Re: [Elecraft] Non-Resonant Antennas and Chokes
Oh dear. Of course I know that stuff. BTW, except for very lossy RX antennas the only thing I know of that gives a "flat" SWR is a dummy load. The OP said, "My antenna, for 40 mtr's, is a 67' wire up about 25'." Why should he make a 40-meter dipole so complicated? Wes N7WS On 1/16/2019 9:37 AM, Ken wrote: On 1/16/19 12:11 AM, Wes Stewart wrote: Why don't you cut the dipole in the center, add an insulator and feed it with coax? Put a CM choke at the feedpoint if you insist and lose the ladderline and tuner. _Prune the wire length for resonance. _ You do realize that "pruning" for resonance will rarely give a flat SWR and pruning for low SWR does not mean the antenna is resonant? Of course, if you cut an end fed Zepp and feed it in the center with coax, it becomes a single band dipole instead of a multi band antenna. Ken WA8JXM __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
Re: [Elecraft] Non-Resonant Antennas and Chokes
You have several choices. 1. Add voltage or current balun at tuner output. Voltage 4:1 is simpler. 2. Connect the ladder line directly but loop the coax by the tuner a few times around a large toroid. This is equivalent to adding a balun. Do not ground the tuner. The first option guarantees balance but may add some losses. The second one is trivial to do. Most likely you will not be able to tell a difference. I used to have 70ft dipole at 25 ft fed with ladderline all the way to the shack. Matched by MFJ-962C, which had internal balun. Never a problem with up to 500W and even worked coast to coast on 160m! Ignacy, NO9E -- Sent from: http://elecraft.365791.n2.nabble.com/ __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
Re: [Elecraft] Non-Resonant Antennas and Chokes
On 1/16/19 12:11 AM, Wes Stewart wrote: Why don't you cut the dipole in the center, add an insulator and feed it with coax? Put a CM choke at the feedpoint if you insist and lose the ladderline and tuner. _Prune the wire length for resonance. _ You do realize that "pruning" for resonance will rarely give a flat SWR and pruning for low SWR does not mean the antenna is resonant? Of course, if you cut an end fed Zepp and feed it in the center with coax, it becomes a single band dipole instead of a multi band antenna. Ken WA8JXM __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
Re: [Elecraft] Non-Resonant Antennas and Chokes
I have three suggestions: 1) If you are feeding the antenna at the precise end of the wire, move the feedpoint out a few feet. That's because you need "the other half of the antenna." Because of the high impedance at this point, only a few feet are needed. 2) Don't connect one side of the ladder line to a ground rod. It doesn't help and will probably hurt. If you want to ground something, ground the shield of the coax at the tuner. 3) Use a good 1:1 balun designed for antenna tuner use between the tuner and the ladder line. Yes, people say you should use a 4:1 balun. Don't listen to them. The impedance at the end of the ladder line will be low on 40m (it's transformed by the near 1/4 wave ladder line). If you use the antenna on other bands it will vary all over the map. 73, Victor, 4X6GP Rehovot, Israel Formerly K2VCO CWops no. 5 http://www.qsl.net/k2vco/ On 16/01/2019 4:15, John Pierce wrote: I haven't read all the comments on this thread and thus may requesting redundant info. My antenna, for 40 mtr's, is a 67' wire up about 25'. I have 30' of 450 ohm ladder line connected to the end of the antenna and bringing the antenna into my basement. So I guess you could say that I have a non-resonant end fed Zepp. I can check the impedance at the end of the ladder line, where one side is the antenna and the other side is connected to a ground rod at the point of entering the house. I can measure the impedance of the antenna in the basement, using Mini60 without an antenna tuner . The ladder line is then connected to a MFJ 926B remote antenna tuner The tuner is fed by about 30' of RG58 coax from my transmitter. When I see a reactive component to the impedance of the ladder line, without the tuner connected, should I have some choke inserted in one side of the ladder line or not? Should the tuner be able to handle the reactive component? John AD2F __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to k2vco@gmail.com __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
Re: [Elecraft] Non-Resonant Antennas and Chokes
Why don't you cut the dipole in the center, add an insulator and feed it with coax? Put a CM choke at the feedpoint if you insist and lose the ladderline and tuner. Prune the wire length for resonance. Wes N7WS On 1/15/2019 7:15 PM, John Pierce wrote: I haven't read all the comments on this thread and thus may requesting redundant info. My antenna, for 40 mtr's, is a 67' wire up about 25'. I have 30' of 450 ohm ladder line connected to the end of the antenna and bringing the antenna into my basement. So I guess you could say that I have a non-resonant end fed Zepp. I can check the impedance at the end of the ladder line, where one side is the antenna and the other side is connected to a ground rod at the point of entering the house. I can measure the impedance of the antenna in the basement, using Mini60 without an antenna tuner . The ladder line is then connected to a MFJ 926B remote antenna tuner The tuner is fed by about 30' of RG58 coax from my transmitter. When I see a reactive component to the impedance of the ladder line, without the tuner connected, should I have some choke inserted in one side of the ladder line or not? Should the tuner be able to handle the reactive component? John AD2F __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to wes_n...@triconet.org __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
[Elecraft] Non-Resonant Antennas and Chokes
I haven't read all the comments on this thread and thus may requesting redundant info. My antenna, for 40 mtr's, is a 67' wire up about 25'. I have 30' of 450 ohm ladder line connected to the end of the antenna and bringing the antenna into my basement. So I guess you could say that I have a non-resonant end fed Zepp. I can check the impedance at the end of the ladder line, where one side is the antenna and the other side is connected to a ground rod at the point of entering the house. I can measure the impedance of the antenna in the basement, using Mini60 without an antenna tuner . The ladder line is then connected to a MFJ 926B remote antenna tuner The tuner is fed by about 30' of RG58 coax from my transmitter. When I see a reactive component to the impedance of the ladder line, without the tuner connected, should I have some choke inserted in one side of the ladder line or not? Should the tuner be able to handle the reactive component? John AD2F __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
Re: [Elecraft] Non-Resonant Antennas and Chokes
On 1/14/2019 6:32 PM, Wes Stewart wrote: You do realize that if the common-mode current on the line is due to asymmetry between the antenna and the line, then choking at the feed-point reduces the CM current at that point, but just like your example said, a quarter wave *down* the line you have a CM peak. Yes, but the choke at the feedpoint prevents that common mode current coupling to/from the antenna, which is how it gets inside the feedline as a differential mode signal. And the common mode current on the line below the choke is that induced by the field from the antenna, not from any imbalance in the antenna. If asymmetry isn't an issue then open wire is just fine with a balun at the tuner and I say this while not being a fan of open wire fed antennas. And it's also not an issue if you have no local noise sources, a luxury that I think you said you have. I'm also somewhat bemused by the cottage industry that's sprung up in the last few years to calm the hand wringing about common mode current. I think there's been a lot of development of cures that are looking for ailments. In my curmudgeonly opinion of course :-) If you had local noise you might view things differently. :) FWIW, I wasn't the first to realize the relationship between common mode current and RX noise -- I picked up on it from a long treatise that W1HIS wrote around the time I was doing my work. And I contributed pieces of the puzzle that he didn't know about, most importantly, the equivalent circuit of a choke and the importance of a high resistive impedance. 73, Jim K9YC __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
Re: [Elecraft] Non-Resonant Antennas and Chokes
You do realize that if the common-mode current on the line is due to asymmetry between the antenna and the line, then choking at the feed-point reduces the CM current at that point, but just like your example said, a quarter wave *down* the line you have a CM peak. If asymmetry isn't an issue then open wire is just fine with a balun at the tuner and I say this while not being a fan of open wire fed antennas. I'm also somewhat bemused by the cottage industry that's sprung up in the last few years to calm the hand wringing about common mode current. I think there's been a lot of development of cures that are looking for ailments. In my curmudgeonly opinion of course :-) Wes N7WS , On 1/13/2019 10:24 PM, Jim Brown wrote: 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. __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
Re: [Elecraft] Non-Resonant Antennas and Chokes
It is one hell of a nice read Jim, THANK YOU for publishing it. 73s and thanks, Dave (NK7Z) https://www.nk7z.net Award Manager, 30MDG Grid Contest ARRL Technical Specialist ARRL Volunteer Examiner ARRL OOC for Oregon On 1/13/19 9:24 PM, Jim Brown wrote: 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:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
Re: [Elecraft] Non-Resonant Antennas and Chokes
On 1/13/19 11:24 PM, Jim Brown wrote: >> 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. The common mode current can be measured 1/2 wavelength from the feedpoint. Here is an example: https://www.kn5l.net/Balun-CM/ MFJ has a clamp-on current meter which can be used. John KN5L __ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
Re: [Elecraft] 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:
Re: [Elecraft] Non-Resonant Antennas and Chokes
>>> 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:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
[Elecraft] Non-Resonant Antennas and Chokes
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:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
