[Elecraft] Artificial RF Ground - Redux
To all who took the time to answer my questions about the MFJ Artificial RF Ground - Thank you! I sincerely appreciate the well thought out replies and references. After digesting as much as I could I decided to go simple and work up. I began with 11 turns of RG-8X around a 4 inch PVC pipe, directly feeding the antenna. Preliminary tests show a much improved situation with the real test coming after the sun goes down and 40 m comes up. Thanks to all and apologies to MFJ. I certainly didn't intend to ignite a fire storm of debate. 72/73 es gd props. Tom, ng3v __ 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
Re: [Elecraft] Artificial RF Ground
David and All, I'm not jumping into the middle of the discussion on RF ground, but I'm certainly enjoying the commentary. It's very thought provoking. However, I do want to take issue, with all due respect, to your point #1. I've used a variety of MFJ accessories over the last 30 years or so, and I find most of them to be quite competent. I might agree that there is the occasional situation where a connection is bad, or something is rattling around inside the case, but I think that is certainly the exception rather than the rule. MFJ products are generally aimed to provide a lower cost approach to something. As such, they tend to be somewhat closer spec'd in order to reduce cost. Nevertheless, when used as designed, they usually work, and work well. Many of their items, like tuners, are relatively simple with few components. There's not much to go wrong, unless the item is abused, which I maintain is why many users end up being unhappy. When MFJ says it's a 200 watt tuner, they usually mean it! I don't think I've ever had a problem with an MFJ tuner, and I've had (have) a bunch of them! But I've never run more than 100 watts through them either. But if you bang one with power over the spec'd rating, or even close to the spec'd rating with a horrible load (you should tune at a reduced power first) for very long, something's probably going to blow! A lot of unhappiness with MFJ products is also due to the fact that they tend to be fairly basic--as in not too many bells and whistles. I can't even count the number of time I've seen others opt for an MFJ item because it was cheaper than brand X, and then discover that it wouldn't do some of the things brand X would do. Of course, brand X was twice as expensive too! But suddenly, the MFJ item becomes junk in their minds, and they give it a 1 or 2 rating on Eham. That's even though the MFJ item did what it said it would do--just not a lot more. MFJ isn't innocent when it comes to claiming great results, but neither are other manufacturers. A good example is their antennas and how wonderful they are. They aren't! But they work, and they are comparatively inexpensive. So, I suggest that most of the whining about MFJ stuff is self induced. Either it's excessive expectation, abuse, or both. After all, in most cases now they give you a no matter what guarantee. As a QRPer, I've found many MFJ items to be extremely beneficial. I think Martin Jue, who owns MFJ, is borderline genius when it comes to identifying a niche and filling it. He's done more to accomodate the QRP community (and QRO folks too) than just about anybody. I remember over 30 years ago, when I lived in Memphis, seeing him selling his gadgets at the local hamfest. Since then he's been incredibly prolific (and successful) , and it's been to our benefit. The problem is some folks want a Cadillac for a Chevrolet price. As for the MFJ artificial ground accessory, I don't have one, and have never used one. But I bet it works--at least it probably does what it is designed to do. However, it's a compromise solution, and not a perfect solution. Accordingly, some folks are going to be disappointed. Dave W7AQK - Original Message - From: David Gilbert xda...@cis-broadband.com To: Ron D'Eau Claire r...@cobi.biz Cc: Elecraft@mailman.qth.net Sent: Thursday, April 02, 2009 5:49 PM Subject: Re: [Elecraft] Artificial RF Ground Sorry, but I believe your posting to be full of bad advice. 1. Most bad reviews for companies that cater to hams are due to bad customer service. MFJ is one of the few that consistently gets bad marks for quality of construction and durability of use. If you don't believe that, you simply haven't been paying attention. 2. An artificial ground doesn't keep RF out of the shack ... it just tunes a low impedance path to something, which may or not be a ground of any sort. In any case, the chances of finding a functioning RF ground in an otherwise insulated basement room are not great. 3. Vertical antennas are notorious for putting RF on the coax shield when the coax is lying right alongside the radials. Why would you expect such an arrangement NOT to couple RF to the coax, and therefore right into the shack? Check around and you'll find all sorts of instances where people had such problems with a vertical antenna and used a current choke to fix it ... there are writeups everywhere on it. I agree with K9YC that a coil of coax will normally do the job just fine, although I'm not sure that the base of the antenna is always the best place to put it. If the coax is lying next to the radials it seems to me that there could still be a lot of coupling to the coax on the run to the shack. I'd be more inclined to put the choke closer to the house, but I'll defer judgment on that to the experts. Dave AB7E Ron D'Eau Claire wrote: Ha, ha
Re: [Elecraft] Artificial RF Ground
On Thu, 02 Apr 2009 17:49:26 -0700, David Gilbert wrote: I agree with K9YC that a coil of coax will normally do the job just fine, Note that I am NOT recommending a coil of coax, I am recommending a coil of coax through a suitable ferrite core. BIG difference. A coil of coax is inductive (and not a big inductance). A feedline less than a quarter wave looks capacitive (as a radial or common mode circuit element), so a simple inductor (the coil of coax) can resonate with the line and INCREASE the current. Not a good thing. A feedline between 3/4 and 1 1/4 wavelength is also inductive. Also, velocity factor does NOT apply to the common mode circuit, but Rudy Severns (N6LF) has shown that elevated radials DO exhibit a velocity factor as they interact with the earth. Google to find his website for really excellent work on this. A ferrite choke is a low Q parallel resonant circuit. When wound to put its resonance at the transmitting frequency, it looks like a BIG resistor in series with the common mode circuit, and it blocks the common mode current, no matter what the length of the coax. As to WHERE the choke should go -- if you're short of radials and WANT the coax to be another radial, then put the choke at the shack. I have a lot of radials and the coax run to my 160 vertical is short (about 35 ft), so my choke is at the feedpoint. BTW -- think about it -- I'm running legal power to that vertical, which is only 25 ft from my operating position, and I've NEVER had even the slightest symptom of RFI in my shack on 160M. I do NOT see coax laying on the ground picking up any more induced current than if it were up in the air. Remember that the EARTH is a LOUSY CONDUCTOR, and the object of radials is to put that current in low loss wire rather than lossy earth. 73, Jim Brown 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
Re: [Elecraft] Artificial RF Ground
Ha, ha! There are MFJ Bashers just like there are those who must complain about just about every company. As my Grandmother used to say of such people, He'd complain if they hung him with a new rope. To your point, yes the MFJ Artificial Ground works FB. One the MFJ tuners I have around is the MFJ-934 which has the AG built in and I've used it quite successfully with a variety of antennas. If you have Moxon's HF Antennas for All Locations he discusses such grounding schemes in detail, but it seems to be something ignored in many texts. What you are doing is arranging a 1/4 wave wire connected to your rig. At 1/4 wave, it presents a rather low impedance to the rig, which keeps it at a low RF voltage. The AG is nothing more than a simple single-wire tuner to establish that condition with almost any wire across the HF spectrum. The tuner circuit is a tapped coil in series with a variable capacitor that can be adjusted to compensate for a wide range of reactance values present at the end of your ground wire attached to the tuner. Since you are working for the lowest impedance at that point, it includes an RF ammeter in series with the circuit. You adjust the coil and cap for maximum reading on the ammeter. At any given power, maximum current equals lowest impedance equals lowest voltage. As Moxon points out, you can do the same thing with any length of wire that is 1/4 wave and a simple loading coil at the rig, just as you might do with a short antenna. (The variable capacitor in the AG covers situations where the ground wire is longer than 1/4 wavelength). A small flashlight bulb in series with the wire will indicated the proper amount of inductance. Adjust the power for some indication and set the coil for maximum brilliance. You can remove the bulb when you find the right setting so it won't burn out at higher powers. It's not a panacea. There are some situations where it doesn't cure the problem. As others took pains to point out in the other recent thread, there is no perfect (zero ohm) ground. As frequency increases, dealing with reactance becomes more of an issue and, at RF, we have significant conductor resistance as skin effect enters into the equation. But that doesn't mean the effort is useless or ineffective. You raise an interesting issue though, saying you're using a vertical with radials on the ground. Those antennas aren't typically RF feedback prone antennas. What are the feedback symptoms. Does the feeder show a low SWR at the Antenna? Is the shield of the coax properly connected to the radials with a good, low resistance connection? Ron AC7AC -Original Message- From: elecraft-boun...@mailman.qth.net [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of NG3V Sent: Thursday, April 02, 2009 2:43 PM To: Elecraft@mailman.qth.net Subject: [Elecraft] Artificial RF Ground Good Evening Group, All this talk of RF grounds has re-kindled my desire to run more than 10 watts without RF feedback in the shack (yeah, I know 10 watts is enough, I just like the ability to have more). Anyway, my shack is in an enclosed room in the basement with no direct access to the outside world. My antenna is a ground-mounted vertical about 15 feet from the house with buried radials. The feedline is laying on the ground. Poor arrangement, but best I can do for the time being. I asked DX Engineering if they thought their Feedline Current Choke might help and Bob, N8QE, replied with a very nice message suggesting that I look at the MFJ-931 artificial RF Ground. Have any of you had experience with this thing? The eham reviews were mostly what we've come to expect about MFJ stuff. Buy it, open it, fix it, use it. I'm interested in the 'use it' part. TIA es 72, Tom, ng3v __ 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 __ 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
Re: [Elecraft] Artificial RF Ground
On Thu, 2 Apr 2009 17:42:43 -0400, NG3V wrote: All this talk of RF grounds has re-kindled my desire to run more than 10 watts without RF feedback in the shack (yeah, I know 10 watts is enough, I just like the ability to have more). There is a MUCH better (and MUCH LESS EXPENSIVE) solution. Simply wind a coaxial choke around a 2.4-inch toroid, following the instructions in the Choke Cookbook section of http://audiosystemsgroup.com/RFI-Ham.pdf Place this choke at the feedpoint of your antenna. It will do a VERY good job of isolating the coax from the antenna. Anyway, my shack is in an enclosed room in the basement with no direct access to the outside world. My antenna is a ground-mounted vertical about 15 feet from the house with buried radials. The feedline is laying on the ground. Poor arrangement Huh? There's nothing wrong with this arrangement that I can think of. It is, of course, important that the shield of the coax have a very short connection to earth for lightning protection (that is, a ground rod), and a short, beefy connection between that rod and the ground rod(s) for your electrical system and telephone system and CATV system. These connections should be outside your building. There is NO need for an ADDITIONAL connection to the earth inside your shack, but the connections listed above are VERY important for lightning safety. The confusion evidenced by your poor arrangement statement is why I so strongly object to ANY use of the words RF ground. The many complex electrical and radio sytems in a modern aircraft have NO connection to the earth, but they work just fine! And that 400 Hz whine you hear when you plug your headset in -- that's a pin 1 problem in their audio system, again the result of fuzzy thinking about ground. 73, Jim Brown 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
Re: [Elecraft] Artificial RF Ground
Tom, The only possible troublesome item that you have talked about is the feedline laying on the ground. That could be picking up radiation from the vertical and bringing it into the shack - that would be from 'common mode' current being induced on the coax shield. Burying the coax may help. Many such problems can be tamed with a balun at the base of the antenna, plus a current choke about a half wavelength away (or at the entry to the building). Normally, only the balun at the antenna is sufficient, but in your case, it may be that the shield of the feedline is picking up radiation from the antenna itself and carrying it into the shack - thus my recommendation for a current choke at the entry point. A ferrite choke will produce the best results, but if you have excess coax length, try coiling the coax into an inductor - 6 to 8 turns about 6 inches in diameter at the shack entry point may help tremendously. If you must resort to the use of an artificial ground, I recommend that it be attached to the coax shield at the point where the coax enters the house - in other words, get rid of the common mode RF before it gets into the shack. OTOH, if your feedline is not coax, other means may be necessary. One could try various incantations while waving chicken carcasses about - it may not help, but it can make one feel better about it. No number of ground rods will fix the problem. RF feedback is best handled at the antenna. or at other select places in the antenna system. The exact location depends on your installation, there are no general answers that will work in every situation. RF moves in ways that appear to be mysterious until one realizes that *anything* (even a ground rod) can become a part of the antenna system - and then one can move forward to control where the RF currents will flow. 73, Don W3FPR NG3V wrote: Good Evening Group, Anyway, my shack is in an enclosed room in the basement with no direct access to the outside world. My antenna is a ground-mounted vertical about 15 feet from the house with buried radials. The feedline is laying on the ground. Poor arrangement, but best I can do for the time being. __ 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
Re: [Elecraft] Artificial RF Ground
Sorry, but I believe your posting to be full of bad advice. 1. Most bad reviews for companies that cater to hams are due to bad customer service. MFJ is one of the few that consistently gets bad marks for quality of construction and durability of use. If you don't believe that, you simply haven't been paying attention. 2. An artificial ground doesn't keep RF out of the shack ... it just tunes a low impedance path to something, which may or not be a ground of any sort. In any case, the chances of finding a functioning RF ground in an otherwise insulated basement room are not great. 3. Vertical antennas are notorious for putting RF on the coax shield when the coax is lying right alongside the radials. Why would you expect such an arrangement NOT to couple RF to the coax, and therefore right into the shack? Check around and you'll find all sorts of instances where people had such problems with a vertical antenna and used a current choke to fix it ... there are writeups everywhere on it. I agree with K9YC that a coil of coax will normally do the job just fine, although I'm not sure that the base of the antenna is always the best place to put it. If the coax is lying next to the radials it seems to me that there could still be a lot of coupling to the coax on the run to the shack. I'd be more inclined to put the choke closer to the house, but I'll defer judgment on that to the experts. Dave AB7E Ron D'Eau Claire wrote: Ha, ha! There are MFJ Bashers just like there are those who must complain about just about every company. As my Grandmother used to say of such people, He'd complain if they hung him with a new rope. To your point, yes the MFJ Artificial Ground works FB. One the MFJ tuners I have around is the MFJ-934 which has the AG built in and I've used it quite successfully with a variety of antennas. If you have Moxon's HF Antennas for All Locations he discusses such grounding schemes in detail, but it seems to be something ignored in many texts. What you are doing is arranging a 1/4 wave wire connected to your rig. At 1/4 wave, it presents a rather low impedance to the rig, which keeps it at a low RF voltage. The AG is nothing more than a simple single-wire tuner to establish that condition with almost any wire across the HF spectrum. The tuner circuit is a tapped coil in series with a variable capacitor that can be adjusted to compensate for a wide range of reactance values present at the end of your ground wire attached to the tuner. Since you are working for the lowest impedance at that point, it includes an RF ammeter in series with the circuit. You adjust the coil and cap for maximum reading on the ammeter. At any given power, maximum current equals lowest impedance equals lowest voltage. As Moxon points out, you can do the same thing with any length of wire that is 1/4 wave and a simple loading coil at the rig, just as you might do with a short antenna. (The variable capacitor in the AG covers situations where the ground wire is longer than 1/4 wavelength). A small flashlight bulb in series with the wire will indicated the proper amount of inductance. Adjust the power for some indication and set the coil for maximum brilliance. You can remove the bulb when you find the right setting so it won't burn out at higher powers. It's not a panacea. There are some situations where it doesn't cure the problem. As others took pains to point out in the other recent thread, there is no perfect (zero ohm) ground. As frequency increases, dealing with reactance becomes more of an issue and, at RF, we have significant conductor resistance as skin effect enters into the equation. But that doesn't mean the effort is useless or ineffective. You raise an interesting issue though, saying you're using a vertical with radials on the ground. Those antennas aren't typically RF feedback prone antennas. What are the feedback symptoms. Does the feeder show a low SWR at the Antenna? Is the shield of the coax properly connected to the radials with a good, low resistance connection? Ron AC7AC -Original Message- From: elecraft-boun...@mailman.qth.net [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of NG3V Sent: Thursday, April 02, 2009 2:43 PM To: Elecraft@mailman.qth.net Subject: [Elecraft] Artificial RF Ground Good Evening Group, All this talk of RF grounds has re-kindled my desire to run more than 10 watts without RF feedback in the shack (yeah, I know 10 watts is enough, I just like the ability to have more). Anyway, my shack is in an enclosed room in the basement with no direct access to the outside world. My antenna is a ground-mounted vertical about 15 feet from the house with buried radials. The feedline is laying on the ground. Poor arrangement, but best I can do for the time being. I asked DX Engineering if they thought their Feedline Current Choke might help and Bob, N8QE
Re: [Elecraft] Artificial RF Ground
Don is right. It's quite possible that your shield *exterior* is acting as another radial and, if the run to your shack is near 1/4 wave or 3/4 wave, you have a high impedance at your rig. The solution is to decouple the shield with a toroid or other device near the 1/4 wave point. As Don suggested, at the entrance to the shack is a good point. Otherwise, a point 1/4 wave from the radial is ideal. Ron AC7AC -Original Message- From: elecraft-boun...@mailman.qth.net [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of Don Wilhelm Sent: Thursday, April 02, 2009 5:47 PM To: n...@comcast.net Cc: Elecraft@mailman.qth.net Subject: Re: [Elecraft] Artificial RF Ground Tom, The only possible troublesome item that you have talked about is the feedline laying on the ground. That could be picking up radiation from the vertical and bringing it into the shack - that would be from 'common mode' current being induced on the coax shield. Burying the coax may help. Many such problems can be tamed with a balun at the base of the antenna, plus a current choke about a half wavelength away (or at the entry to the building). Normally, only the balun at the antenna is sufficient, but in your case, it may be that the shield of the feedline is picking up radiation from the antenna itself and carrying it into the shack - thus my recommendation for a current choke at the entry point. A ferrite choke will produce the best results, but if you have excess coax length, try coiling the coax into an inductor - 6 to 8 turns about 6 inches in diameter at the shack entry point may help tremendously. If you must resort to the use of an artificial ground, I recommend that it be attached to the coax shield at the point where the coax enters the house - in other words, get rid of the common mode RF before it gets into the shack. OTOH, if your feedline is not coax, other means may be necessary. One could try various incantations while waving chicken carcasses about - it may not help, but it can make one feel better about it. No number of ground rods will fix the problem. RF feedback is best handled at the antenna. or at other select places in the antenna system. The exact location depends on your installation, there are no general answers that will work in every situation. RF moves in ways that appear to be mysterious until one realizes that *anything* (even a ground rod) can become a part of the antenna system - and then one can move forward to control where the RF currents will flow. 73, Don W3FPR NG3V wrote: Good Evening Group, Anyway, my shack is in an enclosed room in the basement with no direct access to the outside world. My antenna is a ground-mounted vertical about 15 feet from the house with buried radials. The feedline is laying on the ground. Poor arrangement, but best I can do for the time being. __ 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 __ 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
Re: [Elecraft] Artificial RF Ground
Well, Dave: Let me take it point by point: 1) I am an MFG customer and while I might choose to design my stuff a little differently, I'm not trying to compete in the marketplace with others. I've not found my MFG equipment to be defective or needing repair out of the box. Indeed, their tuners are quite efficient and effective when used as recommended. 2) What do you mean by out of the shack? I assume it means that RF doesn't impair the operation of the rig or other equipment. Putting a low-impedance ground (in quotes with respect to the others who claim there is no such thing) at the rig does that. The Artificial Ground sold by MFJ does just that. I've used it and I know from experience. 3) You are quite right as Don also noted. A coaxial line laying on the ground running away from the antenna will intercept RF and have current flowing on the outside of the coax shield that is quite independent of the current flowing to the antenna on the inside of the shield. In that case, the objective is to stop those currents on the outside of the shield where the coax enters the shack as Don suggested. A coil of coax is a one way, although it's important to make it a coil, not a jumble-wound bunch of coax. In a bunch of coax wound up, it's possible for capacitive coupling between the coax at one end to the coax at the other end to eliminate most of the benefit of the inductance of the coil. Ron AC7AC -- Sorry, but I believe your posting to be full of bad advice. 1. Most bad reviews for companies that cater to hams are due to bad customer service. MFJ is one of the few that consistently gets bad marks for quality of construction and durability of use. If you don't believe that, you simply haven't been paying attention. 2. An artificial ground doesn't keep RF out of the shack ... it just tunes a low impedance path to something, which may or not be a ground of any sort. In any case, the chances of finding a functioning RF ground in an otherwise insulated basement room are not great. 3. Vertical antennas are notorious for putting RF on the coax shield when the coax is lying right alongside the radials. Why would you expect such an arrangement NOT to couple RF to the coax, and therefore right into the shack? Check around and you'll find all sorts of instances where people had such problems with a vertical antenna and used a current choke to fix it ... there are writeups everywhere on it. I agree with K9YC that a coil of coax will normally do the job just fine, although I'm not sure that the base of the antenna is always the best place to put it. If the coax is lying next to the radials it seems to me that there could still be a lot of coupling to the coax on the run to the shack. I'd be more inclined to put the choke closer to the house, but I'll defer judgment on that to the experts. Dave AB7E __ 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
