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PHASING HARNESS FOR 2 BAY CIRCULAR POLARIZED ANTENNA?????
Good Evening,
I'd like some advice on building the phasing harness required for the following antenna: http://members.tripod.com/~AMN92/cp_ant.htm It certainly would use 75 ohm RG-6, and i believe it would be 1 full wavelength, fed in the center, with the two bays on either end (1/2 wave to each antenna). You would also need 4 turns of 3-4" diameter coils in the RG-6 at the feedpoints, to serve as baluns, which decouples antenna currents down the braid. So in theory, this should be two 100 ohm impedances in parallel, for a total of 50 ohms, which can them be fed with garden variety RG-8 or RG-213. Any advice or input is greatly appreciated. Slick |
David
Idid describe an antenna that will do all the things that the new patent claims and it would not surprise me i9n the least if it turns out to be of the same design. I am not a guru so I am not a prisoner to past accomplishments and am able to focus purely on max radiation per unit length. Tome the loop is a prime example in that it produces not only more gain per unit length but also has an additive vector radiation in a similar way centrifugal forces add a vector even tho the electrons have a constant speed. It does not take a rocket scientist to visualise that an antenna with a figure 8 circuit would change the radiation field.( clockwise + counterclockwise radiation ) The other point to remember is that true coupling can increase the current in an element while reducing its voltage and it is current which we hamms are interested in. Ofcourse if your radiator melts then it shows good coupling and the element has to be larger in diameter. Now I know that what I say is basic but it apears that hams are locked into a rut when thinking about new design and if the new patented antenna melted then I suspect he under estimated the increased current created by coupling. Now because feed impedance is so important to the ham it will be important to have a unconnected vertical placed inside the coil perimeters which allows by coupling the ability to meet impedance requirementst Now David I know hams do not like to pursue the idea of maxcimum radiation per unit length but if you have a computor program that can use variable dimensions to meet specs then I have provided enough info for those that are interested but I suspect that most are inately lazy and prefer to0 await the unveiling at the end of this month. By the way in the description of the multi loop I made no reference to use of clockwise and counter clockwise travel a statistic that is very important as is the center pole. Am I sold on the idea of a different aproach to antennas, you bet I am as this aproach provides a flattening of the take off angle on the horizon at the expense of higher angle radiation which for cell type frequency is wasted energy Cheers and beers Art "Dave Platt" wrote in message ... In article uUdSc.237968$%_6.185742@attbi_s01, wrote: Yes, that does seem a reasonable aproach if you want to use the antenna that you describe. It does give me thought about the new revolutionary antenna and the impact it will have on radio. On reading your comments, Art, my feeling is that you've swallowed the glowing claims in the original press releases hook, line, and sinker, and as a result you are greatly over-estimating the likely impact of this "revolutionary" technology. snip |
Dqavid just to clear some things up with reference to "gain"
There is no energy created or destroyed here, all I am doing with this design is "flattening" the 3D pattern that you see when you have a vertical whip style antenna. Thus the radiation can be "pushed" to augment the usefull existing gain on the horizon whether it remains purely omni-directional or slightly directional.. I suppose that this can be seen as the various loops in the column have a redirectiive effect on high angle radiation emanating from the lower coils, I'm off to bed Cheers and beers Art " wrote in message news:XXfSc.241290$IQ4.147543@attbi_s02... David Idid describe an antenna that will do all the things that the new patent claims and it would not surprise me i9n the least if it turns out to be of the same snip968$%_6.185742@attbi_s01, wrote: Yes, that does seem a reasonable aproach if you want to use the antenna that you describe. It does give me thought about the new revolutionary antenna and the impact it will have on radio. On reading your comments, Art, my feeling is that you've swallowed the glowing claims in the original press releases hook, line, and sinker, and as a result you are greatly over-estimating the likely impact of this "revolutionary" technology. snip |
David I feel sure that the new design is what I have described and we will
know in a few weeks. But do yourself a favour and roughly model the following which is not a final design. Create a closed loop and divide it into 20 segments and then cut it in half to make the program shorter. You then place these haf loops at varying heights with a matter of inches between them. When they are connected together starting from the ground the resulting patter will be a cloud warmer with high gain. This does not take long to do and if you scan frequencies you qill find the pattern change interesting. If you then become interest then connect the half loops instead of 1,2,3 vertically move the fthird loop to position one and so forth plus connect them so that you have both clockwise and counter clockwise radiation, you will then see a flattening of radiation to the horizon. Enough said, I am not interested in a verbal snotty battle which will surely happen now that Richard has entered the thread. If the new patent does not follow the scheme of things outlined then I will supply to you a model and a picture of an actual antenna. So let us await the release of details of this new' revolutionary' antenna which I am sure that Chip will supply. Cheers Art "Dave Platt" wrote in message ... In article XXfSc.241290$IQ4.147543@attbi_s02, wrote: David Idid describe an antenna that will do all the things that the new patent claims and it would not surprise me i9n the least if it turns out to be of the same design. Art, You've _talked_ about an antenna design which you claim does these things. You have never, as far as I have seen, "described" it clearly enough for anyone to actually draw a picture of it that you were willing to agree was accurate, despite having been asked for this detail a number of times. Until you actually demonstrate it (so that it can be subjected to unbiased tests), or succeed in describing it clearly and accurately enough that someone else can reproduce it and test the reproduction. Until you do one or the other, I'm afraid that I have no confidence that your design does what you say it does, or works the way you say that it works. Sorry, guy. On this, I'm "from Missouri"... I say "show me!" Now David I know hams do not like to pursue the idea of maxcimum radiation per unit length but if you have a computor program that can use variable dimensions to meet specs then I have provided enough info for those that are interested I disagree, Art. Every discussion or description I've seen you write has been fuzzy, contradictory, unclear, etc. but I suspect that most are inately lazy and prefer to0 await the unveiling at the end of this month. I don't buy it, Art. I've tried pretty hard to make sense of what you've written, and have gotten nowhere at all. By the way in the description of the multi loop I made no reference to use of clockwise and counter clockwise travel a statistic that is very important as is the center pole. So, you've "provided enough info for those that are interested" but have made no reference to two factors that are "very important"? Am I sold on the idea of a different aproach to antennas, you bet I am as this aproach provides a flattening of the take off angle on the horizon at the expense of higher angle radiation which for cell type frequency is wasted energy Fine. Show us. -- Dave Platt AE6EO Hosting the 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! |
On Wed, 11 Aug 2004 12:32:28 GMT, "
wrote: I am not interested in a verbal snotty battle which will surely happen now that Richard has entered the thread. Hi Art, I merely pointed out your antenna shows 17dB loss compared to the standard antenna. Sorry if that rains on your parade. 73's Richard Clark, KB7QHC |
Dr. Slick wrote:
"I`d like some advice on building the phasing harness required for the following antenna," The antenna described has one-wavelength spacing between antenna elements. As velocity in the coax is only about 2/3 that in free space, one-wavelength of phase delay in cable won`t stretch between the antenna bays as shown. Solution is to add another wavelength of phase shift to the interconnection cable or to simply use a pair of identical cables of almost any length to parallel the bays at a driving point. Any desired phase delay can be added to one or the other of the cables to shape the pattern. Best regards, Richard Harrison, KB5WZI |
Jerry Martes wrote:
"Is it clear to you what this array of two CP antennas will be used for?" I hope not fo promote terrorism, but I`m not judging the merits of application nor advising anyone to break the law. I only advised how to feed the antennas in phase as need be for radiation to be aiding around the 360-degrees. As for circular polarization, I would refer any reader to "Antennas" by J.D. Kraus. Best regards, Richard Harrison, KB5WZI |
"Richard Harrison" wrote in message ... Jerry Martes wrote: "Is it clear to you what this array of two CP antennas will be used for?" I hope not fo promote terrorism, but I`m not judging the merits of application nor advising anyone to break the law. I only advised how to feed the antennas in phase as need be for radiation to be aiding around the 360-degrees. As for circular polarization, I would refer any reader to "Antennas" by J.D. Kraus. Best regards, Richard Harrison, KB5WZI Richard I've been amusing myself by trying to develop an antenna that "receives" RHCP polar orbiting satellites. The coverage I trying to obtain would suffer from the stacking refered to in this thread. And, it seems so obvious that feeding the two antennas in phase would produce maximum gain on the horizon, that I thought there was some special requirement for this antenna's pattern. In addition, I have assumed that the impedance mismatch for receiving antennas can be allowed to be much greater than for transmitting. This statement is directed to the fact that transmission line loss is increased by VSWR, and the reciever isnt restricted to being the same impedance of the transmission line. Jerry |
"Jerry Martes" wrote in message ... "Richard Harrison" wrote in message ... Jerry Martes wrote: "Is it clear to you what this array of two CP antennas will be used for?" snipJerry I am presently working with circular polarised antennas for transmit, These antennas are omnidirectional but does produce a choice of TOA What exactly is the pattern that you are looking for? I do not have any knoweledge of satellite work needs Regards Art snip |
"Jim - NN7K" wrote in message .. . Jerry, just because you are wanting to recieve, instead of transmit, doesn't mean that the V.S.W.R. mismatch isn't there-- nor its effect on the subsequent coax loss's being multiplied because of the mismatch! Remember- the antenna , coax, and the mismatch are all bi-directional ! The additional loss in the feedline, because of a high swr, is also present in the recieve direction! If you have copy of ARRL handbook, or other sources, look under "transmission lines" - these have a chart of coax loss, and also the swr effect on that coax loss. For example, coax with 4 dB loss, at swr of 1.5 /1 would raise the line loss by an extra .1 dB, while an swr of 3:1 would rais it by an extra dB, at 5:1, would add about 2.2dB loss! and- keep in mind this is BI-DIRECTIONAL loss ! Another consideration is that the noise figure is optimized for 50 ohm, and this (because of a mismatch), will also be negatively affected! Best to find good impedence match for all considerations-- Jim NN7K Jerry Martes wrote: SNIP In addition, I have assumed that the impedance mismatch for receiving antennas can be allowed to be much greater than for transmitting. This statement is directed to the fact that transmission line loss is increased by VSWR, and the reciever isnt restricted to being the same impedance of the transmission line. Jerry Jim I sure wouldnt argue with your findings. I guess this identifies my sloppy engineering. I didnt think 4 Db was a tolerable feed line loss under any but the most dire circumstances. I wonder what a ~3:1 VSWR would do to add attenuation to feed line loss os, say 1 1/2 db. To further identify my sloppyness, I cant distinguish zero db from 0.1 db. It is even difficult for me to determine with any confidance the difference between Zero db and 1 db. What I'm getting at is -- I still contend that a receiver can tolerate some mismatches that arent tolerable in a sophisticated transmitting station. Richard Fry has pointed out how critical small mismatches are in FM transmitters. This original post referanced a FM transmitting antenna. It would be reasonable (to me) to assume this referanced antenna could have some rigid constraints if it was to be used for transmitting stereo FM. But, as a omniazimuth FM receiver antenna, the phasing might be fairly straightforward. Jerry |
Jim - NN7K wrote:
Jerry, just because you are wanting to recieve, instead of transmit, doesn't mean that the V.S.W.R. mismatch isn't there-- nor its effect on the subsequent coax loss's being multiplied because of the mismatch! Remember- the antenna , coax, and the mismatch are all bi-directional ! The additional loss in the feedline, because of a high swr, is also present in the recieve direction!. . . That's not quite correct. The SWR on the line is determined solely by the transmission line impedance and the load impedance. When transmitting, the antenna is the load. When receiving, the receiver is the load. If the receiver and antenna have different impedances, the line SWR will be different when transmitting than when receiving. Fiddling with the antenna or the antenna/feedline match won't have any effect at all on the line SWR. It will, however, have an effect on the strength of signal arriving at the receiver. Roy Lewallen, W7EL |
Jerry-- at your stated 1.5 dB loss coax, with a 3/1 vswr, the extra loss
would be just under .7dB, but remember that the reflected mismatch, also will affect your recievers front end noise (factot/figure), most likely in a negative way (no way to calculate the impedence presented to your rcvr front end ) also, remember that fm sensitivity (because of its nature) is not near as sensitive as cw/ssb (typical 2 meter rcvr can detect a signal below .01 microvolt cw - fm rcvr does good around .1 microvolt- 20 dB more sensitive! Something to think about-- jim NN7K Jerry Martes wrote: "Jim - NN7K" wrote in message .. . Jerry, just because you are wanting to recieve, instead of transmit, doesn't mean that the V.S.W.R. mismatch isn't there-- nor its effect on the subsequent coax loss's being multiplied because of the mismatch! Remember- the antenna , coax, and the mismatch are all bi-directional ! The additional loss in the feedline, because of a high swr, is also present in the recieve direction! If you have copy of ARRL handbook, or other sources, look under "transmission lines" - these have a chart of coax loss, and also the swr effect on that coax loss. For example, coax with 4 dB loss, at swr of 1.5 /1 would raise the line loss by an extra .1 dB, while an swr of 3:1 would rais it by an extra dB, at 5:1, would add about 2.2dB loss! and- keep in mind this is BI-DIRECTIONAL loss ! Another consideration is that the noise figure is optimized for 50 ohm, and this (because of a mismatch), will also be negatively affected! Best to find good impedence match for all considerations-- Jim NN7K Jerry Martes wrote: SNIP In addition, I have assumed that the impedance mismatch for receiving antennas can be allowed to be much greater than for transmitting. This statement is directed to the fact that transmission line loss is increased by VSWR, and the reciever isnt restricted to being the same impedance of the transmission line. Jerry Jim I sure wouldnt argue with your findings. I guess this identifies my sloppy engineering. I didnt think 4 Db was a tolerable feed line loss under any but the most dire circumstances. I wonder what a ~3:1 VSWR would do to add attenuation to feed line loss os, say 1 1/2 db. To further identify my sloppyness, I cant distinguish zero db from 0.1 db. It is even difficult for me to determine with any confidance the difference between Zero db and 1 db. What I'm getting at is -- I still contend that a receiver can tolerate some mismatches that arent tolerable in a sophisticated transmitting station. Richard Fry has pointed out how critical small mismatches are in FM transmitters. This original post referanced a FM transmitting antenna. It would be reasonable (to me) to assume this referanced antenna could have some rigid constraints if it was to be used for transmitting stereo FM. But, as a omniazimuth FM receiver antenna, the phasing might be fairly straightforward. Jerry |
Roy Lewallen wrote:
. . . Fiddling with the antenna or the antenna/feedline match won't have any effect at all on the line SWR. It will, however, have an effect on the strength of signal arriving at the receiver. Oops. I meant, WHEN RECEIVING, fiddling with the antenna or the antenna/feedline match won't have any effect at all on the line SWR. It will, of course, change the line SWR when transmitting. Roy Lewallen, W7EL |
.... and, though it may not have importance at HF, any loss in the
transmission line (unless it is very cold) will add noise at the same time that the signal is attenuated. Once upon a time, serious consideration was given to using liquid air (might have been Nitrogen) to cool a rather short piece of waveguide (between feed and first receiver stage) in a really high frequency system that was pointing out into space. Such cooling would not have changed the attenuation a noticeable amount, but it would have improved the SNR. ... and further: please do not think of using the maximum-power-transfer theorem to maximize SNR. The first stage needs to see a (small) mismatch, which might not be seen by the transmission line. With a low directivity antenna in the absence of close man-made noise sources, the above issues are usually of no importance at HF and below because the SNR is almost always (in a reasonably well designed system) determined beyond the antenna. [Obviously, a highly directive antenna system could dramatically affect SNR] 73 Mac N8TT -- J. Mc Laughlin - Michigan USA Home: |
"Roy Lewallen" wrote
Oops. I meant, WHEN RECEIVING, fiddling with the antenna or the antenna/feedline match won't have any effect at all on the line SWR. It will, of course, change the line SWR when transmitting. _____________ However that doesn't mean that it's unimportant to match the feedpoint impedance of a rx antenna to the feedline connected there. A rx antenna with a mismatched feedline will not deliver to the rx end of the feedline the maximum possible energy available from the fields in which that antenna is immersed. Whatever received power that is reflected by a mismatch at the rx antenna feedpoint is re-radiated (less losses). RF |
Richard Fry wrote:
"Roy Lewallen" wrote Oops. I meant, WHEN RECEIVING, fiddling with the antenna or the antenna/feedline match won't have any effect at all on the line SWR. It will, of course, change the line SWR when transmitting. _____________ However that doesn't mean that it's unimportant to match the feedpoint impedance of a rx antenna to the feedline connected there. A rx antenna with a mismatched feedline will not deliver to the rx end of the feedline the maximum possible energy available from the fields in which that antenna is immersed. Whatever received power that is reflected by a mismatch at the rx antenna feedpoint is re-radiated (less losses). What both Roy and Richard say is correct in principle, but may be missing the point about what a receiver needs. What's usually important is to present the RX input with the *source* impedance it was designed for. (Most often this is 50 ohms, and let's also assume 50-ohm line for the rest of this discussion.) Likewise the transmitter needs to be presented with a 50-ohm load impedance, so those two requirements coincide. In order to achieve a 50-ohm load impedance for the transmitter, and a 50-ohm source impedance for the receiver, the antenna itself must be matched to 50 ohms - so that's your design aim. Now when Richard says: Whatever received power that is reflected by a mismatch at the rx antenna feedpoint is re-radiated (less losses), that is true in principle, but more important is that if any energy is reflected from the receiver input, that is perfectly OK - that energy was "not wanted" by the receiver. The receiver *does* want a 50-ohm source, but it only takes what it needs from that source. For example, a simple tuned-gate FET amplifier only needs a voltage swing at the input - it doesn't need current as well, so most of the incident power is reflected. That type of situation is very common in receiver design, and completely OK. It is a myth that a receiver input is not optimized unless it presents a 50-ohm load. What it does need is a 50-ohm source impedance. The design details about input reflection coefficient are much more complex, but the underlying principle is simply "The RX input takes whatever it needs from a 50-ohm source, and reflects the rest." RX inputs *can* be designed to present a 50-ohm load impedance, even with FETs, but this requires special design techniques that generally involve feedback. It is usually done when some other device has to be inserted between the feedline and the RX input, eg a filter which requires a 50-ohm load impedance. But that device probably requires a 50-ohm source impedance too, so you still have the same requirement for the antenna to be matched to the feedline. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
"Ian White, G3SEK" wrote
... the underlying principle is simply "The RX input takes whatever it needs from a 50-ohm source, and reflects the rest." ______ The above statement might be read as though there is a threshold voltage limit above which a receiver will not deliver improved performance. A receiver amplifies and detects whatever voltage is present at its input terminals, if just thermal noise. Even if no current flows in the input stage device itself, the wanted signal voltage present there should be as high as possible above the thermal noise voltage in order to maximise SNR. RF |
Richard Fry wrote:
"Ian White, G3SEK" wrote ... the underlying principle is simply "The RX input takes whatever it needs from a 50-ohm source, and reflects the rest." ______ The above statement might be read as though there is a threshold voltage limit above which a receiver will not deliver improved performance. If you truly believe that danger exists, then please don't make it worse by quoting my statement out of the context in which it was made. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Hello again,
Well, surprise, surprise...the thread deviates again! Hehe. If i may add to the discussion: By "reciprocity", a transmit antenna can also be used as a receive antenna, assuming you want the same polar pattern. A mismatch on the receive side will adversely affect the signal to noise ratio, or would increase the noise factor of the system (which is why you always want your low noise amplifiers as close to the receive antenna as possible, usually mast-mounted, to avoid the losses of a long coax). However, at the very least, the mismatch on the receive side will not result in catastrophic destruction of your output transistors, which is what a mismatch on the output of a transmitter can result in. So one mismatch is a bit more serious than the other. Slick |
"Dr. Slick" wrote in message om... Hello again, Well, surprise, surprise...the thread deviates again! Hehe. If i may add to the discussion: By "reciprocity", a transmit antenna can also be used as a receive antenna, assuming you want the same polar pattern. A mismatch on the receive side will adversely affect the signal to noise ratio, or would increase the noise factor of the system (which is why you always want your low noise amplifiers as close to the receive antenna as possible, usually mast-mounted, to avoid the losses of a long coax). However, at the very least, the mismatch on the receive side will not result in catastrophic destruction of your output transistors, which is what a mismatch on the output of a transmitter can result in. So one mismatch is a bit more serious than the other. Slick Slick I'd submit that, in a practical situation, the mismatch of an antenna for receiving can be as high as 2:1 or even 3:1 without degrading the strength of the received signal when the transmission line losses are low. I thought that, if the receiver is tuneable, the actual impedance the transmission line presents to the receiver can be 'accounted for. I thought that, for a given antenna and transmission line, the effects of VSWR are less important that for delivering power by a transmitter. Am I wrong when I consider VSWR to be less important for receivers than for transmitters? Jerry |
Jerry Martes wrote:
Am I wrong when I consider VSWR to be less important for receivers than for transmitters? Depends upon the source of the noise. My 40m vertical couldn't copy stations that gave me an RST of 559. -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Jerry Martes wrote:
I'd submit that, in a practical situation, the mismatch of an antenna for receiving can be as high as 2:1 or even 3:1 without degrading the strength of the received signal when the transmission line losses are low. I thought that, if the receiver is tuneable, the actual impedance the transmission line presents to the receiver can be 'accounted for. I thought that, for a given antenna and transmission line, the effects of VSWR are less important that for delivering power by a transmitter. Am I wrong when I consider VSWR to be less important for receivers than for transmitters? Jerry It sounds like you (and some other posters) might be confusing the transmission line SWR with the impedance seen by the transmitter/receiver, which is often indicated with an SWR meter. The two aren't the same. Consider, for example, a 50 ohm antenna and 50 ohm tx/rx, with a 300 ohm half wavelength transmission line connecting the two. The antenna sees a perfect match (50 ohms) when receiving, and the transmitter sees a perfect match (50 ohms) when transmitting. The transmission line SWR is 6:1 when transmitting and receiving. On the other hand, if the antenna and transmission line are both 300 ohms (+ j0), the line SWR will be 1:1 when transmitting, 6:1 when receiving. And so forth. The effects of impedance mismatch seen by the transmitter when transmitting, the impedance mismatch seen by the antenna when receiving, and the transmission line SWR are three separate issues. Each has its own effect on system performance, and each needs to be treated separately. The importance of one or the other depends on the individual situation. Roy Lewallen, W7EL |
"Roy Lewallen" wrote in message ... Jerry Martes wrote: I'd submit that, in a practical situation, the mismatch of an antenna for receiving can be as high as 2:1 or even 3:1 without degrading the strength of the received signal when the transmission line losses are low. I thought that, if the receiver is tuneable, the actual impedance the transmission line presents to the receiver can be 'accounted for. I thought that, for a given antenna and transmission line, the effects of VSWR are less important that for delivering power by a transmitter. Am I wrong when I consider VSWR to be less important for receivers than for transmitters? Jerry It sounds like you (and some other posters) might be confusing the transmission line SWR with the impedance seen by the transmitter/receiver, which is often indicated with an SWR meter. The two aren't the same. Consider, for example, a 50 ohm antenna and 50 ohm tx/rx, with a 300 ohm half wavelength transmission line connecting the two. The antenna sees a perfect match (50 ohms) when receiving, and the transmitter sees a perfect match (50 ohms) when transmitting. The transmission line SWR is 6:1 when transmitting and receiving. On the other hand, if the antenna and transmission line are both 300 ohms (+ j0), the line SWR will be 1:1 when transmitting, 6:1 when receiving. And so forth. The effects of impedance mismatch seen by the transmitter when transmitting, the impedance mismatch seen by the antenna when receiving, and the transmission line SWR are three separate issues. Each has its own effect on system performance, and each needs to be treated separately. The importance of one or the other depends on the individual situation. Roy Lewallen, W7EL Roy It might be that I'm wrong with my conclusion that when receiving, the loss of received signal wont be appreciably worsened by VSWRs as high as, say, 3:1, when the line atenuation is under a couple DB. But I dont think I've been confused about the Line to Load Mismatch and VSWR. I recognize that the lowest VSWR and lowest line loss and the lowest noise figure are all important. Since I'm always working with systems that arent perfect, I dont get concerned with low VSWR when working with receivers except when there is need for phasing arrays. It is my understanding that the transmission line loss isnt increased excessively when the line loss is under about 2 DB and the VSWR is as high as 3:1, and that S-meter readings arent measureably degraded when the receiver sees these signals thats not coming in with a 50 ohm internal impedance. It occurred to me that the original poster of this "Phasing Harness" might have some sophisticated need for phase or low mismatch. If the two antennas being harnessed are only to maximize radiation toward the horizon, I'd consider the question a simple one to answer. ie Feed them in phase with any convenient equal lengths of line. If bandwidth is a factor, expect sidelobes to vary with frequency. But, my knowledge and experience is very limited, so any 'redirection' of my thinking is appreciated. Jerry |
On Sun, 15 Aug 2004 23:57:02 GMT, "Jerry Martes"
wrote: It is my understanding that the transmission line loss isnt increased excessively when the line loss is under about 2 DB and the VSWR is as high as 3:1, and that S-meter readings arent measureably degraded when the receiver sees these signals thats not coming in with a 50 ohm internal impedance. Hi Jerry, It seems your question isn't going to be answered except to three decimal places. You are right, no one will notice much difference to mismatches such as you describe. I know that your interest is in satellite plots of weather conditions. You may experience some drop out - snow in the picture. However this would be for marginal signals, and I am sure that the uncorrelated noise would only slightly degrade the contrast or detail. I've played with WEFAX over HF to worse conditions and those pictures came out quite readable. 73's Richard Clark, KB7QHC |
'corse, from this "troublemaker", on VHF, (and granted, the noise figure
of a reciever , or preamp is less compromised by swr, THE FURTHER from the SOURCE(antenna), because of the loss's in that line (great dummy load for 432: 200 foot of rg-58u, don't even terminate it! If a Bird reads ANY swr,meter has a problem (because of the loss)! But, then look at the loss's from the standpoint of a (Scatter, Moonbounce, Long Distance VHF (ect)) Operator, trying for the "HOLY GRAIL" of a BI-DIRECTIONAL 20 + dB gain, noise figure of less than , say 1.3 dB (sky noise), and a KW ,+ Minimum FEED LOSS'S, on 2 meters to hear your own echos. When at THAT point, and keep in mind: 1) that when stacking antennas, the MAXIMUM (maybe you know different) GAIN accomplished on a bay, is 3 dB , for 2 antennas, 6dB, for 4 antennas,ect., 2) that The Reciever front end, Maximized for BEST NOISE FIGURE, is adjusted to the that point, by intentionally MISADJUSTING the front end impedence, to obtain THAT optimum point,at 50 OHMS! and 3) that anything that is misadjusted, to add ANY LOSS's to the system means the difference (because bad stacking distances, mis- fed coax(out of phase), change in the front end impedence of the LNA, ect.) means the difference between sucess,or failure!! Perhaps was wrong on initial assumption that swr was bi-directional, but doesn't negate the original premise that the swr has no effect on recieve-- and, btw, will the stacking actually provide THAT 3dB?? (before, or after the added 3:1 mismatch)?? Yours for comment?? Jim NN7K Richard Clark wrote: On Sun, 15 Aug 2004 23:57:02 GMT, "Jerry Martes" wrote: It is my understanding that the transmission line loss isnt increased excessively when the line loss is under about 2 DB and the VSWR is as high as 3:1, and that S-meter readings arent measureably degraded when the receiver sees these signals thats not coming in with a 50 ohm internal impedance. Hi Jerry, It seems your question isn't going to be answered except to three decimal places. You are right, no one will notice much difference to mismatches such as you describe. I know that your interest is in satellite plots of weather conditions. You may experience some drop out - snow in the picture. However this would be for marginal signals, and I am sure that the uncorrelated noise would only slightly degrade the contrast or detail. I've played with WEFAX over HF to worse conditions and those pictures came out quite readable. 73's Richard Clark, KB7QHC |
Jerry Martes wrote:
It might be that I'm wrong with my conclusion that when receiving, the loss of received signal wont be appreciably worsened by VSWRs as high as, say, 3:1, when the line atenuation is under a couple DB. But I dont think I've been confused about the Line to Load Mismatch and VSWR. As long as we make sure the readers understand we're talking about the actual SWR on the transmission line, that's true for both transmitting and receiving. I recognize that the lowest VSWR and lowest line loss and the lowest noise figure are all important. If the VSWR and (matched) line loss are roughly within the bounds you mentioned, the transmission line VSWR isn't really important, since the extra loss it causes is generally negligible. However, as I somewhat indirectly mentioned earlier, the match between the antenna and its load determine the amount of signal that gets to the receiver. And as Ian has mentioned, the receiver noise figure might be best when the receiver is mismatched to the source impedance it sees. Further qualifying the matter is the fact that at HF, losing signal when receiving due to mismatch, line loss or SWR doesn't impact the signal/noise ratio unless the attenuation is very great, since the dominant atmospheric noise is attenuated by the same factor. I do realize, though, that the original posting was directed toward VHF or above, where those things do matter. Since I'm always working with systems that arent perfect, I dont get concerned with low VSWR when working with receivers except when there is need for phasing arrays. It is my understanding that the transmission line loss isnt increased excessively when the line loss is under about 2 DB and the VSWR is as high as 3:1, and that S-meter readings arent measureably degraded when the receiver sees these signals thats not coming in with a 50 ohm internal impedance. It occurred to me that the original poster of this "Phasing Harness" might have some sophisticated need for phase or low mismatch. If the two antennas being harnessed are only to maximize radiation toward the horizon, I'd consider the question a simple one to answer. ie Feed them in phase with any convenient equal lengths of line. If bandwidth is a factor, expect sidelobes to vary with frequency. But, my knowledge and experience is very limited, so any 'redirection' of my thinking is appreciated. Your thinking looks fine to me. Roy Lewallen, W7EL |
On Mon, 16 Aug 2004 02:09:49 GMT, Jim - NN7K
wrote: Perhaps was wrong on initial assumption that swr was bi-directional, but doesn't negate the original premise that the swr has no effect on recieve-- and, btw, will the stacking actually provide THAT 3dB?? (before, or after the added 3:1 mismatch)?? Yours for comment?? Jim NN7K Hi Jim, My experience in the very short wavelengths is confined to RADAR. I have not pursued satellite nor EME. RADAR comes with its own compensations in that if you have one, you can afford to do it right the first time (I pine for the day when the FCC allows Amateur RADAR operation). As for Transmit/Receive, they are so intimately wed, that it is sometimes difficult to separate them and judge their needs on their own merits. A Receiver doesn't need to have an input Z of 50 Ohms, but given that the Receiver of a Transceiver shares the same path ways of the transmitter, it is foolish to go a different direction. Why would you put a 300 Ohm first RF stage after a filter designed for 50 Ohms? A 6:1 SWR from the get-go is simply stupid when you can do it right with so little effort. I've seen some discussion that it doesn't matter because front ends only take voltage and need no current. This is a 0Hz analysis and at 10MHz is thoroughly dead in the water. Stray capacitance negates any claims to an input being Hi-Z and the whole point of low Z inputs is to swamp nature's capacity to send your signal straight to ground before it sees that amplifier. For the mild SWRs such as described by Jerry, most receivers have a lot of head room (capacity) to amplify what makes its way in. The only down-side is degrading S+N/N ratio for very small signals where this capacity fails to make up for information loss. 73's Richard Clark, KB7QHC |
Richard Clark wrote:
I've seen some discussion that it doesn't matter because front ends only take voltage and need no current. This is a 0Hz analysis and at 10MHz is thoroughly dead in the water. Once again, what I said has been thoroughly misquoted. Stray capacitance negates any claims to an input being Hi-Z and the whole point of low Z inputs is to swamp nature's capacity to send your signal straight to ground before it sees that amplifier. Now that analysis really *is* dead in the water! My simplification to "the amplifier takes what it needs from a 50-ohm source" is just that - a simplification. But it is based on actually knowing something about the subject. If you wish to discuss input network design for FET RF stages in terms of Smith-chart circles of constant gain and noise figure, and the device manufacturer's quoted data for gamma-opt, then I'm willing and able. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
On Mon, 16 Aug 2004 23:06:43 +0100, "Ian White, G3SEK"
wrote: Richard Clark wrote: I've seen some discussion that it doesn't matter because front ends only take voltage and need no current. This is a 0Hz analysis and at 10MHz is thoroughly dead in the water. Once again, what I said has been thoroughly misquoted. Hi Ian, If it was you that said it, otherwise you are misquoting me. My simplification to "the amplifier takes what it needs from a 50-ohm source" is just that - a simplification. Ah yes, you are misquoting me. 73's Richard Clark, KB7QHC |
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