| Page 1 of 2 | 1 | 2 |
|
|
short antennae
I have stated my view that short antenna are poor radiatiors based
upon a number of physics and electromagnetics text books in my possession. Some have chosen to disagree, and repeatedly so, but their case is weakened because they produce no evidence and seem to want to rely on aggressive and abusive remarks in order to win the day. (I have no interest in winning the day, only in knowing the truth, which for me at the moment is that short antennae are poor radiators, and I have no need to resort to abuse to state that viewpoint) One is reminded of the religions of the world, firstly Christianity and latterly Islam that rely on violence to put their message across, but that reliance is surely an indication that their messages are false, for, if true, the message would stand up for itself. If you wanted everybody to profess falsehood, such as saying that 1 + 1 = 3, or that short antennae are as good radiatiors as long antennae, then you could only hope to get that message across by the propaganda of abuse, which is regrettably what we are seeing in this NG, by a number of people who, although they might now be wearing long trousers, have minds that are still wearing nappies / diapers. |
short antennae
gareth wrote:
I have stated my view that short antenna are poor radiatiors based upon a number of physics and electromagnetics text books in my possession. Nope, you have made a bunch of arm waving claimed based on the lack of understanding of the difference between feed systems and antennas as well as impedance matching. Some have chosen to disagree, and repeatedly so, but their case is weakened because they produce no evidence and seem to want to rely on aggressive and abusive remarks in order to win the day. (I have no interest in winning the day, only in knowing the truth, which for me at the moment is that short antennae are poor radiators, and I have no need to resort to abuse to state that viewpoint) On the contrary you have been repeatedly shown the results of antenna analysis programs as well as simple Ohms Law which show you just post nonsense. You have yet to respond to such with anything but arm waving and hang wringing over "personal attacks". snip remaining arm waving and babble -- Jim Pennino |
short antennae
|
short antennae
"Mike Tomlinson" wrote in message
... gareth wrote: I have stated my view that short antenna are poor radiatiors based upon a number of physics and electromagnetics text books in my possession. Nope, you have made a bunch of arm waving claimed based on the lack of understanding of the difference between feed systems and antennas as well as impedance matching. You even asked him for a citation from one of those books, which was conspicuously ignored. If you are querying as to why I ignore repeated abusive outbursts from individuals who behave more as a student in the kindergarten school playground than in the style to be expected from grown-ups in an international discussion forum, then I think that you'll find the answer to the question in the question itself. |
short antennae
gareth wrote:
"Mike Tomlinson" wrote in message ... gareth wrote: I have stated my view that short antenna are poor radiatiors based upon a number of physics and electromagnetics text books in my possession. Nope, you have made a bunch of arm waving claimed based on the lack of understanding of the difference between feed systems and antennas as well as impedance matching. You even asked him for a citation from one of those books, which was conspicuously ignored. If you are querying as to why I ignore repeated abusive outbursts from individuals who behave more as a student in the kindergarten school playground than in the style to be expected from grown-ups in an international discussion forum, then I think that you'll find the answer to the question in the question itself. Nope, the question was why is it you never respond to any technical postings. Though it would appear that you concider any technical posting that shows you post arm waving nonsene an abusive outburst. If your ego can't stand you being corrected on obvious fallacies, don't post them. -- Jim Pennino |
short antennae
On 28/10/14 19:04, gareth wrote:
I have stated my view that short antenna are poor radiatiors based upon a number of physics and electromagnetics text books in my possession. Some have chosen to disagree, and repeatedly so, but their case is weakened because they produce no evidence and seem to want to rely on aggressive and abusive remarks in order to win the day. (I have no interest in winning the day, only in knowing the truth, which for me at the moment is that short antennae are poor radiators, and I have no need to resort to abuse to state that viewpoint) One is reminded of the religions of the world, firstly Christianity and latterly Islam that rely on violence to put their message across, but that reliance is surely an indication that their messages are false, for, if true, the message would stand up for itself. If you wanted everybody to profess falsehood, such as saying that 1 + 1 = 3, or that short antennae are as good radiatiors as long antennae, then you could only hope to get that message across by the propaganda of abuse, which is regrettably what we are seeing in this NG, by a number of people who, although they might now be wearing long trousers, have minds that are still wearing nappies / diapers. Maybe I've missed something here, but I would expect a half-wave dipole to out-perform a full-wave dipole at the same frequency, despite being half the size. -- ;-) .. 73 de Frank Turner-Smith G3VKI - mine's a pint. .. http://turner-smith.co.uk .. Ubuntu 12.04 Thunderbirds are go. |
short antennae
On 29/10/14 16:27, Jeff wrote:
Maybe I've missed something here, but I would expect a half-wave dipole to out-perform a full-wave dipole at the same frequency, despite being half the size. Define what you mean by "out perform"!!! A 1/2 wave dipole will half a max gain of about 2.14db in free space broadside to the antenna, a full wave dipole will be a little over 3db. However, if you examine the pattern of the full wave compared to the 1/2 wave the lobes of the full wave will be narrower. ie although more power is radiated in a direction normal to the antenna less is radiated in other directions, or to put is another way all of the power applied to the antenna structure is radiated in both cases, but the full wave concentrates to more into the direction normal to the antenna. Of course this does not take into account any losses associated with matching the full wave. Jeff OK, Jeff, I don't think I worded that too well. I've never bothered to learn complex mathematical formulae just for the sake of it. I prefer to try to visualise things as simply as I can make them. In the case of a half-wave dipole, at the instant the voltage at one end goes to peak positive the other end will be at peak negative, and maximum current will flow. In a full wave dipole the voltage at both ends will always be in phase, so I would expect to see a very high impedance at the feed point. As you point out, matching the full-wave could be difficult and very lossy. -- ;-) .. 73 de Frank Turner-Smith G3VKI - mine's a pint. .. http://turner-smith.co.uk .. Ubuntu 12.04 Thunderbirds are go. |
short antennae
In message , Frank Turner-Smith G3VKI
writes In a full wave dipole the voltage at both ends will always be in phase, Are you sure? Think on't! so I would expect to see a very high impedance at the feed point. Correct. As you point out, matching the full-wave could be difficult and very lossy. Double zepp? -- Ian |
short antennae
On 30/10/14 08:47, Ian Jackson wrote:
In message , Frank Turner-Smith G3VKI writes In a full wave dipole the voltage at both ends will always be in phase, Are you sure? Think on't! so I would expect to see a very high impedance at the feed point. Correct. As you point out, matching the full-wave could be difficult and very lossy. Double zepp? OK, what did I miss? In a full wave dipole, at the instant the voltage at one end is peak positive, the voltage at the other end will also be peak positive. Similarly, at the feed point, both legs would be at peak negative and no current would flow in the feeder, hence the high impedance. There would be a current flowing in each leg of the dipole, but the currents would be in anti-phase. Where have I got it wrong? Do I need another drink? -- ;-) .. 73 de Frank Turner-Smith G3VKI - mine's a pint. .. http://turner-smith.co.uk .. Ubuntu 12.04 Thunderbirds are go. |
short antennae
In message , Frank Turner-Smith G3VKI
writes On 30/10/14 08:47, Ian Jackson wrote: In message , Frank Turner-Smith G3VKI writes In a full wave dipole the voltage at both ends will always be in phase, Are you sure? Think on't! so I would expect to see a very high impedance at the feed point. Correct. As you point out, matching the full-wave could be difficult and very lossy. Double zepp? OK, what did I miss? In a full wave dipole, at the instant the voltage at one end is peak positive, the voltage at the other end will also be peak positive. Similarly, at the feed point, both legs would be at peak negative and no current would flow in the feeder, hence the high impedance. There would be a current flowing in each leg of the dipole, but the currents would be in anti-phase. Where have I got it wrong? Do I need another drink? Maybe I need a drink too. However, all dipoles/doublets have to fed 'push-pull', so when one leg goes +ve, the other leg goes -ve. The voltage at all points along the antenna that are equidistant from the feedpoint will be in antiphase, so if the feedpoint is in the centre, the voltages at the ends will be in antiphase. [Or is my thinking seriously muddled?] -- Ian |
short antennae
On 30/10/14 14:04, Ian Jackson wrote:
In message , Frank Turner-Smith G3VKI writes On 30/10/14 08:47, Ian Jackson wrote: In message , Frank Turner-Smith G3VKI writes In a full wave dipole the voltage at both ends will always be in phase, Are you sure? Think on't! so I would expect to see a very high impedance at the feed point. Correct. As you point out, matching the full-wave could be difficult and very lossy. Double zepp? OK, what did I miss? In a full wave dipole, at the instant the voltage at one end is peak positive, the voltage at the other end will also be peak positive. Similarly, at the feed point, both legs would be at peak negative and no current would flow in the feeder, hence the high impedance. There would be a current flowing in each leg of the dipole, but the currents would be in anti-phase. Where have I got it wrong? Do I need another drink? Maybe I need a drink too. However, all dipoles/doublets have to fed 'push-pull', so when one leg goes +ve, the other leg goes -ve. The voltage at all points along the antenna that are equidistant from the feedpoint will be in antiphase, so if the feedpoint is in the centre, the voltages at the ends will be in antiphase. [Or is my thinking seriously muddled?] Looks like I owe you a pint. You've described the situation where a TX is feeding the dipole. I was trying to visualise the RX conditions, but it reciprocates. One of us has to be wrong, and I strongly suspect it's me. Time for a drink. -- ;-) .. 73 de Frank Turner-Smith G3VKI - mine's a pint. .. http://turner-smith.co.uk .. Ubuntu 12.04 Thunderbirds are go. |
short antennae
"Frank Turner-Smith G3VKI" wrote in message ... On 30/10/14 14:04, Ian Jackson wrote: In message , Frank Turner-Smith G3VKI writes On 30/10/14 08:47, Ian Jackson wrote: In message , Frank Turner-Smith G3VKI writes In a full wave dipole the voltage at both ends will always be in phase, Are you sure? Think on't! so I would expect to see a very high impedance at the feed point. Correct. As you point out, matching the full-wave could be difficult and very lossy. Double zepp? OK, what did I miss? In a full wave dipole, at the instant the voltage at one end is peak positive, the voltage at the other end will also be peak positive. Similarly, at the feed point, both legs would be at peak negative and no current would flow in the feeder, hence the high impedance. There would be a current flowing in each leg of the dipole, but the currents would be in anti-phase. Where have I got it wrong? Do I need another drink? Maybe I need a drink too. However, all dipoles/doublets have to fed 'push-pull', so when one leg goes +ve, the other leg goes -ve. The voltage at all points along the antenna that are equidistant from the feedpoint will be in antiphase, so if the feedpoint is in the centre, the voltages at the ends will be in antiphase. [Or is my thinking seriously muddled?] # Looks like I owe you a pint. You've described the situation where a TX # is feeding the dipole. I was trying to visualise the RX conditions, but # it reciprocates. One of us has to be wrong, and I strongly suspect it's # me. Time for a drink. With drinking involved, I must throw in my 2 cents. I'd go with Frank....for full wave assume positive peak at one end, negative peak in the middle, and positive peak at the other end. (or vice versa) But, I suppose I should think about it a little more.....Laphroig would help :) Wayne W5GIE/6 |
short antennae
"Wayne" wrote in :
Laphroig would help :) Oban. That comes at you like a strong onshore wind. Good stuff. About fullwave dipoles, I read something that said just stay with halfwave for easier matching and an easier time getting it high enough. The writer had a callsign and everything. :) Seriously, I think he's right because those things will make up for any 'advantage' you might gain with a fullwave, apparently. |
short antennae
"Lostgallifreyan" wrote in message . .. "Wayne" wrote in : Laphroig would help :) # Oban. That comes at you like a strong onshore wind. Good stuff. # About fullwave dipoles, I read something that said just stay with halfwave # for easier matching and an easier time getting it high enough. The writer had # a callsign and everything. :) Seriously, I think he's right because those # things will make up for any 'advantage' you might gain with a fullwave, # apparently. IIRC one might choose a full wave over half wave if the pattern lobes are more suitable. I might look at that later today. |
short antennae
In message , Wayne
writes "Frank Turner-Smith G3VKI" wrote in message ... On 30/10/14 14:04, Ian Jackson wrote: In message , Frank Turner-Smith G3VKI writes On 30/10/14 08:47, Ian Jackson wrote: In message , Frank Turner-Smith G3VKI writes In a full wave dipole the voltage at both ends will always be in phase, Are you sure? Think on't! so I would expect to see a very high impedance at the feed point. Correct. As you point out, matching the full-wave could be difficult and very lossy. Double zepp? OK, what did I miss? In a full wave dipole, at the instant the voltage at one end is peak positive, the voltage at the other end will also be peak positive. Similarly, at the feed point, both legs would be at peak negative and no current would flow in the feeder, hence the high impedance. There would be a current flowing in each leg of the dipole, but the currents would be in anti-phase. Where have I got it wrong? Do I need another drink? Maybe I need a drink too. However, all dipoles/doublets have to fed 'push-pull', so when one leg goes +ve, the other leg goes -ve. The voltage at all points along the antenna that are equidistant from the feedpoint will be in antiphase, so if the feedpoint is in the centre, the voltages at the ends will be in antiphase. [Or is my thinking seriously muddled?] # Looks like I owe you a pint. You've described the situation where a TX # is feeding the dipole. I was trying to visualise the RX conditions, but # it reciprocates. One of us has to be wrong, and I strongly suspect it's # me. Time for a drink. With drinking involved, I must throw in my 2 cents. I'd go with Frank....for full wave assume positive peak at one end, negative peak in the middle, and positive peak at the other end. (or vice versa) But, I suppose I should think about it a little more.....Laphroig would help :) See: http://tinyurl.com/q8nxqep ten rows of images down, second from left: This shows the amplitude and the polarity of the voltage and current for a halfwave dipole. [Lots of diagrams only show the amplitude.] You will see that the polarities on each leg are +ve and -ve. For a fullwave, just imagine it continuing on for another halfwave each side. -- Ian |
short antennae
"Ian Jackson" wrote in message ... In message , Wayne writes "Frank Turner-Smith G3VKI" wrote in message ... On 30/10/14 14:04, Ian Jackson wrote: In message , Frank Turner-Smith G3VKI writes On 30/10/14 08:47, Ian Jackson wrote: In message , Frank Turner-Smith G3VKI writes In a full wave dipole the voltage at both ends will always be in phase, Are you sure? Think on't! so I would expect to see a very high impedance at the feed point. Correct. As you point out, matching the full-wave could be difficult and very lossy. Double zepp? OK, what did I miss? In a full wave dipole, at the instant the voltage at one end is peak positive, the voltage at the other end will also be peak positive. Similarly, at the feed point, both legs would be at peak negative and no current would flow in the feeder, hence the high impedance. There would be a current flowing in each leg of the dipole, but the currents would be in anti-phase. Where have I got it wrong? Do I need another drink? Maybe I need a drink too. However, all dipoles/doublets have to fed 'push-pull', so when one leg goes +ve, the other leg goes -ve. The voltage at all points along the antenna that are equidistant from the feedpoint will be in antiphase, so if the feedpoint is in the centre, the voltages at the ends will be in antiphase. [Or is my thinking seriously muddled?] # Looks like I owe you a pint. You've described the situation where a TX # is feeding the dipole. I was trying to visualise the RX conditions, but # it reciprocates. One of us has to be wrong, and I strongly suspect it's # me. Time for a drink. With drinking involved, I must throw in my 2 cents. I'd go with Frank....for full wave assume positive peak at one end, negative peak in the middle, and positive peak at the other end. (or vice versa) But, I suppose I should think about it a little more.....Laphroig would help :) # See: # http://tinyurl.com/q8nxqep # ten rows of images down, second from left: # This shows the amplitude and the polarity of the voltage and current for # a halfwave dipole. [Lots of diagrams only show the amplitude.] You will # see that the polarities on each leg are +ve and -ve. For a fullwave, # just imagine it continuing on for another halfwave each side. # -- # Ian Isn't that figure for a full wave?... lambda |
short antennae
In message , Wayne
writes "Ian Jackson" wrote in message ... In message , Wayne writes "Frank Turner-Smith G3VKI" wrote in message ... On 30/10/14 14:04, Ian Jackson wrote: In message , Frank Turner-Smith G3VKI writes On 30/10/14 08:47, Ian Jackson wrote: In message , Frank Turner-Smith G3VKI writes In a full wave dipole the voltage at both ends will always be in phase, Are you sure? Think on't! so I would expect to see a very high impedance at the feed point. Correct. As you point out, matching the full-wave could be difficult and very lossy. Double zepp? OK, what did I miss? In a full wave dipole, at the instant the voltage at one end is peak positive, the voltage at the other end will also be peak positive. Similarly, at the feed point, both legs would be at peak negative and no current would flow in the feeder, hence the high impedance. There would be a current flowing in each leg of the dipole, but the currents would be in anti-phase. Where have I got it wrong? Do I need another drink? Maybe I need a drink too. However, all dipoles/doublets have to fed 'push-pull', so when one leg goes +ve, the other leg goes -ve. The voltage at all points along the antenna that are equidistant from the feedpoint will be in antiphase, so if the feedpoint is in the centre, the voltages at the ends will be in antiphase. [Or is my thinking seriously muddled?] # Looks like I owe you a pint. You've described the situation where a TX # is feeding the dipole. I was trying to visualise the RX conditions, but # it reciprocates. One of us has to be wrong, and I strongly suspect it's # me. Time for a drink. With drinking involved, I must throw in my 2 cents. I'd go with Frank....for full wave assume positive peak at one end, negative peak in the middle, and positive peak at the other end. (or vice versa) But, I suppose I should think about it a little more.....Laphroig would help :) # See: # http://tinyurl.com/q8nxqep # ten rows of images down, second from left: # This shows the amplitude and the polarity of the voltage and current for # a halfwave dipole. [Lots of diagrams only show the amplitude.] You will # see that the polarities on each leg are +ve and -ve. For a fullwave, # just imagine it continuing on for another halfwave each side. # -- # Ian Isn't that figure for a full wave?... lambda Maybe you're looking at the wrong one. I've had another look, and it's now 9 down, far left. It's the one with the thick black dipole, entitled "Halfwave Dipole Antenna (Hertz)". Ah, I've found the source, here (Fig 1): http://www.digikey.com/en/articles/t...standing-anten na-specifications-and-operation -- Ian |
short antennae
On 30/10/14 20:44, Ian Jackson wrote:
In message , Wayne writes "Ian Jackson" wrote in message ... In message , Wayne writes "Frank Turner-Smith G3VKI" wrote in message ... On 30/10/14 14:04, Ian Jackson wrote: In message , Frank Turner-Smith G3VKI writes On 30/10/14 08:47, Ian Jackson wrote: In message , Frank Turner-Smith G3VKI writes In a full wave dipole the voltage at both ends will always be in phase, Are you sure? Think on't! so I would expect to see a very high impedance at the feed point. Correct. As you point out, matching the full-wave could be difficult and very lossy. Double zepp? OK, what did I miss? In a full wave dipole, at the instant the voltage at one end is peak positive, the voltage at the other end will also be peak positive. Similarly, at the feed point, both legs would be at peak negative and no current would flow in the feeder, hence the high impedance. There would be a current flowing in each leg of the dipole, but the currents would be in anti-phase. Where have I got it wrong? Do I need another drink? Maybe I need a drink too. However, all dipoles/doublets have to fed 'push-pull', so when one leg goes +ve, the other leg goes -ve. The voltage at all points along the antenna that are equidistant from the feedpoint will be in antiphase, so if the feedpoint is in the centre, the voltages at the ends will be in antiphase. [Or is my thinking seriously muddled?] # Looks like I owe you a pint. You've described the situation where a TX # is feeding the dipole. I was trying to visualise the RX conditions, but # it reciprocates. One of us has to be wrong, and I strongly suspect it's # me. Time for a drink. With drinking involved, I must throw in my 2 cents. I'd go with Frank....for full wave assume positive peak at one end, negative peak in the middle, and positive peak at the other end. (or vice versa) But, I suppose I should think about it a little more.....Laphroig would help :) # See: # http://tinyurl.com/q8nxqep # ten rows of images down, second from left: # This shows the amplitude and the polarity of the voltage and current for # a halfwave dipole. [Lots of diagrams only show the amplitude.] You will # see that the polarities on each leg are +ve and -ve. For a fullwave, # just imagine it continuing on for another halfwave each side. # -- # Ian Isn't that figure for a full wave?... lambda Maybe you're looking at the wrong one. I've had another look, and it's now 9 down, far left. It's the one with the thick black dipole, entitled "Halfwave Dipole Antenna (Hertz)". Ah, I've found the source, here (Fig 1): http://www.digikey.com/en/articles/t...standing-anten na-specifications-and-operation The original proposal in this thread was that long antennas performed better than short ones. If that was true you'd get a good 600MHz UHF TV picture using a 132ft end fed longwire. I've not tried it, but it doesn't seem very likely. -- ;-) .. 73 de Frank Turner-Smith G3VKI - mine's a pint. .. http://turner-smith.co.uk .. Ubuntu 12.04 Thunderbirds are go. |
short antennae
In message , Frank Turner-Smith G3VKI
writes On 30/10/14 20:44, Ian Jackson wrote: In message , Wayne writes "Ian Jackson" wrote in message ... In message , Wayne writes "Frank Turner-Smith G3VKI" wrote in message ... On 30/10/14 14:04, Ian Jackson wrote: In message , Frank Turner-Smith G3VKI writes On 30/10/14 08:47, Ian Jackson wrote: In message , Frank Turner-Smith G3VKI writes In a full wave dipole the voltage at both ends will always be in phase, Are you sure? Think on't! so I would expect to see a very high impedance at the feed point. Correct. As you point out, matching the full-wave could be difficult and very lossy. Double zepp? OK, what did I miss? In a full wave dipole, at the instant the voltage at one end is peak positive, the voltage at the other end will also be peak positive. Similarly, at the feed point, both legs would be at peak negative and no current would flow in the feeder, hence the high impedance. There would be a current flowing in each leg of the dipole, but the currents would be in anti-phase. Where have I got it wrong? Do I need another drink? Maybe I need a drink too. However, all dipoles/doublets have to fed 'push-pull', so when one leg goes +ve, the other leg goes -ve. The voltage at all points along the antenna that are equidistant from the feedpoint will be in antiphase, so if the feedpoint is in the centre, the voltages at the ends will be in antiphase. [Or is my thinking seriously muddled?] # Looks like I owe you a pint. You've described the situation where a TX # is feeding the dipole. I was trying to visualise the RX conditions, but # it reciprocates. One of us has to be wrong, and I strongly suspect it's # me. Time for a drink. With drinking involved, I must throw in my 2 cents. I'd go with Frank....for full wave assume positive peak at one end, negative peak in the middle, and positive peak at the other end. (or vice versa) But, I suppose I should think about it a little more.....Laphroig would help :) # See: # http://tinyurl.com/q8nxqep # ten rows of images down, second from left: # This shows the amplitude and the polarity of the voltage and current for # a halfwave dipole. [Lots of diagrams only show the amplitude.] You will # see that the polarities on each leg are +ve and -ve. For a fullwave, # just imagine it continuing on for another halfwave each side. # -- # Ian Isn't that figure for a full wave?... lambda Maybe you're looking at the wrong one. I've had another look, and it's now 9 down, far left. It's the one with the thick black dipole, entitled "Halfwave Dipole Antenna (Hertz)". Ah, I've found the source, here (Fig 1): http://www.digikey.com/en/articles/t...standing-anten na-specifications-and-operation The original proposal in this thread was that long antennas performed better than short ones. If that was true you'd get a good 600MHz UHF TV picture using a 132ft end fed longwire. I've not tried it, but it doesn't seem very likely. A 132' endfed will have one hell of a gain on 600MHz - but it will be almost straight off the ends. -- Ian |
short antennae
"Ian Jackson" wrote in message ... The original proposal in this thread was that long antennas performed better than short ones. If that was true you'd get a good 600MHz UHF TV picture using a 132ft end fed longwire. I've not tried it, but it doesn't seem very likely. A 132' endfed will have one hell of a gain on 600MHz - but it will be almost straight off the ends. -- Ian I know the gain will be off the end of the wire, but still wonder if an antenna that long (in wavelengths) will actually work or will it be too long and the gain does not meet the expectations or if programs like NEC will predict it or fall apart. --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com |
short antennae
In message , Ralph
Mowery writes "Ian Jackson" wrote in message ... The original proposal in this thread was that long antennas performed better than short ones. If that was true you'd get a good 600MHz UHF TV picture using a 132ft end fed longwire. I've not tried it, but it doesn't seem very likely. A 132' endfed will have one hell of a gain on 600MHz - but it will be almost straight off the ends. -- Ian I know the gain will be off the end of the wire, but still wonder if an antenna that long (in wavelengths) will actually work or will it be too long and the gain does not meet the expectations or if programs like NEC will predict it or fall apart. On that point, you'll have to ask the experts! -- Ian |
short antennae
On 30/10/14 22:26, Ian Jackson wrote:
In message , Ralph Mowery writes "Ian Jackson" wrote in message ... The original proposal in this thread was that long antennas performed better than short ones. If that was true you'd get a good 600MHz UHF TV picture using a 132ft end fed longwire. I've not tried it, but it doesn't seem very likely. A 132' endfed will have one hell of a gain on 600MHz - but it will be almost straight off the ends. -- Ian I know the gain will be off the end of the wire, but still wonder if an antenna that long (in wavelengths) will actually work or will it be too long and the gain does not meet the expectations or if programs like NEC will predict it or fall apart. On that point, you'll have to ask the experts! The gain would be at the cost of a very narrow front lobe. You'd need a big rotator. -- ;-) .. 73 de Frank Turner-Smith G3VKI - mine's a pint. .. http://turner-smith.co.uk .. Ubuntu 12.04 Thunderbirds are go. |
short antennae
In message , Frank Turner-Smith G3VKI
writes On 30/10/14 22:26, Ian Jackson wrote: In message , Ralph Mowery writes "Ian Jackson" wrote in message ... The original proposal in this thread was that long antennas performed better than short ones. If that was true you'd get a good 600MHz UHF TV picture using a 132ft end fed longwire. I've not tried it, but it doesn't seem very likely. A 132' endfed will have one hell of a gain on 600MHz - but it will be almost straight off the ends. -- Ian I know the gain will be off the end of the wire, but still wonder if an antenna that long (in wavelengths) will actually work or will it be too long and the gain does not meet the expectations or if programs like NEC will predict it or fall apart. On that point, you'll have to ask the experts! The gain would be at the cost of a very narrow front lobe. You'd need a big rotator. In the UK, would you be wanting to rotate it for TV? Don't forget that one type of antenna used in the very early days of BBC TV (Channel 1, vertical, 45MHz) was a 'sloper. This was an off-centre-fed wire dipole, with the short leg being a quarterwave, and attached as high as possible (maybe to a chimney or a gutter). The other leg was an odd number of quarterwaves, and attached much lower down. As a result, the antenna had one of its major lobes sort-of off the end (say 30 degrees off the wire), in a more-or-less horizontal direction, and responding well to vertically polarized signals. -- Ian |
short antennae
"Ian Jackson" wrote in message In the UK, would you be wanting to rotate it for TV? Don't forget that one type of antenna used in the very early days of BBC TV (Channel 1, vertical, 45MHz) was a 'sloper. This was an off-centre-fed wire dipole, with the short leg being a quarterwave, and attached as high as possible (maybe to a chimney or a gutter). The other leg was an odd number of quarterwaves, and attached much lower down. As a result, the antenna had one of its major lobes sort-of off the end (say 30 degrees off the wire), in a more-or-less horizontal direction, and responding well to vertically polarized signals. -- Ian I have not kept up with TV signals for a long time. In the US they started off as all horizontal. I think that some may have gone to circular, but not sure. It might be the FM stations I am thinking about. Not sure what they are using now on the digital signals. What are they using in other countries ? Horizontal, vertical ? --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com |
short antennae
In message , Ralph
Mowery writes "Ian Jackson" wrote in message In the UK, would you be wanting to rotate it for TV? Don't forget that one type of antenna used in the very early days of BBC TV (Channel 1, vertical, 45MHz) was a 'sloper. This was an off-centre-fed wire dipole, with the short leg being a quarterwave, and attached as high as possible (maybe to a chimney or a gutter). The other leg was an odd number of quarterwaves, and attached much lower down. As a result, the antenna had one of its major lobes sort-of off the end (say 30 degrees off the wire), in a more-or-less horizontal direction, and responding well to vertically polarized signals. -- Ian I have not kept up with TV signals for a long time. In the US they started off as all horizontal. I think that some may have gone to circular, but not sure. It might be the FM stations I am thinking about. Not sure what they are using now on the digital signals. What are they using in other countries ? Horizontal, vertical ? Historically, both. The very first UK TV transmitter (45MHz), launched in 1936, was vertical. This closed down swiftly on 1 September, 1939, and WW2 started two days later. After WW2, TV resumed in 1946. Four more channel frequencies (all BBC only in those days) were added throughout the country in the Low Band (Band 1, as we call it), and the split of vertical and horizontal was around 50/50. In 1954, the independent network (ITV) arrived, all in the High Band (Band 3) - again with around a 50/50 split. In 1963 (?) UHF was launched - initially carrying only a second BBC channel, on 625-lines. Eventually, this expanded to four - and in some places - five channels, all transmissions being co-sited - or almost co-sited (so that only one, fixed antenna was required). Without exception, all high(er) power analogue transmitters were horizontal, and almost without exception, all the low(er)-power fill-in relay stations were vertical. Although these days it's all digital, same pertains - except for (I think) one new fairly high power directional vertical transmitter. BTW, the old 405-line VHF network was totally closed down in the 1980s. Although VHF is no longer used for TV, part of the Band 3 allocation is now digital radio - all vertical. FM was originally all horizontal, but gradually the benefits of circular - and the simpler mixed - polarizations became implemented. Very few are now purely horizontal. Other European countries have had a somewhat different history, but I'm pretty sure that very few TV transmitters were/are vertical - except for local fill-in and low power. For FM, the Irish Republic has always (sensibly) used vertical. -- Ian |
short antennae
"Ian Jackson" wrote in message ... In message , Wayne writes "Ian Jackson" wrote in message ... In message , Wayne writes "Frank Turner-Smith G3VKI" wrote in message ... On 30/10/14 14:04, Ian Jackson wrote: In message , Frank Turner-Smith G3VKI writes On 30/10/14 08:47, Ian Jackson wrote: In message , Frank Turner-Smith G3VKI writes In a full wave dipole the voltage at both ends will always be in phase, Are you sure? Think on't! so I would expect to see a very high impedance at the feed point. Correct. As you point out, matching the full-wave could be difficult and very lossy. Double zepp? OK, what did I miss? In a full wave dipole, at the instant the voltage at one end is peak positive, the voltage at the other end will also be peak positive. Similarly, at the feed point, both legs would be at peak negative and no current would flow in the feeder, hence the high impedance. There would be a current flowing in each leg of the dipole, but the currents would be in anti-phase. Where have I got it wrong? Do I need another drink? Maybe I need a drink too. However, all dipoles/doublets have to fed 'push-pull', so when one leg goes +ve, the other leg goes -ve. The voltage at all points along the antenna that are equidistant from the feedpoint will be in antiphase, so if the feedpoint is in the centre, the voltages at the ends will be in antiphase. [Or is my thinking seriously muddled?] # Looks like I owe you a pint. You've described the situation where a TX # is feeding the dipole. I was trying to visualise the RX conditions, but # it reciprocates. One of us has to be wrong, and I strongly suspect it's # me. Time for a drink. With drinking involved, I must throw in my 2 cents. I'd go with Frank....for full wave assume positive peak at one end, negative peak in the middle, and positive peak at the other end. (or vice versa) But, I suppose I should think about it a little more.....Laphroig would help :) # See: # http://tinyurl.com/q8nxqep # ten rows of images down, second from left: # This shows the amplitude and the polarity of the voltage and current for # a halfwave dipole. [Lots of diagrams only show the amplitude.] You will # see that the polarities on each leg are +ve and -ve. For a fullwave, # just imagine it continuing on for another halfwave each side. # -- # Ian Isn't that figure for a full wave?... lambda # Maybe you're looking at the wrong one. I've had another look, and it's # now 9 down, far left. It's the one with the thick black dipole, entitled # "Halfwave Dipole Antenna (Hertz)". Ah, I've found the source, here (Fig # 1): # http://www.digikey.com/en/articles/t...standing-anten # na-specifications-and-operation # -- # Ian OK, but I'm losing touch with what the point is. The figures referenced both times show voltage peaks of opposite phase at the ends of half wave dipoles, thus voltage peaks of the same phase at the end of full wave dipoles. I'm assuming we have agreement on that. And with that, it currently is time here in CA for Lagavulin 16. |
short antennae
Ralph Mowery wrote:
"Ian Jackson" wrote in message ... The original proposal in this thread was that long antennas performed better than short ones. If that was true you'd get a good 600MHz UHF TV picture using a 132ft end fed longwire. I've not tried it, but it doesn't seem very likely. A 132' endfed will have one hell of a gain on 600MHz - but it will be almost straight off the ends. -- Ian I know the gain will be off the end of the wire, but still wonder if an antenna that long (in wavelengths) will actually work or will it be too long and the gain does not meet the expectations or if programs like NEC will predict it or fall apart. EZNEC handles it just fine. I modeled a 120' (I had forgotten the exact number posted) long wire at 6' over real ground at 600 Mhz: Impedance: 55-j308 Max vertical gain: 21 dBi at 4 degrees Horizontal gain: two 21 dBi lobes at +/- 4 degrees Front/back: 9.5 dB LOTS of little lobes... -- Jim Pennino |
short antennae
In message , Wayne
writes "Ian Jackson" wrote in message ... In message , Wayne writes "Ian Jackson" wrote in message ... In message , Wayne writes "Frank Turner-Smith G3VKI" wrote in message ... On 30/10/14 14:04, Ian Jackson wrote: In message , Frank Turner-Smith G3VKI writes On 30/10/14 08:47, Ian Jackson wrote: In message , Frank Turner-Smith G3VKI writes In a full wave dipole the voltage at both ends will always be in Are you sure? Think on't! so I would expect to see a very high impedance at the feed point. Correct. As you point out, matching the full-wave could be difficult and very lossy. Double zepp? OK, what did I miss? In a full wave dipole, at the instant the voltage at one end is peak positive, the voltage at the other end will also be peak positive. Similarly, at the feed point, both legs would be at peak negative and no current would flow in the feeder, hence the high impedance. There would be a current flowing in each leg of the dipole, but the currents would be in anti-phase. Where have I got it wrong? Do I need another drink? Maybe I need a drink too. However, all dipoles/doublets have to fed 'push-pull', so when one leg goes +ve, the other leg goes -ve. The voltage at all points along the antenna that are equidistant from the feedpoint will be in antiphase, so if the feedpoint is in the centre, the voltages at the ends will be in antiphase. [Or is my thinking seriously muddled?] # Looks like I owe you a pint. You've described the situation where a TX # is feeding the dipole. I was trying to visualise the RX conditions, but # it reciprocates. One of us has to be wrong, and I strongly suspect it's # me. Time for a drink. With drinking involved, I must throw in my 2 cents. I'd go with Frank....for full wave assume positive peak at one end, negative peak in the middle, and positive peak at the other end. (or vice versa) But, I suppose I should think about it a little more.....Laphroig would help :) # See: # http://tinyurl.com/q8nxqep # ten rows of images down, second from left: # This shows the amplitude and the polarity of the voltage and current for # a halfwave dipole. [Lots of diagrams only show the amplitude.] You will # see that the polarities on each leg are +ve and -ve. For a fullwave, # just imagine it continuing on for another halfwave each side. # -- # Ian Isn't that figure for a full wave?... lambda # Maybe you're looking at the wrong one. I've had another look, and it's # now 9 down, far left. It's the one with the thick black dipole, entitled # "Halfwave Dipole Antenna (Hertz)". Ah, I've found the source, here (Fig # 1): # http://www.digikey.com/en/articles/t...standing-anten # na-specifications-and-operation # -- # Ian OK, but I'm losing touch with what the point is. The figures referenced both times show voltage peaks of opposite phase at the ends of half wave dipoles, thus voltage peaks of the same phase at the end of full wave dipoles. I'm assuming we have agreement on that. You assume wrongly. A centre-fed fullwave is also fed 'push-pull', ie in antiphase - except that it's a high voltage feed instead of high current. Just draw a diagram similar to the halfwave, - again showing both the voltage amplitude and polarity. The lines you draw on each side for the amplitudes are copies of each other - not mirror images. However, it seems to a different matter if you feed the fullwave off-centre, a quarterwave from one end. At least on my drawing, the voltages at the ends ARE in phase - so I guess the radiation pattern will be different from the centre-fed. And with that, it currently is time here in CA for Lagavulin 16. No such luck here. FWIW, it's nearly midday, and I'm having a (rather late) strong 'coffee-bag' coffee, with two teaspoons of maple syrup and one teaspoon (heaped) of dark drinking chocolate powder. -- Ian |
short antennae
On 31/10/14 11:56, Ian Jackson wrote:
In message , Wayne writes "Ian Jackson" wrote in message ... In message , Wayne writes "Ian Jackson" wrote in message ... In message , Wayne writes "Frank Turner-Smith G3VKI" wrote in message ... On 30/10/14 14:04, Ian Jackson wrote: In message , Frank Turner-Smith G3VKI writes On 30/10/14 08:47, Ian Jackson wrote: In message , Frank Turner-Smith G3VKI writes In a full wave dipole the voltage at both ends will always be in Are you sure? Think on't! so I would expect to see a very high impedance at the feed point. Correct. As you point out, matching the full-wave could be difficult and very lossy. Double zepp? OK, what did I miss? In a full wave dipole, at the instant the voltage at one end is peak positive, the voltage at the other end will also be peak positive. Similarly, at the feed point, both legs would be at peak negative and no current would flow in the feeder, hence the high impedance. There would be a current flowing in each leg of the dipole, but the currents would be in anti-phase. Where have I got it wrong? Do I need another drink? Maybe I need a drink too. However, all dipoles/doublets have to fed 'push-pull', so when one leg goes +ve, the other leg goes -ve. The voltage at all points along the antenna that are equidistant from the feedpoint will be in antiphase, so if the feedpoint is in the centre, the voltages at the ends will be in antiphase. [Or is my thinking seriously muddled?] # Looks like I owe you a pint. You've described the situation where a TX # is feeding the dipole. I was trying to visualise the RX conditions, but # it reciprocates. One of us has to be wrong, and I strongly suspect it's # me. Time for a drink. With drinking involved, I must throw in my 2 cents. I'd go with Frank....for full wave assume positive peak at one end, negative peak in the middle, and positive peak at the other end. (or vice versa) But, I suppose I should think about it a little more.....Laphroig would help :) # See: # http://tinyurl.com/q8nxqep # ten rows of images down, second from left: # This shows the amplitude and the polarity of the voltage and current for # a halfwave dipole. [Lots of diagrams only show the amplitude.] You will # see that the polarities on each leg are +ve and -ve. For a fullwave, # just imagine it continuing on for another halfwave each side. # -- # Ian Isn't that figure for a full wave?... lambda # Maybe you're looking at the wrong one. I've had another look, and it's # now 9 down, far left. It's the one with the thick black dipole, entitled # "Halfwave Dipole Antenna (Hertz)". Ah, I've found the source, here (Fig # 1): # http://www.digikey.com/en/articles/t...standing-anten # na-specifications-and-operation # -- # Ian OK, but I'm losing touch with what the point is. The figures referenced both times show voltage peaks of opposite phase at the ends of half wave dipoles, thus voltage peaks of the same phase at the end of full wave dipoles. I'm assuming we have agreement on that. You assume wrongly. A centre-fed fullwave is also fed 'push-pull', ie in antiphase - except that it's a high voltage feed instead of high current. Just draw a diagram similar to the halfwave, - again showing both the voltage amplitude and polarity. The lines you draw on each side for the amplitudes are copies of each other - not mirror images. However, it seems to a different matter if you feed the fullwave off-centre, a quarterwave from one end. At least on my drawing, the voltages at the ends ARE in phase - so I guess the radiation pattern will be different from the centre-fed. And with that, it currently is time here in CA for Lagavulin 16. No such luck here. FWIW, it's nearly midday, and I'm having a (rather late) strong 'coffee-bag' coffee, with two teaspoons of maple syrup and one teaspoon (heaped) of dark drinking chocolate powder. Ian, thanks for your help. There's just one bit of this I can't get my head around. I'm happy with the idea of 'push-pull' feeding the antenna. This, as you say, will cause the amplitudes on each side to be copies and not mirror images. This in turn causes the currents flowing in the legs to be in phase, with about 1dB increase in the front lobe and a subsequent decrease in the lobe's width. This has to reciprocate, so what happens in RX mode to cause the same phase inversion at the feed point? -- ;-) .. 73 de Frank Turner-Smith G3VKI - mine's a pint. .. http://turner-smith.co.uk .. Ubuntu 12.04 Thunderbirds are go. |
short antennae
In message , Frank Turner-Smith G3VKI
writes On 31/10/14 11:56, Ian Jackson wrote: In message , Wayne writes "Ian Jackson" wrote in message ... In message , Wayne writes "Ian Jackson" wrote in message ... In message , Wayne writes "Frank Turner-Smith G3VKI" wrote in message ... On 30/10/14 14:04, Ian Jackson wrote: In message , Frank Turner-Smith G3VKI writes On 30/10/14 08:47, Ian Jackson wrote: In message , Frank Turner-Smith G3VKI writes In a full wave dipole the voltage at both ends will always be in Are you sure? Think on't! so I would expect to see a very high impedance at the feed point. Correct. As you point out, matching the full-wave could be difficult and very lossy. Double zepp? OK, what did I miss? In a full wave dipole, at the instant the voltage at one end is peak positive, the voltage at the other end will also be peak positive. Similarly, at the feed point, both legs would be at peak negative and no current would flow in the feeder, hence the high impedance. There would be a current flowing in each leg of the dipole, but the currents would be in anti-phase. Where have I got it wrong? Do I need another drink? Maybe I need a drink too. However, all dipoles/doublets have to fed 'push-pull', so when one leg goes +ve, the other leg goes -ve. The voltage at all points along the antenna that are equidistant from the feedpoint will be in antiphase, so if the feedpoint is in the centre, the voltages at the ends will be in antiphase. [Or is my thinking seriously muddled?] # Looks like I owe you a pint. You've described the situation where a TX # is feeding the dipole. I was trying to visualise the RX conditions, but # it reciprocates. One of us has to be wrong, and I strongly suspect it's # me. Time for a drink. With drinking involved, I must throw in my 2 cents. I'd go with Frank....for full wave assume positive peak at one end, negative peak in the middle, and positive peak at the other end. (or vice versa) But, I suppose I should think about it a little more.....Laphroig would help :) # See: # http://tinyurl.com/q8nxqep # ten rows of images down, second from left: # This shows the amplitude and the polarity of the voltage and current for # a halfwave dipole. [Lots of diagrams only show the amplitude.] You will # see that the polarities on each leg are +ve and -ve. For a fullwave, # just imagine it continuing on for another halfwave each side. # -- # Ian Isn't that figure for a full wave?... lambda # Maybe you're looking at the wrong one. I've had another look, and it's # now 9 down, far left. It's the one with the thick black dipole, entitled # "Halfwave Dipole Antenna (Hertz)". Ah, I've found the source, here (Fig # 1): # http://www.digikey.com/en/articles/t...standing-anten # na-specifications-and-operation # -- # Ian OK, but I'm losing touch with what the point is. The figures referenced both times show voltage peaks of opposite phase at the ends of half wave dipoles, thus voltage peaks of the same phase at the end of full wave dipoles. I'm assuming we have agreement on that. You assume wrongly. A centre-fed fullwave is also fed 'push-pull', ie in antiphase - except that it's a high voltage feed instead of high current. Just draw a diagram similar to the halfwave, - again showing both the voltage amplitude and polarity. The lines you draw on each side for the amplitudes are copies of each other - not mirror images. However, it seems to a different matter if you feed the fullwave off-centre, a quarterwave from one end. At least on my drawing, the voltages at the ends ARE in phase - so I guess the radiation pattern will be different from the centre-fed. And with that, it currently is time here in CA for Lagavulin 16. No such luck here. FWIW, it's nearly midday, and I'm having a (rather late) strong 'coffee-bag' coffee, with two teaspoons of maple syrup and one teaspoon (heaped) of dark drinking chocolate powder. Ian, thanks for your help. There's just one bit of this I can't get my head around. I'm happy with the idea of 'push-pull' feeding the antenna. This, as you say, will cause the amplitudes on each side to be copies and not mirror images. This in turn causes the currents flowing in the legs to be in phase, with about 1dB increase in the front lobe and a subsequent decrease in the lobe's width. This has to reciprocate, so what happens in RX mode to cause the same phase inversion at the feed point? Well..... Errrrrr....... It.... just sort-of does? As you say, it has to reciprocate, therefore the incoming wavefront(s) from where the 'radiation' lobes are pointing, induce volts and amps in the antenna such that they end up being in anti-phase at the feeder connection points. They then slide, in antiphase, all the way down the feeder and into the receiver. Or something like that. -- Ian |
short antennae
On 31/10/14 00:44, Ian Jackson wrote:
In message , Ralph Mowery writes "Ian Jackson" wrote in message In the UK, would you be wanting to rotate it for TV? Don't forget that one type of antenna used in the very early days of BBC TV (Channel 1, vertical, 45MHz) was a 'sloper. This was an off-centre-fed wire dipole, with the short leg being a quarterwave, and attached as high as possible (maybe to a chimney or a gutter). The other leg was an odd number of quarterwaves, and attached much lower down. As a result, the antenna had one of its major lobes sort-of off the end (say 30 degrees off the wire), in a more-or-less horizontal direction, and responding well to vertically polarized signals. -- Ian I have not kept up with TV signals for a long time. In the US they started off as all horizontal. I think that some may have gone to circular, but not sure. It might be the FM stations I am thinking about. Not sure what they are using now on the digital signals. What are they using in other countries ? Horizontal, vertical ? Historically, both. The very first UK TV transmitter (45MHz), launched in 1936, was vertical. This closed down swiftly on 1 September, 1939, and WW2 started two days later. After WW2, TV resumed in 1946. Four more channel frequencies (all BBC only in those days) were added throughout the country in the Low Band (Band 1, as we call it), and the split of vertical and horizontal was around 50/50. In 1954, the independent network (ITV) arrived, all in the High Band (Band 3) - again with around a 50/50 split. In 1963 (?) UHF was launched - initially carrying only a second BBC channel, on 625-lines. Eventually, this expanded to four - and in some places - five channels, all transmissions being co-sited - or almost co-sited (so that only one, fixed antenna was required). Without exception, all high(er) power analogue transmitters were horizontal, and almost without exception, all the low(er)-power fill-in relay stations were vertical. Although these days it's all digital, same pertains - except for (I think) one new fairly high power directional vertical transmitter. BTW, the old 405-line VHF network was totally closed down in the 1980s. Although VHF is no longer used for TV, part of the Band 3 allocation is now digital radio - all vertical. FM was originally all horizontal, but gradually the benefits of circular - and the simpler mixed - polarizations became implemented. Very few are now purely horizontal. Other European countries have had a somewhat different history, but I'm pretty sure that very few TV transmitters were/are vertical - except for local fill-in and low power. For FM, the Irish Republic has always (sensibly) used vertical. VHF FM radio was indeed horizontally polarised at first, and was aimed at fixed receivers with rooftop aerials. I believe horizontal polarisation (HP) was first chosen in an attempt to provide greater coverage than vertical polarisation (VP). AIUI, over a distance from the TX, a VP transmission twists toward HP as it "grazes" the curvature of the Earth. In doing so energy gets absorbed and the signal is attenuated. The arrival of FM car radios meant a growing number of listeners were using vertical antennas so a change to slant or circular polarisation was introduced. The convention for UHF TV stations in the UK was for high powered "main stations" to use HP and low power relay stations to use VP. This cross polarisation provided about 26dB protection against co-channel interference. -- ;-) .. 73 de Frank Turner-Smith G3VKI - mine's a pint. .. http://turner-smith.co.uk .. Ubuntu 12.04 Thunderbirds are go. |
short antennae
Frank Turner-Smith G3VKI wrote in
: This cross polarisation provided about 26dB protection against co-channel interference. That's a useful figure. I asked a few weeks ago about the prosects of wiring an external vertical dipole for FM VFH broadcasts, via a MAR6 based amplifier boosting by maybe 20dB, to an internal horizontal dipole to overcome local digital hash from nearby flats that gets in to degrade the signal from a portable radio with a telescopic whip. As it is the SNR rather than the raw strength which is an issue, an ideal situation would be to allow thwe whip to be in its resting horizontal, cotracted position, while still allowing clear use of radios carried around the flat while I work. Various possible problems have been discussed, and I haven't pushed for this with a trial, but if 26 or more dB are cut in the difference between antenna based on polarisation, and the amp boosts only by 20dB, it seems that feedback can be avoided, AND also the risk of interference to other FM VHF radios in other flats. (Which might even benefit, if my own would.) This is the first time anyone's mentioned a figure for isaolation (for want of a better word) between similar dipoles based on 90° difference in orientation, so I'm taking this moment to reopen the subject in passing... |
short antennae
Frank Turner-Smith G3VKI wrote:
On 31/10/14 01:17, wrote: Ralph Mowery wrote: "Ian Jackson" wrote in message ... The original proposal in this thread was that long antennas performed better than short ones. If that was true you'd get a good 600MHz UHF TV picture using a 132ft end fed longwire. I've not tried it, but it doesn't seem very likely. A 132' endfed will have one hell of a gain on 600MHz - but it will be almost straight off the ends. -- Ian I know the gain will be off the end of the wire, but still wonder if an antenna that long (in wavelengths) will actually work or will it be too long and the gain does not meet the expectations or if programs like NEC will predict it or fall apart. EZNEC handles it just fine. I modeled a 120' (I had forgotten the exact number posted) long wire at 6' over real ground at 600 Mhz: Impedance: 55-j308 Max vertical gain: 21 dBi at 4 degrees Horizontal gain: two 21 dBi lobes at +/- 4 degrees Front/back: 9.5 dB LOTS of little lobes... Interesting, and I assume the -j308 is due to the capacitance between the wire and ground. Since we are discussing a 50cm wavelength I would imagine a change of only a cm or so in the overall length of the wire would cause a significant change in impedance. Another problem could be finding a big enough plot of land facing in the right direction. I did a little playinng around... Changing the length from 120' to 132' has little effect on the pattern as this is on the order of 80 wavelenths. As the wavelength is so short, the impedance varies greatly with small changes in length and the reactive part is heavily influenced by the height above ground and the quality of the ground. So in addition to finding enough land, you would also have to keep it always wet or always dry otherwise you would be constantly retuning as the ground moisture changed. However, if one lived right on a beach and set up a series of floats across the water... Then your only problem is waves causing changes in height. -- Jim Pennino |
short antennae
On 31/10/14 18:19, Frank Turner-Smith G3VKI wrote:
VHF FM radio was indeed horizontally polarised at first, and was aimed at fixed receivers with rooftop aerials. I believe horizontal polarisation (HP) was first chosen in an attempt to provide greater coverage than vertical polarisation (VP). AIUI, over a distance from the TX, a VP transmission twists toward HP as it "grazes" the curvature of the Earth. In doing so energy gets absorbed and the signal is attenuated. The arrival of FM car radios meant a growing number of listeners were using vertical antennas so a change to slant or circular polarisation was introduced. http://www.bbc.co.uk/rd/publications/rdreport_1960_16 -- ;-) .. 73 de Frank Turner-Smith G3VKI - mine's a pint. .. http://turner-smith.co.uk .. Ubuntu 12.04 Thunderbirds are go. |
short antennae
On 31/10/14 18:40, Lostgallifreyan wrote:
Frank Turner-Smith G3VKI wrote in : This cross polarisation provided about 26dB protection against co-channel interference. That's a useful figure. I asked a few weeks ago about the prospects of wiring an external vertical dipole for FM VHF broadcasts, via a MAR6 based amplifier boosting by maybe 20dB, to an internal horizontal dipole to overcome local digital hash from nearby flats that gets in to degrade the signal from a portable radio with a telescopic whip. As it is the SNR rather than the raw strength which is an issue, an ideal situation would be to allow the whip to be in its resting horizontal, contracted position, while still allowing clear use of radios carried around the flat while I work. Various possible problems have been discussed, and I haven't pushed for this with a trial, but if 26 or more dB are cut in the difference between antenna based on polarisation, and the amp boosts only by 20dB, it seems that feedback can be avoided, AND also the risk of interference to other FM VHF radios in other flats. (Which might even benefit, if my own would.) This is the first time anyone's mentioned a figure for isolation (for want of a better word) between similar dipoles based on 90° difference in orientation, so I'm taking this moment to reopen the subject in passing... 26dB cross-polarisation protection was the "rule of thumb" figure in use by the BBC service Planning Section in the 1970s when planning TV relay stations. I know, I was there. "Active Deflectors", relaying TV signals down into valleys without changing channels, have been doing exactly what you describe for many years. For safety's sake, keep the TX and RX aerials as far apart as you can. -- ;-) .. 73 de Frank Turner-Smith G3VKI - mine's a pint. .. http://turner-smith.co.uk .. Ubuntu 12.04 Thunderbirds are go. |
short antennae
In message ,
Lostgallifreyan writes Frank Turner-Smith G3VKI wrote in : This cross polarisation provided about 26dB protection against co-channel interference. That's a useful figure. I asked a few weeks ago about the prosects of wiring an external vertical dipole for FM VFH broadcasts, via a MAR6 based amplifier boosting by maybe 20dB, to an internal horizontal dipole to overcome local digital hash from nearby flats that gets in to degrade the signal from a portable radio with a telescopic whip. As it is the SNR rather than the raw strength which is an issue, an ideal situation would be to allow thwe whip to be in its resting horizontal, cotracted position, while still allowing clear use of radios carried around the flat while I work. Various possible problems have been discussed, and I haven't pushed for this with a trial, but if 26 or more dB are cut in the difference between antenna based on polarisation, and the amp boosts only by 20dB, it seems that feedback can be avoided, AND also the risk of interference to other FM VHF radios in other flats. (Which might even benefit, if my own would.) This is the first time anyone's mentioned a figure for isaolation (for want of a better word) between similar dipoles based on 90° difference in orientation, so I'm taking this moment to reopen the subject in passing... I've often heard this '26dB protection' quoted, but I'm sure that it's just a 'wet finger in the air' figure. Even if it is sort-of a 'typical average', at any location it could equally turn out to be almost anything between 'not a lot', and a lot more than 26dB - mainly depending on reflections. -- Ian |
short antennae
Frank Turner-Smith G3VKI wrote in
: 26dB cross-polarisation protection was the "rule of thumb" figure in use by the BBC service Planning Section in the 1970s when planning TV relay stations. I know, I was there. "Active Deflectors", relaying TV signals down into valleys without changing channels, have been doing exactly what you describe for many years. For safety's sake, keep the TX and RX aerials as far apart as you can. Nice. That looks like bearing out my hopes for the scheme after all. The distance isn't huge, but I've already decided to aim for the internal horizontal dipole to be central to the flat, so I can use the smallest boost that will do the deed. Small extra question.. Would I get away with a simple low loss coax to dipole with ferrites on the coax to prevent leaky signals running outside on the screen, or should the internal dipole get a balun or other appropriate treatment for a transmitting antenna, despits the tiny signals and extreme locality involved? |
short antennae
Ian Jackson wrote in
: at any location it could equally turn out to be almost anything between 'not a lot', and a lot more than 26dB - mainly depending on reflections. Good point. In optics reflections wreak merry hell with polarisation (often intentionally for good purpose) so I assume that it's the same with radio. Even so it has to beat putting both in same plane, and I'll use no more boost than will reduce local RF noise acceptably. Actually I have a bunch of PV panels and such out there on grounded mounts that any major reflections that occur won't make it into the flat, it's more likely there would be a few narrow opportunities for a direct signal to get past rather than many reflections getting here. |
short antennae
Frank Turner-Smith G3VKI wrote in
: For safety's sake, keep the TX and RX aerials as far apart as you can. I can quantify that relatively, which may help.. The distance between the internal dipole to the radio will be about a tenth of the distance between dipoles. Would that offer enough scope for avoiding feedback while boosting 20dB in the antenna amp? (If not I estimate I can get useful help with 10dB). |
| All times are GMT +1. The time now is 06:08 AM. |
|
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
RadioBanter.com