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Modified Vector 4000
- Thread starter bob85
- Start date
Alexis
if you can put the analyser right at the antenna thats what i would do, if not as short a coax jumper as possible,
if been stood near it seems to mess with readings you can always stand back & look through binoculars to see if the reading moves,
edit
look at page 35 of the mfj user manual, it tells you how to tune odd 1/4wave coax & 1/2wave multiples,
and suggests a second method by using the distance to fault function
if you can put the analyser right at the antenna thats what i would do, if not as short a coax jumper as possible,
if been stood near it seems to mess with readings you can always stand back & look through binoculars to see if the reading moves,
edit
look at page 35 of the mfj user manual, it tells you how to tune odd 1/4wave coax & 1/2wave multiples,
and suggests a second method by using the distance to fault function
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Dr_Dx,
if it was easy people would not be tuning for resonance through coax,
its that line of thinking over simplifying a complex subject that has gotten us in this mess to start with,
taking half an idea "resonance" which is a good thing and making our own laws up about how it works,
you don't have to trawl through Cebik's reflections, it tells you in the user manual about coax shifting resonance & people still don't get it,
even jumping on folk that are doing it right telling them they are doing it wrong when the opposite is true,
so it can't be easy for many people
1/2wave of coax is 2x 1/4wave multiple,
we are not talking odd 1/4wave multiples that invert a mismatched impedance looking from the rig end,
its any 1/4wave multiples for resonance but not for impedance,
1/2wave tuned for the frequency of choice will repeat the antennas impedance at the input when its matched to the coax AND when its not matched to the coax,
none 1/2wave multiples will transform the impedance to something other than the antennas impedance whenever the antenna is mismatched to the coax,
odd 1/4waves multiples will invert the impedance whenever the antenna is mismatched to the coax,
BUT we are not talking about impedance,
people are tuning for resonance through coax thinking its showing the antennas resonant frequency which its not, you are measuring system resonance coax & antenna,
and that causes a worse match between coax and antenna with higher reflect and the antenna is not resonant,
1/2wave multiple is OK for impedance at your chosen frequency, but you still don't know where the antenna is resonant,
if its not resonant on the same frequency as the coax is cut for exact 1/4wave multiples the coax cancels antenna reactance making X=0 somewhere other than where the antenna is resonant.
And your back to where you started with antenna resonance not where you just tuned it to and that causes higher reflect & lower efficiency than if you tuned for minimum vswr,
since minimum vswr on the line occurs at the self resonant frequency of the element its feeding and adding reactance to the antenna to move resonance seen at the rig end causes higher reflect.
hope this helps.
if it was easy people would not be tuning for resonance through coax,
its that line of thinking over simplifying a complex subject that has gotten us in this mess to start with,
taking half an idea "resonance" which is a good thing and making our own laws up about how it works,
you don't have to trawl through Cebik's reflections, it tells you in the user manual about coax shifting resonance & people still don't get it,
even jumping on folk that are doing it right telling them they are doing it wrong when the opposite is true,
so it can't be easy for many people
1/2wave of coax is 2x 1/4wave multiple,
we are not talking odd 1/4wave multiples that invert a mismatched impedance looking from the rig end,
its any 1/4wave multiples for resonance but not for impedance,
1/2wave tuned for the frequency of choice will repeat the antennas impedance at the input when its matched to the coax AND when its not matched to the coax,
none 1/2wave multiples will transform the impedance to something other than the antennas impedance whenever the antenna is mismatched to the coax,
odd 1/4waves multiples will invert the impedance whenever the antenna is mismatched to the coax,
BUT we are not talking about impedance,
people are tuning for resonance through coax thinking its showing the antennas resonant frequency which its not, you are measuring system resonance coax & antenna,
and that causes a worse match between coax and antenna with higher reflect and the antenna is not resonant,
1/2wave multiple is OK for impedance at your chosen frequency, but you still don't know where the antenna is resonant,
if its not resonant on the same frequency as the coax is cut for exact 1/4wave multiples the coax cancels antenna reactance making X=0 somewhere other than where the antenna is resonant.
And your back to where you started with antenna resonance not where you just tuned it to and that causes higher reflect & lower efficiency than if you tuned for minimum vswr,
since minimum vswr on the line occurs at the self resonant frequency of the element its feeding and adding reactance to the antenna to move resonance seen at the rig end causes higher reflect.
hope this helps.
Eddie
look at the current in the radiator at the bottom,
its far from near zero as it would be if you model a wire dipole with the source in the middle,
i could be wrong as i never saw anybody model a dipole with a choke at the bottom end but that to me looks like the choke is not presenting a high enough impedance & current will likely flow on the choke and any mast or coax you connect to it.
you could try adding a mast under the choke, make it 1/4wave & see if it carries significant current, just an idea.
look at the current in the radiator at the bottom,
its far from near zero as it would be if you model a wire dipole with the source in the middle,
i could be wrong as i never saw anybody model a dipole with a choke at the bottom end but that to me looks like the choke is not presenting a high enough impedance & current will likely flow on the choke and any mast or coax you connect to it.
you could try adding a mast under the choke, make it 1/4wave & see if it carries significant current, just an idea.
Just to try and be clear on the important issue you raise here. When I posted this model in post #537, I suggested this very point. See my post below where I said:
So, I think this means exactly the point you're making here Bob, a 2 wire insertion object at the end of a wire, is not and will not work in making a model that is close to correct.
I use to make this very mistake as noted in the model you referenced, and it was wrong them, but I did not understand it at the time.
This model, as it is, is just a work in progress, and it has flaws. The coil is just a coil, and it only adds inductance as you noted. Now I need to use the Transmission Line tool and make the coil simulate coax with 2 wires, and the bottom wire of the GM is also coax with 2 wires. Just haven't got to the point of doing that yet.
But I'm still not sure either, that I can get the model to act like a GM, and for sure that might not happen unless I can also include the stub and the capacitor in the model.
I think if I can get the choke to work like a choke and place the feed point at the bottom of the coil, like the real GM, and the Average Gain is = 1, I might not have to deal with figuring out the match.
I'm still working on this model. I probably will also have to add a mast in order to have something to make the connection for the 2 wire insertion object...the feed point.
I wish I had checked my blown GM out a bit closer to see how all this was put together at the coil end. Got any suggestions?
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If you mean just the choke i can measure the former on one of mine that's stripped down,
won't be today i just came back from the other place where it is & not sure when i will be going again,
The snapped Gain-Master i had here i gave the bottom section & cable parts to a local who had a burned out one in need of fixing,
The coax is bieffe cavi foam dielectric, its about the same as rg58 but better specs higher VF around 81% if i remember correctly,
Their website seems to be down at the moment so can't post a link to the specs,
from the so239 you have a couple of inch then through the former wraps around then back through former & through a hole in the bottom tube then up the antenna,
theres nothing special going on in the choke area other than its dimensionss.
won't be today i just came back from the other place where it is & not sure when i will be going again,
The snapped Gain-Master i had here i gave the bottom section & cable parts to a local who had a burned out one in need of fixing,
The coax is bieffe cavi foam dielectric, its about the same as rg58 but better specs higher VF around 81% if i remember correctly,
Their website seems to be down at the moment so can't post a link to the specs,
from the so239 you have a couple of inch then through the former wraps around then back through former & through a hole in the bottom tube then up the antenna,
theres nothing special going on in the choke area other than its dimensionss.
Ok Bob, I got some rest and I started back checking the GM dimensions.
Below is what I think are the GM measurements based in part with the spec sheet and watching a video I posted a while back, where a man named Mike Momsn builds a homemade GM:
.
If you can help me here, check these specs out and let me know if I'm on the right track. Sorry for the PDF file written in pencil, use the Zoom if necessary.
Below is what I think are the GM measurements based in part with the spec sheet and watching a video I posted a while back, where a man named Mike Momsn builds a homemade GM:
.
If you can help me here, check these specs out and let me know if I'm on the right track. Sorry for the PDF file written in pencil, use the Zoom if necessary.
That's alright Bob, I used the dimensions in the Schematic diagram I posted above. I didn't really intend for you to go to the trouble of actually measuring the GM.
Here is the model...it is still a work in progress. I have not tried to add the stub yet.
It is in Free Space. The AG reports 1.082 = 0.34 db, which is not too good. Eznec also shows the model contain Loss. This loss is due to my adding the loss for the coax in the radiator and the coil at the 10 meters level. You will see this in the Transmission Line tool details report.
The AG error value of 0.34 db is the overstated error for the maximum gain and angle. In this case, we subtracted the error correction value from the reported gain since the correction value is a (+). If the value was (-) the value would be added to the reported maximum gain and angle value.
2.76 dbi - 0.34 = 2.42 dbi, for the real maximum gain and angle.
Bob, this model is to specs as best I can tell.
The PDF file below also includes the wires description report that shows all the wires, the Transmission Line tool details, and the capacitor details I added at the feed point.
The main thing I see is what happened with the coil when I made it coaxial instead of just a wire helix as I posted earlier. Then the coil started acting like a choke. The inductance, which was over >600 ohms, in the model to start with, dropped the inductancee down to +27.14 ohms at the feed point as soon as I setup the details using the Transmission Line tool on the coil and the bottom radiating element.
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Eddie,
from what im looking at in this latest model which may not be what it looks like from my side,
the dimensions drawing with pencil annotations could be giving you the wrong idea of what a gainmaster is,
the red foam coax goes from the so239 under the choke all the way up to the feedpoint at the middle of the antenna, electrically there is no 2.5cm gap in the braid where the stub attaches,
i posted images on here of the stub connection board and feed-point board years ago but can't find them,
if you use the tl function to make the coax parts in the model you must add a 3rd wire in parallel with the coax so it carries current and radiates,,
if you did that i can't see it here from the model images.
have you got coax made with the tl function plus a 3rd wire to simulate the outside of the braid that i can't see Eddie?
it looks like a wire dipole but that does not mean that's is what i am looking at as i never saw a coax dipole in nec with the extra wire to simulate the braid.
from what im looking at in this latest model which may not be what it looks like from my side,
the dimensions drawing with pencil annotations could be giving you the wrong idea of what a gainmaster is,
the red foam coax goes from the so239 under the choke all the way up to the feedpoint at the middle of the antenna, electrically there is no 2.5cm gap in the braid where the stub attaches,
i posted images on here of the stub connection board and feed-point board years ago but can't find them,
if you use the tl function to make the coax parts in the model you must add a 3rd wire in parallel with the coax so it carries current and radiates,,
if you did that i can't see it here from the model images.
have you got coax made with the tl function plus a 3rd wire to simulate the outside of the braid that i can't see Eddie?
it looks like a wire dipole but that does not mean that's is what i am looking at as i never saw a coax dipole in nec with the extra wire to simulate the braid.
I think I've made models with the Feed Line where I also off-set the 2 wires so folks could see the coaxial application. Don't know if I tried to explain, but if I did it was likely in vain.
This 2 wire Transmission Line business has been a jumble of misundersting regarding modeling, since Henry's did his work on the Sigma 4. The TL is hard to see and thus it is even harder to explain. I don't believe I'll be modeling a 3rd wire.
Below are some exhibits of the GM up as close as I can get, trying to show the "T" in a little box that is indicating the wire is coaxial.
Bob, when the Eznec manual explains a Transmission Line and says we have to add a wire to the model...it only means we have to have a conductor and that is a wire. Eznec cannot make a Transmission Line from point A to point B without a without a wire conductor to enter the data into the TML Tool spreadsheet. The TL tool allows me to lay a coaxial line over this wire and sometimes it shows-up (easy to see) and sometimes it doesn't.
This is why I included the Wire Description Report...so you could see the addition of all the TL tool data that I added to the GM model. Sometimes if you use a magnifying glass you can see the two wires...similar to when you are able to see the segments being in a symmetrical-line-up on the A/P and the S4 designs for the cones.
I think MMANA-GAL might allow folks to create a Feed Line with the click of a button.
I meant to number the pages so as to make for easy referencing, but I forgot to complete those page #'s.
I added a few notes, but rather than trying to explain in words, take a look, an if you have a specific question just ask.
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Eddie
what i have read from more multiple sources when people ask about coax dipoles using the transmissionline fuction to model coax to include coax losses it treats it as a 2 wire transmissionline with no outside surface of the braid to carry current & radiate unless you add a 3rd wire,
the vector is a coaxial arrangement but its not created using the tl function,
its wires close spaced at an accute angle & theres a few ways it can radiate which i have talked about before,
you don't need any extra wires to simulate the outside of the cone if you are not using the tl function to simulate coax cable that radiates unless you are modeling coax that radiates,
if you terminate the vector cone with a resistive load all you will see is some small radiation due to the wires not been parallel like any tapered line terminated with a resistor, which is what Henry's model shows,
in the vector the cone is not terminated with a reistive load its unbalanced and must radiate more than the radiation due to none parallel wires however large or small that is,
heres what the eznec manual tells us about coax
"A non-radiating coaxial cable is electrically identical to a non-radiating two-wire line, so an ordinary transmission line model can be used for modeling a non-radiating coaxial line.
A radiating coaxial cable can be modeled quite well with a combination of transmission line model and a wire.
The transmission line model represents the inside of the coax, and the wire represents the outside of the shield.
The wire is the diameter of the shield, and connected where the shield of the actual cable is.
It should follow the same physical path as the real coaxial cable.
One of the example files, DipTL.ez, shows how this is done. (See its Antenna Notes file for more information.)
If the coax cable doesn't radiate, the current on the additional wire will be zero, and the additional wire contributes nothing to the analysis so can be removed."
and about open wire feedlines
"There's no way to use the transmission line model to accurately model a radiating two-wire transmission line (one with common mode current present). If it's necessary to do this, the line will have to be modeled as two parallel wires. See Closely Spaced Wires for precautions"
"EZNEC transmission line objects don't interact with the antenna fields.
That is, the currents in the model's two conductors are always equal and opposite, so the line doesn't radiate or have current induced by coupling.
In many real-life cases, a transmission line does interact with the antenna fields, and becomes a radiating part of the antenna.
This can be caused by coupling to the antenna due to non-symmetrical placement of the feedline, or by presenting the feedline with an unbalanced source or load.
EZNEC transmission line objects are suitable for modeling symmetrically-placed balanced lines such as a log-periodic feed distribution line or W8JK phasing line.
They aren't suitable for modeling, for example, a quarter-wavelength phasing stub extending outward from a collinear antenna, since the stub wires in a real collinear antenna have unbalanced currents and will radiate.
Coaxial lines laid on the ground, such as in a vertical phased array feed system, may carry current on the outside just like a ground radial wire.
In many cases this can be ignored and the EZNEC Pro model used.
A coaxial cable with field interaction (that is, with current flowing on the outside of the shield) can be modeled with a combination of a wire and transmission line object as described in Modeling Coaxial Cable.
A two-wire line which interacts with the antenna's fields has to be modeled as wires.
what i have read from more multiple sources when people ask about coax dipoles using the transmissionline fuction to model coax to include coax losses it treats it as a 2 wire transmissionline with no outside surface of the braid to carry current & radiate unless you add a 3rd wire,
the vector is a coaxial arrangement but its not created using the tl function,
its wires close spaced at an accute angle & theres a few ways it can radiate which i have talked about before,
you don't need any extra wires to simulate the outside of the cone if you are not using the tl function to simulate coax cable that radiates unless you are modeling coax that radiates,
if you terminate the vector cone with a resistive load all you will see is some small radiation due to the wires not been parallel like any tapered line terminated with a resistor, which is what Henry's model shows,
in the vector the cone is not terminated with a reistive load its unbalanced and must radiate more than the radiation due to none parallel wires however large or small that is,
heres what the eznec manual tells us about coax
"A non-radiating coaxial cable is electrically identical to a non-radiating two-wire line, so an ordinary transmission line model can be used for modeling a non-radiating coaxial line.
A radiating coaxial cable can be modeled quite well with a combination of transmission line model and a wire.
The transmission line model represents the inside of the coax, and the wire represents the outside of the shield.
The wire is the diameter of the shield, and connected where the shield of the actual cable is.
It should follow the same physical path as the real coaxial cable.
One of the example files, DipTL.ez, shows how this is done. (See its Antenna Notes file for more information.)
If the coax cable doesn't radiate, the current on the additional wire will be zero, and the additional wire contributes nothing to the analysis so can be removed."
and about open wire feedlines
"There's no way to use the transmission line model to accurately model a radiating two-wire transmission line (one with common mode current present). If it's necessary to do this, the line will have to be modeled as two parallel wires. See Closely Spaced Wires for precautions"
"EZNEC transmission line objects don't interact with the antenna fields.
That is, the currents in the model's two conductors are always equal and opposite, so the line doesn't radiate or have current induced by coupling.
In many real-life cases, a transmission line does interact with the antenna fields, and becomes a radiating part of the antenna.
This can be caused by coupling to the antenna due to non-symmetrical placement of the feedline, or by presenting the feedline with an unbalanced source or load.
EZNEC transmission line objects are suitable for modeling symmetrically-placed balanced lines such as a log-periodic feed distribution line or W8JK phasing line.
They aren't suitable for modeling, for example, a quarter-wavelength phasing stub extending outward from a collinear antenna, since the stub wires in a real collinear antenna have unbalanced currents and will radiate.
Coaxial lines laid on the ground, such as in a vertical phased array feed system, may carry current on the outside just like a ground radial wire.
In many cases this can be ignored and the EZNEC Pro model used.
A coaxial cable with field interaction (that is, with current flowing on the outside of the shield) can be modeled with a combination of a wire and transmission line object as described in Modeling Coaxial Cable.
A two-wire line which interacts with the antenna's fields has to be modeled as wires.
Eddie i can't see the wires even blown up many times,
i think what you are saying is the wire you add before you overlay the tl model is effectively the outside of the coax,
so if you were modeling a two wire open line using the tl function you would just put the ends in and we would see nothing between the two end points ,
is that what you mean ?
i have seen people talk about coax dipoles but only ever seen you model one so i have nothing to compare against.
i think what you are saying is the wire you add before you overlay the tl model is effectively the outside of the coax,
so if you were modeling a two wire open line using the tl function you would just put the ends in and we would see nothing between the two end points ,
is that what you mean ?
i have seen people talk about coax dipoles but only ever seen you model one so i have nothing to compare against.