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1:1coax balun Help Please
- Thread starter LightFoot
- Start date
Hello All:
Ok think I got my last posting here a little messed up. I was answering Thundir and Booty Monster, but only replied to Booty Mobster. And was technically incorrect to some point that Doc caught me on.
Yes the Coiled up Coax is a Choke, meaning that is will show a Impedance or Resistance to current flow on the outer shield. Thats why they call it a choke. But a Balun is a Transmission Line Transformer that will force the Coax Unbalanced Currents, to feed the antenna with equal and opposite currents, such as in a center feed dipole.
The ARRL Antenna Book has a list of Coax Chokes made up and measured for the Impedance values, in the "Coupling the Line to the Antenna" chapter, chapter 26. The list shows the highest Inpedance for 27 MHz as 2000 Zo Ohms, with a Coax Coil having 6 Turns, on a 4 1/4 inch diameter, using a single layer of coax. Good reading.
The MaCo V5/8 that Thundir was talking about, could use the 7/8 inch OD Aluminum Tubing installed over the antenna coax and PL-259 connector, as a choke. If you wanted to go thru all that, with a good posibility of no real changes in the antennas performance.
The wire type antenna Booty Monster was talking about, I would think that you would want to install the coax choke, or in line choke a 1/4 wavelength down the coax, from the antenna connection or about 9 feet. Allowing the coax to be a counter poise, but not allowing the RF Energy down the coax. This will work with other none ground plane antennas.
The Ground Plane Antennas usually have very good isolation from the RF Energy being radiated in a down angle, or down the coax, from the vertical element. So usually a Choke is not needed, but some operators want a choke or Balun at the antennas connection. Again you don't want or need a Balun with Coax feeding a vertical ground plane antenna, that is a unbalanced antenna. Yeah sure for a Quad, Dipole, or Yagi with a split and insulated driven element.
I have had Interceptor 10K customers want to use a inline choke made from DX Engineering. No noticeable difference was seen.
Using a splice or double SO-239 female connector like I said, I have used because someone was too smart to measure the coax length for a antenna installation. They start showing up as a measurable impedance mismatch in the 400 MHz frequency range, as seen on a Network Analyzer. But the slight mismatch they have is insignificant at the lower HF Frequencies. But at the same time, I have seen guys have several splices in there coax, again its a dime holding up a dollar.
I have not seen any different with the grounding of antennas using the base of the metal mast or tower, as compared to using a heavy gauge wire connected directly to the antenna, and run to a grounding rod. But grounding the antenna is always a good call. I have not measured antennas insulated or not insulated from the mast. Seems like there won't be a lot of difference as you always have the coax running down the mast that is a metal object.
Jay in the Mojave
Ok think I got my last posting here a little messed up. I was answering Thundir and Booty Monster, but only replied to Booty Mobster. And was technically incorrect to some point that Doc caught me on.
Yes the Coiled up Coax is a Choke, meaning that is will show a Impedance or Resistance to current flow on the outer shield. Thats why they call it a choke. But a Balun is a Transmission Line Transformer that will force the Coax Unbalanced Currents, to feed the antenna with equal and opposite currents, such as in a center feed dipole.
The ARRL Antenna Book has a list of Coax Chokes made up and measured for the Impedance values, in the "Coupling the Line to the Antenna" chapter, chapter 26. The list shows the highest Inpedance for 27 MHz as 2000 Zo Ohms, with a Coax Coil having 6 Turns, on a 4 1/4 inch diameter, using a single layer of coax. Good reading.
The MaCo V5/8 that Thundir was talking about, could use the 7/8 inch OD Aluminum Tubing installed over the antenna coax and PL-259 connector, as a choke. If you wanted to go thru all that, with a good posibility of no real changes in the antennas performance.
The wire type antenna Booty Monster was talking about, I would think that you would want to install the coax choke, or in line choke a 1/4 wavelength down the coax, from the antenna connection or about 9 feet. Allowing the coax to be a counter poise, but not allowing the RF Energy down the coax. This will work with other none ground plane antennas.
The Ground Plane Antennas usually have very good isolation from the RF Energy being radiated in a down angle, or down the coax, from the vertical element. So usually a Choke is not needed, but some operators want a choke or Balun at the antennas connection. Again you don't want or need a Balun with Coax feeding a vertical ground plane antenna, that is a unbalanced antenna. Yeah sure for a Quad, Dipole, or Yagi with a split and insulated driven element.
I have had Interceptor 10K customers want to use a inline choke made from DX Engineering. No noticeable difference was seen.
Using a splice or double SO-239 female connector like I said, I have used because someone was too smart to measure the coax length for a antenna installation. They start showing up as a measurable impedance mismatch in the 400 MHz frequency range, as seen on a Network Analyzer. But the slight mismatch they have is insignificant at the lower HF Frequencies. But at the same time, I have seen guys have several splices in there coax, again its a dime holding up a dollar.
I have not seen any different with the grounding of antennas using the base of the metal mast or tower, as compared to using a heavy gauge wire connected directly to the antenna, and run to a grounding rod. But grounding the antenna is always a good call. I have not measured antennas insulated or not insulated from the mast. Seems like there won't be a lot of difference as you always have the coax running down the mast that is a metal object.
Jay in the Mojave
Hi Jay,your suggesting replacing an rf choke balun with a sleeve balun (which serves basically the same purpose) on an antenna you claim doesn't need a balun or rf choke.not to mention you say connect the sleeve balun at the wrong end,ie the pl plug and not as is normal connected a 1/4 wavelength from the pl plug directly to the sheath.
My question is very simple,WTF?:headbang
please elaborate.
Lots of good responses here, and even though the Jazzsinger takes a somewhat gratuitous approach in his question, he is totally correct as was 'Doc, C2, and Loosecannon.
I agree that Jay's idea may have merit under specific circumstances and good construction as he describes even though arguable to working at 27 mhz. I hear the arguments raised that are due to support problems, the size of the device, and being able to keep the feed line centered well (symmetry)---like the center conductor in the coax we use.
Recently, my friend Bob85 advised me, that among other things the top portion of this type of sleeved device really needs to be open (not attached) at the SO239 feed point and maybe it should be insulated from the antenna as well. The whole element, I call a bazooka balun, needs to be very large relative to the feed line inside with lots of air volume available, the feed line inside needs to be symmetrically centered and maintained just like the center conductor in the coax we use, and it must be an appropriate 1/4 wave length. Maybe I should call it a bazooka choke instead of balun, like 'Doc suggests. If these principals of construction are not followed, then things might not work out as planned. Additionally, the bottom end shield of the coax inside must be conveniently brought to ground with the supporting mast.
Bob85 set me straight of these practical aspects that very likely need to be followed in design to the “T,” as I was trying a similar and new idea for me on my Marconi project---a homemade Startduster using all 102" ss whips. The aim of this project however, was to see how adding radials elements helped improve the effectiveness of the antenna and what effects this had on tuning of the 102” ss whips or any other resonant ¼ wave antenna available to us for 11 meters.
Thank you Bob.
At this point, I have concluded that due to the problems associated with such a bazooka balun design and the lack of symmetry that I was able to achieve---maybe the simple Starduster design is about as good as I could hope for in such an application. So in my case, I now think maybe two simple coiled chokes, one at the feed point, and one at the bottom end of the slanted down radials is a more practical and effective solution to any such problems my Marconi might have with CMC.
Thundir, you claim the addition of your choke added problems, which you did not expect and wondered why. It is obvious to me that you may not have a good tune or a loose connection somewhere in the construction of your antenna. I can give you examples of construction problems and their possible effects on interference if you wish, but my thoughts about your tune are; if you don’t have a good tune then the coax is having some transforming effects, and when you added the necessary length to the line with your choke, the problems showed up. Since removing the choke brought things back to normal, then simply add a similar length back to your feed line at the radio end and see if the problems return. If so, then your feed line is doing more than radiating and for me the fix is a good tune. When tuning, try to do an occasional bandwidth test, and do not look for a flat curve all the way across the band with a low SWR. Look instead for a long smooth curve as one might expect. I suspect these curves that are long and flat are not an indication of a good tune, and thus your system response will change every time you change the feed line length.
Good luck and keep us posted.
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Hello Jazz Singer:
The Ground Plane Antennas do not need a RF Choke or Balun to help isolate the RF Energy from the vertical element to below the ground plane radials. Or RF radiating down the Coax.
The question was raised as what could be done further to increase or eliminate the RF Energy from the coax, my interruption. I never called it a Sleeve Balun, but it is installed that way to reduce any radiation, and decouple any RF Energy. One could install this addition and measure the changes in SWR and Field Strength, if any.
Using a "Sleeve Balun" will force the currents to be equal and opposite, which is not desired for an unbalanced ground plane antenna. Which was designed from day one to be feed with unbalanced Coax.
Using a RF Choke at 1/4 wavelength down the coax for none ground plane antenna has been used for some time and I have seen this shown in a few Antenna Books. Some customers that I have talked to have installed chokes a 1/4 WL down the coax for their none ground plane antennas. And beloved they saw a increase in the antennas performance. I have not done this or measured its results.
When CB and Ham Radio was getting off the ground with newer radios, testing equipment and such, in the 50's and 60's. Some antenna manufactures supplied another set of ground plane radials mounted below the normal set of ground plane radials. I believe this was to increase the RF Isolation from the lower part of the ground plane antenna. After the RF Energy was measured, the design concept was dropped. It wasn't needed.
Hope this is a help.
Jay in the Mojave
The Ground Plane Antennas do not need a RF Choke or Balun to help isolate the RF Energy from the vertical element to below the ground plane radials. Or RF radiating down the Coax.
The question was raised as what could be done further to increase or eliminate the RF Energy from the coax, my interruption. I never called it a Sleeve Balun, but it is installed that way to reduce any radiation, and decouple any RF Energy. One could install this addition and measure the changes in SWR and Field Strength, if any.
Using a "Sleeve Balun" will force the currents to be equal and opposite, which is not desired for an unbalanced ground plane antenna. Which was designed from day one to be feed with unbalanced Coax.
Using a RF Choke at 1/4 wavelength down the coax for none ground plane antenna has been used for some time and I have seen this shown in a few Antenna Books. Some customers that I have talked to have installed chokes a 1/4 WL down the coax for their none ground plane antennas. And beloved they saw a increase in the antennas performance. I have not done this or measured its results.
When CB and Ham Radio was getting off the ground with newer radios, testing equipment and such, in the 50's and 60's. Some antenna manufactures supplied another set of ground plane radials mounted below the normal set of ground plane radials. I believe this was to increase the RF Isolation from the lower part of the ground plane antenna. After the RF Energy was measured, the design concept was dropped. It wasn't needed.
Hope this is a help.
Jay in the Mojave
Hi Jay,your suggesting replacing an rf choke balun with a sleeve balun (which serves basically the same purpose) on an antenna you claim doesn't need a balun or rf choke.not to mention you say connect the sleeve balun at the wrong end,ie the pl plug and not as is normal connected a 1/4 wavelength from the pl plug directly to the sheath.
My question is very simple,WTF?:headbang
please elaborate.
Update for Jay and Bob
Jay raises a very good point that I did not address in my post above.
You might be right Jay, IMHO I'm not totally convinced this type of balun, bazooka balun, choke, or whatever the idea we may all be talking about here---really works to prevent CMC's, period. I think it could simply work in the case of the Starduster just to shield the first 1/4 wave of the feed line from the inductive RF (not CMC) of the ground radials and that may account entirely for the little help in TVI we noticed years ago.
My first experience with this idea came from a friend named Johnny. He had a good friend that was a big wholesaler in CB business in Houston, all during the roaring 80's. The article is entitled "An Analysis of the Balun" dated April 1980. The important words for us were just in a snippet of the first full paragraph of the front page right hand column, and that is all we thought we could understand. However, Johnny convinced me that something like the bazooka was like the mast part of our Starduster antennas and that making a modest change, adding a tuned 1/4 coax jumper from the feed point inside a shortened portion of the support mast and grounding the shield at the bottom of the bazooka tube---made a big difference to his performance and completely eliminated his TVI---even with his little 200 watt hooten-nater on (means amplifier for you guys in Rio Linda). We swore by this idea even until today. This article is from the ARRL Antenna Handbook and surprisingly, much later on, the article became freestanding on the Internet at: http://www.arrl.org/tis/info/pdf/8004019.pdf
Again, much later on in 2008 for me, I was researching some reference material on an old Avanti patent and ran across this patent at the US Patent site at reference #2,184,729. You can check it our at: http://patft.uspto.gov/ Use the Patent Number Search and enter the number above for a 1939 reference to a similar idea and likely the for-runner to the idea for the Starduster design. This is some very old nity-grity (means details for CB’s) about sleeved systems that isolate well---the feed lines of vertical antenna systems, etc.
At least read the front page to get the gist of the idea (means idea for Dem's) in the images available, and you will find that these good old principles presented, as usual, go back a long ways in radio work related to antenna problems. Ain’t it amazing the answers and questions raised back in the good old days.
So Jay, you might find some additional support or maybe some new ideas as it applies to Thundir’s post. Sorry to always be so long-winded.
Very good thread.
Jay raises a very good point that I did not address in my post above.
You might be right Jay, IMHO I'm not totally convinced this type of balun, bazooka balun, choke, or whatever the idea we may all be talking about here---really works to prevent CMC's, period. I think it could simply work in the case of the Starduster just to shield the first 1/4 wave of the feed line from the inductive RF (not CMC) of the ground radials and that may account entirely for the little help in TVI we noticed years ago.
My first experience with this idea came from a friend named Johnny. He had a good friend that was a big wholesaler in CB business in Houston, all during the roaring 80's. The article is entitled "An Analysis of the Balun" dated April 1980. The important words for us were just in a snippet of the first full paragraph of the front page right hand column, and that is all we thought we could understand. However, Johnny convinced me that something like the bazooka was like the mast part of our Starduster antennas and that making a modest change, adding a tuned 1/4 coax jumper from the feed point inside a shortened portion of the support mast and grounding the shield at the bottom of the bazooka tube---made a big difference to his performance and completely eliminated his TVI---even with his little 200 watt hooten-nater on (means amplifier for you guys in Rio Linda). We swore by this idea even until today. This article is from the ARRL Antenna Handbook and surprisingly, much later on, the article became freestanding on the Internet at: http://www.arrl.org/tis/info/pdf/8004019.pdf
Again, much later on in 2008 for me, I was researching some reference material on an old Avanti patent and ran across this patent at the US Patent site at reference #2,184,729. You can check it our at: http://patft.uspto.gov/ Use the Patent Number Search and enter the number above for a 1939 reference to a similar idea and likely the for-runner to the idea for the Starduster design. This is some very old nity-grity (means details for CB’s) about sleeved systems that isolate well---the feed lines of vertical antenna systems, etc.
At least read the front page to get the gist of the idea (means idea for Dem's) in the images available, and you will find that these good old principles presented, as usual, go back a long ways in radio work related to antenna problems. Ain’t it amazing the answers and questions raised back in the good old days.
So Jay, you might find some additional support or maybe some new ideas as it applies to Thundir’s post. Sorry to always be so long-winded.
Very good thread.
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Hi Jay,thanks for clarifying your position,but over here in the uk we have seen evidence that decoupling the coax with a rf choke and insulating the mast from the antenna in effect decoupling it too, does infact make a difference,both on performance and on rfi issues.
W8JI also seems to disagree with your position as I believe also does the late great LB Cebik.Its common knowledge that 4 1/4 wave ground plane radials are insufficient alone to stop common mode currents although do go a fair way in reducing them.
The add on you say you didn't call a sleeve balun but i would say obviously is intended to act like one is of insufficient proportions to be effective,as there should be much more air around the coax from a larger diameter sleeve than what you have recommended to achieve the desired results,its also meant to be connected to the coax 1/4 wave away from the feedpoint.
Another point i'd like to add is as you are sleeving coax with a sleeve balun then surely it would make more sense for it to be 6ft long and not 9 feet long as a 1/4 wavelength in coax like rg213/u or rg58c/u is only 6 feet due to the cable's velocity factor.it would be around 7ft 2" with low loss foam dielectric style coaxial cables.If you make it 9 feet you are technically sleeving more than a 1/4 wave.
An rf choke is supposed to be added right at the feedpoint,not a 1/4 wavelength away,if you add it a 1/4 wavelength away you are effectively adding a 1/4 wavelength to the radiating system.
Adding a ground wire to the antenna at the antenna would also be another addition to the radiating system which is why it is recommended all ground wires should be less than 1/4 wavelength at operating frequencies.if added to an antenna that is choked /isolated it would effectively defeat the purpose of isolating the antenna,what you really want is the coax grounded at ground level and also the mast grounded at ground level,at a push you could also ground the coax to the mast below the rf choke and just ground the mast,which would give a low resistance path for lightning and a high impedance path to rf.your way would give a low resistance path for lightning but also a low impedance path for rf too.
Don't get me wrong I rate your antenna very highly,among the best for performance and build quality,but your theory to go with it doesn't add up,If i'm not mistaken your design has been borrowed from the penetrator/devant antennas and combined whats good about both of them.
Excuse my gratuitous approch as marconi puts it,but I find getting straight to the point brings response from which will yield debate and we can all learn.It wasn't intended to offend just to spark debate and learning.
Jay have you ever considered inverting the radials upwards in a v shape and stacking the 5/8 wave over a 1/4 wave in a similar vein to the vector 4000/sigma 1V antennas? these colinear arrays show much improved radiation and gain in the far field than a 5/8 wave alone.I'm sure your inherent build quality would improve beyond the vector 4000 capabilities as although it is a fine transmitting antenna it is inherently weak,or piss weak as we say in the UK.Feeding it with a trombone style matching system instead of a gamma match may also improve it.
Some food for thought for you.
All the best,George (Jazzsinger)
W8JI also seems to disagree with your position as I believe also does the late great LB Cebik.Its common knowledge that 4 1/4 wave ground plane radials are insufficient alone to stop common mode currents although do go a fair way in reducing them.
The add on you say you didn't call a sleeve balun but i would say obviously is intended to act like one is of insufficient proportions to be effective,as there should be much more air around the coax from a larger diameter sleeve than what you have recommended to achieve the desired results,its also meant to be connected to the coax 1/4 wave away from the feedpoint.
Another point i'd like to add is as you are sleeving coax with a sleeve balun then surely it would make more sense for it to be 6ft long and not 9 feet long as a 1/4 wavelength in coax like rg213/u or rg58c/u is only 6 feet due to the cable's velocity factor.it would be around 7ft 2" with low loss foam dielectric style coaxial cables.If you make it 9 feet you are technically sleeving more than a 1/4 wave.
An rf choke is supposed to be added right at the feedpoint,not a 1/4 wavelength away,if you add it a 1/4 wavelength away you are effectively adding a 1/4 wavelength to the radiating system.
Adding a ground wire to the antenna at the antenna would also be another addition to the radiating system which is why it is recommended all ground wires should be less than 1/4 wavelength at operating frequencies.if added to an antenna that is choked /isolated it would effectively defeat the purpose of isolating the antenna,what you really want is the coax grounded at ground level and also the mast grounded at ground level,at a push you could also ground the coax to the mast below the rf choke and just ground the mast,which would give a low resistance path for lightning and a high impedance path to rf.your way would give a low resistance path for lightning but also a low impedance path for rf too.
Don't get me wrong I rate your antenna very highly,among the best for performance and build quality,but your theory to go with it doesn't add up,If i'm not mistaken your design has been borrowed from the penetrator/devant antennas and combined whats good about both of them.
Excuse my gratuitous approch as marconi puts it,but I find getting straight to the point brings response from which will yield debate and we can all learn.It wasn't intended to offend just to spark debate and learning.
Jay have you ever considered inverting the radials upwards in a v shape and stacking the 5/8 wave over a 1/4 wave in a similar vein to the vector 4000/sigma 1V antennas? these colinear arrays show much improved radiation and gain in the far field than a 5/8 wave alone.I'm sure your inherent build quality would improve beyond the vector 4000 capabilities as although it is a fine transmitting antenna it is inherently weak,or piss weak as we say in the UK.Feeding it with a trombone style matching system instead of a gamma match may also improve it.
Some food for thought for you.
All the best,George (Jazzsinger)
theres no such thing as flat swr,this is just an indication that lossy coax is being used and depriving the swr meter of the true reflected power it needs to give accurate readings.
B
BOOTY MONSTER
Guest
i dont pretend to or think that im in a league with you guys by any means ......... but i do have a few questions .. as usuall , LOL .
im of the impression that 1/2wgp and 5/8wgp antennas radiate from the middle section of the verticle where 1/4wgp's radiate from the ends or the full length . IF...... if ive correctly understood what ive read could that be why the larger antennas dont need or benefit as much from a choke as a 1/4wgp for CMC's ?
as for the sleveing or tubing around the coax below the feedpoint . since its connected to the ground side of the coax why would that stop CMC's ? would using doubled shielded coax (foil and braid) have a similar effect ?
in another bid to prove my lack of reading comprehension skills (LOL) arnt antennas with no ground radials (antron/imax/etc.) and antennas with drooping down ground radials much more likely to encounter CMC problems than ones with horozontal full length grounds ?
jazz the other guys know im just a cb'er so be easy on me , hahahaha .
just letting you know .
im thinking (the source of many of my problems) that a flat vswr is relative to the bandwith youre measuring it on . doesnt every antenna tune flat at some frequency ? even if its not the one intended ?
awesome thread guys !!! thanks i love this technical mumbo-jumbo stuff (means quality/correct information to hams) . marconi
im of the impression that 1/2wgp and 5/8wgp antennas radiate from the middle section of the verticle where 1/4wgp's radiate from the ends or the full length . IF...... if ive correctly understood what ive read could that be why the larger antennas dont need or benefit as much from a choke as a 1/4wgp for CMC's ?
as for the sleveing or tubing around the coax below the feedpoint . since its connected to the ground side of the coax why would that stop CMC's ? would using doubled shielded coax (foil and braid) have a similar effect ?
in another bid to prove my lack of reading comprehension skills (LOL) arnt antennas with no ground radials (antron/imax/etc.) and antennas with drooping down ground radials much more likely to encounter CMC problems than ones with horozontal full length grounds ?
jazz the other guys know im just a cb'er so be easy on me , hahahaha .
just letting you know .
im thinking (the source of many of my problems) that a flat vswr is relative to the bandwith youre measuring it on . doesnt every antenna tune flat at some frequency ? even if its not the one intended ?
awesome thread guys !!! thanks
i love this technical mumbo-jumbo stuff (means quality/correct information to hams) . marconiim of the impression that 1/2wgp and 5/8wgp antennas radiate from the middle section of the verticle where 1/4wgp's radiate from the ends or the full length . IF...... if ive correctly understood what ive read could that be why the larger antennas dont need or benefit as much from a choke as a 1/4wgp for CMC's ?
Not really. Any antenna can produce CMCs if it's not tuned correctly. An antenna radiates over it's entire length. That radiation may be concentrated at particular heights/distances from it's feed point, but the whole length is required for it to do so.
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as for the sleveing or tubing around the coax below the feedpoint . since its connected to the ground side of the coax why would that stop CMC's ? would using doubled shielded coax (foil and braid) have a similar effect ?
No. That sleeve is a particular electrical length and that length has properties that affect what current happens to be conducted on it. One of the properties of a 1/4 wave length is that it offers very high resistance (impedance actually) to RF at it's design frequency. So, as that current gets to the 'end' of that sleeve it's all used up, gone. Is it really like that? No, but it works as a non-electrical explanation.
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in another bid to prove my lack of reading comprehension skills (LOL) arnt antennas with no ground radials (antron/imax/etc.) and antennas with drooping down ground radials much more likely to encounter CMC problems than ones with horozontal full length grounds ?
No. Those radials don't stop CMCs as such. The only thing the 'tilt' of those radials affect is the input impedance of the antenna.
---
jazz the other guys know im just a cb'er so be easy on me , hahahaha .
just letting you know .
im thinking (the source of many of my problems) that a flat vswr is relative to the bandwith youre measuring it on . doesnt every antenna tune flat at some frequency ? even if its not the one intended ?
Yes to both of those questions. 'Flat' is a matter of definition. What's the upper/lower limits for that 'bandwidth, 2:1, 1.5:1, or 1.2:1? The common standard is a 2:1 SWR bandwidth limit. Is there a "standard" spread for that bandwidth? No, there isn't. Depends on the particular antenna and how/where it's mounted. They are all gonna be different to some extent.
SWR will be 'flat', or very close to 1.0:1 at some frequency. May not be the one you want (seldom is), but it'll be 'flat' somewhere. The 'catch' to that 'flat' SWR is that resonance has nothing to do with SWR. An SWR meter can't tell you anything about an antenna being resonant. It can only tell you the degree of 'match' between the feed line's and antenna's impedances. That 'nice' SWR doesn't, and typically is never close to resonance with out some type of impedance matching device being used. That 'tilting' of the radials from horizontal is a matching device just like a gamma match, or any other such device.
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awesome thread guys !!! thanks i love this technical mumbo-jumbo stuff (means quality/correct information to hams) . marconi[/QUOTE]
If you will delete that "to hams" part, I agree with you here too. That correctness applies to everybody, whether they like it or not.
- 'Doc
Quit 'bad mouthing' yourself! You ain't dumb. There were only two of us who were born knowing all this stuff. I can't tell you the other one is, she'll get mad at me...
Not really. Any antenna can produce CMCs if it's not tuned correctly. An antenna radiates over it's entire length. That radiation may be concentrated at particular heights/distances from it's feed point, but the whole length is required for it to do so.
---
as for the sleveing or tubing around the coax below the feedpoint . since its connected to the ground side of the coax why would that stop CMC's ? would using doubled shielded coax (foil and braid) have a similar effect ?
No. That sleeve is a particular electrical length and that length has properties that affect what current happens to be conducted on it. One of the properties of a 1/4 wave length is that it offers very high resistance (impedance actually) to RF at it's design frequency. So, as that current gets to the 'end' of that sleeve it's all used up, gone. Is it really like that? No, but it works as a non-electrical explanation.
---
in another bid to prove my lack of reading comprehension skills (LOL) arnt antennas with no ground radials (antron/imax/etc.) and antennas with drooping down ground radials much more likely to encounter CMC problems than ones with horozontal full length grounds ?
No. Those radials don't stop CMCs as such. The only thing the 'tilt' of those radials affect is the input impedance of the antenna.
---
jazz the other guys know im just a cb'er so be easy on me , hahahaha .
just letting you know .
im thinking (the source of many of my problems) that a flat vswr is relative to the bandwith youre measuring it on . doesnt every antenna tune flat at some frequency ? even if its not the one intended ?
Yes to both of those questions. 'Flat' is a matter of definition. What's the upper/lower limits for that 'bandwidth, 2:1, 1.5:1, or 1.2:1? The common standard is a 2:1 SWR bandwidth limit. Is there a "standard" spread for that bandwidth? No, there isn't. Depends on the particular antenna and how/where it's mounted. They are all gonna be different to some extent.
SWR will be 'flat', or very close to 1.0:1 at some frequency. May not be the one you want (seldom is), but it'll be 'flat' somewhere. The 'catch' to that 'flat' SWR is that resonance has nothing to do with SWR. An SWR meter can't tell you anything about an antenna being resonant. It can only tell you the degree of 'match' between the feed line's and antenna's impedances. That 'nice' SWR doesn't, and typically is never close to resonance with out some type of impedance matching device being used. That 'tilting' of the radials from horizontal is a matching device just like a gamma match, or any other such device.
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awesome thread guys !!! thanks
i love this technical mumbo-jumbo stuff (means quality/correct information to hams) . marconi[/QUOTE]If you will delete that "to hams" part, I agree with you here too. That correctness applies to everybody, whether they like it or not.
- 'Doc
Quit 'bad mouthing' yourself! You ain't dumb. There were only two of us who were born knowing all this stuff. I can't tell you the other one is, she'll get mad at me...
B
BOOTY MONSTER
Guest
thanks Doc. you said electrical length in the line about the sleve . i know coax has a velocity factor that affects its electrial wavelength and fractions of it . would something like aluminium tubing or copper tubing have a velocity factor to take into account ? jay mentioned 9 foot which is a physical 1/4 wave length . i know it would weigh much more but would there be any difference in length or effect/results wsing 3/4 copper pipe/tube vs. 3/4 aluninium ?
wasnt bad mouth myself , just showing a lil playful humility and respect to folks that know much more than me . but thanks for the vote of confidence .
dont want that better half mad now do we ! hehehe
wasnt bad mouth myself , just showing a lil playful humility and respect to folks that know much more than me
. but thanks for the vote of confidence . dont want that better half mad now do we ! hehehe
I have no idea what the actual length of that 1/4 wave section of pipe would be. I'd have to 'dip' it to make sure. As for copper, aluminum, stainless steel, cast iron, whatever, the conductivity of any of them just isn't going to make enough difference to worry about. I would worry about the weight of it and supporting it.
Got to thinking about that double shielded coax. That wouldn't work because of both layers of shielding would be in contact with each other. That 'sleeve' should only make contact with the shield in one place.
- 'Doc
Oh, and you'd better hope 'she' doesn't see you calling her my 'better half', you'll be in more trouble than me! And no 'she' isn't!
Got to thinking about that double shielded coax. That wouldn't work because of both layers of shielding would be in contact with each other. That 'sleeve' should only make contact with the shield in one place.
- 'Doc
Oh, and you'd better hope 'she' doesn't see you calling her my 'better half', you'll be in more trouble than me! And no 'she' isn't!
doc booty,
its not the electrical length of the pipe or the coax you want to know about,
when you put the tube around the coax the inside surface of the tube and outside braid of the coax running inside form a new coaxial transmissionline,
it is the electrical properties of that newly formed transmissionline that determines just how long the sleeve needs to be for optimal choking impedance,
the relative diameters, exact spacing, the coax outer jacket and anything else between the two determine the newly formed transmissionline's impedance and velocity factor just like it does in regular coax,
change any of the above and your optimum sleeve length will change for a given frequency,
small coax with a low loss jacket in a large tube or large coax in a VERY large tube will give you a high choking impedance,
large coax in a small tube will give poor choking performance especially if the coax outer jacket is pvc or some other none low loss dielectric material,
shorting it at the antenna end will give you a radiating sleeve,
shorting it at the lower end will give a none radiating sleeve,
hope this helps
its not the electrical length of the pipe or the coax you want to know about,
when you put the tube around the coax the inside surface of the tube and outside braid of the coax running inside form a new coaxial transmissionline,
it is the electrical properties of that newly formed transmissionline that determines just how long the sleeve needs to be for optimal choking impedance,
the relative diameters, exact spacing, the coax outer jacket and anything else between the two determine the newly formed transmissionline's impedance and velocity factor just like it does in regular coax,
change any of the above and your optimum sleeve length will change for a given frequency,
small coax with a low loss jacket in a large tube or large coax in a VERY large tube will give you a high choking impedance,
large coax in a small tube will give poor choking performance especially if the coax outer jacket is pvc or some other none low loss dielectric material,
shorting it at the antenna end will give you a radiating sleeve,
shorting it at the lower end will give a none radiating sleeve,
hope this helps
B
BOOTY MONSTER
Guest
i have 50 feet of 8x tramflex so it would have a lot more room to shift around in a 3/4 tube so it seems that would be asking for trouble . but if i used some of that courrgrated plastic wire loom (like is used on car wiring systems) on the coax inside the tube that might keep it fairly consistantly centered.........but what are the odds that spacing would be remotely acceptable for 27MHz. ? from what i understand a roll of coax can be off 10 or more ohms in some places so it seems proper spacing is critical for correct impedance . is there a formula to figgure that out ? trial and error ? antenna analizer ? luck ?
lmr-400 is only .405 inches outside diameter so it still has .345 of play to shift around inside the 3/4 tubing .
marconi , you mentioned "I think it could simply work in the case of the Starduster just to shield the first 1/4 wave of the feed line from the inductive RF (not CMC) of the ground radials......."
since the tube is clamped to the outside/ground of the 259 doesnt it itself become a ground element that could radiate inductive RF in very close proximity to the coax essentally encapsulating it inside it ?
since ill be upgrading to a 5/8 that really shouldnt need this im %99 sure i wont be attempting it , but id still like to try to understand it a lil more and ill thank you guys again for the great info y'all have shared here and putting up with me . seems every answer i get leads to more questions . but i think thats a good thing .... as long as i dont get on peoples nerves looking for answers ! hahahahaha .
lmr-400 is only .405 inches outside diameter so it still has .345 of play to shift around inside the 3/4 tubing .
marconi , you mentioned "I think it could simply work in the case of the Starduster just to shield the first 1/4 wave of the feed line from the inductive RF (not CMC) of the ground radials......."
since the tube is clamped to the outside/ground of the 259 doesnt it itself become a ground element that could radiate inductive RF in very close proximity to the coax essentally encapsulating it inside it ?
since ill be upgrading to a 5/8 that really shouldnt need this im %99 sure i wont be attempting it , but id still like to try to understand it a lil more and ill thank you guys again for the great info y'all have shared here and putting up with me . seems every answer i get leads to more questions . but i think thats a good thing .... as long as i dont get on peoples nerves looking for answers ! hahahahaha .