Hi all...
Wondering if builders have changed over to the Coaxial Dipole yagi feeds, or are sticking with the traditional methods.
If stickng is there a tech. reason for the decision....?
John
Coaxial Dipole and Split dipole Feeds
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VK2FAK
Coaxial Dipole and Split dipole Feeds
Last edited by VK2FAK on Fri Jan 24, 2014 1:27 pm, edited 1 time in total.
Re: coaxil dipole feeds
Hi John,
I have used the coaxial diapole on a couple of 70cm yagis,works brilliant ,no matching ,baluns or chokes used.
i'm going to try it on 2m using hard draw copper tube.
cheers
Frank
I have used the coaxial diapole on a couple of 70cm yagis,works brilliant ,no matching ,baluns or chokes used.
i'm going to try it on 2m using hard draw copper tube.
cheers
Frank
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VK2FAK
Re: coaxil dipole feeds
Hi all....
Hi Frank....did the antennas still work as expected, as I am guessing you could not model the antenna with the Coaxial Dipole....or did you ?
John
Hi Frank....did the antennas still work as expected, as I am guessing you could not model the antenna with the Coaxial Dipole....or did you ?
John
Re: coaxil dipole feeds
Hi John
Not sure what you mean but I suspect you mean folded dipoles with the coax feed going through inside the dipole.
This is a system that works very well. It is commonly used for commercial application antennas but does not seem to be used outside Australia.
It is based on using a quarter wave section of coax to achieve the match required (odd multiples often used on UHF to avoid the need for a transition conection).
eg: On a 25 ohm yagi, a quarter wave length of 75 hm coax is used, for a folded dipole, 125 ohm. This achieves the desired impedance transformation. The coax running inside the dipole performs the function of a balun to complete the system.
The above may not alway be apparent. This is because, for example, a length of 75 ohm RG11 inner may be soldered onto the inner of RG213 and then pulled through into the RG213 outer sheath so as to have one end of the coax sheath with the RG11 sticking out. The displaced RG213 inner pulled out is discarded. The end result is a continuous length of RG213 outer sheath but not inner conductor. You will only notice this is you carefully take note of the centre conductor diameters.
Not all brands of RG213 allows you to pull the inner through so you need to use one that does.
A drain hole is used at one end of the dipole to let any moisture/water that might enter the dipole to get out. The hole needs to be at the bottom with the coax/dipole connection at the top.
Not sure what you mean but I suspect you mean folded dipoles with the coax feed going through inside the dipole.
This is a system that works very well. It is commonly used for commercial application antennas but does not seem to be used outside Australia.
It is based on using a quarter wave section of coax to achieve the match required (odd multiples often used on UHF to avoid the need for a transition conection).
eg: On a 25 ohm yagi, a quarter wave length of 75 hm coax is used, for a folded dipole, 125 ohm. This achieves the desired impedance transformation. The coax running inside the dipole performs the function of a balun to complete the system.
The above may not alway be apparent. This is because, for example, a length of 75 ohm RG11 inner may be soldered onto the inner of RG213 and then pulled through into the RG213 outer sheath so as to have one end of the coax sheath with the RG11 sticking out. The displaced RG213 inner pulled out is discarded. The end result is a continuous length of RG213 outer sheath but not inner conductor. You will only notice this is you carefully take note of the centre conductor diameters.
Not all brands of RG213 allows you to pull the inner through so you need to use one that does.
A drain hole is used at one end of the dipole to let any moisture/water that might enter the dipole to get out. The hole needs to be at the bottom with the coax/dipole connection at the top.
73s
Igor
VK6ZFG
Igor
VK6ZFG
Re: coaxil dipole feeds
John, you imply that one needs a reason to "stick with traditional methods", but I would contend that you need a reason to do whatever, and that includes a reason to do something different (a more substantial reason than "why not").VK2FAK wrote:Hi all...
Wondering if builders have changed over to the Coaxial Dipole yagi feeds, or are sticking with the traditional methods.
If stickng is there a tech. reason for the decision....?
John
There are a bunch of claims made for the G4CQM dipole (if that is what you are talking about), I wrote an article analysing the dipole and came to a view that it didn't work like the inventor claimed. The article is no longer online following my forced call sign change.
The thing shares some 'features' with a Double Bazooka, both are praised by some and criticised by others.,, that is the nature of things.
Owen
PS: we probably all though we knew what a 'coaxial dipole' was, at least until G4CQM applied the term to something different.
Last edited by VK2OMD on Sat Jan 18, 2014 2:26 pm, edited 1 time in total.
Re: coaxil dipole feeds
Hi John,
Antennas were built of the YU7EF,Pops website.24 element 70cms.They worked very well.
More info on G4CQM website about the coaxial diapoles.
Only built the two so far but very pleased with the performance.
cheers
Frank
Antennas were built of the YU7EF,Pops website.24 element 70cms.They worked very well.
More info on G4CQM website about the coaxial diapoles.
Only built the two so far but very pleased with the performance.
cheers
Frank
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VK2FAK
Re: Coaxial Dipole Feeds
Hi all...
no implying anything, just wanted to keep it short and simple...
weather it works or not, I was also curious if the modeling requires a certain length Driven element at fixed distances from other elements......how could you just replace with a coaxial dipole and the antenna works the same....
unless you first model with the coax dipole..
Igor..i was asking about a straight dipole feed...similar to a standard spilt-dipole but using coax..
John
no implying anything, just wanted to keep it short and simple...
weather it works or not, I was also curious if the modeling requires a certain length Driven element at fixed distances from other elements......how could you just replace with a coaxial dipole and the antenna works the same....
unless you first model with the coax dipole..
Igor..i was asking about a straight dipole feed...similar to a standard spilt-dipole but using coax..
John
Re: coaxil dipole feeds
Yep, he explains "The simple answer is all about resonance and matching, in short, RF energy will take the path of least opposition. A truly resonant and well matched dipole or driven element will draw most of the power from its source and radiate it."VK7DX wrote:...
More info on G4CQM website about the coaxial diapoles.
John, don't try to use NEC to support that proposition. If you want to get results from NEC, you need to think beyond the resonance myth.
The G4CQM dipole can be modelled in NEC. The steps are:
1. Determine Zo, vf and loss characteristics of the coaxial line formed by the wire inside the dipole tube. You might want to simplify it and consider it a uniform line, and you might want to consider it lossless initially. You could make a half dipole up and measure vf using an antenna analyser. I calculated Zo and vf for G4CQM's implementation as 76 and 71% if you want to start with some numbers.
2. Make an ordinary GW card to model the outside surface of the dipole tubes, and feed it with a source in the centre.
3. Model the interior like any s/c stub, but here you have two connected in series across the feed point. Pretending they are lossless allows you to use TL cards for this. Otherwise, you need to assess the loss characteristic and build NT cards for each frequency.
Those who insist that it cannot be modelled are simply wrong.
As far a proving that it is inherently self balancing, measurement of common mode current in a range of scenarious would be the best evidence.
Good luck.
Owen
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VK2FAK
Re: Coaxial Dipole and Split dipole Feeds
Hi all....
something that always messes with me is....when using the software such as NEC to model an antenna and your using a split feed, when it comes to actual construction and you leave a gap say 10mm ....how does this effect modeling.
And do you have to reduce the lengths of each side to compensate for the length of the coax feeding each side of the split dipole........you can read so many opinions on this it gets confusing...
john
something that always messes with me is....when using the software such as NEC to model an antenna and your using a split feed, when it comes to actual construction and you leave a gap say 10mm ....how does this effect modeling.
And do you have to reduce the lengths of each side to compensate for the length of the coax feeding each side of the split dipole........you can read so many opinions on this it gets confusing...
john
Re: Coaxial Dipole and Split dipole Feeds
Well, you won't want any more opinions... though I note that in online forums, popularity is commonly held to determine fact (ie scores of posters can't be wrong), or repetition determines fact (say it often enough and loud enough and it must be fact).VK2FAK wrote:Hi all....
something that always messes with me is....when using the software such as NEC to model an antenna and your using a split feed, when it comes to actual construction and you leave a gap say 10mm ....how does this effect modeling.
And do you have to reduce the lengths of each side to compensate for the length of the coax feeding each side of the split dipole........you can read so many opinions on this it gets confusing...
john
Perhaps you need to read the NEC documentation and understand how it treats sources and that although you modelled a conductor from end to end, segmented, that one of those segments contains a source and it is not 'short circuited' by the conductor segment, a common misconception by beginners.
As far as your last question, draw a physical configuration on a piece of paper, and think about where currents flow and their magnitude, and how they contribute to radiation and reactance effects. In a sense all conductors are transmission lines, conductors that are coaxial or parallel with balanced currents have low radiation, whereas the dipole (which can be thought of as a transmission line with the two conductors bent outwards) have poor balance in that region in most directions, much less cancellation and more radiation.
Owen
Re: Coaxial Dipole and Split dipole Feeds
If you think about it, your second question has something to do with the first one. Also from the practical perspective, don't get too wound up about the finer things in computer antenna modelling like 10mm gaps replaced with coax tails. There will be many many other things introduced into a Yagi when it is physically constructed that minor computer modelling details will pale into insignificance. For instance ...VK2FAK wrote:Hi all....
something that always messes with me is....when using the software such as NEC to model an antenna and your using a split feed, when it comes to actual construction and you leave a gap say 10mm ....how does this effect modeling.
And do you have to reduce the lengths of each side to compensate for the length of the coax feeding each side of the split dipole........you can read so many opinions on this it gets confusing...
john
... almost no one models a conductive boom, but most people build on one.
... same for the "H" frame in an array.
... thru-boom element correction factors are available, but there are different tables/calculations on the Internet that give different answers!
... what are those above-boom plastic mounting 'insulators' doing? (some have VERY significant effects)
... and what is that bolt doing when going through the middle of the element to hold it and the insulator on to the boom?
... and the effect of the 'plastic' box used to enclose the feed point connection?
... if a box is not used, there most be something to hold the DE in place.
... the nice antenna is assembled, now we must mount it ... usually with a metal plate and some U-bolts stuffed between two of the directors
... now lets dress it up a bit with some plastic caps on the element tips ... hmmm, the SWR dip moved.
... and for the really pedantic ... what is happening at the element tips with an open end of a tube rather than a solid rod element?
(computer models are using a solid 'wire')
Those ones just came off the top of my head, there are probably more ... and then there are factors within the NEC modelling, like how many segments are being used (more is not necessarily "better" when it comes to translating/transferring to the real world).
As well as a lot of time going into computer modelling, a lot of time can go into experimenting with the physical items used in the construction of a real antenna and their effect(s) / compensation factors, but keep in mind the antenna is then no longer the same thing as the computer showed on the screen (usually just a NEC analysis of some 'wires'). In the end, with the simple modelling software we use and no matter how many compensation factors are applied, at best we can only have something that has characteristics that "look like" the computer model ... but I've built dozens of VHF and UHF Yagi antennas from computer models and haven't seen ONE in real life that has exactly the same pattern, SWR, or SWR curve shape!
A pragmatic approach is required.
73,
Bob, ZL1RS in the Bay of Islands at RF64vs
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VK2FAK
Re: Coaxial Dipole and Split dipole Feeds
Hi all...
I wanted to play with a simple 5 element yagi using nothing but PVC (white) tube, 6.3mm al rod, and some hot glue.
Surprised that it actually looks good..elements pass thru the PVC and glued in place and the D.E is mounted on top but using 12mm Al tube...and is not long enough to have bending issues....
I made a silly assumption that with the PVC maybe the model element lengths may be right.....but the SWR was running around 2:1 ....it was directive so showing signs of what the model was showing me..
There was no plastic near the ends of elements...
I modelled the D.E above the other elements , it makes a change but mainly minor pattern and SWR...changes.
John
I wanted to play with a simple 5 element yagi using nothing but PVC (white) tube, 6.3mm al rod, and some hot glue.
Surprised that it actually looks good..elements pass thru the PVC and glued in place and the D.E is mounted on top but using 12mm Al tube...and is not long enough to have bending issues....
I made a silly assumption that with the PVC maybe the model element lengths may be right.....but the SWR was running around 2:1 ....it was directive so showing signs of what the model was showing me..
There was no plastic near the ends of elements...
I modelled the D.E above the other elements , it makes a change but mainly minor pattern and SWR...changes.
John
Re: Coaxial Dipole and Split dipole Feeds
That is a pretty scan description John, and it doesn't mention a balun or the feed arrangement.VK2FAK wrote:Hi all...
I wanted to play with a simple 5 element yagi using nothing but PVC (white) tube, 6.3mm al rod, and some hot glue.
Surprised that it actually looks good..elements pass thru the PVC and glued in place and the D.E is mounted on top but using 12mm Al tube...and is not long enough to have bending issues....
I made a silly assumption that with the PVC maybe the model element lengths may be right.....but the SWR was running around 2:1 ....it was directive so showing signs of what the model was showing me..
There was no plastic near the ends of elements...
I modelled the D.E above the other elements , it makes a change but mainly minor pattern and SWR...changes.
John
I think we are meant to assume that it is a 5 el Yagi of your design, uses 1/4" elements but for a 12mm DE which is fed in the centre from 50 ohm coax, and using a PVC tube boom.
Owen
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VK2FAK
Re: Coaxial Dipole and Split dipole Feeds
Hi all..
You are right Owen..... supposed to have a 50ohm direct feed....just using some clip on ferrite cores for the experiment.
Pretty much anything is up for change...
I am curious as to the effect of the PVC on element lengths, Including the D.E.....note the D.E. is actually sitting on a small square section of the plastic breadboard material...which is suppose to be and good insulator in itself.
As mentioned this is simple, but seems to be very strong......basically because of its size.
John
You are right Owen..... supposed to have a 50ohm direct feed....just using some clip on ferrite cores for the experiment.
Pretty much anything is up for change...
I am curious as to the effect of the PVC on element lengths, Including the D.E.....note the D.E. is actually sitting on a small square section of the plastic breadboard material...which is suppose to be and good insulator in itself.
As mentioned this is simple, but seems to be very strong......basically because of its size.
John
Re: Coaxial Dipole and Split dipole Feeds
Do you have any reason to think that the balun works as it needs to? You couldn't even give the number of clip on cores!VK2FAK wrote:Hi all..
You are right Owen..... supposed to have a 50ohm direct feed....just using some clip on ferrite cores for the experiment.
Pretty much anything is up for change...
I am curious as to the effect of the PVC on element lengths, Including the D.E.....note the D.E. is actually sitting on a small square section of the plastic breadboard material...which is suppose to be and good insulator in itself.
As mentioned this is simple, but seems to be very strong......basically because of its size.
John
Your model is probably limited in not including feed line, balun, environment. It is my experience that this type of antenna if accurately built to a valid model and suitably isolated, is normally pretty close in behaviour. I know that differs to Bob's experience.
You didn't take the trouble to describe how you measured it, and believe it or not, that is relevant. I have wasted lots of time trying to help people who turned out eventually to be testing the antenna inside the metal garage / workshop, or in one instance, lying on the ground. In one instance, the guy emailed me a humungous 10min video to reveal that the Yagi was inside his metal shed... and it takes just a few words to say that, but he just didn't know enough to know that was relevant.
Another partial picture is you gave us the min VSWR, but didn't think the freq compared to the model freq is relevant, is it high or low?
So often, solving these kind of problems is about firstly recognising what is relevant, and taking stock of what you have, and taking time to compose a relevant and complete description of the problem. You might even solve the problem yourself when you set it out clearly and logically.
Owen
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VK2FAK
Re: Coaxial Dipole and Split dipole Feeds
Hi all....
Your point taken Owen....but I am the only one that will solve the issues..as you said..
But you did make the point "properly modelled",it should behave as designed......so take that one step, if ignoring the feed balun....and all other external factors......a simple design from Al and PVC should behave as designed...in other words the PVC wont have an effect on element lengths...? as I have never seen modelling using a plastic boom..
John
Your point taken Owen....but I am the only one that will solve the issues..as you said..
But you did make the point "properly modelled",it should behave as designed......so take that one step, if ignoring the feed balun....and all other external factors......a simple design from Al and PVC should behave as designed...in other words the PVC wont have an effect on element lengths...? as I have never seen modelling using a plastic boom..
John
Re: Coaxial Dipole and Split dipole Feeds
Hi John,
The list I gave and the point I was making was that various factors can cause changes because they are different to what was modeled. As Owen mentions, the 'rods through PVC' construction eliminates most of the construction differences, so I'm with Owen ... your Yagi should not be far off the mark. Assuming it was designed to be 50 ohms, a result with a 2:1 SWR is a BIG surprise.
Could you provide your .nec file to first see if there is a problem back at the modeling stage?
Regards,
The list I gave and the point I was making was that various factors can cause changes because they are different to what was modeled. As Owen mentions, the 'rods through PVC' construction eliminates most of the construction differences, so I'm with Owen ... your Yagi should not be far off the mark. Assuming it was designed to be 50 ohms, a result with a 2:1 SWR is a BIG surprise.
Could you provide your .nec file to first see if there is a problem back at the modeling stage?
Regards,
Bob, ZL1RS in the Bay of Islands at RF64vs
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VK2FAK
Re: Coaxial Dipole and Split dipole Feeds
Hi all....
Thanks for the comments, its time to just go measure and remeasure, and check cables and such.....I think that is going to be my task tomorrow morning....
John
Thanks for the comments, its time to just go measure and remeasure, and check cables and such.....I think that is going to be my task tomorrow morning....
John
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VK2FAK
Re: Coaxial Dipole and Split dipole Feeds
Hi all...
Problem turned out to be cable....not the actual antenna....was using a short 1.5m length of RG58....simply because it was lite and easy to handle for some tests.....I still think the cable should have been better than that....but...lesson learned....
I was trying the little antenna in picking up the Balloon on the weekend, got a few decodes just not complete....the end kept messing up....But hey what can you expect inside, pointing the 5 element beam through a window to the south...lol
Made some changes to the antenna last night and does seem to follow what the predictions are on NEC.....as mentioned in this thread as it should do,, based on construction..
john
Problem turned out to be cable....not the actual antenna....was using a short 1.5m length of RG58....simply because it was lite and easy to handle for some tests.....I still think the cable should have been better than that....but...lesson learned....
I was trying the little antenna in picking up the Balloon on the weekend, got a few decodes just not complete....the end kept messing up....But hey what can you expect inside, pointing the 5 element beam through a window to the south...lol
Made some changes to the antenna last night and does seem to follow what the predictions are on NEC.....as mentioned in this thread as it should do,, based on construction..
john
Re: Coaxial Dipole and Split dipole Feeds
Thanks for letting us all know the answer John.
FWIW I use an electrically measured 1 wavelength of LMR400 for testing my 2m antennas, and do an 'OSL' calibration on the analyser before testing a new antenna to make sure the obvious measuring imponderables are taken care of. With confidence that the measuring side is OK, I place faith in NEC2 being right and any differences are as a result of the antenna construction. Almost always, after a little adjustment to the DE length to compensate for the coax to DE transition and minimise reactance, the SWR plot result is good enough right away and a rough pattern check is made to confirm we are 'in the ball park'.
73,
FWIW I use an electrically measured 1 wavelength of LMR400 for testing my 2m antennas, and do an 'OSL' calibration on the analyser before testing a new antenna to make sure the obvious measuring imponderables are taken care of. With confidence that the measuring side is OK, I place faith in NEC2 being right and any differences are as a result of the antenna construction. Almost always, after a little adjustment to the DE length to compensate for the coax to DE transition and minimise reactance, the SWR plot result is good enough right away and a rough pattern check is made to confirm we are 'in the ball park'.
73,
Bob, ZL1RS in the Bay of Islands at RF64vs