Modified Vector 4000

[Shockwave]

ok. Thanks. Clears that up quite a bit.

Homer,

If your main radiator was not grounded at the base, then when you ground it expect to have to retune the antenna. For example if grounding the base makes the VSWR go up higher, don't get discouraged. You will have to adjust the gamma match again for the new condition and may find adding length to the main radiator helps now that it's grounded.

I know it doesn't appear to make sense at first that the radiator should be grounded at the base. This is common practice with a shunt fed gamma match and allows the entire antenna to be at DC ground for lightning protection. RF currents do not see this as a direct short at the resonant frequency.

 

[HomerBB]

Ok. Between the on and off rain here today, I went up on the roof prepared to ground the main vertical as explained. What I discovered is that in spite of my doubts at the time of construction, I had a very good ground between all the parts. I recall debating with myself about it, but knew some driven elements on Yagis were so constructed. I guess that put my mind at least in the try it and see mode.

I guess the next thing is an adjustment to the vertical length. I have a little room to play so I'll get after it when the weather will let me. Also I may look at rechecking the gamma for comparison to the exact dimensions recommended in the original specs.

Thanks again.

 

[bob85]

eddie,
i was looking at the current plot ( middle pic ) the colored lines show current phase and magnitude in the monopole and radials,
when i did the ez-bob plots to show where i think the antenna mode currents flow and in which direction they flow i had them correct and in the correct phase,
i made current magnitude equal to monopole current for easy viewing of current maxima/minima where real radials share the current giving the lower radial currents shown in the ez-nec plot,
the elevation plot ( bottom pic ) also agrees with what i have argued about the vector vs 5/8wave gain and radiation angle

 

[MrSuburban]

Marconi,

I think there are two reasons Henry's model shows a 24 foot radiator. EZNEC considers the portion of the main radiator below the gamma tap point to be a separate conductor. It's length is about 1.6 feet and needs to be added to the total main radiator. When you count the total length from the grounded connector bracket to the tip of the main radiator it's closer to 27 feet.

My changes to Henry's model are basically changing the cone dimensions to fit the new Vector. When Henry did his model he did not have the actual dimensions of the antenna. I'm very impressed with the model he made because it's gain is .02db higher then the new Vectors actual measurements produce. No changes I've made have effected the wonderful radiation pattern the antenna produces. They do effect gain and VSWR.

What I see is the when you lengthen the main radiator you have to move the gamma tap point up and the generator source to the same point. This also causes the minimum current point shown by the dip in the pink line to move upwards. Notice in the model that radiation currents are shown in pink with a separate current in phase coming from the base. Maximum gain occurs when the top of the radials and loop are placed right inline with the minimum current point. So they too should be lengthened with the main radiator.

I'm a Sirio distributor and I know they keep all of their box sizes at 56 inches long so they can be stacked on pallets and placed in containers. This means that the maximum size they can make the radials in two pieces is about 106.5 inches and still have a little overlap at the seam. Modeling shows that more gain can be had with a little more radial length. Expanding the loop diameter like I originally did, reduces gain.

What shocked me most about the computer model of this antenna was seeing a real radiation pattern for the first time ever. It shows why the antenna is a great performer for both line of sight and sky wave propagation. The radiation plot shows that the primary lobe actually consists of two major lobes. One centered at 8 degrees above the horizon and a second centered around 21 degrees. They combine and form a uniform signal (+/-3db) from 4 degrees to 27 degrees above the horizon. The lower angles are ideal for line of sight while the range of other angles will fit varying conditions of sky wave propagation.

I think Homer had to add the loop to get a good VSWR because his radials were not long enough to match without the loop. The radials and loop also play a part in the resonant frequency of the antenna. He surely could have found a length and angle that would match without the loop. We should also expect there will be some differences between the ideal computer model and the ideal antenna. For example the model suggests larger diameter main radiators will destroy the gain while real world tests prove it has much less impact. The model shows more gain with the radiator shorter then field tests do.

On the other hand the model shows more gain with longer radials. That is true and probably why Sirio lengthened them. The model gain at 5.55dbi is only off about a half db from Avanti's claim of 6.14dbi. The test equipment from 30 years ago probably had a +/- 0.5db tolerance.

I have used Eznec It doesnt look like he is starting the antenna above ground. the way it is design the bottom of antenna doesnt look to be above ground. does anyone have the eznec file they would like to share. im gonna input the wire's from the pic I see and see what that looks like.

 

[MrSuburban]

I believe my loop is too big on my Vector. When you read the original Avanti patent for the Sigma IV it suggests that increased gain can be had with a larger distance between the radial ends and the main radiator with a larger hoop. Extensive field testing an experimenting with the antenna in EZNEC+ are confirming the exact opposite!

Most of my work has been done perfecting an FM broadcast version of this antenna. It just so happens that it's radials are just over 1/4 wavelength with a 1/4 wave loop. Those radials are longer then the original Sigma and early Vector. I've been told the new Vector has radials that are longer then 1/4 wave. Bob85 has confirmed an increase in gain with the longer radials and is what I see at 98 MHz too.

I think we will find the new Vector has the right cone section and may be similar to Bob's 11 meter design. I question shortening the radiator from 7/8 wave to 3/4 wave on the new Vector because in field tests on 98 MHz .82 wave always shows the max gain. EZNEC+ however, does show the shorter radiator to give more gain. It also shows that the tighter the radials are to the main radiator, the higher the gain. This can present design problems at high power because the radials can arc over to the radiator if they are too close.

Henry has been kind enough to send me a model he made of the Vector in EZNEC+. I am new with working in this program and find building the model difficult. I am able to easily change any of the dimensions in the model to experiment. If anyone else out here works with EZNEC I would like to share this model with them in an effort to confirm the model is the most accurate representation of the basic Vector design.

Shockwave i have EzNec+ I have been working with Bob on this as well if you like send me the file, I see some things with the model that dont seem 100% looking at the few pics

 

[DXER]

Check the wire diameter for wires 2 and 9 on the wire sheet Bob posted.
Very interesting info from the group. Lets get the exact tube diameter,taper and lengths to update the nec file then adjust segmentation. Pulling up a chair.

 

[MrSuburban]

here you go, intersting plot,
i agree tube diameter ratios effect the antenna but i don't believe that gain falls like a stone.

thanks

Why do the element diameters look small? isnt the main radiator over 1.25" in model it seems as if they are all smaller than 1" is this MM instead of inches?? .039??

 

[Shockwave]

There does seem to be a problem with EZNEC when modeling this antenna. As has been said, the program reports close to unity gain or 0dbd when the antenna is modeled with diameters other then wire size. This result is in contradiction to in field tests with the antenna compared to a dipole in field strength tests. When modeled as a wire the antenna closely matches the results in the field from Avanti's original gain claims to what I've seen also.

Until someone can figure out this discrepancy, this seems to be the only way to get the model to match real world results. I think most people will agree radiator diameter effects bandwidth, not gain. The only logical way this should effect gain in the Sigma design is if the diameter change causes a change in distance between the cone and the radiator. Even when you keep this distance the same, the model still shows a huge drop in gain that is false.

 

[Marconi]

I think Dxer has a point about wires 2 & 9. Shockwave, could this problem also have anything to do with the way you have selected diameter in the "Control Center Units" section of Eznec?

 

[bob85]

shockwave,
look at how ez-nec handles transmissionlines and closely spaced wires, it shows a j-pole as an example,
heres what nec says about closely spaced wires,


Closely Spaced Wires Closely Spaced Wires

When modeling parallel or nearly parallel wires which are closely spaced, it can be very important to align the segment junctions. That is, they should be directly across from each other. This is particularly true if the segment length is greater than the line spacing. As the segment length gets shorter compared to the wire spacing, this requirement becomes less important. When segment junctions are misaligned but need to be aligned, results can change dramatically as segmentation is changed.
The easiest way to assure that aligned parallel wires have aligned segment junctions is to give the wires identical lengths and numbers of segments. If the wires are staggered or different lengths, break them into more wires. Make the portions of the wires which are directly across from each other into separate wires of equal length and number of segments. An example of this technique is shown below, where the long wire of a J-pole antenna was broken into two wires. Wire 4 is made the same length and given the same number of segments as parallel wire 6. The upper wire is segmented to make its segment length approximately equal to the segment length on wire 4. (This is a close up view – some of the long wire, and its number, aren't shown.)

 

[Marconi]

Bob, your point here is for sure true in the case of the Vector model. The 33 segments distributed over the 9' radials have to be much smaller and thus out of line compared to the same 33 segments distributed over the 26' radiator.

 

[Shockwave]

I'll admit I still have my EZNEC training wheels on. This is my first experience working in the program to any depth. I have been relying on the basic model that Henry made and have not changed any of the segments or diameters other then the main radiator diameter since this seemed to have the most impact on results.

I see where wires 2 & 9 are thinner then the rest. I'm not sure if Henry did that or if it was an oversight. The "Control Center Units" just seems to change from metric to inches or feet. I understand what Bob85 and Marconi are saying about segment lengths for the most part but still am not familiar with the program enough to make the changes.

That was my reason for wanting the work done so far to be on this site. In the hopes people with more EZNEC experience would have interest in making the model represent the actual antenna as closely as possible. Lets see where this goes.

 

[Henry HPSD]

Hello All,

Good to see the discussion continues !
The 1st model was made in a couple minutes as the sizes of the antenne were not known yet.
At this moment i have a model with the "true" lengths.
I still have problems with getting honest results as the program (no mather what segmentation length etc) keeps giving me "error". Or resson to believe it could be off.

The Problem with eznec is more or less a issue with any programme. It has limitations.
As one pumps up now.

Now, I have taken the liberty to ask Kirk Mcdonald at Princeton to look into it,
Told him about the discussion going on at this froum. He at his turn came up with mr Alan Boswell who is very familiar with Nec engineering.

My knoweldge of the programm is limited. I know how to use it not how it works.
Those guys know how it works. So if anyone can come up with a solution they can.

... my hopes up for them in getting "true" readings.

Just bougth a new house and very busy with that, havnt read all the other post yet..
But surely will do when time permits.

Kind regards,

Henry

 

[Marconi]

Hello All,

Good to see the discussion continues !
The 1st model was made in a couple minutes as the sizes of the antenne were not known yet.
At this moment i have a model with the "true" lengths.
I still have problems with getting honest results as the program (no mather what segmentation length etc) keeps giving me "error". Or resson to believe it could be off.

The Problem with eznec is more or less a issue with any programme. It has limitations.
As one pumps up now.

Now, I have taken the liberty to ask Kirk Mcdonald at Princeton to look into it,
Told him about the discussion going on at this froum. He at his turn came up with mr Alan Boswell who is very familiar with Nec engineering.

My knoweldge of the programm is limited. I know how to use it not how it works.
Those guys know how it works. So if anyone can come up with a solution they can.

... my hopes up for them in getting "true" readings.

Just bougth a new house and very busy with that, havnt read all the other post yet..
But surely will do when time permits.

Kind regards,

Henry

Henry could you post the new "wires" input details for your new model with "true" lengths, like Bob posted the other day as shown below? I would like to speculate some on the possible solution to this modeling problem from a common sense view point. It would also be nice if you could post any other output this model produced as well, like BW curve, pattern, antenna image including current flow and wire numbers.

Thanks for all your help.

 

[Henry HPSD]

Perhaps that isnt wise in this case,

I guess the file already has between 100...200 wires. ( a lot of wires take up for the ring) .
So that would be a very large Jpeg?..

I can send the eznec file to who ever wants it no problem just contact me at:
19sd348@planet.nl and ill send the file as a .ez file.

The file sofar has limitation. As mentioned i have asked kirk who at his turn asked allen to verify the file. Now to my believe these guys are the top of the antenna world..
They are at least a lot closer than where we are standing lol...

If that is done we can say with certainty what the antenna does.
So first confimation of the file (since i am not a expert either!)
And than we can post findings...
Trust is good....confirmation is better !

73 H.