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Author Topic: home-brew sigma 4 11 meter antenna  (Read 71643 times)
KB1GMX
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Posts: 773




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« Reply #45 on: January 30, 2013, 04:24:53 PM »

I'm with Dale and Tom.

To the thread author, good for you on building and testing.  Now do the rest of the work and
find out what and why this behave differently.

As a youngin I beat my head against antennas for 11M.  In that time frame I was willing to
experiment, a lot. The best designs that were reproducible and worked were standard 1/4wave
ground plane, properly constructed and tuned vertical halfwave, 3 element beam 8ft boom, and a
4 element beam with a 16ft boom.  Most all the rest were variations on a theme and didn't make
the meter move much.   One thing I'd learned well then is SWR meters can LIE and textbook
built it designs were often incorrect or not complete.    There were also the failures and high
claims that didn't pan out.  I was lucky enough to have someone that could loan me the basic
gear to make measurements and calibrate my crude instruments.  I also understood enough
about propagation to know it was not an indicator but an engineer ham/CB  friend 3 miles
away was a reliable signal indicator of better or worse and Texas was not.

If cutting the coax tunes the antenna, impedance mismatch, shield currents[balance issues],
coax issues, measurement error.  Learned them all by the reliable way, doing it wrong and
learning what it was. 

The first issue measurement of tuning was by far the most problematic as back then
(late 60s early 70s) a SWR bridge was big deal.   When I played with initial antennas I
found the few meager watts worked better to the right length vertical than the wrong. 
When I started getting and reading "Ham Radio" back in '68 I learned how to measure
impedance, and wheatstone bridges and how to apply it at RF (return loss bridge) and
that was eye opening as I was still in high school then.  Thank ARRL for the Handbook,
and a long list of others from the library. 

After about 40+ years nothing has changed in advertising.   But with serious computing power
now and good models and rigorous testing using accurate network analyzers to see if the real
thing verifies against the models.

Most all the antennas I know of have theory and cites dating back to before the 50s and some
the 20s and every time I see something claimed as new its a rehash of the old or totally bogus.
Its science not magic.  Physics and theory often crushes wild guesses and claims.

Small differences in small antennas are not going to be strikingly different.  However lousy
implementations are rampant.   To this day I use well known antenna types properly matched
and installed as well as possible.  I get the results I expect, the rest is propagation.  But I still
experiment a lot and I still create (and recreate) failures.


Allison
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KE1IZ
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Posts: 30




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« Reply #46 on: January 30, 2013, 10:34:53 PM »

I was an engineer at Avanti when the Sigma IV design was being built. It's a J pole- nothing more and nothing less. No magic,

Dale W4OP

Dale, I know this is an old thread but I'm hopeful you may see this and have the time to respond. I see that you did indeed work for Avanti and have progressed well beyond that today. I am fascinated by the old Sigma design and currently work with Sirio on the model scaled to the FM broadcast band. I see there are lots of people here that I could learn from including yourself. With that in mind please don't take my questions as offensive or doubting your honesty. The goal is to have a better understanding of how this antenna works since there are such widely different views of how it works or doesn't.

I'm curious as to why your view is not closer to what Herb claims in his patent on the design? I know there are similarities to the J-pole however, this is a Coaxial J-pole that also shares similar characteristics to the Skeleton Sleeve monopole. The key differences here are why L.B. Cebik called it a "non apparent collinear". Just like the 5/8 wave ground plane has the bottom 1/8 wave current inverting, the 3/4 wave would have the bottom 1/4 wave current inverted on the vertical radiator. When you sweep the radials upwards more like the Skeleton Sleeve, you are now confining the out of phase radiation along the lower 1/4 wave section of the vertical radiator so that it is not able to combine with the pattern in the far field.

The currents along the radials of the 5/8 wave are out of phase with those at the base of the vertical radiator. Why is it so hard to think that Herb scored two birds with one stone simply by sweeping those radials upwards on the 3/4 wave? First he used those radials to stop that pesky out of phase base radiation on the vertical that Tom speaks of in the 5/8 wave. By having the radials close to the same vertical axis as the main radiator, he scored his second bird using those currents to form the second current node that reinforces the top 1/2 wave. That makes the company claim that the "entire antenna radiates effectively" plausible and is something that I'm able to prove today. It also falls in line with the brief comments that L.B. was willing to make when he called it the "non apparent collinear".
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MARCONI390
Member

Posts: 14




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« Reply #47 on: February 04, 2013, 06:35:01 PM »

I was an engineer at Avanti when the Sigma IV design was being built. It's a J pole- nothing more and nothing less. No magic,

Dale W4OP

Dale, I know this is an old thread but I'm hopeful you may see this and have the time to respond. I see that you did indeed work for Avanti and have progressed well beyond that today. I am fascinated by the old Sigma design and currently work with Sirio on the model scaled to the FM broadcast band. I see there are lots of people here that I could learn from including yourself. With that in mind please don't take my questions as offensive or doubting your honesty. The goal is to have a better understanding of how this antenna works since there are such widely different views of how it works or doesn't.

I'm curious as to why your view is not closer to what Herb claims in his patent on the design? I know there are similarities to the J-pole however, this is a Coaxial J-pole that also shares similar characteristics to the Skeleton Sleeve monopole. The key differences here are why L.B. Cebik called it a "non apparent collinear". Just like the 5/8 wave ground plane has the bottom 1/8 wave current inverting, the 3/4 wave would have the bottom 1/4 wave current inverted on the vertical radiator. When you sweep the radials upwards more like the Skeleton Sleeve, you are now confining the out of phase radiation along the lower 1/4 wave section of the vertical radiator so that it is not able to combine with the pattern in the far field.

The currents along the radials of the 5/8 wave are out of phase with those at the base of the vertical radiator. Why is it so hard to think that Herb scored two birds with one stone simply by sweeping those radials upwards on the 3/4 wave? First he used those radials to stop that pesky out of phase base radiation on the vertical that Tom speaks of in the 5/8 wave. By having the radials close to the same vertical axis as the main radiator, he scored his second bird using those currents to form the second current node that reinforces the top 1/2 wave. That makes the company claim that the "entire antenna radiates effectively" plausible and is something that I'm able to prove today. It also falls in line with the brief comments that L.B. was willing to make when he called it the "non apparent collinear".

KE1IZ, I agree with your comments above regarding a 5/8 wave radiator. I also see the 4 radials on the Vector are pretty much parallel to the radiator like you describe, but it looks to me like they are out of phase too. I would bet...if the maximum currents in the base of these 4 radials were summed that result would be very close to equaling the maximum currents in the base of the radiator as well. Isn't this condition what we have when cancellation occurs...just like we hear described regarding the 5/8 wave radiator?
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W4OP
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Posts: 422


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« Reply #48 on: February 05, 2013, 05:57:07 AM »

The antenna is  a Jpole and has no relationship to the coaxial sleeve antenna. The bottom of the sleeve on a coaxial antenna is high impedance, the top is a current maxima- just the opposite of the Sigma 4. In addition the sleeve on the coaxial antenna has, because of skin effect opposite currents on the inside and outside.

The reason Herb flared the 1/4 wave arms on the Sigma IV was threefold fold:
1. It made it mechanically easier to build the antenna
2. It likely acts as a transmission line whose characteristic impedance increases as the separation increases. This would be more important if this were a more traditional transmission line with dielectric, as it would reduce the losses in this matching section.
3.It looked cooler

It could still benefit from a means to choke off common mode currents as there is no reason, the support mast and feedline would not have current on them. A quick EZNEC model will show this.
Model the antenna and then put a quarter wave wire from the bottom of the antenna extending downward (admittedly, a worst case scenario, but done to prove a point). That wire would represent the mast and/or the outside  of the coaxial shield. Rerun the model and look at the currents on that quarter wave wire.

Dale W4OP
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MARCONI390
Member

Posts: 14




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« Reply #49 on: February 05, 2013, 07:09:26 AM »

The antenna is  a Jpole and has no relationship to the coaxial sleeve antenna. The bottom of the sleeve on a coaxial antenna is high impedance, the top is a current maxima- just the opposite of the Sigma 4. In addition the sleeve on the coaxial antenna has, because of skin effect opposite currents on the inside and outside.

The reason Herb flared the 1/4 wave arms on the Sigma IV was threefold fold:
1. It made it mechanically easier to build the antenna
2. It likely acts as a transmission line whose characteristic impedance increases as the separation increases. This would be more important if this were a more traditional transmission line with dielectric, as it would reduce the losses in this matching section.
3.It looked cooler

It could still benefit from a means to choke off common mode currents as there is no reason, the support mast and feedline would not have current on them. A quick EZNEC model will show this.
Model the antenna and then put a quarter wave wire from the bottom of the antenna extending downward (admittedly, a worst case scenario, but done to prove a point). That wire would represent the mast and/or the outside  of the coaxial shield. Rerun the model and look at the currents on that quarter wave wire.

Dale W4OP

Thanks Dale, I agree.

I also happen to disagree with the idea this antenna has some collinear aspects that provides improved gain, but if it does it comes no where near producing the type of gain advantage that a true 1/2 wave collinear should produce.

As noted above, some feel the bottom basket cancels the current for the bottom of the radiator, and provides commom mode currents on the out side of the radials that radiate in-phase with the top 1/2 wave radiator. I don't see that possible, but I do think the antenna provides a little improvement in gain over the shorter 5/8 wave antenna, just by the simple fact the maximum 1/2 wave current node is raised higher by a few feet. I see that in my own testing with feed points at the same height.

I see no mystery or magic in this antenna, but I appreciate the design and seeming effectiveness at my locations all the same.
« Last Edit: February 05, 2013, 07:34:06 AM by MARCONI390 » Logged
KE1IZ
Member

Posts: 30




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« Reply #50 on: February 05, 2013, 08:22:10 AM »

The antenna is  a Jpole and has no relationship to the coaxial sleeve antenna. The bottom of the sleeve on a coaxial antenna is high impedance, the top is a current maxima- just the opposite of the Sigma 4. In addition the sleeve on the coaxial antenna has, because of skin effect opposite currents on the inside and outside.

The reason Herb flared the 1/4 wave arms on the Sigma IV was threefold fold:
1. It made it mechanically easier to build the antenna
2. It likely acts as a transmission line whose characteristic impedance increases as the separation increases. This would be more important if this were a more traditional transmission line with dielectric, as it would reduce the losses in this matching section.
3.It looked cooler

It could still benefit from a means to choke off common mode currents as there is no reason, the support mast and feedline would not have current on them. A quick EZNEC model will show this.
Model the antenna and then put a quarter wave wire from the bottom of the antenna extending downward (admittedly, a worst case scenario, but done to prove a point). That wire would represent the mast and/or the outside  of the coaxial shield. Rerun the model and look at the currents on that quarter wave wire.

Dale W4OP

It may not be magic but it's the only omni antenna I've ever seen that develops two in phase current nodes with no insulator, phasing coil, or delay line. All one needs to do to see this is stop playing with EZNEC and look at the CST models. Alternately, anyone who might happen to have CST Microwave Studio is strongly invited to build their own model that would confirm beyond any doubt this antenna has two in phase current nodes. The currents on the 1/4 wave counterpoise you speak of would work just like a coax or mast and would be in phase with the radiation currents of the antenna along its 1/4 wave length.
« Last Edit: February 05, 2013, 08:28:27 AM by KE1IZ » Logged
W4OP
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« Reply #51 on: February 05, 2013, 09:29:14 AM »

My only comment is your phrase "current nodes" - this refers to a point of current minima or voltage maxima- is this what you are referring to? I would think you mean current loop. If we don't have the terminology correct then we can't make any progress.

There is nothing in the EZNEC model that violates any of its rules.
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KE1IZ
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Posts: 30




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« Reply #52 on: February 05, 2013, 09:55:17 AM »

With respect to "current nodes", I may very well have used the incorrect term. I was referring to a pair of separate in phase current peaks on the Sigma. One on the cone and one on the upper 1/2 wave radiator.

It may also be helpful to examine the CST model at the following link: http://fmbroadcastantenna.com/images/Dominator%20NWE-34%20in%20CST.gif
This is an animation that is produced entirely  by the CST software based on the antenna dimensions inputted in the program. It displays radiation current, phase and magnitude along the radiator throughout one complete 360 degree sinewave of drive.
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W4OP
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« Reply #53 on: February 05, 2013, 11:09:04 AM »

Something doesn't seem right with that model as it shows a current node at the base, if I understand what I am looking at. We know this is a low Z point and should be a current loop.

Dale
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KE1IZ
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Posts: 30




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« Reply #54 on: February 05, 2013, 11:20:15 AM »

Something doesn't seem right with that model as it shows a current node at the base, if I understand what I am looking at. We know this is a low Z point and should be a current loop.

Dale

Please look at the model again as it shows exactly what you're expecting in this area. Currents at the base of the vertical reach a current maxima of 2.37 amps per meter as indicated by the CST chart on the right.
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WB6BYU
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Posts: 13248




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« Reply #55 on: February 05, 2013, 11:53:39 AM »

After watching the image, then stopping it for a closer analysis, I don't think it
shows anything different than EZNEC does.

Yes, current is maximum at the bottom of the vertical radiator (inside the bottom
"cage".  But the currents are out of phase between the "cage" and the bottom
portion of the radiator:  there is a high field strength in the space between them,
but not nearly as much actually radiated.

The field strength on the outside of the "cage" is much lower than inside, and
extends outwards more due to the proximity to the outside of the cage than
anything else.  (So if you measure the field strength at a fixed distance from
the main radiator, it will appear to have a maximum near the top of the cage
simply because the cage extends out closer to the measurement point.)  The
fact that the field strength around the cage is maximum at the top (a high
voltage point) rather than at the bottom (a high current point) would support
this.  The display doesn't show the net effect in free space, or even
as much as 1/4 wave from the radiator itself, where I would expect the fields
around the bottom section to be much lower than around the upper half wave.

So, in spite of the high currents in the bottom matching section and the high
field strength inside the cage, the actual field strength that it contributes to
overall radiation appears to be rather low (though difficult to tell from the narrow
plot).  That's what I would expect any good modeling program to show.  At
that point you are down to analyzing how the exact construction of the cage
affects the phase and amplitude of the radiation from it relative to the main
conductor.

But from studying the plot I'd say that it does NOT support the being any
significant gain advantage over a half wave radiator.
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KE1IZ
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Posts: 30




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« Reply #56 on: February 05, 2013, 01:13:43 PM »

After watching the image, then stopping it for a closer analysis, I don't think it
shows anything different than EZNEC does.

Yes, current is maximum at the bottom of the vertical radiator (inside the bottom
"cage".  But the currents are out of phase between the "cage" and the bottom
portion of the radiator:  there is a high field strength in the space between them,
but not nearly as much actually radiated.

The field strength on the outside of the "cage" is much lower than inside, and
extends outwards more due to the proximity to the outside of the cage than
anything else.  (So if you measure the field strength at a fixed distance from
the main radiator, it will appear to have a maximum near the top of the cage
simply because the cage extends out closer to the measurement point.)  The
fact that the field strength around the cage is maximum at the top (a high
voltage point) rather than at the bottom (a high current point) would support
this.  The display doesn't show the net effect in free space, or even
as much as 1/4 wave from the radiator itself, where I would expect the fields
around the bottom section to be much lower than around the upper half wave.

So, in spite of the high currents in the bottom matching section and the high
field strength inside the cage, the actual field strength that it contributes to
overall radiation appears to be rather low (though difficult to tell from the narrow
plot).  That's what I would expect any good modeling program to show.  At
that point you are down to analyzing how the exact construction of the cage
affects the phase and amplitude of the radiation from it relative to the main
conductor.

But from studying the plot I'd say that it does NOT support the being any
significant gain advantage over a half wave radiator.

We still have deep red and blue colors on the outside of the cone that indicates the maximum radiation current of 2.37 amps is still there in spite of the opposing currents inside the cone. Closer examination of these colors as they expand outward show they are closer in magnitude than they appear on the radiator itself. This is due in part because the currents on the cone are split between the 4 upward radials but combine as the field spreads out. If we know the cone is a 1/4 wave then the width of the image suggests we are seeing what happens approximately 1/8 wavelength away. Granted that's not far field but one can get a sense of how these two in phase currents are combining.
« Last Edit: February 05, 2013, 01:15:50 PM by KE1IZ » Logged
WB6BYU
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Posts: 13248




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« Reply #57 on: February 05, 2013, 02:36:56 PM »

Quote from: KE1IZ

We still have deep red and blue colors on the outside of the cone that indicates the maximum radiation current of 2.37 amps is still there in spite of the opposing currents inside the cone...



But that field strength is only when you are very close to one of the conductors in
the cone.  In the same way you can have high field strength around each wire in
a length of open wire line, but the net radiation is nearly zero when the currents are
properly balanced.  The same applies to the conductors in the cone:  when you are
much closer to one wire than the rest (whether on the inside or the outside) the field
strengths will be high, but it drops off quickly with distance.  That, and the fact that
the field is stronger near the end of the cone where the current are lower, would
suggest that it doesn't correspond with far-field radiation.

However, EZNEC will give you near-field levels and currents in each element - it would
be interesting to compare those with the results from the CST model to see if there
are any significant differences.
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KE1IZ
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« Reply #58 on: February 05, 2013, 04:41:36 PM »

The conductors on the cone are not parallel with the vertical and the antenna is not a balanced design. Not to be redundant but if you look at the currents in the cone as they radiate away from the antenna, the colors that represent magnitude and phase are effectively combining with the top 1/2 wave as we approach the far field.

I've also experimented with EZNEC and field tests long enough to realize EZNEC cannot model this design accurately at all. It's a complete failure when it comes to identifying radiation currents on the outside of the cone. EZNEC sees the cone as nothing more than transmission line that alters impedance.

When I brought this to Roy's attention he insisted it was impossible for his program to misidentify any currents. That made me determined to get to the truth behind this design. If your thoughts are it's only a half wave radiator, why doesn't a half wave phasing section allow you to effectively stack another 1/2 wave radiator on top of the existing antenna to form a collinear antenna? The phasing section must be shrunk all the way down to 1/4 wave before the added top 1/2 wave enters the same constructive phase as the lower 3/4 wave.

This is only possible because the cone not only confines out of phase radiation at the base of the vertical, it also adds its own radiation current into the pattern that is in phase with the portion above the cone. If the cone were only a method of feeding the antenna that did not effectively add to the gain, then you would have to invert the phase 180 degrees to drive a second half wave collinear radiator.

If you place a folded 180 degree phase line with a second half wave vertical radiator above, on top of an existing Sigma antenna and take the time to readjust all the newly introduced reactance out of the system at the gamma match, there is virtually no change in gain from the original antenna. This is because 50% of the 1/2 wave you added to the top will now be bucking the phase of the cones radiation and it takes the second 50% of the top 1/2 wave to return you to a point close to where you started.

On the other hand, if you place a 90 degree phasing section with a second 1/2 wave radiator on top of a properly tuned Sigma, it takes about 2 db of attenuation to return the signal back to where it was with the original Sigma. There is no noticeable change in the resonant frequency or increase in reactance and almost no retuning of the matching network is required.

The collinear design is not practicle on most wavelengths and in applications where it is, testing shows you'll achieve more gain with less complications by just stacking a pair of separate antennas driven in phase through a divider. Non the less, a copy of the design I described works and is on file with the PTO.
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KE1IZ
Member

Posts: 30




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« Reply #59 on: February 05, 2013, 08:53:29 PM »

The antenna is  a Jpole and has no relationship to the coaxial sleeve antenna. The bottom of the sleeve on a coaxial antenna is high impedance, the top is a current maxima- just the opposite of the Sigma 4. In addition the sleeve on the coaxial antenna has, because of skin effect opposite currents on the inside and outside.

The Coaxial sleeve antenna is not the one I was comparing the Sigma with. It was the Skeleton sleeve monopole shown here: http://k6mhe.com/files/ssfm.pdf
When I first learned of this antenna it became much easier for me to see why the Sigma is often mistaken for a 1/2 wave J-Pole. There are only 3 small differences in the designs.

1) The Sigma uses a gamma to feed the 3/4 wave vertical.

2) The Sigma uses a loop to connect the tops of the four "radials".

3) The Sigma uses a tapered cone that surrounds the vertical.

Of these 3 differences what one could cause the base of the Sigma to radiate? Simple RF theory tells us it would be number 3. The 1/4 wave elements would have to be parallel and have equal but opposite currents for cancellation to occur. They obviously are not parallel and there is strong evidence that they may not be equal either. It's possible CMC flowing over this 1/4 wave counterpoise is contributing to an imbalance. Tapering is the key reason behind the radiation from the cone.
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