5 ...... 1.4:1
27.0 ...... 1.2:1
27.5 ...... 1.2:1
28.0 ...... 1.3:1
28.5 ...... 1.35:1
29.0 ...... 1.45:1
29.550 .. 2:1

...and it seems to radiate the same signal strength from 23mHz to 29.5mHz in testing with a local buddy @ 10miles, iCOM ProIII, who followed me with his broadband vertical & tuner.

I only put it up to have something to use while I moved the Penetrator(or Saliut...?) to the 70' pine tree, but now with such a broad band spread I hate to touch it. :blink:

 

[HomerBB]

Unbelievable. I've never seen SWR like that.

 

[HomerBB]

Okay, I put together the tip over pole. It needs more to weather proof it - I chose non-pressure treated wood to restrict the weight for lifting, but I at least have the Qv4k up to 34' 8" at the feedpoint. Next I will put the (other) 5/8 back up on the house and attempt to get some comparisons between the two.

BTW, raising the antenna has not affected SWR. It remained as it was at ten feet off the ground.

 

[CDX-007]

Unbelievable. I've never seen SWR like that.

NO SH!T!!!

The first thing I did was check to see if I had the dummy load in line or if I had hooked up the 80m Inverted Vee and it was somehow sympathetically tuning or WhoTF knows!?!?

So I then called my radio buddy with the ProIII and asked him to give me a Sig Rep on several bands. Works perfectly... Something has to be setting up an auto-tuning scenario with the Imax inductive match. I don't want to touch it! It was actually a little wider and flatter in the 26-28MHz range before I lowered it and raised it back up, maybe an inch or two higher than before.

I'm wondering if I could somewhat duplicate it with the Saliut when I get it up there, except perhaps with better performance, though this Imax doesn't suck! :mellow:

Homer, How does the Qv4k 'seem' to be performing... noticeably better than the 5/8 (is it higher than was the 5/8?) or nothing so far to write home about?

That looks cool! Funky, but I like it!

 

[Shockwave]

How The Sigma / Vector really works!

When placing my last order for stock from Sirio I was able to correspond with one of their design engineers. The new Vector was computer modeled using CST Microwave Studio. We now have the answers behind how the Sigma / Vector radiates. Here are the facts behind how it works. It turns out Avanti did not lie about the entire antenna radiating, it just doesn't accomplish this like one might think. The lower 1/4 wave of the main radiator is indeed out of phase with the top 1/2 wave. The upward radials effectively confine the out of phase radiation within the cone. The confined out of phase radiation is replaced by in phase radiation from the radial cone.

The radiation angle is also lower on the horizon then the common 5/8 wave. They have modeled the antenna in a real environment with a mast. Many of us know that it's not unusual for a vertical end fed antenna to cause the mast to radiate. With the 5/8 wave antenna, the mast radiation is out of phase near the base of the antenna where it matters most. With the Sigma / Vector design, the mast radiation is in phase and desirable. DO NOT isolate this design from the mast, you will lose gain on the horizon. You can use a choke to stop the coax from radiating but let the mast radiate in this case.

 

[HomerBB]

Homer, How does the Qv4k 'seem' to be performing... noticeably better than the 5/8 (is it higher than was the 5/8?) or nothing so far to write home about?

I just got it up before dark last night so have had no opportunity to see. i will be putting the other 5/8 I have up and see. it is at the same height as the 5/8 once was. i will get them both up at similar heights and se how things go.

 

[HomerBB]

With the Sigma / Vector design, the mast radiation is in phase and desirable. DO NOT isolate this design from the mast, you will lose gain on the horizon. You can use a choke to stop the coax from radiating but let the mast radiate in this case.

thanks for the info, SW.
I do not have the mast isolated from the antenna on its current perch. However, the metal portion of the mast is only the upper 10' of the mast, all the rest being wood.

 

[Robb]

When placing my last order for stock from Sirio I was able to correspond with one of their design engineers. The new Vector was computer modeled using CST Microwave Studio. We now have the answers behind how the Sigma / Vector radiates. Here are the facts behind how it works. It turns out Avanti did not lie about the entire antenna radiating, it just doesn't accomplish this like one might think. The lower 1/4 wave of the main radiator is indeed out of phase with the top 1/2 wave. The upward radials effectively confine the out of phase radiation within the cone. The confined out of phase radiation is replaced by in phase radiation from the radial cone.

The radiation angle is also lower on the horizon then the common 5/8 wave. They have modeled the antenna in a real environment with a mast. Many of us know that it's not unusual for a vertical end fed antenna to cause the mast to radiate. With the 5/8 wave antenna, the mast radiation is out of phase near the base of the antenna where it matters most. With the Sigma / Vector design, the mast radiation is in phase and desirable. DO NOT isolate this design from the mast, you will lose gain on the horizon. You can use a choke to stop the coax from radiating but let the mast radiate in this case.

Mystery solved!
Nice!

 

[1342]

Sorry to semi-hijack this thread

- but I just have to share this.

I put the Imax back up this morning in a hurry because I was having breakfast with a friend, so I haven't yet measured the exact lengths of mast & guy wire.
OK, with one 23 foot guy wire attached to the top Imax U-bolt, and about 8 feet of mast down to 3 @ 9 foot guy wires going to insulators, a total of about 34 feet of Rohn telescoping mast atop an 11 foot high, grounded galvanized vent pipe, with ~89 feet of RG-213...

- The Imax with 100 watts shows 2:1 at 23.150 & 29.550.

...or, 6.4 MHz wide at 2:1 swr.

I'm using three meters, an Heathkit 2040, Drake W7 and the internal meter, all of which were within .1 of the others.

It's got to be something in the various lengths of guy wire and mast / coax length that's causing some sort of an active resonance tuning circuit, but I've never before seen anything this wide and I'd love it if someone else tried this same combination of variables then posted their findings, especially if they attained similar results.

How about <1.5:1 from 24.650 to 29.050MHz!!!

MHz ...... SWR

23.0 ...... 2.5:1
23.150 .. 2:1
23.5 ...... 1.75:1
24.0 ...... 1.7:1
24.5 ...... 1.6:1
25.0 ...... 1.3:1
25.5 ...... 1.25:1
26.0 ...... 1.5:1 (back up a bit)
26.5 ...... 1.4:1
27.0 ...... 1.2:1
27.5 ...... 1.2:1
28.0 ...... 1.3:1
28.5 ...... 1.35:1
29.0 ...... 1.45:1
29.550 .. 2:1

...and it seems to radiate the same signal strength from 23mHz to 29.5mHz in testing with a local buddy @ 10miles, iCOM ProIII, who followed me with his broadband vertical & tuner.

I only put it up to have something to use while I moved the Penetrator(or Saliut...?) to the 70' pine tree, but now with such a broad band spread I hate to touch it. :blink:

thats great cdx what do you think the chances are of taking a picture of that baby?

 

[bob85]

shockwave,
thanks for corresponding with the sirio engineers, thats music to my ears.

i agree with what sirio say about how the monopole and radial sleeve operate to produce a lower radiation angle than any 5/8wave, i agree you can manipulate radiation angle through changing radial-monopole length ratios,
thats what i have been saying since we started talking about the design about 6 years ago and imho it explains cebik's none aparent colinear array statement to me,

i have a problem understanding how the mast radiates in phase,
i don't doubt there will be some mast/feedline radiation when the antenna is elevated and not isolated, i don't understand how that radiation could be in phase on a long mast but i am always looking to learn ,
i have some field tests and current measurements to do in this area of mast/feedline radiation,

avanti did not lie, they were indeed ahead of the cb antenna game,
the patent is not just salesman speel as claimed by the j-pole snakeoil camp,
they should understand that a 5/8wave with fat conductors is nothing special with regards to performance and forget the lossy gamma/j-pole nonesense before they will ever catch up,
i guess backpeddling is harder than it looks even when theres people out here willing to help,

taking the brute force and ignorance method of building grid driven tetrode amps to compensate for antenna performance is like putting a blower on your car because the brakes are binding .

 

[Shockwave]

shockwave,
thanks for corresponding with the sirio engineers, thats music to my ears.

i agree with what sirio say about how the monopole and radial sleeve operate to produce a lower radiation angle than any 5/8wave, i agree you can manipulate radiation angle through changing radial-monopole length ratios,
thats what i have been saying since we started talking about the design about 6 years ago and imho it explains cebik's none aparent colinear array statement to me,

i have a problem understanding how the mast radiates in phase,
i don't doubt there will be some mast/feedline radiation when the antenna is elevated and not isolated, i don't understand how that radiation could be in phase on a long mast but i am always looking to learn ,
i have some field tests and current measurements to do in this area of mast/feedline radiation,

avanti did not lie, they were indeed ahead of the cb antenna game,
the patent is not just salesman speel as claimed by the j-pole snakeoil camp,
they should understand that a 5/8wave with fat conductors is nothing special with regards to performance and forget the lossy gamma/j-pole nonesense before they will ever catch up,
i guess backpeddling is harder than it looks even when theres people out here willing to help,

taking the brute force and ignorance method of building grid driven tetrode amps to compensate for antenna performance is like putting a blower on your car because the brakes are binding .

Here is exactly what the engineer said about the mast with the Vector. "Considering also the installation, the small current that descends on the first part (the most important) of the mast, near the antenna, it's in phase (another small extra gain) That is not true for a standard 5/8 Lambda". If I understand this correctly he's not saying the entire length of a long mast would be in phase. I think he's saying that the currents are strongest where the antenna meets the mast and this section would be in phase. That kind of indicates a "tuned" length of mast might be ideal.

 

[bob85]

i could agree with that idea,
my thoughts were if you isolate and choke then any current that would usually flow on the mast/feedline would be forced to flow in the radials,
a tuned mast length and coaxial choke are something to experiment with

 

[jazzsinger]

Here is exactly what the engineer said about the mast with the Vector. "Considering also the installation, the small current that descends on the first part (the most important) of the mast, near the antenna, it's in phase (another small extra gain) That is not true for a standard 5/8 Lambda". If I understand this correctly he's not saying the entire length of a long mast would be in phase. I think he's saying that the currents are strongest where the antenna meets the mast and this section would be in phase. That kind of indicates a "tuned" length of mast might be ideal.

reading between the lines it sounds to me like a mast that is a multiple of a 1/4 wave might be optimal.

the current being strongest where the antenna meets the mast seems reasonable for a 3/4 wave vector as its well known I max occurs 1/4 wave below the tip and every half wave after that.but doesn't seem right for a 7/8 wave version where the second I max point would be 1/8 wave above the base.

It would also suggest that I max is the point of maximum radiation.

 

[CDX-007]

I'm wondering how a mirror image (downward) radial cone + a tuned (3/4?) section of mast might add to the performance, making it a semi-center-gamma fed 1.5wl dipole.

Perhaps an experiment for a 3 meter version first!