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Question on dipole SWR problem
Our ARES station has had a NVIS antenna on the roof of the building we are in. It is a large, one story structure with a rubber coated on steele roof. The dual dipole type antenna, center at 14' and ends at about 3' high, was fed with a long run of coax and has historically exhibited a large SWR when operating below about 3825 KHz. We recently replaced the entire antenna assy with a 122' half wave dipole, fed with 600 ohm ladder line to an SGC antenna coupler at the base of the center mast. Heights are the same as previous. The SGC 237 coupler tunes the antenna well above 3825, and all other bands, too. However, we still exhibit a very high SWR when going below about 3825 and the tuner fails to tune... in fact the Radio's power drops way down, possibly preventing the tuner from working properly. A different radio shows the same problem. QUESTION: Can anyone offer reasons we may be having this tuning problem below 3825? Thanks. Ed K7AAT |
Question on dipole SWR problem
"Ed G" wrote in message 92.196... Our ARES station has had a NVIS antenna on the roof of the building we are in. It is a large, one story structure with a rubber coated on steele roof. The dual dipole type antenna, center at 14' and ends at about 3' high, was fed with a long run of coax and has historically exhibited a large SWR when operating below about 3825 KHz. We recently replaced the entire antenna assy with a 122' half wave dipole, fed with 600 ohm ladder line to an SGC antenna coupler at the base of the center mast. Heights are the same as previous. The SGC 237 coupler tunes the antenna well above 3825, and all other bands, too. However, we still exhibit a very high SWR when going below about 3825 and the tuner fails to tune... in fact the Radio's power drops way down, possibly preventing the tuner from working properly. A different radio shows the same problem. QUESTION: Can anyone offer reasons we may be having this tuning problem below 3825? Thanks. Ed K7AAT I ran EZnec on it, and the impedance at resonance is about 5 Ohms (at about 3.8 MHz). Most of the energy goes straight up, which might not be bad for local contacts. If you have your heart set on it, try a 9:1 balun at the feedpont. Tam/WB2TT |
Question on dipole SWR problem
"Jimmie D" wrote in message ... "Ed G" wrote in message 92.196... Our ARES station has had a NVIS antenna on the roof of the building we are in. It is a large, one story structure with a rubber coated on steele roof. The dual dipole type antenna, center at 14' and ends at about 3' high, was fed with a long run of coax and has historically exhibited a large SWR when operating below about 3825 KHz. We recently replaced the entire antenna assy with a 122' half wave dipole, fed with 600 ohm ladder line to an SGC antenna coupler at the base of the center mast. Heights are the same as previous. The SGC 237 coupler tunes the antenna well above 3825, and all other bands, too. However, we still exhibit a very high SWR when going below about 3825 and the tuner fails to tune... in fact the Radio's power drops way down, possibly preventing the tuner from working properly. A different radio shows the same problem. QUESTION: Can anyone offer reasons we may be having this tuning problem below 3825? Thanks. Ed K7AAT Not surprising with a dipole located that close to a counterpoise. The feedpoint impedance should be pretty low, I would guess 10 ohms or less. Jimmie It occurs to me a bandaid fix might be a 4:1 balun. Just hook it up backwards; that is the 200 Ohm side to the tuner and the 50 Ohm side to the antenna. Tam/WB2TT |
Question on dipole SWR problem
On 16 oct, 03:00, Ed G wrote:
Our ARES station has had a NVIS antenna on the roof of the building we are in. It is a large, one story structure with a rubber coated on steele roof. The dual dipole type antenna, center at 14' and ends at about 3' high, was fed with a long run of coax and has historically exhibited a large SWR when operating below about 3825 KHz. We recently replaced the entire antenna assy with a 122' half wave dipole, fed with 600 ohm ladder line to an SGC antenna coupler at the base of the center mast. Heights are the same as previous. The SGC 237 coupler tunes the antenna well above 3825, and all other bands, too. However, we still exhibit a very high SWR when going below about 3825 and the tuner fails to tune... in fact the Radio's power drops way down, possibly preventing the tuner from working properly. A different radio shows the same problem. QUESTION: Can anyone offer reasons we may be having this tuning problem below 3825? Thanks. Ed K7AAT Hi Ed, I assume that the metal roof extends over the full length of the dipole. Your antenne is very close to ground, that results (as mentioned by other posters) in a very low radiation resistance (about 5..10 Ohms). When using this construction at 40m, the impedance will be very High (certainly above 6 kOhms, when losses are low). I would recommend you to raise the antenna. This increases the radiation resistance at 80m significantly. This lowers the impedance at 40m also (making it easier for the tuner, resulting in higher overall radiation efficiency). When raising the antenna is not possible: Construct the quarter wave sections of about 3 wires in parallel, about 1m separated. Connect the wires at the feed point. This "emulates" a thick strip dipole. The result is that the radiation resistance at 80m does not change, but the input impedance at 40m reduces significantly (a factor 6 is possible). This enables the use of a transformer (for example 2(feed line) : 1(dipole side) ). Now the 5..10 Ohms becomes 20..40 Ohms and the 6 kOhms will remain, or drops a little. As the impedance ratio has been reduced now, matching is easier. Please note that your transformer experiences high voltage (so high core flux) when operated on 40m. Other option: use the transformer on 80m only, and remove it (relay) when operating on 40m. I hope this helps a bit. Best regards, Wim PA3DJS www.tetech.nl |
Question on dipole SWR problem
Never used a BALUN like that. Maybe they could make a BALBAL.
Is there such a thing?? ========== Balanced to balanced means an RF transformer with separate windings . Preferably on a toroid ,otherwise on a ferrite bar (ex medium wave rx) Frank GM0CSZ / KN6WH |
Question on dipole SWR problem
"Jimmie D" wrote in message ... Ok, now I see your post where you modeled it. Something is weird here, I have to leave Outlook Express before it will update. Never used a BALUN like that. Maybe they could make a BALBAL. Is there such a thing?? Jimmie I guess I should have called it a transformer. Although, it could be a balun, like they use on output stages of RF amplifiers; 5.6 Ohm balanced to 50 Ohm unbalanced. Tam |
Question on dipole SWR problem
Ed G wrote in
92.196: Our ARES station has had a NVIS antenna on the roof of the building we are in. It is a large, one story structure with a rubber coated on steele roof. The dual dipole type antenna, center at 14' and ends at about 3' high, was fed with a long run of coax and has historically exhibited a large SWR when operating below about 3825 KHz. We recently replaced the entire antenna assy with a 122' half wave dipole, fed with 600 ohm ladder line to an SGC antenna coupler at the base of the center mast. Heights are the same as previous. The SGC 237 coupler tunes the antenna well above 3825, and all other bands, too. However, we still exhibit a very high SWR when going below about 3825 and the tuner fails to tune... in fact the Radio's power drops way down, possibly preventing the tuner from working properly. A different radio shows the same problem. QUESTION: Can anyone offer reasons we may be having this tuning problem below 3825? Thanks. Ed K7AAT Ed, The way in which you dealt with the transition from the open wire feedline to the SGC237 unbalanced tuner is relevant. As is whether or not you bonded the tuner 'ground' terminal to the roof. Your description is short on relevant detail. Owen |
Question on dipole SWR problem
Highland Ham wrote:
Never used a BALUN like that. Maybe they could make a BALBAL. Is there such a thing?? ========== Balanced to balanced means an RF transformer with separate windings . Preferably on a toroid ,otherwise on a ferrite bar (ex medium wave rx) If you take a regular transformer and connect it between a "balanced" (symmetrical) system and "unbalanced" (asymmetrical) system, is it a BALUN? Then does it change to an UNUN if both sides are asymmetrical and a BALBAL if both are symmetrical? What does a BALBAL or UNUN do? What does a BALUN do? Roy Lewallen, W7EL |
Question on dipole SWR problem
Ed, The way in which you dealt with the transition from the open wire feedline to the SGC237 unbalanced tuner is relevant. As is whether or not you bonded the tuner 'ground' terminal to the roof. Your description is short on relevant detail. Owen SG237 tuner is not "bonded" to the roof ground, SGC does not suggest doing such a thing, and in this case, it would not be possible anyway since the "ground" side of the tuner is also one side of the balanced antenna feedpoint. The SG237 is feeding about 14' of 600 ohm open ladder line, spaced about 1 foot off the aluminum mast, up to the feedpoint of the antenna, a 75M dipole ( 61' per leg). The tuner is sitting in a plastic box, about 1 foot off the roof. Ed |
Question on dipole SWR problem
"Ed G" wrote in message .89... Ed, The way in which you dealt with the transition from the open wire feedline to the SGC237 unbalanced tuner is relevant. As is whether or not you bonded the tuner 'ground' terminal to the roof. Your description is short on relevant detail. Owen SG237 tuner is not "bonded" to the roof ground, SGC does not suggest doing such a thing, and in this case, it would not be possible anyway since the "ground" side of the tuner is also one side of the balanced antenna feedpoint. The SG237 is feeding about 14' of 600 ohm open ladder line, spaced about 1 foot off the aluminum mast, up to the feedpoint of the antenna, a 75M dipole ( 61' per leg). The tuner is sitting in a plastic box, about 1 foot off the roof. Ed If I had a huge metal roof like that, I would be tempted to try one of the shortened verticals. The kind that requires radials - which would be your roof. You are running coax from the tuner to the shack, right; with just the 14 feet of ladder line? Tam/WB2TT |
Question on dipole SWR problem
Tam/WB2TT wrote:
If I had a huge metal roof like that, I would be tempted to try one of the shortened verticals. The kind that requires radials - which would be your roof. Unfortunately, a vertical makes a poor NVIS antenna. . . . Roy Lewallen, W7EL |
Question on dipole SWR problem
On Tue, 16 Oct 2007 16:07:40 -0400, "Tam/WB2TT"
wrote: I guess I should have called it a transformer. Although, it could be a balun, like they use on output stages of RF amplifiers; 5.6 Ohm balanced to 50 Ohm unbalanced. Hi Tam, Lift one wire from ground and you've got that BalBal. 73's Richard Clark, KB7QHC |
Question on dipole SWR problem
Ed G wrote in
.89: Ed, The way in which you dealt with the transition from the open wire feedline to the SGC237 unbalanced tuner is relevant. As is whether or not you bonded the tuner 'ground' terminal to the roof. Your description is short on relevant detail. Owen SG237 tuner is not "bonded" to the roof ground, SGC does not suggest doing such a thing, and in this case, it would not be possible anyway since the "ground" side of the tuner is also one side of the balanced antenna feedpoint. The SG237 is feeding about 14' of 600 ohm open ladder line, spaced about 1 foot off the aluminum mast, up to the feedpoint of the antenna, a 75M dipole ( 61' per leg). The tuner is sitting in a plastic box, about 1 foot off the roof. Ed The feedline is doing more than transporting energy to the dipole centre, it almost certainly carries a significant common mode current and in that case is just as much a part of the radiating system as the dipole itself. So, isn't the coax and control line to the SG237 carrying current mode RF current, ie contibuting to radiation. Inch by inch, the detail unfolds. Owen |
Question on dipole SWR problem
"Ed G" Huckleberry_ large SWR when operating below about 3825 KHz. We recently replaced the entire antenna assy with a 122' half wave dipole, fed with 600 ohm ladder line to an SGC antenna coupler at the base of the center mast. The SGC 237 coupler tunes the antenna well above 3825, and all other bands, too. However, we still exhibit a very high SWR when going below about 3825 and the tuner fails to tune... in fact the Radio's power drops way down, possibly preventing the tuner from working properly. A different radio shows the same problem. Ed, sounds like your radios are into VSWR turndown from the inability of the tuner to match at the low end. It could be that the Z at 3825 is going high which a lot of tuners can't handle--or could be very low, same problem, no match. Try adding some more length to the dipole if you can and see if it tunes better at the low end. Even if you only drape pieces down at the ends would help if you can't extend more horizontally. 73,Bill W0BVR |
Question on dipole SWR problem
"Roy Lewallen" wrote in message ... Tam/WB2TT wrote: If I had a huge metal roof like that, I would be tempted to try one of the shortened verticals. The kind that requires radials - which would be your roof. Unfortunately, a vertical makes a poor NVIS antenna. . . . Roy Lewallen, W7EL Still, a waste of a ground plane. Tam/WB2TT |
Question on dipole SWR problem
Owen Duffy wrote in
: .... So, isn't the coax and control line to the SG237 carrying current mode RF current, ie contibuting to radiation. I will answer my own question... In the absence of an effective device to prevent common mode feedline current, your configuration where one side of the ladder line connects to the 'ground' side of the nominally unbalanced tuner, and that same tuner terminal connects to the outside of the feedline coax and control wires (at RF), then you have a continuous RF common mode path to wherever the coax and control lines connect and so on, they are just as much a part of your radiator as the dipole wire. Analysis of the 122' entre fed wire in isolation of the rest of the antenna system are inadequate in explaining the difficulties you encounter. Owen |
Question on dipole SWR problem
If I had a huge metal roof like that, I would be tempted to try one of the shortened verticals. The kind that requires radials - which would be your roof. That is something we had not considered. Will keep that in mind. Tnx. You are running coax from the tuner to the shack, right; with just the 14 feet of ladder line? Yes, 14' ladderline from tuner to antenna center. About 100 ' of RG-213 on roof and another 30' or so going down to station inside building. We have a coax choke balun located in line on the roof about 8' before the coax goes down in the building. Ed |
Question on dipole SWR problem
Is the 237 designed for balanced antennas or single random wires, or either? bob k5qwg Not sure what SGC recommends regarding balanced antennas.... will review their info. However, I have read numerous reports from hams who have used SGC and other similar tuners to feed balanced lines. The only important condition was to use a choke balun to keep RF from coming back down the coax shield. Ed |
Question on dipole SWR problem
The feedline is doing more than transporting energy to the dipole centre, it almost certainly carries a significant common mode current and in that case is just as much a part of the radiating system as the dipole itself. So, isn't the coax and control line to the SG237 carrying current mode RF current, ie contibuting to radiation. Inch by inch, the detail unfolds. Yeah, inch by inch. I hate to fill up the post with too many details as some people grab something insignifcant and go off on a tangent.... however, the devil may be in the details..... You're right on the current in the shield. We have a choke balun, currently located 85' down the coax toward the transmitter. I have it scheduled, when it stops raining, to move it up to the input of the SG- 237 coupler unit. The Choke balun is homemade, 6" diamter, about 11 turns of RG-8X coax. Does that sound sufficient? I didn't make it or provide it. There is no control line for the SGC coupler unit. It only requires 12VDC @ 300ma and we are providing that by using a couple MFJ DC adapter units to feed the 12VDC up the coax. Ed |
Question on dipole SWR problem
Ed, sounds like your radios are into VSWR turndown from the inability of the tuner to match at the low end. It could be that the Z at 3825 is going high which a lot of tuners can't handle--or could be very low, same problem, no match. Try adding some more length to the dipole if you can and see if it tunes better at the low end. Even if you only drape pieces down at the ends would help if you can't extend more horizontally. 73,Bill W0BVR You, and others earlier on in this thread, have suggested such a thing. I have that on my list of things to try when the weather allows. Tnx. |
Question on dipole SWR problem
"Ed G" wrote in message .81... The feedline is doing more than transporting energy to the dipole centre, it almost certainly carries a significant common mode current and in that case is just as much a part of the radiating system as the dipole itself. So, isn't the coax and control line to the SG237 carrying current mode RF current, ie contibuting to radiation. Inch by inch, the detail unfolds. Yeah, inch by inch. I hate to fill up the post with too many details as some people grab something insignifcant and go off on a tangent.... however, the devil may be in the details..... You are right here. I have a feeling some people think you are running ladder line to the radio. You're right on the current in the shield. We have a choke balun, currently located 85' down the coax toward the transmitter. I have it scheduled, when it stops raining, to move it up to the input of the SG- 237 coupler unit. This has got to be a step in the right direction. Tam/WB2TT The Choke balun is homemade, 6" diamter, about 11 turns of RG-8X coax. Does that sound sufficient? I didn't make it or provide it. There is no control line for the SGC coupler unit. It only requires 12VDC @ 300ma and we are providing that by using a couple MFJ DC adapter units to feed the 12VDC up the coax. Ed |
Question on dipole SWR problem
Ed G wrote in
.81: The feedline is doing more than transporting energy to the dipole centre, it almost certainly carries a significant common mode current and in that case is just as much a part of the radiating system as the dipole itself. So, isn't the coax and control line to the SG237 carrying current mode RF current, ie contibuting to radiation. Inch by inch, the detail unfolds. Yeah, inch by inch. I hate to fill up the post with too many details as some people grab something insignifcant and go off on a tangent.... however, the devil may be in the details..... Yes, the people who modelled your antenna based on half the information got an incorrect answer. You're right on the current in the shield. We have a choke balun, currently located 85' down the coax toward the transmitter. I have it scheduled, when it stops raining, to move it up to the input of the SG- 237 coupler unit. The Choke balun is homemade, 6" diamter, about 11 turns of RG-8X coax. Does that sound sufficient? I didn't make it or provide it. There is no control line for the SGC coupler unit. It only requires 12VDC @ 300ma and we are providing that by using a couple MFJ DC adapter units to feed the 12VDC up the coax. You need to visualise that you what you have looks like a dipole with a wire from one side of the centre following the path of the feedlines to the choke and from the choke on to wherever. The choke balun introduces a common mode impedance in that equivalent wire, but it does not necessarily eliminate common mode current, rather it modifies the amplitude and distribution of the common mode current. Owen |
Question on dipole SWR problem
On 17 Oct 2007 22:40:44 GMT, Ed G
wrote: It only requires 12VDC @ 300ma and we are providing that by using a couple MFJ DC adapter units to feed the 12VDC up the coax. Hi Ed, This is another antenna element unless you choke it too. 73's Richard Clark, KB7QHC |
Question on dipole SWR problem
Ed G wrote:
Not sure what SGC recommends regarding balanced antennas.... will review their info. However, I have read numerous reports from hams who have used SGC and other similar tuners to feed balanced lines. The only important condition was to use a choke balun to keep RF from coming back down the coax shield. My SG-230 manual says NOT to connect any transmission line to the tuner output - to connect only an antenna. -- 73, Cecil http://www.w5dxp.com |
Question on dipole SWR problem
..
Hi Ed, This is another antenna element unless you choke it too. Richard, I don't think this one is. Its just a "barrel" type device located in the coax run to the SGC coupler, about 1 foot from the input to the SGC coupler.. There are no other connections or long runs of anything going away from the coupler or the antenna. The MFJ adapter merely filters the 12VDC off the coax center conductor and routes it through a wire a couple feet long into the coupler 12VDC input. Ed |
Question on dipole SWR problem
Not sure what SGC recommends regarding balanced antennas.... will review their info. However, I have read numerous reports from hams who have used SGC and other similar tuners to feed balanced lines. The only important condition was to use a choke balun to keep RF from coming back down the coax shield. My SG-230 manual says NOT to connect any transmission line to the tuner output - to connect only an antenna. SGC manual for the SG-237 says it is recommended to mount the coupler at the antenna feedpoint, but it will work on anything, including balanced line. They show a similar example to what we are doing in the manual. Ed |
Question on dipole SWR problem
Bob Miller wrote:
Maybe one could say the ground side of the balanced line and its dipole quarter wave are acting as an elevated counterpoise to the "random wire" on the other side? Antenna systems are often a lot easier to analyze and understand if you put aside concepts like "counterpoise" and "ground". An antenna is a two terminal device, even if it's "end fed". The transmitter is also a two terminal device. Connect the two together and you have an electrical circuit. Like any circuit, the current leaving one terminal has to equal the current going into the other terminal. So what happens with an end fed antenna? Well, whatever the current going into the antenna (and current must go into it, since the power into it is I^2 * R, where R is the sum of radiation and loss resistance), an equal and opposite current must go somewhere else. If you succeed in completely choking off the current going somewhere else, you've also succeeded in choking off the current going to the antenna. So you don't want to do that. Conductors don't care what label you put on them -- calling one a "ground" or "counterpoise" doesn't give it magical properties. When a current flows on a conductor, it creates a field. This field will radiate unless canceled by other fields. So the "somewhere else" that the current flows is just as much an antenna as the supposed antenna is. If the current has nowhere else to go, it'll go down the outside of the coax, which will effectively become the other half of a dipole. If you choke off the current on the outside of the coax, it'll go somewhere else if it can. But if there is no other place, then the current to the antenna will drop -- the feedpoint impedance will increase. If the current goes into two or more radial wires which are symmetrically placed, the fields from the wires will largely (but not completely) cancel, so the net radiation from the radials will be small. This can reasonably called a "counterpoise" -- a place for the current to flow without creating much radiation. Or you can connect the antenna to a buried radial field ("ground"), which behaves much the same way, but with even better field cancellation. But people often put these names on other configurations, expecting the currents or fields to behave differently than on other conductors. But the things to remember are that all antennas have two terminals, and the current into one equals the current out of the other. And current flowing along any conductor creates a field, whether you consider it to be an "antenna" or not. Keeping this in mind helps a lot in understanding end fed and other antennas. Roy Lewallen, W7EL |
Question on dipole SWR problem
On 18 Oct 2007 01:39:41 GMT, Ed G
wrote: . Hi Ed, This is another antenna element unless you choke it too. Richard, I don't think this one is. Its just a "barrel" type device located in the coax run to the SGC coupler, about 1 foot from the input to the SGC coupler.. There are no other connections or long runs of anything going away from the coupler or the antenna. The MFJ adapter merely filters the 12VDC off the coax center conductor and routes it through a wire a couple feet long into the coupler 12VDC input. Hi Ed, I stand corrected. You seem to have that one under control. 73's Richard Clark, KB7QHC |
Question on dipole SWR problem
Ed G wrote:
w5dxp wrote: My SG-230 manual says NOT to connect any transmission line to the tuner output - to connect only an antenna. SGC manual for the SG-237 says it is recommended to mount the coupler at the antenna feedpoint, but it will work on anything, including balanced line. They show a similar example to what we are doing in the manual. Perhaps the SG-237 has some protection that the SG-230 didn't have? The wording in the SG-230 manual suggests that, since the autotuner is capable of developing voltages high enough to cause open-circuited coax to arc, discretion is advised. -- 73, Cecil http://www.w5dxp.com |
Question on dipole SWR problem
You're right on the current in the shield. We have a choke balun, currently located 85' down the coax toward the transmitter. I have it scheduled, when it stops raining, to move it up to the input of the SG- 237 coupler unit. This has got to be a step in the right direction. Tam/WB2TT I think I misunderstood by what you meant by INPUT. The choke goes between the antenna and the tuner. Generally at the antenna feed point. Tam/WB2TT |
Question on dipole SWR problem
I think I misunderstood by what you meant by INPUT. The choke goes between the antenna and the tuner. Generally at the antenna feed point. Tam/WB2TT Tam, Wouldln't that would be a very bad idea, at least in our particular case? As I pointed out in the start of this thread, we are feeding the antenna with about 14' of 600 ohm ladder line from the output of the SGC antenna coupler. Isn't a choke balan used to prevent the flow of current in the outer braid of a coax feedline? We certainly wouldn't want to be choking any RF coming out of the SGC coupler going to the antenna! Ed |
Question on dipole SWR problem
"Tam/WB2TT" wrote in
: You're right on the current in the shield. We have a choke balun, currently located 85' down the coax toward the transmitter. I have it scheduled, when it stops raining, to move it up to the input of the SG- 237 coupler unit. This has got to be a step in the right direction. Tam/WB2TT I think I misunderstood by what you meant by INPUT. The choke goes between the antenna and the tuner. Generally at the antenna feed point. Tam/WB2TT Tam, Can you explain the difference between putting the choke adjacent to and on either side of the tuner? Owen |
Question on dipole SWR problem
Owen Duffy wrote:
Can you explain the difference between putting the choke adjacent to and on either side of the tuner? How about a tutorial on choking common-mode current on ladder-line? -- 73, Cecil http://www.w5dxp.com |
Question on dipole SWR problem
"Ed G" wrote in message . 192.196... I think I misunderstood by what you meant by INPUT. The choke goes between the antenna and the tuner. Generally at the antenna feed point. Tam/WB2TT Tam, Wouldln't that would be a very bad idea, at least in our particular case? As I pointed out in the start of this thread, we are feeding the antenna with about 14' of 600 ohm ladder line from the output of the SGC antenna coupler. Isn't a choke balan used to prevent the flow of current in the outer braid of a coax feedline? We certainly wouldn't want to be choking any RF coming out of the SGC coupler going to the antenna! Ed The choke suppresses the common mode signal. There will still be current flowing on the shield which will have the same magnitude as the current flowing in the center conductor. You are not throwing away any energy by doing this. If you put the choke at the antenna feed point, the transmission line will not radiate and act as part of the antenna. The choke as you described it could also be called a 1:1 current balun. You might want to take a look at the ARRL Handbook. As for the 600 Ohm line, it is not obvious that 14 feet of 600 Ohm line with an SWR of 120:1 is better than 14 feet of RG8 with an SWR of 10:1. (I came up with an impedance at 3800 of 5 Ohms, assuming a perfect ground. If the roof is not that good a ground, you will want to add a couple of Ohms to that). You guys could solve a lot of the mystery by measuring the SWR as close to the feedpoint of the antenna as possible. Tam/WB2TT |
Question on dipole SWR problem
"Tam/WB2TT" wrote in
: .... The choke suppresses the common mode signal. There will still be current flowing on the shield which will have the same magnitude as the current flowing in the center conductor. You are not throwing away .... This might just be really loose language, but assuming fully effective skin effect (which is a reasonable assumption for most practical coaxial cables at HF): The current flowing on the outside of the inner conductor is accompanied by a current equal in magnitude and opposite in direction flowing on the inside of the outer conductor. Skin effect isolates the inner of the outer conductor from the outer of the outer conductor, but current on the inner of the outer conductor may contribute to current on the outer of the outer conductor depending on the treatment of the shield at the ends of the cable. So, a choke formed by coiling the coaxial cable or placing ferrite sleeves on the cable affects the impedance in the current path of the outer of the outer conductor and does not directly affect what is happening inside the coax. Mind you, this concept is not universally accepted by hams. In the case of coax, so-called common mode current flows only on the outside of the outer conductor, and differential mode current flows only on the inside of the outer conductor and outside of the inner conductor. Owen |
Question on dipole SWR problem
Let me add a little to Owen's excellent explanation.
We can mathematically separate any two currents into a "common mode" (or even mode) current and a "differential mode" (or odd mode) current. If the two currents are equal in magnitude and opposite in direction, the common mode component is zero; if they're equal in magnitude and in the same direction, the differential mode component is zero. This mathematical trickery is very useful in analyzing transmission lines, because superposition allows us to treat the effects of the two mode currents separately and sum the results. In a transmission line, the differential mode current is sometimes appropriately called the "transmission line" current, and the common mode current the "antenna" current. This is because the differential mode current conforms to all the transmission line rules -- that is, it behaves as though it and it alone is being carried by the transmission line, and its properties can be found by applying normal transmission line equations and analysis. No radiation results from the transmission line currents. (In practice, a very small amount of radiation results from the differential current on a non-coax line, but if it's significant, a poor choice of transmission line was made.) And the common mode current behaves just like any other current on a single conductor (or identical currents on two parallel conductors) - it creates a radiating field. The conductor carrying the current is, by any definition, an antenna. So if we want to eliminate feedline radiation we need to eliminate (or, practically speaking, reduce to a small value) the common mode current. To do this analysis with a symmetrical line such as twinlead or open wire line, we use the currents on the two conductors as the two currents to separate into common and differential mode components. We can do exactly the same thing with coax, using the current on the inner conductor as one of the currents to be separated, and the vector total current on the inside and outside of the shield to be the other. If we do this, we find that the two types of line behave identically: If the common mode current is zero, the line won't radiate (and can be considered balanced). If it isn't, the line will. Equations and analysis are identical. Either type of line can be balanced or unbalanced. Coaxial lines do, however, have an interesting characteristic not shared by other kinds -- the differential and common mode components aren't simply a mathematical convenience, but are actually physically separate. If we do the analysis described above, we find that the common mode current equals the current on the outside of the shield and the differential current equals the current on the inside. As Owen pointed out, the differential current is solely on the inside of the shield and the common mode current solely on the outside. While this makes the effects of each mode current easier to visualize and sometimes to measure, the net effects of common mode and differential currents are exactly the same for coaxial and non-coaxial lines. Roy Lewallen, W7EL Owen Duffy wrote: "Tam/WB2TT" wrote in : ... The choke suppresses the common mode signal. There will still be current flowing on the shield which will have the same magnitude as the current flowing in the center conductor. You are not throwing away ... This might just be really loose language, but assuming fully effective skin effect (which is a reasonable assumption for most practical coaxial cables at HF): The current flowing on the outside of the inner conductor is accompanied by a current equal in magnitude and opposite in direction flowing on the inside of the outer conductor. Skin effect isolates the inner of the outer conductor from the outer of the outer conductor, but current on the inner of the outer conductor may contribute to current on the outer of the outer conductor depending on the treatment of the shield at the ends of the cable. So, a choke formed by coiling the coaxial cable or placing ferrite sleeves on the cable affects the impedance in the current path of the outer of the outer conductor and does not directly affect what is happening inside the coax. Mind you, this concept is not universally accepted by hams. In the case of coax, so-called common mode current flows only on the outside of the outer conductor, and differential mode current flows only on the inside of the outer conductor and outside of the inner conductor. Owen |
Question on dipole SWR problem
"Roy Lewallen" wrote in message
... [...] Coaxial lines do, however, have an interesting characteristic not shared by other kinds -- the differential and common mode components aren't simply a mathematical convenience, but are actually physically separate. If we do the analysis described above, we find that the common mode current equals the current on the outside of the shield and the differential current equals the current on the inside. As Owen pointed out, the differential current is solely on the inside of the shield and the common mode current solely on the outside. While this makes the effects of each mode current easier to visualize and sometimes to measure, the net effects of common mode and differential currents are exactly the same for coaxial and non-coaxial lines. Owen Duffy wrote: In the case of coax, so-called common mode current flows only on the outside of the outer conductor, and differential mode current flows only on the inside of the outer conductor and outside of the inner conductor. I assume that you are talking about a length of coax that is attached to a free-space antenna. What about the case where the coax shield is grounded at both ends? (make it a non-ideal ground if you like.) Wouldn't this create a ground-loop that will cause some of the signal current to flow through the ground-connection, thus unbalancing the center-conductor/shield current? In this case, the common-mode current isn't necessarily flowing on the outside of the shield. (I am asking a question here). Also, consider the case at frequencies low enough that skin-effect doesn't apply. Here there is no inside or outside of the coax shield. Still, the magnetic fields caused by imbalance between center-conductor and shield currents are the same, with or without skin effect. At least these are the thoughts I had while I was discussing the installation of antennas and tuners on boats. There is no end to the controversy surrounding the grounding of radio equipment on a boat. There the antennas are typically end-fed wires (usually part of the rigging), and some combination of radials and seawater connection for the RF counterpoise. -Paul |
Question on dipole SWR problem
Paul wrote:
"Roy Lewallen" wrote in message ... [...] Coaxial lines do, however, have an interesting characteristic not shared by other kinds -- the differential and common mode components aren't simply a mathematical convenience, but are actually physically separate. If we do the analysis described above, we find that the common mode current equals the current on the outside of the shield and the differential current equals the current on the inside. As Owen pointed out, the differential current is solely on the inside of the shield and the common mode current solely on the outside. While this makes the effects of each mode current easier to visualize and sometimes to measure, the net effects of common mode and differential currents are exactly the same for coaxial and non-coaxial lines. Owen Duffy wrote: In the case of coax, so-called common mode current flows only on the outside of the outer conductor, and differential mode current flows only on the inside of the outer conductor and outside of the inner conductor. I assume that you are talking about a length of coax that is attached to a free-space antenna. What about the case where the coax shield is grounded at both ends? (make it a non-ideal ground if you like.) Wouldn't this create a ground-loop that will cause some of the signal current to flow through the ground-connection, thus unbalancing the center-conductor/shield current? In this case, the common-mode current isn't necessarily flowing on the outside of the shield. (I am asking a question here). A tricky part in answering this is determining what you mean by "shield current". There are separate and distinct currents on the inside and outside of the shield. I'll assume that by "shield current" you mean the vector sum of these two currents. The first part of the answer is that the current on the outside of the inner conductor is always equal to the current on the inside of the shield, and in the opposite direction (that is to say, they comprise a pure differential current), provided that the shield is at least several skin depths thick. This is a consequence of the confinement of the field by the shield, and has nothing to do with what we connect the cable to. Connections only impact the current on the outside. Now consider what happens when the coax is connected to a free-space dipole, for example. All the current from the center conductor flows into one half the dipole. But the current on the inside of the shield has two possible paths: to the other half of the dipole or around the end of the shield to the outside of the shield. I won't go into more detail about this, since I've already done so -- you can see what I've written at http://eznec.com/Amateur/Articles/Baluns.pdf. If you "ground" both ends of the coax, that is, connect them to conductors which provide another path between the two ends, you have a third path the inner shield can follow -- along the "ground" path. So it splits three ways instead of two. If you use a "pigtail" wire for grounding or connecting to the load, it adds inductance to the desired path to the load, which makes the path back along the outside of the coax more desirable, so you end up with more common mode current than you would with a low impedance connection. Also, consider the case at frequencies low enough that skin-effect doesn't apply. Here there is no inside or outside of the coax shield. Still, the magnetic fields caused by imbalance between center-conductor and shield currents are the same, with or without skin effect. I'm not sure I follow this. When the frequency gets low enough that the field can penetrate the shield, the line behaves more like a twinlead line behaves at HF. As I mentioned in my earlier posting, the line can still have common and differential mode currents -- they're just no longer physically separated. At least these are the thoughts I had while I was discussing the installation of antennas and tuners on boats. There is no end to the controversy surrounding the grounding of radio equipment on a boat. There the antennas are typically end-fed wires (usually part of the rigging), and some combination of radials and seawater connection for the RF counterpoise. Grounding would be much easier to understand if people would realize that calling a conductor or connection "ground" doesn't impart magical qualities. And that currents flow wherever the impedance dictates. Roy Lewallen, W7EL |
Question on dipole SWR problem
"Roy Lewallen" wrote in message ... Paul wrote: I assume that you are talking about a length of coax that is attached to a free-space antenna. What about the case where the coax shield is grounded at both ends? (make it a non-ideal ground if you like.) Wouldn't this create a ground-loop that will cause some of the signal current to flow through the ground-connection, thus unbalancing the center-conductor/shield current? In this case, the common-mode current isn't necessarily flowing on the outside of the shield. (I am asking a question here). A tricky part in answering this is determining what you mean by "shield current". There are separate and distinct currents on the inside and outside of the shield. I'll assume that by "shield current" you mean the vector sum of these two currents. The first part of the answer is that the current on the outside of the inner conductor is always equal to the current on the inside of the shield, and in the opposite direction (that is to say, they comprise a pure differential current), provided that the shield is at least several skin depths thick. This is a consequence of the confinement of the field by the shield, and has nothing to do with what we connect the cable to. Connections only impact the current on the outside. Now consider what happens when the coax is connected to a free-space dipole, for example. All the current from the center conductor flows into one half the dipole. But the current on the inside of the shield has two possible paths: to the other half of the dipole or around the end of the shield to the outside of the shield. I won't go into more detail about this, since I've already done so -- you can see what I've written at http://eznec.com/Amateur/Articles/Baluns.pdf. If you "ground" both ends of the coax, that is, connect them to conductors which provide another path between the two ends, you have a third path the inner shield can follow -- along the "ground" path. So it splits three ways instead of two. If you use a "pigtail" wire for grounding or connecting to the load, it adds inductance to the desired path to the load, which makes the path back along the outside of the coax more desirable, so you end up with more common mode current than you would with a low impedance connection. Yes, this agrees with what I have been thinking, and what I was trying to say. Also, consider the case at frequencies low enough that skin-effect doesn't apply. Here there is no inside or outside of the coax shield. Still, the magnetic fields caused by imbalance between center-conductor and shield currents are the same, with or without skin effect. I'm not sure I follow this. When the frequency gets low enough that the field can penetrate the shield, the line behaves more like a twinlead line behaves at HF. As I mentioned in my earlier posting, the line can still have common and differential mode currents -- they're just no longer physically separated. Grounding would be much easier to understand if people would realize that calling a conductor or connection "ground" doesn't impart magical qualities. And that currents flow wherever the impedance dictates. Again, I agree. The reason I suggested a "non-ideal ground" was to de-magic it. We do need to recognize some non-zero impedances on the grounds and shields, and their connections, if we are to analyze how the current splits. The reason for the low-frequency question is that on the boat installations I've been discussing, there are multiple signal sources and some very non-ideal grounds. The signal sources include DC (due to how the electrical equipment is usually wired), and relatively low frequency signals, sometimes carried on cables in very close proximity to the coax. There is also the RF field from the close-in antenna and RF grounding system. I've been stating that the coax shield does *not* provide a magic shield. Your comparison to twinlead at low frequencies (for current, and thus for inductive coupling), confirms what I have been saying. These superimposed low-frequency currents don't affect the RF situation, but they can affect the equipment itself. Regards, -Paul (wb6cxc) |
Question on dipole SWR problem
Getting back to the original problem.
Most autotuners can't cope with impedances of less than 5 or 6 ohms. As you are near that limit even a slight increase in antenna height may facilitate correct tuning. In order to improve the antenna balance and RX S/N ratio. Put a good quality ferrite common mode chokes on the Coax and control cables going into the tuner (I suggest either 50 ferrite beads over the coax or 10 turns of coax on a ferrite ring as a minimum) If you wish to earth the coax do it on the transmitter side of the choke. A few turns of coax will not provide sufficient choking impedance at 3.8MHz. The purpose of this is to 'float' the tuner above RF earth so that the output apears to be balanced. By putting the choke on the input side of the tuner, it is always working at a constant impedance thus minimising losses. UKM On Oct 30, 2:42 pm, "Paul" wrote: "Roy Lewallen" wrote in message ... Paul wrote: I assume that you are talking about a length of coax that is attached to a free-space antenna. What about the case where the coax shield is grounded at both ends? (make it a non-ideal ground if you like.) Wouldn't this create a ground-loop that will cause some of the signal current to flow through the ground-connection, thus unbalancing the center-conductor/shield current? In this case, the common-mode current isn't necessarily flowing on the outside of the shield. (I am asking a question here). A tricky part in answering this is determining what you mean by "shield current". There are separate and distinct currents on the inside and outside of the shield. I'll assume that by "shield current" you mean the vector sum of these two currents. The first part of the answer is that the current on the outside of the inner conductor is always equal to the current on the inside of the shield, and in the opposite direction (that is to say, they comprise a pure differential current), provided that the shield is at least several skin depths thick. This is a consequence of the confinement of the field by the shield, and has nothing to do with what we connect the cable to. Connections only impact the current on the outside. Now consider what happens when the coax is connected to a free-space dipole, for example. All the current from the center conductor flows into one half the dipole. But the current on the inside of the shield has two possible paths: to the other half of the dipole or around the end of the shield to the outside of the shield. I won't go into more detail about this, since I've already done so -- you can see what I've written at http://eznec.com/Amateur/Articles/Baluns.pdf. If you "ground" both ends of the coax, that is, connect them to conductors which provide another path between the two ends, you have a third path the inner shield can follow -- along the "ground" path. So it splits three ways instead of two. If you use a "pigtail" wire for grounding or connecting to the load, it adds inductance to the desired path to the load, which makes the path back along the outside of the coax more desirable, so you end up with more common mode current than you would with a low impedance connection. Yes, this agrees with what I have been thinking, and what I was trying to say. Also, consider the case at frequencies low enough that skin-effect doesn't apply. Here there is no inside or outside of the coax shield. Still, the magnetic fields caused by imbalance between center-conductor and shield currents are the same, with or without skin effect. I'm not sure I follow this. When the frequency gets low enough that the field can penetrate the shield, the line behaves more like a twinlead line behaves at HF. As I mentioned in my earlier posting, the line can still have common and differential mode currents -- they're just no longer physically separated. Grounding would be much easier to understand if people would realize that calling a conductor or connection "ground" doesn't impart magical qualities. And that currents flow wherever the impedance dictates. Again, I agree. The reason I suggested a "non-ideal ground" was to de-magic it. We do need to recognize some non-zero impedances on the grounds and shields, and their connections, if we are to analyze how the current splits. The reason for the low-frequency question is that on the boat installations I've been discussing, there are multiple signal sources and some very non-ideal grounds. The signal sources include DC (due to how the electrical equipment is usually wired), and relatively low frequency signals, sometimes carried on cables in very close proximity to the coax. There is also the RF field from the close-in antenna and RF grounding system. I've been stating that the coax shield does *not* provide a magic shield. Your comparison to twinlead at low frequencies (for current, and thus for inductive coupling), confirms what I have been saying. These superimposed low-frequency currents don't affect the RF situation, but they can affect the equipment itself. Regards, -Paul (wb6cxc)- Hide quoted text - - Show quoted text - |
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