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Request EZNEC computation
This is the first time I've done this. If the need arises again, I'll have to download and try EZNEC, I guess. Meanwhile, I am concerned about the efficiency of our ARES Command Center HF antenna system. Could someone please advise me what feedpoint SWRs would be seen on a wire antenna, 15 feet above a flat metal roofed building, (it a NVIS antenna), and resonant at 6.2 MHz. It is operated at 3.98 MHZ, 5.4 MHz, and 7.2 MHz The 100 watt radio has one of those built in tuners that can only handle 3:1 SWR. I suspect we are terribly inefficient. Thanks for any response. Ed K7AAT |
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Ed wrote in
92.196: This is the first time I've done this. If the need arises again, I'll have to download and try EZNEC, I guess. Meanwhile, I am concerned about the efficiency of our ARES Command Center HF antenna system. Could someone please advise me what feedpoint SWRs would be seen on a wire antenna, 15 feet above a flat metal roofed building, (it a NVIS antenna), and resonant at 6.2 MHz. It is operated at 3.98 MHZ, 5.4 MHz, and 7.2 MHz The 100 watt radio has one of those built in tuners that can only handle 3:1 SWR. I suspect we are terribly inefficient. Ed, I am not sure exactly what you mean by a "a wire antenna, 15 feet above a flat metal roofed building, (it a NVIS antenna), and resonant at 6.2 MHz". I assume you mean a centre fed dipole that is an electrical half wave at 6.2MHz. The efficiency of the radiator component can be estimated from an NEC model, it is probably very high and not the real issue. Components of an antenna system interact with each other in a complex way, and it is important to analyse the entire antenna system (radiator, earth, transmission line, balun, ATU etc) to obtain a correct understanding of how the antenna system works overall. In your case, the feedline and ATU are the likely main contributors to antenna system loss. Antenna system loss, transmitter behaviour, and antenna pattern are the main contibutors to station performance. To illustrate with an example. One Saturday a few months ago, I had 7MHz three QSOs in a row with our new six hour hams who were each using an 80m half wave dipole fed with a substantial run of RG58C/U coax and an ATU. I explained to them that their antenna would work ten times better on 7MHz if their antenna was half the length. A difficult concept for people with six hours investment in ham radio to understand. A fourth person who had heard one of the QSOs (and had apparently called me, but I couldn't dig him out of the noise) emailed me saying he was using the same setup and now knew the problem, it had to be the ATU. I replied to him that the ATU probably wasn't losing all that much power, the high coax losses protected the ATU from an extreme load and extreme loss. Again a hard concept to swallow when the ATU is closer to the transmitter but something downstream "protects" it from higher loss. Owen |
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I am not sure exactly what you mean by a "a wire antenna, 15 feet above a flat metal roofed building, (it a NVIS antenna), and resonant at 6.2 MHz". I assume you mean a centre fed dipole that is an electrical half wave at 6.2MHz. Yes. My appologies for leaving out a critical description. Its a wire centerfed halfwave antenna, 15 feet above a flat metal roof, and resonant at 6.2 MHz. The people who installed it expected the tuner in the FT900 radio to handle the matching. I explained that the tuner was never intended to match an antenna that would be so far off resonance. To compound matters, they are using at least 100 feet of 50 ohm coax... mostly RG- 8, I think. The efficiency of the radiator component can be estimated from an NEC model, it is probably very high and not the real issue. Well, in this case, it IS a necessary issue. I wish to present the probably actual feedpoint SWR to these people so they will realize how far beyond tolerance they are trying to use that built in tuner. That is why I was asking for SWR estimates for 3.95MHz, 7.2MHz, and 5.4MHz. I intend to propose going to a resonant antenna, and use traps or multiple dipoles to present a near 50ohm feedpoint to the coax and tuner for those three frequencies of operation. ... so.... if anyone could run those figures, I'd sure appreciate it. Ed PS: Thanks for the info, though, Owen. |
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Ed wrote in
. 192.196: I am not sure exactly what you mean by a "a wire antenna, 15 feet above a flat metal roofed building, (it a NVIS antenna), and resonant at 6.2 MHz". I assume you mean a centre fed dipole that is an electrical half wave at 6.2MHz. Yes. My appologies for leaving out a critical description. Its a wire centerfed halfwave antenna, 15 feet above a flat metal roof, and resonant at 6.2 MHz. The people who installed it expected the tuner in the FT900 radio to handle the matching. I explained that the tuner was never intended to match an antenna that would be so far off resonance. To compound matters, they are using at least 100 feet of 50 ohm coax... mostly RG- 8, I think. The efficiency of the radiator component can be estimated from an NEC model, it is probably very high and not the real issue. Well, in this case, it IS a necessary issue. I wish to present Whilst neither of us KNOW whether it is an issue, I suggest to you that based on experience radiator loss in this type of configurationis probably less than 2%. the probably actual feedpoint SWR to these people so they will realize how far beyond tolerance they are trying to use that built in tuner. That is why I was asking for SWR estimates for 3.95MHz, 7.2MHz, and 5.4MHz. You seem to be equating feed point VSWR and that seen by the built in tuner. That might be the case if the transmission line was lossless. It is unlikely to be, and we don't know again due to lack of information. You might find my article at http://www.vk1od.net/LOLL/index.htm which looks at multi-frequency use of a coax fed centre fed dipole of interest. Though it models a 66' dipole at 30' height, some (most) of the messages are relevant to your situation. Perhaps on reviewing the article, you will see my point that radiator loss is less important, and that understanding transmission line loss is very important to your problem. The article might even suggest some other options that you haven't nominated. Owen |
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You seem to be equating feed point VSWR and that seen by the built in tuner. That might be the case if the transmission line was lossless. It is unlikely to be, and we don't know again due to lack of information. Owen, I appreciate your responses, but I am at a loss as to what additional information one would need to model a center fed 75 foot long half wave antenna, resosnant at 6.2 MHz, positioned 15 feet above a flat metal roof, fed with 100 feet of 50 ohm coax (RG8), and operated on any one of three frequencies.... 3.95MHz, 5.4MHz, and 7.2 MHz. What parameter is missing to provide a good model of this? Ed |
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but I am at a loss as to what
additional information one would need to model a center fed 75 foot long half wave antenna, resosnant at 6.2 MHz, positioned 15 feet above a flat metal roof, fed with 100 feet of 50 ohm coax (RG8), and operated on any one of three frequencies.... 3.95MHz, 5.4MHz, and 7.2 MHz. Small correction.... the feedline is also not important since all I am asking for is essentially the feedpoint impedance of this antenna at those three frequencies of operation. Ed K7AAT |
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Ed wrote in
. 192.196: but I am at a loss as to what additional information one would need to model a center fed 75 foot long half wave antenna, resosnant at 6.2 MHz, positioned 15 feet above a flat metal roof, fed with 100 feet of 50 ohm coax (RG8), and operated on any one of three frequencies.... 3.95MHz, 5.4MHz, and 7.2 MHz. Small correction.... the feedline is also not important since all I am asking for is essentially the feedpoint impedance of this antenna at those three frequencies of operation. If the metal roof was infinite (or very large compared to the dipole), the feedpoint Z looks to be around 3-j711 at 3.95MHz. (This is probably not a good model of your scenario though.) Is that all you really need to assess the outcome? 100' of RG8 with such a load will lose 20.6dB. The ATU will see around 4-j55, so you may well be losing 3dB Radiator loss (with with you seemed concerned) depends on the wire (which you haven't told us about), and it might be a dB with such a low feedpoint R. The coax loss dominates the problem. There is a lot more to quantifying the problem, and designing a solution than knowing feedpoint Z at three spot frequencies for a dipole over an inadequately specified metal roof. However, we do know that a coax fed centre fed dipole is not an efficient multi-frequency antenna if the coax is of significant length (meaning basically more than nearly zero). Owen |
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"Ed" wrote in message 92.196... This is the first time I've done this. If the need arises again, I'll have to download and try EZNEC, I guess. Meanwhile, I am concerned about the efficiency of our ARES Command Center HF antenna system. Could someone please advise me what feedpoint SWRs would be seen on a wire antenna, 15 feet above a flat metal roofed building, (it a NVIS antenna), and resonant at 6.2 MHz. It is operated at 3.98 MHZ, 5.4 MHz, and 7.2 MHz The 100 watt radio has one of those built in tuners that can only handle 3:1 SWR. I suspect we are terribly inefficient. Thanks for any response. Ed K7AAT Hi Ed I probably dont quite understand your situation. But, if your question is referenced to a 75 foot long center fed dipole 15 feet above a metal ground, EZNEC shows the terminal impedance will be 72 ohms at 6.4 MHz. The VSWR is under 3:1 from about 6.05 MHz to 6.5 MHz. Jerry (who used 1 mm diameter copper wire) |
Request EZNEC computation
"Jerry Martes" wrote in
news:ScTqi.3733$8u1.2359@trnddc07: "Ed" wrote in message 92.196... This is the first time I've done this. If the need arises again, I'll have to download and try EZNEC, I guess. Meanwhile, I am concerned about the efficiency of our ARES Command Center HF antenna system. Could someone please advise me what feedpoint SWRs would be seen on a wire antenna, 15 feet above a flat metal roofed building, (it a NVIS antenna), and resonant at 6.2 MHz. It is operated at 3.98 MHZ, 5.4 MHz, and 7.2 MHz The 100 watt radio has one of those built in tuners that can only handle 3:1 SWR. I suspect we are terribly inefficient. Thanks for any response. Ed K7AAT Hi Ed I probably dont quite understand your situation. But, if your question is referenced to a 75 foot long center fed dipole 15 feet above a metal ground, EZNEC shows the terminal impedance will be 72 ohms at 6.4 MHz. The VSWR is under 3:1 from about 6.05 MHz to 6.5 MHz. Jerry, are you saying that your model suggests the Z~=72+j0 at resonance, and the half wave dipole is ~0.1m above a perfectly conducting plane. It doesn't seem right. Owen |
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Owen Duffy wrote in
: "Jerry Martes" wrote in news:ScTqi.3733$8u1.2359@trnddc07: "Ed" wrote in message 92.196... This is the first time I've done this. If the need arises again, I'll have to download and try EZNEC, I guess. Meanwhile, I am concerned about the efficiency of our ARES Command Center HF antenna system. Could someone please advise me what feedpoint SWRs would be seen on a wire antenna, 15 feet above a flat metal roofed building, (it a NVIS antenna), and resonant at 6.2 MHz. It is operated at 3.98 MHZ, 5.4 MHz, and 7.2 MHz The 100 watt radio has one of those built in tuners that can only handle 3:1 SWR. I suspect we are terribly inefficient. Jerry, are you saying that your model suggests the Z~=72+j0 at resonance, and the half wave dipole is ~0.1m above a perfectly conducting plane. It doesn't seem right. That should read "0.1 wl above..." |
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"Owen Duffy" wrote in message ... Owen Duffy wrote in : "Jerry Martes" wrote in news:ScTqi.3733$8u1.2359@trnddc07: "Ed" wrote in message 92.196... This is the first time I've done this. If the need arises again, I'll have to download and try EZNEC, I guess. Meanwhile, I am concerned about the efficiency of our ARES Command Center HF antenna system. Could someone please advise me what feedpoint SWRs would be seen on a wire antenna, 15 feet above a flat metal roofed building, (it a NVIS antenna), and resonant at 6.2 MHz. It is operated at 3.98 MHZ, 5.4 MHz, and 7.2 MHz The 100 watt radio has one of those built in tuners that can only handle 3:1 SWR. I suspect we are terribly inefficient. Jerry, are you saying that your model suggests the Z~=72+j0 at resonance, and the half wave dipole is ~0.1m above a perfectly conducting plane. It doesn't seem right. That should read "0.1 wl above..." The answer to the question would seem to be that it would not work very well just due to experiance. The swr would be off the charts and a rig that had an internal tuner would not tune that kind of antenna over that frequency range if the swr had to be under 3:1. Also the loss in the coax due to the high swr would be very large.. |
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.."
The answer to the question would seem to be that it would not work very well just due to experiance. The swr would be off the charts and a rig that had an internal tuner would not tune that kind of antenna over that frequency range if the swr had to be under 3:1. Also the loss in the coax due to the high swr would be very large.. Well, that was my original surmise of the situation. I was just trying to have someone with expertize on EZNEC run the antenna at the three frequencies listed in order to have the feedpoint SWR calculated for each of those three frequencies. So far, no one, except Owen, has come even close. I can work with the figures Owen provided for the one frequency ( 3975) and do the rest of the calculations manually to determine the SWR on that frequency, but had hoped for actual feedpoint SWR data for all three actual transmit frequencies listed. Thanks to all. Ed |
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Ed wrote in
.92: .... Well, that was my original surmise of the situation. I was just trying to have someone with expertize on EZNEC run the antenna at the three frequencies listed in order to have the feedpoint SWR calculated for each of those three frequencies. So far, no one, except Owen, has come even close. I can work with the figures Owen provided for the one frequency ( 3975) and do the rest of the calculations manually to determine the SWR on that frequency, but had hoped for actual feedpoint SWR data for all three actual transmit frequencies listed. Ed, I am giving you information that is not sufficient to solve the problem. Nevertheless, I defer to your opinion. Read my earlier caveats about my reading of your limited information and interpretation in a model. Freq VSWR at feedpoint (RG8/U) 3.98 959 5.4 115 7.2 34 Owen |
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"Owen Duffy" wrote in message ... Owen Duffy wrote in : "Jerry Martes" wrote in news:ScTqi.3733$8u1.2359@trnddc07: "Ed" wrote in message 92.196... This is the first time I've done this. If the need arises again, I'll have to download and try EZNEC, I guess. Meanwhile, I am concerned about the efficiency of our ARES Command Center HF antenna system. Could someone please advise me what feedpoint SWRs would be seen on a wire antenna, 15 feet above a flat metal roofed building, (it a NVIS antenna), and resonant at 6.2 MHz. It is operated at 3.98 MHZ, 5.4 MHz, and 7.2 MHz The 100 watt radio has one of those built in tuners that can only handle 3:1 SWR. I suspect we are terribly inefficient. Jerry, are you saying that your model suggests the Z~=72+j0 at resonance, and the half wave dipole is ~0.1m above a perfectly conducting plane. It doesn't seem right. That should read "0.1 wl above..." Hi Owen I was kinda surprised too. But, I have alot of faith in EZNEC. The problem of determining the input impedance of such a simple antenna over a perfectly conducting ground seemed too easy to model, so I did that. EZNEC tells me that the 75 foot copper wire thats 0.040 diameter and 15 feet above ground looks like 72.33 +j0.5972 ohms.at 6.4 MHz. My experience with Roy's program has always indicated that it is more accurate than my actual measurements. I would appreciate hearing where EZNEC cannot be trusted. Jerry |
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Ed, I am giving you information that is not sufficient to solve the problem. Nevertheless, I defer to your opinion. Read my earlier caveats about my reading of your limited information and interpretation in a model. It is apparent to me that we have not been communicating effectively. I did leave out some info, such as the wire type, ( #18 stranded and insulated) and the size of the flat metal roof ( can probably be treated as infinite given its size), but only because I knew the basic losses due to antenna size itself ( non-resonant ) would be so huge as to mask any much smaller effects of these other items. Freq VSWR at feedpoint (RG8/U) 3.98 959 5.4 115 7.2 34 My thanks on the above. That is something basic I can give to the operators of this station to convince them we need to change the antenna system. ( Giving them complex impedance figures, etc, would not fly with that group!) It is also obvious I need to get EZNEC. Soon as I have time to play with it, I will download the freebie version for starters. I've seen too much discussion here on this group not to realize it is worthwhile to have, even if only occasionally used. Ed K7AAT |
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"Ed" wrote in message .92... ." The answer to the question would seem to be that it would not work very well just due to experiance. The swr would be off the charts and a rig that had an internal tuner would not tune that kind of antenna over that frequency range if the swr had to be under 3:1. Also the loss in the coax due to the high swr would be very large.. Well, that was my original surmise of the situation. I was just trying to have someone with expertize on EZNEC run the antenna at the three frequencies listed in order to have the feedpoint SWR calculated for each of those three frequencies. So far, no one, except Owen, has come even close. Hi Ed Did you miss my post??? Or, did misunderstand your question. This antenna is so easy to model with Roy's EZNEC that I suspect it would be an excellant "first try" for you. Send Roy a few $$ and you will soon realize that you have spent some money well. I'd be willing to show you how to model it, and I am a novice user of EZNEC. Your question certainly doesnt require an expert on EZNEC to be answered very clearly. Jerry KD6JDJ I can work with the figures Owen provided for the one frequency ( 3975) and do the rest of the calculations manually to determine the SWR on that frequency, but had hoped for actual feedpoint SWR data for all three actual transmit frequencies listed. Thanks to all. Ed |
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"Jerry Martes" wrote in
news:_XUqi.1027$FO1.53@trnddc05: "Owen Duffy" wrote in message ... Owen Duffy wrote in : "Jerry Martes" wrote in news:ScTqi.3733$8u1.2359@trnddc07: "Ed" wrote in message 92.196... This is the first time I've done this. If the need arises again, I'll have to download and try EZNEC, I guess. Meanwhile, I am concerned about the efficiency of our ARES Command Center HF antenna system. Could someone please advise me what feedpoint SWRs would be seen on a wire antenna, 15 feet above a flat metal roofed building, (it a NVIS antenna), and resonant at 6.2 MHz. It is operated at 3.98 MHZ, 5.4 MHz, and 7.2 MHz The 100 watt radio has one of those built in tuners that can only handle 3:1 SWR. I suspect we are terribly inefficient. Jerry, are you saying that your model suggests the Z~=72+j0 at resonance, and the half wave dipole is ~0.1m above a perfectly conducting plane. It doesn't seem right. That should read "0.1 wl above..." Hi Owen I was kinda surprised too. But, I have alot of faith in EZNEC. The problem of determining the input impedance of such a simple antenna over a perfectly conducting ground seemed too easy to model, so I did that. EZNEC tells me that the 75 foot copper wire thats 0.040 diameter and 15 feet above ground looks like 72.33 +j0.5972 ohms.at 6.4 MHz. Ok, I get around 20 ohms at resonance. We have obviously made some different interpretations somewhere... but I expect a number smaller than 72 due to the proximity of the conducting plane. My experience with Roy's program has always indicated that it is more accurate than my actual measurements. I would appreciate hearing where EZNEC cannot be trusted. I didn't say that, nor did I imply so, and to infer so is a YKW trick. It is not the tool, it is the difference in the way we have each built the model. As it happens, I am using EZNEC 3, but I expect I would get exactly the same answer from any NEC-2 based tool, 4NEC2 for instance. (I have mailed you my model, you can compare them and see where they are different.) Owen Glossary: YKW=You Know Who |
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Hi Ed Did you miss my post??? Or, did misunderstand your question. This antenna is so easy to model with Roy's EZNEC that I suspect it would be an excellant "first try" for you. Send Roy a few $$ and you will soon realize that you have spent some money well. I'd be willing to show you how to model it, and I am a novice user of EZNEC. Your question certainly doesnt require an expert on EZNEC to be answered very clearly. No, Jerry, I did not miss your post.... but it did not answer my question. You modeled the antenna I described, apparently, but not at the frequencies of intended operation. Yes, it is a 75foot halfwave center fed, etc, etc, but I was looking for either feedpoint impedence or a calculated feedpoint SWR into 50 ohm transmission line on three frequencies, 3993KHz, 5400 KHz, and 7240KHz. Owen has subsequently provided the figures I was looking forward, although he wasn't happy with it. He wanted more info to provide a more accurate model, and I was only looking for a "close" figure. Thanks for your input, Jerry. Ed |
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Owen Duffy wrote: Ed wrote in . 192.196: but I am at a loss as to what additional information one would need to model a center fed 75 foot long half wave antenna, resosnant at 6.2 MHz, positioned 15 feet above a flat metal roof, fed with 100 feet of 50 ohm coax (RG8), and operated on any one of three frequencies.... 3.95MHz, 5.4MHz, and 7.2 MHz. Small correction.... the feedline is also not important since all I am asking for is essentially the feedpoint impedance of this antenna at those three frequencies of operation. If the metal roof was infinite (or very large compared to the dipole), the feedpoint Z looks to be around 3-j711 at 3.95MHz. (This is probably not a good model of your scenario though.) Is that all you really need to assess the outcome? 100' of RG8 with such a load will lose 20.6dB. The ATU will see around 4-j55, so you may well be losing 3dB Radiator loss (with with you seemed concerned) depends on the wire (which you haven't told us about), and it might be a dB with such a low feedpoint R. The coax loss dominates the problem. There is a lot more to quantifying the problem, and designing a solution than knowing feedpoint Z at three spot frequencies for a dipole over an inadequately specified metal roof. However, we do know that a coax fed centre fed dipole is not an efficient multi-frequency antenna if the coax is of significant length (meaning basically more than nearly zero). Owen |
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15 feet above a flat metal roofed building, (it a NVIS
antenna), and resonant at 6.2 MHz. It is operated at 3.98 MHZ, 5.4 MHz, and 7.2 MHz Here's your answer: Freq SWR 3.98 3,240 (three thousand !!) 6.2 2.08 7.2 39.3 Details: Used 4NEC2 (free) 5 meters above ground Length of antenna = 11.62 m Impedance at 3.98 MHz = 3-j710 Impedance at 6.2 MHz = 24 +j3 Impedance at 7.2 MHz = 54+j318 Limitation - real earth used vs flat metal roof File Setup and computation/optimization time - 7.8 minutes Rate $60/hour Your billable cost $7.80 Rebate for first time user $7.80 Net cost $0.00 73, Rick K2XT |
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..
Jerry Martes wrote: "Ed" wrote in message .92... ." The answer to the question would seem to be that it would not work very well just due to experiance. The swr would be off the charts and a rig that had an internal tuner would not tune that kind of antenna over that frequency range if the swr had to be under 3:1. Also the loss in the coax due to the high swr would be very large.. Well, that was my original surmise of the situation. I was just trying to have someone with expertize on EZNEC run the antenna at the three frequencies listed in order to have the feedpoint SWR calculated for each of those three frequencies. So far, no one, except Owen, has come even close. Ed, several years ago I had to find out why an almost identical antenna system as you have discribed operating on similar frequencies didnt work very well. System also used an ATU. Analysis showed data very similar to the EZNEC values. I t was not practical to increase the hiegth of the antenna so I went vertical using a 30ft fiberglass whip, gov surplus. Also used inductors and capacitors switched in to bring the antenna to resonance at the desired freq. Although we only really used one the others had to be available. Although the ATU was still used it was completely unneeded as only 3 frequencies were being used seies inductance and capacitance selected by remotely controled relays provide a match with the VSWR less than 1:.5 on any frequency . Communication from NC to GA, FL, and DC were routinely possible with 100watts PEP SSB. Jimmie |
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Ed, several years ago I had to find out why an almost identical antenna system as you have discribed operating on similar frequencies didnt work very well. System also used an ATU. Analysis showed data very similar to the EZNEC values. I t was not practical to increase the hiegth of the antenna so I went vertical using a 30ft fiberglass whip, gov surplus. Also used inductors and capacitors switched in to bring the antenna to resonance at the desired freq. Although we only really used one the others had to be available. Although the ATU was still used it was completely unneeded as only 3 frequencies were being used seies inductance and capacitance selected by remotely controled relays provide a match with the VSWR less than 1:.5 on any frequency . Communication from NC to GA, FL, and DC were routinely possible with 100watts PEP SSB. That could certainly be an option.... especially since this antenna is on top of a government building. Aesthetics is an issue here. However, the pre-existing antenna can easily be made to work well in our situation simply by shortening it to resonance on 3975, and possibly just adding traps or another dipole off the feedpoint for the other two frequencies of operation. There is room. The reason for this silly situation to begin with is just the misconception held by our ARES people that the internal tuner of the FT900 could handle the mismatch. Ed |
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Ed wrote in
. 192.196: other two frequencies of operation. There is room. The reason for this silly situation to begin with is just the misconception held by our ARES people that the internal tuner of the FT900 could handle the mismatch. Ed, I worked up some numbers for you at 3.98MHz, and the numbers demonstrated that the main cause of system inefficiency was coax loss (and not the ATU). If you are focussed on VSWR and bandwidth as you seem, you need a radiating dummy load. They have certain merit in this application. Google for "TTFD antenna" for an example. Owen |
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Owen Duffy wrote in
: Bushcomm's BBA-100 is designed for the type of application you have Ed: http://www.bushcomm.info/index.php?a...od&productId=3 |
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Ed, I worked up some numbers for you at 3.98MHz, and the numbers demonstrated that the main cause of system inefficiency was coax loss (and not the ATU). If you are focussed on VSWR and bandwidth as you seem, you need a radiating dummy load. They have certain merit in this application. Google for "TTFD antenna" for an example. As I indicated earlier, I am not communicating effectively with you! Sorry if I have not been making myself clear, Owen. I am not "focused" on those issues you mentioned, especially the ATU. I am aware of its limitations and proper uses. I only wanted the data to help "prod" the people in charge of the need to change the antenna system. SWR is a figure that they recognize. The Bushcomm looks interesting, but probably a little bit more wire than we want to hang on this public building. The ARES group ONLY operaties on three frequencies/bands. It is my opinion that we can very easily add traps to the 75 foot halfwave antenna and shorten it at the same time, giving us a resonant antenna with close to 50 ohm match on two of the three operating frequencies. The third operating frequency can be taken care of with another perpendicular dipole off the feedpoint. We easily have end mounting points for both dipoles. That should give us a much improved system for very little work. Thanks, Owen. Ed K7AAT |
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Ed wrote in
. 192.196: The Bushcomm looks interesting, but probably a little bit more wire than we want to hang on this public building. The ARES group ONLY The 'S' model is 66' overall, a little shorter than your current antenna. These antennas are well suited to an application where the operator or not a radio tech, or unfamiliar with the station, frequency agility is needed, particularly rapid change in frequency with confidence that the system is setup and ready to go, and where comms technology is second to the job to be done. No doubt there are similar things made in the USA, and I think Bushcomm stuff is sold there (see Array Solutions). BTW, someone suggested a vertical. Vertical's have very low gain at high angles, so relatively unsuited to NVIS. Owen |
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On 29 Jul 2007 22:25:04 GMT, Ed
wrote: I only wanted the data to help "prod" the people in charge of the need to change the antenna system. SWR is a figure that they recognize. Hi Ed, After having heard the third chorus of: I am not communicating effectively with you! it is time to analyze why. First and foremost, the correspondents here are focused on technical solutions. You are seeking a political answer. The two do not generally reside on the same page. Technical correspondents here would reasonably expect that an association composed of amateur radio operators would appreciate a technical analysis to solve a technical problem. Owen and others have provided that, and several have been specific to exactly the numbers you have asked for. By and large, those numbers don't seem compelling if your audience fails to appreciate the limitations of the internal tuner's abilities. SWR is not in and of itself loss. It does not always represent a quid pro quo for efficiency. Your reliance on these numbers to sway a group's rather myopic view of the problem gives all the appearance of a magic beans solution suited for yokels. Returning to the work we do here, there remain a number of missing details to help us provide solutions, not political answers. I've noted a number of correspondents have presumed this metal roof of yours stretching out in an infinite plain. Clearly no such roof exists. However, I have seen no further amplification of the details by you. At a minimum, and as a gesture of good will towards those you ask favors of, it would be useful to know the size of that roof, if it is flat (or reasonably so); how high it is, and how high the antenna is above earth (I presume you have only described how high the antenna is above the roof). These parameters have a bearing on the results of the EZNEC computation you ask for. The metal roof has no particular effect in terms of loss, and hence doesn't particularly effect SWR if resonance is not perturbed. However, the proximity of earth does tend to broaden bandwidth and to lower resistance of dipoles (shunt resistance doing the job in both situations). This does impact EZNEC computations you ask for. Insofar as actually remedying the problem, that too has a technical solution easily achieved courtesy of your metal roof. You simply convert the dipole into an Inverted F. Consult: http://www.qsl.net/kb7qhc/antenna/In...%20F/index.htm Its discussion is couched in terms of UHF, but HF is merely a matter of scaling. Also, you don't strictly need the wide, bent radiator as single wires will do the same job to a narrower bandwidth (or you can build a skeleton version as illustrated). The Inverted F is suited to a long low horizontal section. This is something that already exists and is probably driven by the lack of higher supports. The metal roof conforms to the expected metal ground conditions (UHF would make this simple to supply, you already have it handy). The amount of wire is less than you already have, so no new burden in that regard. The need for NVIS operation is clearly supported. The design match obviates the transmission line losses driven through the roof by existing mismatches. 73's Richard Clark, KB7QHC |
Request EZNEC computation
Richard, Everything you said makes sense to me, but as I keep trying to convey, accuracy is not necessarily needed in my particular situation. The very basic issue here is the operation of a halfwave dipole antenna, (resonant at 6.2 MHz), by this orgaznization and expected to operate properly through over 100 feet of RG8 on those three specific frequencies I mentioned. It is their expectation that the internal tuner of the radio will handle any mis-match. Losses in the coax due to SWR do not seem to occurr to them. I was not seeking a "technical solution" to the antenna situation, as I already have that... I would simply convert this antenna to resonance on the operating frequencies. My intent in the original post was simply to get some ball park figure for either antenna feedpoint impedance, or SWR so that I could finish the loss calculations in the coax.,... I certainly didn't intend to stir you guys up with a whole raft of necessary details that would refine the data, but not really change it in a way that would be significant to this particular situation! I think I need to just haul my Wattmeter up to the feedpoint and measure the loss for myself! Access to this roof is somewhat difficult, but not impossible. It Certainly will provide the info I need to convince these guys. As far as the roof goes, it is a FLAT rubber covered metal roof on a large 1 story building.... 14 feet above ground. The roof extends for at least 50 feet beyond the ends of the antenna, and appears to be well grounded at a number of points. I really hope you all do realize I appreciate your responses... while my replies to you may "appear" to be unthankful, that is far from the case. I do read all that you discuss and learn much. Thank you. Ed ( sorry for the top post - thought I'd leave Richard's post below for reference.) Richard Clark wrote in : On 29 Jul 2007 22:25:04 GMT, Ed wrote: I only wanted the data to help "prod" the people in charge of the need to change the antenna system. SWR is a figure that they recognize. Hi Ed, After having heard the third chorus of: I am not communicating effectively with you! it is time to analyze why. First and foremost, the correspondents here are focused on technical solutions. You are seeking a political answer. The two do not generally reside on the same page. Technical correspondents here would reasonably expect that an association composed of amateur radio operators would appreciate a technical analysis to solve a technical problem. Owen and others have provided that, and several have been specific to exactly the numbers you have asked for. By and large, those numbers don't seem compelling if your audience fails to appreciate the limitations of the internal tuner's abilities. SWR is not in and of itself loss. It does not always represent a quid pro quo for efficiency. Your reliance on these numbers to sway a group's rather myopic view of the problem gives all the appearance of a magic beans solution suited for yokels. Returning to the work we do here, there remain a number of missing details to help us provide solutions, not political answers. I've noted a number of correspondents have presumed this metal roof of yours stretching out in an infinite plain. Clearly no such roof exists. However, I have seen no further amplification of the details by you. At a minimum, and as a gesture of good will towards those you ask favors of, it would be useful to know the size of that roof, if it is flat (or reasonably so); how high it is, and how high the antenna is above earth (I presume you have only described how high the antenna is above the roof). These parameters have a bearing on the results of the EZNEC computation you ask for. The metal roof has no particular effect in terms of loss, and hence doesn't particularly effect SWR if resonance is not perturbed. However, the proximity of earth does tend to broaden bandwidth and to lower resistance of dipoles (shunt resistance doing the job in both situations). This does impact EZNEC computations you ask for. Insofar as actually remedying the problem, that too has a technical solution easily achieved courtesy of your metal roof. You simply convert the dipole into an Inverted F. Consult: http://www.qsl.net/kb7qhc/antenna/In...%20F/index.htm Its discussion is couched in terms of UHF, but HF is merely a matter of scaling. Also, you don't strictly need the wide, bent radiator as single wires will do the same job to a narrower bandwidth (or you can build a skeleton version as illustrated). The Inverted F is suited to a long low horizontal section. This is something that already exists and is probably driven by the lack of higher supports. The metal roof conforms to the expected metal ground conditions (UHF would make this simple to supply, you already have it handy). The amount of wire is less than you already have, so no new burden in that regard. The need for NVIS operation is clearly supported. The design match obviates the transmission line losses driven through the roof by existing mismatches. 73's Richard Clark, KB7QHC |
Request EZNEC computation
In article . 196,
Ed wrote: As far as the roof goes, it is a FLAT rubber covered metal roof on a large 1 story building.... 14 feet above ground. The roof extends for at least 50 feet beyond the ends of the antenna, and appears to be well grounded at a number of points. Ed, I hope that once you figure out a more efficient matching system (traps, or feedpoint coils, or whatever) and get your TX power up to where you want it to be, you don't regret having succeeded :-) Our ARES/RACES group has its shack located in our city's police-and- fire admin building, which also hosts the city's 911 dispatch center. We have a 40/80-meter trap dipole, mounted perhaps 20' above the roof, running the length of the building. We haven't had any matching problems with it. What we *have* had, is a severe problem with RF incursion into the building's phone system, including the 911 lines. At first we thought it might be due to RF on the outside of the coax feedline (which runs near the phone wiring) or RF leakage from the shack into the phone wiring in the closet next-door. Feedline chokes didn't help, unfortunately. In the end, we ran a test in which we opened up the feedline near the base of the central antenna tower (disconnecting the antenna from the shack and from all of the in-building coax), and transmitted through the antenna using a portable HF radio on battery power. The phone lines still picked up the signal. Grounding or "floating" the HF radio made no difference at all. The problem is apparently due to RF near-field pickup by the phone wiring. Unfortunately, the city comms folks weren't interested in making the large effort (and handout of cash) needed to actually diagnose the deficiencies in their in-house phone system, and install filters and ferrites on the lines to choke off the RF pickup. Our only practical solution was to limit our TX power to below the point which causes RF incursion (varies by band... 20 watts usually seems to be safe). I can't really blame the city folks, as our HF setup is used only infrequently and is about a sixth-level backup to the city's other communication systems. If we were operating independently, out of a non-city-owned building using non-city equipment, we could make the case that the incursion is their problem, not ours (according to the FCC), but since they own the building and the gear we can't do that. If you're lucky, your increased TX power won't cause you any such problems... but you might want to consider the implications of having this sort of incursion problem. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
Request EZNEC computation
Richard Clark wrote in
: On 29 Jul 2007 22:25:04 GMT, Ed wrote: I only wanted the data to help "prod" the people in charge of the need to change the antenna system. SWR is a figure that they recognize. Hi Ed, After having heard the third chorus of: I am not communicating effectively with you! it is time to analyze why. Hi Richard, This all seems much ado about nothing since it emerges that the real objective is to inform a committe in VSWR speak that the VSWR at the antenna side of the radio's ATU is more than the specified 3:1 capability. It seems to me that could be done very easily by measurement, and all the uncertainty of models that resemble to some greater or lesser extent is not only unnecessary, but introduces significant error due to the assumptions used to simplify the model. It seems that the reduction of the modelled feed point impedance of a centre fed dipole over an infinite metal plane using an efficient and effective balun to a single un-qualified metric, the holy VSWR, is a bit of a fraud. There is a certain element of "it doesn't matter if it is accurate or valid, so long as it is compelling". Owen |
Request EZNEC computation
On 30 Jul 2007 18:50:13 GMT, Ed
wrote: I think I need to just haul my Wattmeter up to the feedpoint and measure the loss for myself! Hi Ed, That won't fully demonstrate the magnitude of the problem either. The Wattmeter will only be calibrated into a matched load and if you provide the matched load, then you won't suffer the Mismatch Loss (where the drama of this soap opera resides). You will then be back into a technical discussion complete with loss multipliers or transmission line mechanics that will act as a narcotic to your audience. If you supply the mismatch then you are going to have to juggle the forward and reverse powers (and explain why forward isn't as high coming out as it is going in the line). The result is the appearance of a game of three card Monte . You may as well treat them like full-fledged technicians and lay out the entire problem from the beginning with a technical solution at the end. 73's Richard Clark, KB7QHC |
Request EZNEC computation
I think Richard is unnecessarily complicating the matter. Assuming you
have a directional wattmeter, you determine the power at the line input by subtracting the "reverse power" from the "forward power" at that point. The power at the line output is determined the same way. 10 times the logarithm of the ratio of the input power to output power is the line loss in dB. Use or even knowledge of mismatch loss isn't necessary. In fact, attempting to apply it to the problem at hand will very likely lead to incorrect conclusions. Roy Lewallen, W7EL Richard Clark wrote: On 30 Jul 2007 18:50:13 GMT, Ed wrote: I think I need to just haul my Wattmeter up to the feedpoint and measure the loss for myself! Hi Ed, That won't fully demonstrate the magnitude of the problem either. The Wattmeter will only be calibrated into a matched load and if you provide the matched load, then you won't suffer the Mismatch Loss (where the drama of this soap opera resides). You will then be back into a technical discussion complete with loss multipliers or transmission line mechanics that will act as a narcotic to your audience. If you supply the mismatch then you are going to have to juggle the forward and reverse powers (and explain why forward isn't as high coming out as it is going in the line). The result is the appearance of a game of three card Monte . You may as well treat them like full-fledged technicians and lay out the entire problem from the beginning with a technical solution at the end. 73's Richard Clark, KB7QHC |
Request EZNEC computation
On Mon, 30 Jul 2007 18:51:07 -0700, Roy Lewallen
wrote: Use or even knowledge of mismatch loss isn't necessary. In fact, attempting to apply it to the problem at hand will very likely lead to incorrect conclusions. Hi Roy, Well, I suppose offering a palliative often suits both sides of the podium. 73's Richard Clark, KB7QHC |
Request EZNEC computation
Owen Duffy wrote: BTW, someone suggested a vertical. Vertical's have very low gain at high angles, so relatively unsuited to NVIS. Owen This would explain why the vertical would work into GA,Fl, VA, from NC but not into a station in SC less than 100 miles away. Jimmie |
Request EZNEC computation
JIMMIE wrote:
Owen Duffy wrote: BTW, someone suggested a vertical. Vertical's have very low gain at high angles, so relatively unsuited to NVIS. Owen This would explain why the vertical would work into GA,Fl, VA, from NC but not into a station in SC less than 100 miles away. On what band? That would be pretty normal on 20 meters. - 73 de Mike KB3EIA - |
Request EZNEC computation
"Michael Coslo" wrote in message ... JIMMIE wrote: Owen Duffy wrote: BTW, someone suggested a vertical. Vertical's have very low gain at high angles, so relatively unsuited to NVIS. Owen This would explain why the vertical would work into GA,Fl, VA, from NC but not into a station in SC less than 100 miles away. On what band? That would be pretty normal on 20 meters. - 73 de Mike KB3EIA - Sorry its been years since its been in use. I was on a gov shortwave freq and I dont remember. Normally we answered a net that was being controlled from Atlanta. I had always assumed we couldnt here the station in SC because of its antenna orientation but after I had a chance to visit the station I realized there pattern should have been pretty much omni=directional. Jimmie |
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