Slightly Unbalanced dipole?
Locked
12
12
 
|
I was asked a question by a newcomer to HF today. After thinking about it, I realized it had never really occurred to me. So I’ll pass it on to the group and see what the antenna gurus think.
Is there ever a time where a slight imbalance in a dipole could be beneficial? My first inclination was, No, other than the obvious OCF Dipole. But considering that you could have very different conditions under an antenna, both above the ground and under the ground, not to mention a difference in height above ground. What do you think. Could a couple of inches or so difference in the length of the legs of a dipole ever work in your favor? Ron Genovesi [hidden email] 541-761-1103 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
|
|
Ron,
With a slight difference, I cannot see that it would either benefit or degrade. The current distribution on the radiator might be a little off-center, but it will still work. But it is one reason for using a very good current choke on the feedline - to keep RF off the outer braid of the coax. The OCF antennas are infamous for conducting RF into the shack. 73, Don W3FPR On 1/13/2019 3:07 PM, Ron Genovesi wrote: > I was asked a question by a newcomer to HF today. After thinking about it, I realized it had never really occurred to me. So I’ll pass it on to the group and see what the antenna gurus think. > Is there ever a time where a slight imbalance in a dipole could be beneficial? My first inclination was, No, other than the obvious OCF Dipole. But considering that you could have very different conditions under an antenna, both above the ground and under the ground, not to mention a difference in height above ground. What do you think. Could a couple of inches or so difference in the length of the legs of a dipole ever work in your favor? ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
Re: Slightly Unbalanced dipole?
|
|
In reply to this post by Ron Genovesi
To answer the question "Could a couple of inches or so difference in the
length of the legs of a dipole ever work in your favor?"......depends on the frequency. For lower frequencies likely NO, for higher frequencies maybe YES. Thus 2" for a 160M dipole would be 0.066% while 2" for a 10M dipole would be 1.02%. In either case..........I doubt that it would make a difference due to local installation conditions. i.e. Different height above ground for the 2 sections, different ground conductivity under the 2 sections, different location / proximity to surrounding objects, different types of end insulators, and etc. As pointed out earlier the current may shift a wee bit and thus distort the "ideal" pattern but in the real world of ham antennas........NAH. 73 Bob, K4TAX On 1/13/2019 2:07 PM, Ron Genovesi wrote: > I was asked a question by a newcomer to HF today. After thinking about it, I realized it had never really occurred to me. So I’ll pass it on to the group and see what the antenna gurus think. > Is there ever a time where a slight imbalance in a dipole could be beneficial? My first inclination was, No, other than the obvious OCF Dipole. But considering that you could have very different conditions under an antenna, both above the ground and under the ground, not to mention a difference in height above ground. What do you think. Could a couple of inches or so difference in the length of the legs of a dipole ever work in your favor? > > Ron Genovesi > [hidden email] > 541-761-1103 > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to [hidden email] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
|
|
In reply to this post by Ron Genovesi
A friend once told me, "Almost none of us has a balanced dipole."
What he meant is, few of us are lucky enough to be able to put a dipole high up and completely in the clear - most of us have to put it up where we can, and one side will be closer to houses, trees, gutters, chain link fences..... than the other. Probably true! 73 Ray K2HYD ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
Re: Slightly Unbalanced dipole?
|
|
That's why I use the statement "xxx ft center fed wire with balanced feed". I do have the ability to measure the current in each side of the feedline. As long as it is within 5%, about the limits of accuracy of measurements, I'm happy.
Bob, K4TAX Sent from my iPhone > On Jan 13, 2019, at 3:55 PM, Ray Albers <[hidden email]> wrote: > > A friend once told me, "Almost none of us has a balanced dipole." > > What he meant is, few of us are lucky enough to be able to put a dipole > high up and completely in the clear - most of us have to put it up where we > can, and one side will be closer to houses, trees, gutters, chain link > fences..... than the other. > > Probably true! > > 73 > Ray K2HYD > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to [hidden email] > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
|
|
In reply to this post by Ray Albers-2
Your friend is a wise man. That is, indeed, reality. There's a
discussion about this in one of my tutorials. http://k9yc.com/CoaxChokesPPT.pdf The PRIMARY reason for putting a very good common mode choke at the feedpoint of an antenna is to prevent the feedline from radiating into your neighbor's living room and from picking up noise from his living room on receive. The reason this works is that the feedline probably runs closer to his living room (and yours) than the antenna itself. A choke will not eliminate all noise, because the antenna is picking it up, and it won't eliminate all of your TX RF in his living room, because your antenna radiates it. 73, Jim K9YC On 1/13/2019 1:55 PM, Ray Albers wrote: > A friend once told me, "Almost none of us has a balanced dipole." > > What he meant is, few of us are lucky enough to be able to put a dipole > high up and completely in the clear - most of us have to put it up where we > can, and one side will be closer to houses, trees, gutters, chain link > fences..... than the other. ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
|
|
In reply to this post by Don Wilhelm
On 1/13/2019 12:23 PM, Don Wilhelm wrote:
> The OCF antennas are infamous for conducting RF into the shack. And for being very noisy on receive. 73, Jim K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
|
|
In reply to this post by Jim Brown-10
By the way, whenever Jim says, "...a very good common mode choke at the feedpoint of an antenna...," he means an antenna fed with coax. For the rest of us, of course, that choke would (should) go at the output of the antenna tuner, whether in the shack or close by.
Al W6LX ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
|
|
Al,
If not placed at the antenna feedpoint (up at the antenna), then the feedline may radiate due to common mode current. For feedlines using parallel conductors, the problem is not as bad if the feedline exits at right angles from the radiator for at least a half wavelength. In that case, the common mode choke can be placed at the junction of the parallel line and coax. In all cases of a radiator fed with coax, a good common mode choke at the radiator feedpoint is necessary. Even though the RF on the inside of the coax is balanced, when the coax reaches the radiator, the side of the antenna connected to the coax shield sees two paths. One is the side of the radiator, and the other is the outside of the coax braid - it will dutifully split to follow both paths depending on the impedance of each one. A good common mode choke will prevent the RF current from seeing the coax shield as another conductor. A similar situation exists at the transition between parallel feedline and coax. 73, Don W3FPR On 1/13/2019 6:42 PM, Al Lorona wrote: > By the way, whenever Jim says, "...a very good common mode choke at the feedpoint of an antenna...," he means an antenna fed with coax. For the rest of us, of course, that choke would (should) go at the output of the antenna tuner, whether in the shack or close by. > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
Re: Slightly Unbalanced dipole?
|
|
In reply to this post by Ray Albers-2
One of the things that has been hinted at is the OCF dipole. One of the really great attributes is that you can adjust the matching impedance of the antenna by moving the feed point away from center. If you have a SWR of 1.5:1 and the R is 75 and the j is 0, move the feed point away from center and you can find 75 ohms. Of course you will have to address the 75 ohm, but the match will be 1:1.
Mel, K6KBE From: Ray Albers <[hidden email]> To: [hidden email] Sent: Sunday, January 13, 2019 1:58 PM Subject: Re: [Elecraft] Slightly Unbalanced dipole? A friend once told me, "Almost none of us has a balanced dipole." What he meant is, few of us are lucky enough to be able to put a dipole high up and completely in the clear - most of us have to put it up where we can, and one side will be closer to houses, trees, gutters, chain link fences..... than the other. Probably true! 73 Ray K2HYD ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
|
|
In reply to this post by Don Wilhelm
You guys keep talking about coax-fed antennas. Yes, although it may be easy to wind coax around a toroid and put it up at the antenna midpoint, that's not so easy with open-wire line. This discussion was very coax-centric and I wanted to open your minds that not everybody feeds their antenna with coax.
Balanced feedlines ("window line" or "open-wire" line) can have significant common-mode current. They're not immune to the effect. (I know you know this; I'm saying it for the benefit of all.) A device with high common-mode impedance, located right at the output of the tuner, suppresses common-mode current on a (balanced) feedline. That's it's purpose. I'm in agreement with you about the 1/ cause of common-mode current and the 2/ cure for it. I'm simply saying that for convenience's sake, consider putting the choke at the tuner output. A choke with high Z to common-mode current, external to the tuner, in a non-conductive box, with as short a connection to the tuner as possible, does quite well at suppressing common-mode current. I'll be happy to share my measurements with you. There is no coax anywhere in my antenna system. So my solution was to put a choke with high impedance to common-mode current right at the output of my antenna tuner. I encourage users of open-wire line to do that. Al W6LX >>> In all cases of a radiator fed with coax, ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
Non-Resonant Antennas and Chokes
|
|
In reply to this post by alorona
On 1/13/2019 3:42 PM, Al Lorona wrote:
> By the way, whenever Jim says, "...a very good common mode choke at the feedpoint of an antenna...," he means an antenna fed with coax. What I actually mean is an antenna matched to its feedline at the operating frequency(ies). > For the rest of us, of course, that choke would (should) go at the output of the antenna tuner, whether in the shack or close by. Actually, for the rest of you, a choke at the shack ONLY reduces coupling common mode current into the shack, but does NOTHING to reduce noise pickup on the line, because the feedline is not isolated from the antenna. Remember -- unless it's choked AT THE FEEDPOINT, that feedline is part of the antenna. If it's ONLY choked at the tuner, the feedline is a wire dangling from the antenna that isn't connected to anything on the other end. AND if the antenna is not matched, a choke that doesn't fry with TX power probably isn't doing anything useful. For a discussion of this, see "Don't Burn Up Your Balun" by retired Antenna Book editor N6BV, from QST something like 3-4 years ago. Bottom line -- if you live miles from anywhere with no noise sources pumping trash, all-band non-resonant antennas with open wire feeders can be a decent solution -- they transmit just fine if you've done everything right. But they're a sitting duck for noise because there's no practical way to choke them. 73, Jim K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
|
|
In reply to this post by Ron Genovesi
For HF, I doubt that many (if any) dipoles are fully balanced. On higher
frequencies (VHF), the antenna can be far enough away from other objects and the ground that it could be considered unaffected by the environment. But a 40m dipole at 40 feet is less than a half wave from the ground and other objects so i doubt that it is not influenced by it's unbalanced surroundings. Heck, even the grass is greener on one side than the other (more moisture.) Does it make a difference in operational abilities? Probably not. Sure it might be 3 db better to the north than to the south, but with 6 or 10 db fading, that will never be noticed. Just my opinion. Ken WA8JXM On 1/13/19 3:07 PM, Ron Genovesi wrote: > I was asked a question by a newcomer to HF today. After thinking about it, I realized it had never really occurred to me. So I’ll pass it on to the group and see what the antenna gurus think. > Is there ever a time where a slight imbalance in a dipole could be beneficial? My first inclination was, No, other than the obvious OCF Dipole. But considering that you could have very different conditions under an antenna, both above the ground and under the ground, not to mention a difference in height above ground. What do you think. Could a couple of inches or so difference in the length of the legs of a dipole ever work in your favor? > > Ron Genovesi > [hidden email] > 541-761-1103 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
Re: Slightly Unbalanced dipole?
|
|
In reply to this post by alorona
Al:
Al et al; I do agree. And in fact my 250 ft center fed antenna with 75 ft of window feedline terminates at the operating position into a Balun Designs 1171T. Then a coax jumper of about 2 ft long, which is actually a model 8232 from The Wireman, Common Mode Choke between the balun and the output of the KAT500 tuner. I was asked recently on another discussion group why I used both a balun and a common mode choke. The balun itself does not provide adequate common mode choking ability for all bands 160M - 10M. Since I must use a coax jumper between the balun and tuner, why not just make it a common mode choke. All seems reasonable and simple...... like me. Thus one cable and 2 connectors and a quantity of ferrite beads does double duty. And yes for my coax center fed antennas I use a 1:1 current balun, which is no more than a length of coax with a number of ferrite beads at the feed point in a PVC enclosure that serves as the center insulator. I forget the brands and models. They have been up on the tower for several years. 73 Bob, K4TAX On 1/13/2019 5:42 PM, Al Lorona wrote: > By the way, whenever Jim says, "...a very good common mode choke at the feedpoint of an antenna...," he means an antenna fed with coax. For the rest of us, of course, that choke would (should) go at the output of the antenna tuner, whether in the shack or close by. > > Al W6LX > > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
|
|
In reply to this post by Don Wilhelm
Don et al,
I sense this discussion is targeting the use of a dipole on a single band. What is being missed is that many of us use a dipole cut for for one band and use it for the bands above. What we see are high SWRs on all of the higher frequency bands. We use balanced feeds, open wire or ladder line, as far as we can before we transition to coax. That transition is a good current balun. What this does is control losses due to high SWR and keeps RF currents off the outside of the coax shield. What has been said in previous notes might be true, but they will prevent or make use of balanced feeders quite difficult. An 80 meter dipole used on 40 or 20, for example, is a pretty good antenna. However, it begs to be fed with open wire or equal. That means no balun can be used at the antenna feed point. The exception is to use very expensive coax. :-) 73, Barry K3NDM ------ Original Message ------ From: "Don Wilhelm" <[hidden email]> To: [hidden email] Sent: 1/13/2019 6:59:18 PM Subject: Re: [Elecraft] Slightly Unbalanced dipole? >Al, > >If not placed at the antenna feedpoint (up at the antenna), then the feedline may radiate due to common mode current. >For feedlines using parallel conductors, the problem is not as bad if the feedline exits at right angles from the radiator for at least a half wavelength. In that case, the common mode choke can be placed at the junction of the parallel line and coax. > >In all cases of a radiator fed with coax, a good common mode choke at the radiator feedpoint is necessary. Even though the RF on the inside of the coax is balanced, when the coax reaches the radiator, the side of the antenna connected to the coax shield sees two paths. One is the side of the radiator, and the other is the outside of the coax braid - it will dutifully split to follow both paths depending on the impedance of each one. A good common mode choke will prevent the RF current from seeing the coax shield as another conductor. > >A similar situation exists at the transition between parallel feedline and coax. > >73, >Don W3FPR > > >On 1/13/2019 6:42 PM, Al Lorona wrote: >>By the way, whenever Jim says, "...a very good common mode choke at the feedpoint of an antenna...," he means an antenna fed with coax. For the rest of us, of course, that choke would (should) go at the output of the antenna tuner, whether in the shack or close by. >> >______________________________________________________________ >Elecraft mailing list >Home: http://mailman.qth.net/mailman/listinfo/elecraft >Help: http://mailman.qth.net/mmfaq.htm >Post: mailto:[hidden email] > >This list hosted by: http://www.qsl.net >Please help support this email list: http://www.qsl.net/donate.html >Message delivered to [hidden email] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
|
|
In reply to this post by Jim Brown-10
>>> the feedline is a wire dangling from the antenna
>>> that isn't connected to anything on the other end. That "dangling wire" is actually *two* wires, and the field of one cancels the field of the other for no net radiation or reception -- at least that is the condition we're trying to achieve. That isn't the same as a single wire which by definition would be a common-mode conductor as you correctly say. Once common-mode current is reduced to a small enough value, then the open-wire line isn't radiating nor receiving. But... you gotta measure the common-mode current to know. I have done so. The instrument used to measure this is quite simple. >>> a choke that doesn't fry with TX >>> power probably isn't doing anything useful. A choke dissipates power only in it's resistance, not it's reactance, and only due to the common-mode current. If the parameters of the choke are chosen correctly for the frequency band, and if the choke reduces common-mode current to a low enough value, then the power dissipated in the choke can be very low even when you're operating at high power. A choke that burns up at high power is certainly not inevitable nor normal and can be fixed by re-designing the choke. There are a number of ferrite mixes available and they seem to each be optimum for a slightly different part of the HF spectrum. There's also the twist that permeability is actually a complex quantity (real and imaginary parts) which has a direct bearing on the resistance and reactance of the choke, but I won't go into that at this time. The posting limit is fast approaching; I'll let you have the last word. Al W6LX ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
Re: Non-Resonant Antennas and Chokes
|
|
On 1/13/2019 6:46 PM, Al Lorona wrote:
>>>> the feedline is a wire dangling from the antenna >>>> that isn't connected to anything on the other end. > That "dangling wire" is actually *two* wires, and the field of one cancels the field of the other for no net radiation or reception -- at least that is the condition we're trying to achieve. That isn't the same as a single wire which by definition would be a common-mode conductor as you correctly say. You're confusing common mode with differential mode. Differential mode is the nicely equal currents at every point on the line, which do, indeed, cancel. But common mode current is the DIFFERENCE between currents that are not equal. > > Once common-mode current is reduced to a small enough value, then the open-wire line isn't radiating nor receiving. But... you gotta measure the common-mode current to know. I have done so. WHERE are you measuring it? You do realize that, like any antenna, current varies along the wires that make up the antenna. A choke attempts to force the current to zero AT THE POINT WHERE THE CHOKE IS PLACED. You're putting it at the tuner, so it forces current to zero there. But a quarter wave up the feedline, the current reaches a maximum value. > The instrument used to measure this is quite simple. I'd be very interested in how you're measuring the current at the antenna feedpoint. :) That is NOT so simple. > >>>> a choke that doesn't fry with TX >>>> power probably isn't doing anything useful. > A choke dissipates power only in it's resistance, not it's reactance, and only due to the common-mode current. If the parameters of the choke are chosen correctly for the frequency band, and if the choke reduces common-mode current to a low enough value, then the power dissipated in the choke can be very low even when you're operating at high power. Yes. Because power is I squared R, it is changing twice as fast as R is changing, and current is determined by R. So making R very large makes current very small, which minimizes power dissipated by the choke. > A choke that burns up at high power is certainly not inevitable nor normal and can be fixed by re-designing the choke. There are a number of ferrite mixes available and they seem to each be optimum for a slightly different part of the HF spectrum. There are, in fact, nearly two dozen different ferrite mixes in the Fair-Rite catalog, but only a small fraction of them are useful for common mode chokes on frequencies that we care about. #31 material is the most useful between 160M and 2M; #75 is useful between 630M and 40M. I'm currently investigating a relatively new material that MAY be useful on the higher HF bands, but it's a fairly high Q material, and Fair-Rite's cores are a pretty wide-tolerance part. My recent Cookbook is based on having measured more than 200 cores, then winding chokes on cores that are at the limits of those I measured, and making recommendations on the basis of worst-case results from those cores at the limits. That works for #31, because it's a very low Q part in this range. Well designed ferrite chokes are very low-Q parallel resonant circuits are resonant (or near resonant) in the frequency ranges where they will be used, and it is the very high value of resistance that makes it a good choke. Why? Because a choke that is mostly inductive, with very little resistance, can resonate with the rest of the transmission line in the common mode circuit, INCREASING the common mode current rather that decreasing it. But resistance ALWAYS reduces the common mode current. A choke without a lot of resistance is quite sensitive to the ELECTRICAL length of the feedline, which, of course, increases with increasing frequency. That's why a coil of coax, or coax wound on a low loss toroid like Fair-Rite #61 is a lousy choke. > There's also the twist that permeability is actually a complex quantity (real and imaginary parts) which has a direct bearing on the resistance and reactance of the choke, but I won't go into that at this time. Yep. Folks can read about it in k9yc.com/RFI-Ham.pdf which is a tutorial that I wrote more than ten years ago. There are two components, mu' and mu''; the first is what we've always called mu, and describes the inductance; the second tells us the value of a resistance in series with the inductor that, from a circuit analysis point of view, defines the loss coupled to the wire from the core. And both mu' and mu'' both vary (a LOT) with frequency. I'm not going to go beyond this on the reflector, but will refer interested readers to that tutorial, and to my latest work, a major update of the Choke Cookbook published in the first edition of the tutorial. The update is k9yc.com/2018Cookbook.pdf 73, Jim K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
Re: Slightly Unbalanced dipole?
|
|
In reply to this post by alorona
Why would a common mode choke at the antenna feedpoint present more problems than the common practice of feeding a balanced line through a single-ended tuner and balun?
I’ve done a lot of experimenting with the latter and have learned that a) the best configuration is with a 1:1 balun, and b) the balun will become lossy and heat up if the impedance it sees is highly reactive. The solution to b) is to cancel the reactance by putting the appropriate opposite reactance in both sides of the line between the balun and the antenna. Of course this means added complexity. I would love to figure out how to make a choke that would work with balanced line at the antenna feedpoint, on multiple bands without any additional components. FWIW I’ve switched to a real balanced tuner (Johnson Matchbox). Victor 4X6GP > On 14 Jan 2019, at 1:42, Al Lorona <[hidden email]> wrote: > > By the way, whenever Jim says, "...a very good common mode choke at the feedpoint of an antenna...," he means an antenna fed with coax. For the rest of us, of course, that choke would (should) go at the output of the antenna tuner, whether in the shack or close by. > > Al W6LX > > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to [hidden email] Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
|
|
In reply to this post by Barry LaZar
An array of dipoles are common for high power international broadcast stations. They accept a VSWR of about 2:1 from aprox 20 DB gain curtain arrays They almost always use open wire line. It’s mostly about TX output matching.
Sent from my iPad > On Jan 13, 2019, at 9:40 PM, Barry <[hidden email]> wrote: > > Don et al, > I sense this discussion is targeting the use of a dipole on a single band. What is being missed is that many of us use a dipole cut for for one band and use it for the bands above. What we see are high SWRs on all of the higher frequency bands. We use balanced feeds, open wire or ladder line, as far as we can before we transition to coax. That transition is a good current balun. What this does is control losses due to high SWR and keeps RF currents off the outside of the coax shield. What has been said in previous notes might be true, but they will prevent or make use of balanced feeders quite difficult. > > An 80 meter dipole used on 40 or 20, for example, is a pretty good antenna. However, it begs to be fed with open wire or equal. That means no balun can be used at the antenna feed point. The exception is to use very expensive coax. :-) > > 73, > Barry > K3NDM > > ------ Original Message ------ > From: "Don Wilhelm" <[hidden email]> > To: [hidden email] > Sent: 1/13/2019 6:59:18 PM > Subject: Re: [Elecraft] Slightly Unbalanced dipole? > >> Al, >> >> If not placed at the antenna feedpoint (up at the antenna), then the feedline may radiate due to common mode current. >> For feedlines using parallel conductors, the problem is not as bad if the feedline exits at right angles from the radiator for at least a half wavelength. In that case, the common mode choke can be placed at the junction of the parallel line and coax. >> >> In all cases of a radiator fed with coax, a good common mode choke at the radiator feedpoint is necessary. Even though the RF on the inside of the coax is balanced, when the coax reaches the radiator, the side of the antenna connected to the coax shield sees two paths. One is the side of the radiator, and the other is the outside of the coax braid - it will dutifully split to follow both paths depending on the impedance of each one. A good common mode choke will prevent the RF current from seeing the coax shield as another conductor. >> >> A similar situation exists at the transition between parallel feedline and coax. >> >> 73, >> Don W3FPR >> >> >> On 1/13/2019 6:42 PM, Al Lorona wrote: > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
Re: Slightly Unbalanced dipole?
|
|
In reply to this post by alorona
I’ve measured common mode current when feeding a balanced line with a good DX Engineering 1:1 balun and an unbalanced tuner, versus a real balanced link-coupled tuner (Johnson Matchbox). To my surprise, the currents are closer to equal with the unbalanced tuner and the balun! But the Matchbox seems quieter.
More investigation needed... Victor 4X6GP > On 14 Jan 2019, at 2:44, Al Lorona <[hidden email]> wrote: > > You guys keep talking about coax-fed antennas. Yes, although it may be easy to wind coax around a toroid and put it up at the antenna midpoint, that's not so easy with open-wire line. This discussion was very coax-centric and I wanted to open your minds that not everybody feeds their antenna with coax. > > Balanced feedlines ("window line" or "open-wire" line) can have significant common-mode current. They're not immune to the effect. (I know you know this; I'm saying it for the benefit of all.) A device with high common-mode impedance, located right at the output of the tuner, suppresses common-mode current on a (balanced) feedline. That's it's purpose. > > I'm in agreement with you about the 1/ cause of common-mode current and the 2/ cure for it. I'm simply saying that for convenience's sake, consider putting the choke at the tuner output. > > A choke with high Z to common-mode current, external to the tuner, in a non-conductive box, with as short a connection to the tuner as possible, does quite well at suppressing common-mode current. I'll be happy to share my measurements with you. > > There is no coax anywhere in my antenna system. So my solution was to put a choke with high impedance to common-mode current right at the output of my antenna tuner. I encourage users of open-wire line to do that. > > Al W6LX > >>>> In all cases of a radiator fed with coax, > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to [hidden email] Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
«
Return to Elecraft
|
1 view|%1 views
| Free forum by Nabble | Edit this page |