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| Coax loop or not; Help for new instalation | |
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| Tweet Topic Started: Wednesday, 16. September 2015, 00:31 (907 Views) | |
| plowman | Wednesday, 16. September 2015, 00:31 Post #1 |
9 Charlie Tango 016
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Hi everyone I'm in the process of installing a new Shootin Gunn 2 beam and was wondering if I should put a coax loop at antenna for horizontal and vertical polarization and how big how I should make it. The antenna is going on a 40 foot tower then rotor and 10 feet of pipe. It will be used on 26,000 to 27,999 Mhz. Your expertise would be greatly appreciated. Thanks Plowman 9CT016 |
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| RadioDaze | Wednesday, 16. September 2015, 09:54 Post #2 |
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Coax loop ? That is called a choke and needs to be specific to the coax used and frequency of operation.No idea what a "Shootin Gunn 2 beam" is what is it a Yagi, Quad or some other ? I am not sure you need one on a centre fed balanced dipole (which is often what the driven element is on a Yagi) + the parasitic elements. It may not be as critical as a thoughtlessly thrown up end fed could be without consideration for ground planes and/or isolation from the mounting pole. The only goal would be stopping common mode currents on the coax and that may be a non issue. I would get it up without and see if everything is VSWRing and working nicely first before bothering. |
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| Deleted User | Wednesday, 16. September 2015, 10:30 Post #3 |
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"Shootin Gunn 2 Antenna" ![]() |
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| Nubster | Wednesday, 16. September 2015, 11:00 Post #4 |
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26 Charlie Tango 050
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How to make one take your pick of any of these videos normally a ugly balun to give it its more common name is normally only used to stop stray RF getting back in to your radio shack/room/space under the stairs wherever you are LOL, and to help with stray RF to stop interfering with others/neighbours ie: T.V's and what ever they want to blame you for if all three of what I mention is OK dont bother making one unless you want to for peace of mind video link https://youtu.be/tVVCbaRAUWk?list=PLCbSjoFBfQFcM89O488wc1Wkr9-MnEeY3 https://youtu.be/zpUSe0yrxjs
Edited by Nubster, Wednesday, 16. September 2015, 11:15.
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Life is too short, so ave a laugh now n then never know you might like it | |
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| RadioDaze | Wednesday, 16. September 2015, 11:15 Post #5 |
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I never seen one of those before. It appears to be some kind of gamma matched ground plane antenna with a reflector. You will have to defer to someone with greater antenna system knowledge as that is not like any of the basic antennas I have ever seen.
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| Northern Crusader | Wednesday, 16. September 2015, 11:55 Post #6 |
26 Charlie Tango 1760
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They need to be used on all antennas. Even dipoles have common mode on the coax. (courtesy of G3TXQ) Dipole without choke. ![]() Dipole with air wound choke that someone got wrong so its very reactive at the frequency of interest, +j200Ω. ![]() Dipole with resistive choke how it should be done.
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| 26CT1760 / M0GVZ | |
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| RadioDaze | Wednesday, 16. September 2015, 13:17 Post #7 |
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Thanks for explaining that. I thought balanced antennas tended not cause common mode currents in feeders. Are they less susceptible to cause them or reduce common mode currents? To my eye if I am reading the models correctly graphically there does not seem to be great difference between with and without a choke. |
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| The DB | Wednesday, 16. September 2015, 15:06 Post #8 |
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That can change with coax length. The models just happened to use a length of coax that was naturally resistant to common mode currents. If the feedline wire was a quarter electrical wavelength longer or shorter, the without choke image would look much more like the wrong choke image. The DB |
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| RadioDaze | Wednesday, 16. September 2015, 15:15 Post #9 |
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Thanks for clarifying The DB. |
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| RadioDaze | Wednesday, 16. September 2015, 15:26 Post #10 |
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As I gather common mode currents can cause RF feedback, "RF in the shack", noisy RX, and troubles getting a low SWR (especially 5/8 waves). If you are using an end fed vertical with reasonable power and you have no such problems should you just carry on regardless ? I tried a choke on an IMAX2000 the other day and the SWR hit 2:1 Comparatively when there was no choke it was 1.4:1 (though I could not easily move the choke away from the metal supporting structure the antenna was bolted on which could have been affecting the chokes value i.e. my gate) There seems to be no fixed rule as such when it comes to choke practice and you have to see what you have in any specific situation. I would not like to put a choke on in this specific mounting method due to the increased VSWR that was measured. It might have been that at very close to the feed point I could not get the choke away from the gate by very far. In that situation I would be much happier operating with no choke at all. Would there have been common mode on the coax? Irrelevant of the reasonable VSWR reading and no negative effects to the radio (I was only using QRP at the time when testing). Or would the sufficient ground plane have stopped common mode currents forming to a degree where it would have caused any troubles. When do you apply the coaxial choke and when do you leave seemingly well alone ? |
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| Nik Gnashers | Wednesday, 16. September 2015, 15:28 Post #11 |
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26 Charlie Tango 573
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![]() So, 5 turns on a 4.25" diameter tube for 11m band. Of course, an AIR CHOKE is ok, but to really completely kill any RF from the feedline, you need a 1:1 isolator such as this : http://www.ebay.co.uk/itm/M0CVO-ANTENNAS-1-1-LINE-ISOLATOR-/331370794083 |
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M6GFT ::: QRZ Page- http://www.qrz.com/db/M6GFT [imgw=150]http://i.imgur.com/RJjZCOw.jpg[/imgw] All CT members are expected to use good operating procedure, good manners, and be friendly...Please do not chat on any calling frequencies, or use bad language on air, thank you it is appreciated. | |
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| RadioDaze | Wednesday, 16. September 2015, 15:30 Post #12 |
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I was using the 5T on a 4.25inch former as suggested by the chart Nik posted. |
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| Nubster | Wednesday, 16. September 2015, 15:34 Post #13 |
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26 Charlie Tango 050
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Nik was it you that found out that the plastic hole cup for the hole on a golf course where the perfect diameter for making a balun ? |
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Life is too short, so ave a laugh now n then never know you might like it | |
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| RadioDaze | Wednesday, 16. September 2015, 15:45 Post #14 |
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I am finding this is quite complicated (as always) but this seems quite good: http://www.dj0ip.de/common-mode-chaos/ |
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| RadioDaze | Wednesday, 16. September 2015, 16:16 Post #15 |
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Though in time honored tradition it pooh pooh's air cored chokes and even suggests common mode currents can in fact be increased by their use. Clear as mud There is a power point presentation on the subject here: http://www.dj0ip.de/common-mode-chaos/cmc-theory-simple/ http://www.ifwtech.co.uk/g3sek/in-prac/ This is why I reckon on see if you have an actual problem first. Fortunately for whatever reasons I have yet to encounter an issue. That is not to say I or you won't though of course. |
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| The DB | Wednesday, 16. September 2015, 18:35 Post #16 |
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...
If you put a choke on the coax and it changes your SWR from when there was no choke, then you had common mode currents that were affecting how the antenna appeared to be tuned. You should leave the choke on the coax and rettune the antenna to a low SWR. This is of course assuming the choke is effective for said frequency, and not like the modeling example above where an improperly designed choke made the common mode currents problem worse.
A proper groundplane will also do a lot to lessen common mode currents, and for an elevated antenna.
I don't use coaxial chokes, I much prefer resistive chokes (essentially a few ferrite beads I put on a coax before putting an end on it). They don't really cost any more and have many advantages over coaxial based chokes. That being said, a properly made choke of any kind will not hurt your antennas performance. The DB |
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| The DB | Wednesday, 16. September 2015, 19:14 Post #17 |
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Lets see if I can add some water to that mud for you and at least thin it out. Unfortunately, the why behind such things isn't always easy to explain. Coaxial chokes, and really any coil of wire, have two forms of reactance at play. There is inductance, which is what coils are known for, and there is capacitance. A coaxial choke will work when it is designed in such a way that both inductive and capacitive reactances are canceled (or nearly so) for a given frequency. When this happens, the choke will resist the passage of common mode currents no matter how long the coax run is. This is the optimal use of a choke. Unfortunately, this is only true for a limited range of frequencies. If you go outside of said range of frequencies the choke is effective for, or simply made the choke wrong, something else happens. In this case, the impedances present in the choke, which now do not cancel, not only combine with themselves, but also with a third impedance presented by the length of coax used as well. This impedance being present on the outside of the coax, has nothing to do with the coax's listed impedance, and depending on how long the coax is can be effectively anything, including open and short circuits electrically. If these three combined impedances end up creating a high impedance, they will resist common mode currents just like a properly made choke. However, if these combined impedances combine to create a low impedance, it will allow significant common mode currents to flow. Like I said, it isn't the easiest thing in the world to try and explain. An easier way of explaining it is it is like a balancing act, there are multiple components that have to be balanced just right to guarantee the desired result. That is why specific directions for sizing and how many windings and such are given for multiple different types of coax. And one more thing, a late comment from something that was stated previously. just because you don't notice any of the obvious effects of common mode currents doesn't mean they don't exist, and doesn't mean they aren't negatively effecting your radio station. The DB |
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| Nik Gnashers | Wednesday, 16. September 2015, 19:25 Post #18 |
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26 Charlie Tango 573
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Yes mate ![]() The plastic cups used inside the hole on a golf course, are exactly 4.25" diameter, and are £4.99 each on ebay. You can make 2 chokes with one cup as they are quite long... |
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M6GFT ::: QRZ Page- http://www.qrz.com/db/M6GFT [imgw=150]http://i.imgur.com/RJjZCOw.jpg[/imgw] All CT members are expected to use good operating procedure, good manners, and be friendly...Please do not chat on any calling frequencies, or use bad language on air, thank you it is appreciated. | |
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| RadioDaze | Wednesday, 16. September 2015, 20:17 Post #19 |
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Thanks as always The DB. The choke I used was supposed to be correct given the coax used and the correct turns and core size 5T 4.25 inch air cored bit of waste pipe. So as far as I know it was correct for the application. I tried using the tuning rings on the IMAX when I did not have a choke and they did nothing whatsoever, the SWR remains the same 1.4:1 It suggests that there was something (like close proximity to ground affecting the antenna impedance ?) or the gate to which it was attached was not well matched as a ground plane. I cannot prove there was not common mode on the coax. But when the choke was put on the SWR went up to 2:1.. I am having to guess that the choke value was negatively affected by it's close proximity to the gate. I understand that ideally you need to offset the choke as far from a mounting pole or gate as the case may be ,lol as possible which was not really possible to do. |
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| Staffy 48 | Wednesday, 16. September 2015, 20:59 Post #20 |
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I have a 6"- 6 coil air choke in RG8mini feed to my I-Max 2000 that seems to work exceedingly well, why I don't know as I have no way of checking other than my SWR meter which reads:- 26.500 megs 1.5 to 1..........27.500 megs 1.05 to 1.............28.500 megs 1.5 to 1, and that's using 40 watts on an AM dead key to take the readings, surely that's about as good a reading as I can expect on 10 - 11 metres? |
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It appears to be some kind of gamma matched ground plane antenna with a reflector. You will have to defer to someone with greater antenna system knowledge as that is not like any of the basic antennas I have ever seen.






8:48 AM Jul 11