Modified Vector 4000

[Marconi]

It was the late L.B. Cebik that said the Sigma was a non apparent collinear radiator. Bob 85 has posts in this forum with regards Cebik's comments. Cebik suggests that this collinear is somehow formed by folding the radials upward. All I can say is if you place the radials in any orientation other then folded up, this antenna is a complete flop! I think the gain is all about the radials causing the bottom section to radiate in phase with the top. Fold the radials down on the same 3/4 wave radiator and the antenna has less gain on the horizon then a 1/4 wave.

I agree SW, I don't think anybody is confused on the issue that the radials need to be raised for the Sigma 4 to work like it does. Seeing as I don't know for sure whether the Sigma/Vector works based on compression or due to its being collinear like Bob tells us Cebik told him, I wouldn't argue either way. Both ways seem plausible to me.

I would like to see the actual amp readings per segment though, just maybe it might indicate if there is a normal magnitude of currents flowing in the top 1/2 wave or not. At the very least it might indicate that the 1/4 wave currents are stronger and then we could wonder about that. Such a consideration might lend some credence to the Sigma being collinear or not. Why else would the current magnitude in the 1/4 wave appear, in most of the images so far, to be almost double that of the 1/2 wave up above? Maybe the guy claiming the current in the top 1/2 wave is less, is correct.

Dxer or the others, does your output have a report that records the actual current in amperes per segment?

Just and idea guys, cause I really don't know the truth.

 

[Shockwave]

I could be wrong here but I believe the magnitude of the current is indicated by how far the pink current curve moves away from the radiator. Notice the currents in the radials are much lower in magnitude. It seems to me that the current is equal in both the lower portion and upper portion of the main radiator. The difference I see is that the current in the top half completes a half wave cycle while the lower current only forms a 1/4 wave cycle with both in phase. I also notice that there are two low current points on the main radiator. One near the loop and one near the top of the radiator. Both of these points should have very high RF voltage and show strong radiation. In my opinion Bob 85 was fortunate to be able to have input from L.B. Cebik. I wish I had that opportunity because I believe he is one of the brightest people to examine the Sigma design. Further comment would have been awesome but I understand his reluctance to get involved with this controversial design.

 

[Marconi]

I could be wrong here but I believe the magnitude of the current is indicated by how far the pink current curve moves away from the radiator. Notice the currents in the radials are much lower in magnitude. It seems to me that the current is equal in both the lower portion and upper portion of the main radiator. The difference I see is that the current in the top half completes a half wave cycle while the lower current only forms a 1/4 wave cycle with both in phase. I also notice that there are two low current points on the main radiator. One near the loop and one near the top of the radiator. Both of these points should have very high RF voltage and show strong radiation. In my opinion Bob 85 was fortunate to be able to have input from L.B. Cebik. I wish I had that opportunity because I believe he is one of the brightest people to examine the Sigma design. Further comment would have been awesome but I understand his reluctance to get involved with this controversial design.

Well SW, you're wrong, else you made another type-O. RF is emitted from the maximum current node, not the voltage node.

Edit: You are right the current in the radials is small, but I'm not talking about the radial current. I'm talking about the current from the bottom 1/4 wave radiator. IMO this is the part that should be radiating the 1/4 wave signal and if not what is the red line to the left of the radials that is supposed to be in phase with the top 1/2 wave. Is that just a fluke that means nothing. Maybe we just have a difference in what is radiating at the bottom the radiator part or the radials. You are right if the radials are doing all the radiating. I didn't realize you were talking about the current on the radials until Bob and I exchanged words. I would argue that the radials do like radials do, they have current flowing, but the RF is cancelled due to the rule that applies, as equal and opposite current flows.

You are right about the magnatitude being in red I believe. I did say most of the images showed greater current flow in the 1/4 wave, except for the image of your model that Bob posted, and I didn't see any current flow in Henry's model, it was too small.

 

[Shockwave]

Marconi, I do make mistakes at times. I can't blame it on a typo because it is what I thought. I based this on two things I've noticed over the years with antennas. Even before I had a field strength meter, I used a fluorescent light bulb. The bulb always glowed more intense towards the top of the antenna and away from the feed point. With most shorter verticals like the 1/4 wave and 1/2 wave the current is reduced uniformly as we move away from the feed point as I'm sure you know. If maximum radiation occurs at maximum current, why does the light bulb and field strength meter show more energy near the points of high RF voltage. In the back of my mind I'm remembering something about the electromagnetic and electrostatic components of RF radiation and how it relates to phase that is causing me to reconsider. If someone can refresh my memory here it would be helpful.

 

[bob85]

eddie,
the current magnitude in the ex-nec plot is higher in the upper 1/2wave than it is in each radial as expected, i drew current mag equal in my drawing just to make it easy to see where i thought current maximas were, as you can see when real mag is plotted its much more difficult to see where current maxima is with forum sized pictures,

omnidirectional colinears get their gain by effectively compressing the pattern from two or more elements into a narrower vertical plane but stronger lobe or lobes,

after what cebik told me and reading about some of necs limitations including the ones you pointed out, i don't have much faith at the moment in necs ability to accurately model the vector using tapered tubes and dissimilar diameter radials,

however, the models presentsed here do show a pattern that seems to agree with how the antenna performs vs a conventional 5/8 over 1/4wave colinear,
the modified sigma gets very close to the ham big-mac at distance without having a notably very narrow major lobe which i have talked about in the past,
is it a coincidense that the big-mac signals sway up and down markedly when the wind sways the antenna while signal fluctuation in wind when using the modded sigma is less pronounced,

heres a plot for a 5/8 over 1/4 colinear,

another clue may be in avanti's trade marked CO-INDUCTIVE, heres something i found about coinduction,

coinduction is a technique for defining and proving properties of systems of concurrent interacting objects
As a definition or specification, coinduction describes how an object may be "observed", "broken down" or "destructed" into simpler objects.

 

[Marconi]

Marconi, I do make mistakes at times. I can't blame it on a typo because it is what I thought. I based this on two things I've noticed over the years with antennas. Even before I had a field strength meter, I used a fluorescent light bulb. The bulb always glowed more intense toward the top of the antenna and away from the feed point.

I am speculating, but I would figure seeing a response using a fluorescent has something to do with needing a source of high voltage to get started. The Field Strength meter detector should have an RF detector, but if you get it close to the voltage end of an antenna too, it might also respond better or worse, I'm not sure. I do know that the FS meter does not require you to virtually touch the source of power like it does with the fluorescent bulb to see a response. I've seen the FBulb remain lighted out for some feet once it gets started, but I've never seen one start unless one end touches or is very close to the antenna and maybe that needs to be at the top where the high voltage occurs. How say you?

With most shorter verticals like the 1/4 wave and 1/2 wave the current is reduced uniformly as we move away from the feed point as I'm sure you know. If maximum radiation occurs at maximum current, why does the light bulb and field strength meter show more energy near the points of high RF voltage.

This is true. It might be the nature of the type of detector for the FS Meter or the requirement to start the FBulb.

In the back of my mind I'm remembering something about the electromagnetic and electrostatic components of RF radiation and how it relates to phase that is causing me to reconsider. If someone can refresh my memory here it would be helpful.

I would be curious to know what you think about this issue, it sounds interesting.

 

[Marconi]

Bob, you're looking at the current on the radials and I'm looking at the RF radiated by the 1/4 wave part of the radiator below the hoop. The part you talk about when you tell me these two elements are in phase.

The current in the radials probably cancels out if theory is important. If we assume the magnitude of the current is represented by how far the red line is from the radiator, then which currnet shows the max current in the image below? Dxer shows other antenna images that are similar.

Bob if you check SW's model for the same space between the mast and the red line for the 1/2 wave it measures 2cm and the 1/4 wave measures between 3-4cm. The magnitude of curren is greater in the 1/4 wave here also. For me, this is why we see the bottom lobe, created by the 1/4 wave section, with greater gain.

 

[Shockwave]

Marconi, It is easier to see in your model that the current is higher in the lower section. That is also characteristic of a collinear antenna. As you move away from the feed point, each section would have less energy to feed the next. It's been a while since I used a fluorescent light bulb but as I recall the distance from the tip of the antenna required to ignite the bulb depended entirely on the power level used. I could get the bulb to light with no contact. The field strength meter used the standard germanium diode RF detector. Both methods show stronger results towards the top of the antenna (102 inch whip). I briefly searched around for information that would confirm the point of maximum radiation with respect to voltage and current finding nothing so far. Now you have me curious for the correct answer.

 

[bob85]

eddie,
you must be looking at the current in the lower portion of monopole as that is shown as having the highest magnitude, the currents shown in the radials are smaller than monopole current, i think it should be monopole base current divided by the number of radials in an isolated system,

antenna pattern is a fuction of how the two elements of the antenna interact, the pattern having a strong lower lobe does not mean that the antenna radiates most from its lower portion,
having high radiating currents in the base of a regular 3/4wave monopole would spoil the pattern as you know, its longer than .64wave,
avanti advertised that the sigma radiates most of its signal from its upper portion,
it is my beliefe that it also produces some radiation from the radials in phase with the upper 1/2wave-5/8wave and prevents the currents in the lower 1/4wave of monopole disrupting the pattern,

there is also a question mark over contra flowing transmissionline mode currents which im sure flow at least in some part of the design even if its only within the gamma section,
ez-nec seems to ignore this and im not sure if it needs to be included as in open sleeve antennas.

 

[bob85]

shockwave, you will find maximum voltage at the tip and max current at the base,
longer antennas like 5/8waves ect have max voltage at the tip and current max 1/4wave below it, i think max radiation comes from the current maxima

 

[DXER]

Hope this info may help with the discussion on the Vector. I should have taken the info from the Sigma4 file since it uses know dimensions but I used the 26.6' Vector. The 1st pic is with currents on. The 2nd is current and phase. Also attached is the output file showing more info.

Post edit. Just added the file with the wire #'s turned back on for the text file.
1-9 is the main Vert
10-13 are the radials
14 and up the hoop.

 

[Shockwave]

shockwave, you will find maximum voltage at the tip and max current at the base,
longer antennas like 5/8waves ect have max voltage at the tip and current max 1/4wave below it, i think max radiation comes from the current maxima

The part we are curious about is where is the point of maximum radiation in relation to current and voltage along the radiator? From what I've seen with the field strength meter, it seems to be where RF voltage is highest. I don't want to be spreading misinformation if I'm wrong.

 

[bob85]

i have read that max radiation comes from current maxima point but i don't think near field measurements tell the true story of radiation or at least the distant kind we are interested in,

http://www.majr.com/docs/Understanding_Electromagnetic_Fields_And_Antenna.pdf

"The near and far fields have other
characteristics. The shape of the near
field is closely related to the structure of
the source,whereas the far field becomes
independent of the source, taking the
form of spherical waves. At large distances,
the far field takes the form of traveling
plane waves"

 

[Shockwave]

Thanks Bob,

That was an interesting read. Makes me want to install a loooong wire under a transmission line and siphon off some power from the electric company. I realized that there are big differences between what the near field looks like around the antenna and what the usable far field pattern would look like. I must admit I'm still having difficulty grasping how more radiation is emitted from the maximum current base feed point on antennas like the 1/4 wave and 1/2 wave.

Beyond the field strength meter and primitive light bulb test, there are still other issues that keep me lost on this one. Over the years working with antennas I have made some stupid mistakes and earned a few minor RF burns. Putting aside direct contact with an antenna, I've seen where close proximity to the antenna can cause RF heating in your hand. Also noticing a direct interaction with more thermal heating being present higher up on the radiator. Hasn't anyone else ever experienced something similar to this with hand held VHF or UHF transceivers that make several watts or more?

Not recommended, but putting your hand around the insulated loading coil of a base loaded 1/4 wave will usually produce no thermal heating at around 5 watts. If the antenna is top loaded, you can often feel heat. Unkey the hand held and feel the coil with your other hand and it's cold. Isn't this heating produced by RF radiation into your hand since you are insulated from the antenna?

I think I just figured this out and how it fits with the link Bob posted. It's not radiated energy into your hand, it's reactive energy because you just formed a capacitor by placing your hand around the insulated coil. Further confirmed by the fact this causes a change that can be seen at the source. Then it begins to make sense why the effect is more pronounced with increased RF voltage.

I learned something new again here on this forum. Thanks Marconi and Bob on the follow-up. Now it has me asking all types of new questions like how far away from the radiator do you need to be in terms of wavelength before the field is no longer reactive and it becomes radiated energy? I'm guessing a wavelength or more since that's where I begin to lose the ability to measure change at the source.

 

[bob85]

i can feel the effects of cellphones handietalkies or standing close to several hundred watts from a 1/4wave mobile antenna,

"Now it has me asking all types of new questions like how far away from the radiator do you need to be in terms of wavelength before the field is no longer reactive and it becomes radiated energy"

good question, i would say just beyond the fresnel region though there does not seem to be complete agreement on where that stops and the farfield starts, i have seen it said that nearfield measurements are usually taken @ 3-10 wavelengths out so you would want to be well beyond that to ensure you are past the fresnel region.