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dB relation TX/RX
Hi to all,
Hope that my question has some sense...: If we double the power radiated by an antenna (+3 dB), how does it translate on the S-meter at a receiver 'far' away.. i.e. is the difference in dB on that meter has some mathematical relation to the 3 dB change at the transmitter? 73 de Pierre |
dB relation TX/RX
On Sat, 18 Jul 2009 19:20:04 -0700 (PDT), ve2pid
wrote: Hope that my question has some sense...: If we double the power radiated by an antenna (+3 dB), how does it translate on the S-meter at a receiver 'far' away.. i.e. is the difference in dB on that meter has some mathematical relation to the 3 dB change at the transmitter? One S unit is allegedly 6dB. Doubling the power would therefore be about 1/2 an S unit. http://en.wikipedia.org/wiki/S_meter -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
dB relation TX/RX
ve2pid wrote:
Hi to all, Hope that my question has some sense...: If we double the power radiated by an antenna (+3 dB), how does it translate on the S-meter at a receiver 'far' away.. i.e. is the difference in dB on that meter has some mathematical relation to the 3 dB change at the transmitter? 73 de Pierre No, none at all. S-meters almost always just measure the AGC voltage, which is only approximately logarithmic. And manufacturers adjust the sensitivity of the meter to please the customers, who don't like "dead" meters. So the sensitivity of S-meters varies greatly from rig type to type and from one end of the scale to the other. I measured the sensitivity of the S-meter on my Icom 730 on one band. Here's how big an S-unit is on my rig: S1 - S2 1.4 dB S2 - S3 1.3 dB S3 - S4 1.6 dB S4 - S5 2.3 dB S5 - S6 1.8 dB S6 - S7 3.2 dB S7 - S8 3.1 dB S8 - S9 4.0 dB S9 - "S9 + 10 dB" 5.6 dB "S9 + 10 dB" - "S9 + 20 dB" 7.3 dB "S9 + 20 dB" - "S9 + 30 dB" 6.6 dB "S9 + 30 dB" - "S9 + 40 dB" 10.5 dB "S9 + 40 dB" - "S9 + 50 dB" 11.3 dB "S9 + 50 dB" - "S9 + 60 dB" 13.5 dB Now let's suppose that you built a new 5 element Yagi antenna and I honestly reported that your signal went from S2 to S6 when you switched to it from your dipole. The gain is really 7 dB, about par for the beam, but you read the postings on the Web and decide that an S-unit is "defined" as 6 dB, so the gain improvement must be 24 dB. Wow! Your modest beam has the same gain as a beam with a 25 WAVELENGTH boom and more than 50 elements! By making that incorrect assumption about the sensitivity of my S-meter, you've overestimated the gain by a factor of 63. S-meter sensitivities vary all over the map, so any assumption you make about how many dB in an S-unit is very likely wrong, and often grossly wrong. Roy Lewallen, W7EL |
dB relation TX/RX
"Jeff Liebermann" wrote in message ... On Sat, 18 Jul 2009 19:20:04 -0700 (PDT), ve2pid wrote: Hope that my question has some sense...: If we double the power radiated by an antenna (+3 dB), how does it translate on the S-meter at a receiver 'far' away.. i.e. is the difference in dB on that meter has some mathematical relation to the 3 dB change at the transmitter? One S unit is allegedly 6dB. Doubling the power would therefore be about 1/2 an S unit. http://en.wikipedia.org/wiki/S_meter I've heard the same thing, but the reality is that the S-meter usually follows the AGC and the signal for S9 could be different for every design. But Flex radio and others define S9 as 50 uV. Long time past, I heard that S9 was a noise free signal. But "noise free" is undefined. 30 dB SNR? 40 dB SNR? Imagine that your S-meter is perfectly logarithmic and your SNR is 54 dB at S9. That's one way to get 6 dB per S-unit. |
dB relation TX/RX
Sal M. Onella wrote:
I've heard the same thing, but the reality is that the S-meter usually follows the AGC and the signal for S9 could be different for every design. But Flex radio and others define S9 as 50 uV. . . Adjusting the S-meter to read a specific value for one signal strength is simple, and there's often an adjustment for doing it. I think it's fairly common to find S9 to be around 50 uV -- on one band at least -- although I wouldn't be surprised to see a fair amount of variation from band to band. But that has nothing to do with what the difference is between S7 and S8, S8 and S9, etc. Roy Lewallen, W7EL |
dB relation TX/RX
"Roy Lewallen" wrote in message
... ve2pid wrote: Hi to all, Hope that my question has some sense...: If we double the power radiated by an antenna (+3 dB), how does it translate on the S-meter at a receiver 'far' away.. i.e. is the difference in dB on that meter has some mathematical relation to the 3 dB change at the transmitter? 73 de Pierre No, none at all. S-meters almost always just measure the AGC voltage, which is only approximately logarithmic. And manufacturers adjust the sensitivity of the meter to please the customers, who don't like "dead" meters. So the sensitivity of S-meters varies greatly from rig type to type and from one end of the scale to the other. I measured the sensitivity of the S-meter on my Icom 730 on one band. Here's how big an S-unit is on my rig: S1 - S2 1.4 dB S2 - S3 1.3 dB S3 - S4 1.6 dB S4 - S5 2.3 dB S5 - S6 1.8 dB S6 - S7 3.2 dB S7 - S8 3.1 dB S8 - S9 4.0 dB S9 - "S9 + 10 dB" 5.6 dB "S9 + 10 dB" - "S9 + 20 dB" 7.3 dB "S9 + 20 dB" - "S9 + 30 dB" 6.6 dB "S9 + 30 dB" - "S9 + 40 dB" 10.5 dB "S9 + 40 dB" - "S9 + 50 dB" 11.3 dB "S9 + 50 dB" - "S9 + 60 dB" 13.5 dB Now let's suppose that you built a new 5 element Yagi antenna and I honestly reported that your signal went from S2 to S6 when you switched to it from your dipole. The gain is really 7 dB, about par for the beam, but you read the postings on the Web and decide that an S-unit is "defined" as 6 dB, so the gain improvement must be 24 dB. Wow! Your modest beam has the same gain as a beam with a 25 WAVELENGTH boom and more than 50 elements! By making that incorrect assumption about the sensitivity of my S-meter, you've overestimated the gain by a factor of 63. S-meter sensitivities vary all over the map, so any assumption you make about how many dB in an S-unit is very likely wrong, and often grossly wrong. Roy Lewallen, W7EL Nail on the head. A step attenunator in line will be the fastest way to determine the linearity of your scale if you can't afford a calibrated generator. FM rigs are all over the map on this because some radios have better limiting than others, and it might be actual limiter reading or some other derived reading. |
dB relation TX/RX
In message , Jitt
writes In article 753fee20-bfec-4b9a-81b2- , says... Hi to all, Hope that my question has some sense...: If we double the power radiated by an antenna (+3 dB), how does it translate on the S-meter at a receiver 'far' away.. i.e. is the difference in dB on that meter has some mathematical relation to the 3 dB change at the transmitter? 73 de Pierre Only in theory. It is possible to write an expression which relates an increase in Tx power to Rx antenna power at the remote site, but I doubt if it would agree with observed values in the field. I don't know what that 'expression' might be, but if an increase of TX power was not matched by a corresponding equal increase of RX power, I would immediately suspect that a rift was occurring in the space-time continuum. Whether that corresponds to the 'correct' S-meter reading is another matter. -- Ian |
dB relation TX/RX
In message , JB
writes "Roy Lewallen" wrote in message m... ve2pid wrote: Hi to all, Hope that my question has some sense...: If we double the power radiated by an antenna (+3 dB), how does it translate on the S-meter at a receiver 'far' away.. i.e. is the difference in dB on that meter has some mathematical relation to the 3 dB change at the transmitter? 73 de Pierre No, none at all. S-meters almost always just measure the AGC voltage, which is only approximately logarithmic. And manufacturers adjust the sensitivity of the meter to please the customers, who don't like "dead" meters. So the sensitivity of S-meters varies greatly from rig type to type and from one end of the scale to the other. I measured the sensitivity of the S-meter on my Icom 730 on one band. Here's how big an S-unit is on my rig: S1 - S2 1.4 dB S2 - S3 1.3 dB S3 - S4 1.6 dB S4 - S5 2.3 dB S5 - S6 1.8 dB S6 - S7 3.2 dB S7 - S8 3.1 dB S8 - S9 4.0 dB S9 - "S9 + 10 dB" 5.6 dB "S9 + 10 dB" - "S9 + 20 dB" 7.3 dB "S9 + 20 dB" - "S9 + 30 dB" 6.6 dB "S9 + 30 dB" - "S9 + 40 dB" 10.5 dB "S9 + 40 dB" - "S9 + 50 dB" 11.3 dB "S9 + 50 dB" - "S9 + 60 dB" 13.5 dB Now let's suppose that you built a new 5 element Yagi antenna and I honestly reported that your signal went from S2 to S6 when you switched to it from your dipole. The gain is really 7 dB, about par for the beam, but you read the postings on the Web and decide that an S-unit is "defined" as 6 dB, so the gain improvement must be 24 dB. Wow! Your modest beam has the same gain as a beam with a 25 WAVELENGTH boom and more than 50 elements! By making that incorrect assumption about the sensitivity of my S-meter, you've overestimated the gain by a factor of 63. S-meter sensitivities vary all over the map, so any assumption you make about how many dB in an S-unit is very likely wrong, and often grossly wrong. Roy Lewallen, W7EL Nail on the head. A step attenunator in line will be the fastest way to determine the linearity of your scale if you can't afford a calibrated generator. FM rigs are all over the map on this because some radios have better limiting than others, and it might be actual limiter reading or some other derived reading. You'll need an adequately-screened signal generator to do this test. If it isn't, the signals which leak out can bypass the attenuator and enter a not-too-well-screened receiver. If this happens, the results you get will be false. -- Ian |
dB relation TX/RX
In message , Roy Lewallen
writes Sal M. Onella wrote: I've heard the same thing, but the reality is that the S-meter usually follows the AGC and the signal for S9 could be different for every design. But Flex radio and others define S9 as 50 uV. . . Adjusting the S-meter to read a specific value for one signal strength is simple, and there's often an adjustment for doing it. I think it's fairly common to find S9 to be around 50 uV -- on one band at least -- although I wouldn't be surprised to see a fair amount of variation from band to band. But that has nothing to do with what the difference is between S7 and S8, S8 and S9, etc. Isn't there a school of thought which says that, above 1000MHz, an S-point is 3dB? [Don't ask me why!] -- Ian |
dB relation TX/RX
"Ian Jackson" wrote in message
... In message , JB writes "Roy Lewallen" wrote in message m... ve2pid wrote: Hi to all, Hope that my question has some sense...: If we double the power radiated by an antenna (+3 dB), how does it translate on the S-meter at a receiver 'far' away.. i.e. is the difference in dB on that meter has some mathematical relation to the 3 dB change at the transmitter? 73 de Pierre No, none at all. S-meters almost always just measure the AGC voltage, which is only approximately logarithmic. And manufacturers adjust the sensitivity of the meter to please the customers, who don't like "dead" meters. So the sensitivity of S-meters varies greatly from rig type to type and from one end of the scale to the other. I measured the sensitivity of the S-meter on my Icom 730 on one band. Here's how big an S-unit is on my rig: S1 - S2 1.4 dB S2 - S3 1.3 dB S3 - S4 1.6 dB S4 - S5 2.3 dB S5 - S6 1.8 dB S6 - S7 3.2 dB S7 - S8 3.1 dB S8 - S9 4.0 dB S9 - "S9 + 10 dB" 5.6 dB "S9 + 10 dB" - "S9 + 20 dB" 7.3 dB "S9 + 20 dB" - "S9 + 30 dB" 6.6 dB "S9 + 30 dB" - "S9 + 40 dB" 10.5 dB "S9 + 40 dB" - "S9 + 50 dB" 11.3 dB "S9 + 50 dB" - "S9 + 60 dB" 13.5 dB Now let's suppose that you built a new 5 element Yagi antenna and I honestly reported that your signal went from S2 to S6 when you switched to it from your dipole. The gain is really 7 dB, about par for the beam, but you read the postings on the Web and decide that an S-unit is "defined" as 6 dB, so the gain improvement must be 24 dB. Wow! Your modest beam has the same gain as a beam with a 25 WAVELENGTH boom and more than 50 elements! By making that incorrect assumption about the sensitivity of my S-meter, you've overestimated the gain by a factor of 63. S-meter sensitivities vary all over the map, so any assumption you make about how many dB in an S-unit is very likely wrong, and often grossly wrong. Roy Lewallen, W7EL Nail on the head. A step attenunator in line will be the fastest way to determine the linearity of your scale if you can't afford a calibrated generator. FM rigs are all over the map on this because some radios have better limiting than others, and it might be actual limiter reading or some other derived reading. You'll need an adequately-screened signal generator to do this test. If it isn't, the signals which leak out can bypass the attenuator and enter a not-too-well-screened receiver. If this happens, the results you get will be false. -- Ian Good point. If the signal is off the air it is less likely to be an issue. You need to seriously work on your setup to get better than 60 db of resolution. I homebrewed the 3, 6, 10, 20 step attenuator from one of the ARRL publications and find it quite useful. Output cable is double shielded. |
dB relation TX/RX
In message , JB
writes "Ian Jackson" wrote in message ... In message , JB writes "Roy Lewallen" wrote in message m... ve2pid wrote: Hi to all, Hope that my question has some sense...: If we double the power radiated by an antenna (+3 dB), how does it translate on the S-meter at a receiver 'far' away.. i.e. is the difference in dB on that meter has some mathematical relation to the 3 dB change at the transmitter? 73 de Pierre No, none at all. S-meters almost always just measure the AGC voltage, which is only approximately logarithmic. And manufacturers adjust the sensitivity of the meter to please the customers, who don't like "dead" meters. So the sensitivity of S-meters varies greatly from rig type to type and from one end of the scale to the other. I measured the sensitivity of the S-meter on my Icom 730 on one band. Here's how big an S-unit is on my rig: S1 - S2 1.4 dB S2 - S3 1.3 dB S3 - S4 1.6 dB S4 - S5 2.3 dB S5 - S6 1.8 dB S6 - S7 3.2 dB S7 - S8 3.1 dB S8 - S9 4.0 dB S9 - "S9 + 10 dB" 5.6 dB "S9 + 10 dB" - "S9 + 20 dB" 7.3 dB "S9 + 20 dB" - "S9 + 30 dB" 6.6 dB "S9 + 30 dB" - "S9 + 40 dB" 10.5 dB "S9 + 40 dB" - "S9 + 50 dB" 11.3 dB "S9 + 50 dB" - "S9 + 60 dB" 13.5 dB Now let's suppose that you built a new 5 element Yagi antenna and I honestly reported that your signal went from S2 to S6 when you switched to it from your dipole. The gain is really 7 dB, about par for the beam, but you read the postings on the Web and decide that an S-unit is "defined" as 6 dB, so the gain improvement must be 24 dB. Wow! Your modest beam has the same gain as a beam with a 25 WAVELENGTH boom and more than 50 elements! By making that incorrect assumption about the sensitivity of my S-meter, you've overestimated the gain by a factor of 63. S-meter sensitivities vary all over the map, so any assumption you make about how many dB in an S-unit is very likely wrong, and often grossly wrong. Roy Lewallen, W7EL Nail on the head. A step attenunator in line will be the fastest way to determine the linearity of your scale if you can't afford a calibrated generator. FM rigs are all over the map on this because some radios have better limiting than others, and it might be actual limiter reading or some other derived reading. You'll need an adequately-screened signal generator to do this test. If it isn't, the signals which leak out can bypass the attenuator and enter a not-too-well-screened receiver. If this happens, the results you get will be false. -- Ian Good point. If the signal is off the air it is less likely to be an issue. You need to seriously work on your setup to get better than 60 db of resolution. I homebrewed the 3, 6, 10, 20 step attenuator from one of the ARRL publications and find it quite useful. Output cable is double shielded. But, with homebrew attenuators, beware of the RF 'jumping over' the switches at the higher frequencies. This can limit the maximum attenuation attainable to something which is a lot less than the switches indicate. [Well, mine did, anyway!!] -- Ian |
dB relation TX/RX
In article ,
Ian Jackson wrote: In message , Roy Lewallen writes Sal M. Onella wrote: I've heard the same thing, but the reality is that the S-meter usually follows the AGC and the signal for S9 could be different for every design. But Flex radio and others define S9 as 50 uV. . . Adjusting the S-meter to read a specific value for one signal strength is simple, and there's often an adjustment for doing it. I think it's fairly common to find S9 to be around 50 uV -- on one band at least -- although I wouldn't be surprised to see a fair amount of variation from band to band. But that has nothing to do with what the difference is between S7 and S8, S8 and S9, etc. Isn't there a school of thought which says that, above 1000MHz, an S-point is 3dB? [Don't ask me why!] there are "Many Schools of thought".... but unless someone rewrites the Laws of Physics, and increase in Tx Output will show and Increase of Rx Input, on the same path, period. S-meters are NOT necessarily actually showing Rx Receive Signal Strength, and they are rarely, either linear, or even Logrythmnic in presentation of what they do represent. In the Microwave Field if you measure a path, and also calculate that path, comparing the two will show that they are very close, if you did it right.... |
dB relation TX/RX
In message , You
writes In article , Ian Jackson wrote: In message , Roy Lewallen writes Sal M. Onella wrote: I've heard the same thing, but the reality is that the S-meter usually follows the AGC and the signal for S9 could be different for every design. But Flex radio and others define S9 as 50 uV. . . Adjusting the S-meter to read a specific value for one signal strength is simple, and there's often an adjustment for doing it. I think it's fairly common to find S9 to be around 50 uV -- on one band at least -- although I wouldn't be surprised to see a fair amount of variation from band to band. But that has nothing to do with what the difference is between S7 and S8, S8 and S9, etc. Isn't there a school of thought which says that, above 1000MHz, an S-point is 3dB? [Don't ask me why!] there are "Many Schools of thought".... but unless someone rewrites the Laws of Physics, and increase in Tx Output will show and Increase of Rx Input, on the same path, period. I'm not sure what statement you are arguing against. Has anyone said otherwise? S-meters are NOT necessarily actually showing Rx Receive Signal Strength So what DO they 'show'? OK, they may not 'show' it very accurately, and the way they work is usually a very indirect way of 'measuring' the signal power or voltage at the receiver input, but I'm sure that this is what the do 'show'. For convenience, most S-meters use the AGC voltage to provide an indication of incoming signal level. However, some FM-only CB sets have no AGC, and have to pick off an input level dependent voltage from somewhere else in the circuit. These are usually VERY inaccurate. What alternative methods do you suggest? , and they are rarely, either linear No one is disagreeing. S-meters are renowned liars. Mind you, I have a CB set (which has an AGC line) which is converted to 10m, and this has an S-meter which has remarkably consistent 6dB S-points between S2 and S9 +20dB. , or even Logrythmnic No one's disagreeing. in presentation of what they do represent. In the Microwave Field if you measure a path, and also calculate that path, comparing the two will show that they are very close, if you did it right.... You are undoubtedly correct. But did anyone say that this was not the case? -- Ian |
dB relation TX/RX
ve2pid wrote in news:753fee20-bfec-4b9a-81b2-
: Hi to all, Hope that my question has some sense...: If we double the power radiated by an antenna (+3 dB), how does it translate on the S-meter at a receiver 'far' away.. i.e. is the difference in dB on that meter has some mathematical relation to the 3 dB change at the transmitter? 73 de Pierre Pierre, There is a convention that S meters are calibrated for S9 at 50µV (CW) at the receiver terminals, and the other S units are spaced out at 6dB/Sunit. (Some of the other conventions mean the same thing, eg 100µV EMF as one manufacturer is want to specify). So, if S9 is -73dBm, then S0 ought to be -127dBm. In a typical receiver with a sensitivity of around -135dBm for 10dB S/N, the AGC will be delayed until the signal is about 20dB higher than that, or about -115dBm... so an AGC derived solution cannot indicate less than S2 on that scale. Nevertheless they do. At the high end, extremely strong signals are a challenge for nice scale shape. In my experience, modern transceivers that provide the facility for three point calibration of the S meter are reasonalby good between around S6 and S9+20dB... give or take. Reasonably means withing a dB or two per Sunit. S meter calibration is invariably dependent on a particular setup of preamps and front end attenuators. Some of the outrageous 'measurements' one hears of are done in an 'uncalibrated' state (eg preamp ON when it is meant to be OFF. The S meter behaves fairly similarly to a quasi peak detector, responding to most of the peaks, but not all, and much higher than average power. So applying the S meter to impulse type signals introduces another issue. http://vk1od.net/software/fsm/ details another method of making more accurate measurements of signal level and field strength using a comms receiver. The system compared favourably in a field comparison with a commercial EMC receiver and active loop on a BPL measurement exercise. Owen |
dB relation TX/RX
Jitt wrote:
Only in theory. It is possible to write an expression which relates an increase in Tx power to Rx antenna power at the remote site, but I doubt if it would agree with observed values in the field. And this is one place (of pretty much all in RF) where theory absolutely matches reality. Your doubts are misplaced. Could you please give us an example, with all relevant data, where they did not agree? tom K0TAR |
dB relation TX/RX
"Ian Jackson" wrote in message
... In message , JB writes "Ian Jackson" wrote in message ... In message , JB writes "Roy Lewallen" wrote in message m... ve2pid wrote: Hi to all, Hope that my question has some sense...: If we double the power radiated by an antenna (+3 dB), how does it translate on the S-meter at a receiver 'far' away.. i.e. is the difference in dB on that meter has some mathematical relation to the 3 dB change at the transmitter? 73 de Pierre No, none at all. S-meters almost always just measure the AGC voltage, which is only approximately logarithmic. And manufacturers adjust the sensitivity of the meter to please the customers, who don't like "dead" meters. So the sensitivity of S-meters varies greatly from rig type to type and from one end of the scale to the other. I measured the sensitivity of the S-meter on my Icom 730 on one band. Here's how big an S-unit is on my rig: S1 - S2 1.4 dB S2 - S3 1.3 dB S3 - S4 1.6 dB S4 - S5 2.3 dB S5 - S6 1.8 dB S6 - S7 3.2 dB S7 - S8 3.1 dB S8 - S9 4.0 dB S9 - "S9 + 10 dB" 5.6 dB "S9 + 10 dB" - "S9 + 20 dB" 7.3 dB "S9 + 20 dB" - "S9 + 30 dB" 6.6 dB "S9 + 30 dB" - "S9 + 40 dB" 10.5 dB "S9 + 40 dB" - "S9 + 50 dB" 11.3 dB "S9 + 50 dB" - "S9 + 60 dB" 13.5 dB Now let's suppose that you built a new 5 element Yagi antenna and I honestly reported that your signal went from S2 to S6 when you switched to it from your dipole. The gain is really 7 dB, about par for the beam, but you read the postings on the Web and decide that an S-unit is "defined" as 6 dB, so the gain improvement must be 24 dB. Wow! Your modest beam has the same gain as a beam with a 25 WAVELENGTH boom and more than 50 elements! By making that incorrect assumption about the sensitivity of my S-meter, you've overestimated the gain by a factor of 63. S-meter sensitivities vary all over the map, so any assumption you make about how many dB in an S-unit is very likely wrong, and often grossly wrong. Roy Lewallen, W7EL Nail on the head. A step attenunator in line will be the fastest way to determine the linearity of your scale if you can't afford a calibrated generator. FM rigs are all over the map on this because some radios have better limiting than others, and it might be actual limiter reading or some other derived reading. You'll need an adequately-screened signal generator to do this test. If it isn't, the signals which leak out can bypass the attenuator and enter a not-too-well-screened receiver. If this happens, the results you get will be false. -- Ian Good point. If the signal is off the air it is less likely to be an issue. You need to seriously work on your setup to get better than 60 db of resolution. I homebrewed the 3, 6, 10, 20 step attenuator from one of the ARRL publications and find it quite useful. Output cable is double shielded. But, with homebrew attenuators, beware of the RF 'jumping over' the switches at the higher frequencies. This can limit the maximum attenuation attainable to something which is a lot less than the switches indicate. [Well, mine did, anyway!!] -- Ian http://www.arrl.org/notes/hbk-templates/stepatt.pdf This is the one I based mine on. Fewer sections. The Slide switches are superior because of lower inductance and the bulkheads help with isolation. I found there was another version in the TIS section: http://www.arrl.org/tis/info/pdf/9506033.pdf I looked again at mine and it is actually 10, 10 and 20 built on G10 scraps. The blue foam Color TV tuner cleaner helped with insertion loss performance (if you can find it anymore). It seems to work well and in testing with several FM service monitors, seems to be within 1db to UHF. I had built it with T-hunting in mind. For more attenuation I use fixed BNC attenuators. |
dB relation TX/RX
In message , JB
writes "Ian Jackson" wrote in message ... In message , JB writes "Ian Jackson" wrote in message ... In message , JB writes "Roy Lewallen" wrote in message m... ve2pid wrote: Hi to all, Hope that my question has some sense...: If we double the power radiated by an antenna (+3 dB), how does it translate on the S-meter at a receiver 'far' away.. i.e. is the difference in dB on that meter has some mathematical relation to the 3 dB change at the transmitter? 73 de Pierre No, none at all. S-meters almost always just measure the AGC voltage, which is only approximately logarithmic. And manufacturers adjust the sensitivity of the meter to please the customers, who don't like "dead" meters. So the sensitivity of S-meters varies greatly from rig type to type and from one end of the scale to the other. I measured the sensitivity of the S-meter on my Icom 730 on one band. Here's how big an S-unit is on my rig: S1 - S2 1.4 dB S2 - S3 1.3 dB S3 - S4 1.6 dB S4 - S5 2.3 dB S5 - S6 1.8 dB S6 - S7 3.2 dB S7 - S8 3.1 dB S8 - S9 4.0 dB S9 - "S9 + 10 dB" 5.6 dB "S9 + 10 dB" - "S9 + 20 dB" 7.3 dB "S9 + 20 dB" - "S9 + 30 dB" 6.6 dB "S9 + 30 dB" - "S9 + 40 dB" 10.5 dB "S9 + 40 dB" - "S9 + 50 dB" 11.3 dB "S9 + 50 dB" - "S9 + 60 dB" 13.5 dB Now let's suppose that you built a new 5 element Yagi antenna and I honestly reported that your signal went from S2 to S6 when you switched to it from your dipole. The gain is really 7 dB, about par for the beam, but you read the postings on the Web and decide that an S-unit is "defined" as 6 dB, so the gain improvement must be 24 dB. Wow! Your modest beam has the same gain as a beam with a 25 WAVELENGTH boom and more than 50 elements! By making that incorrect assumption about the sensitivity of my S-meter, you've overestimated the gain by a factor of 63. S-meter sensitivities vary all over the map, so any assumption you make about how many dB in an S-unit is very likely wrong, and often grossly wrong. Roy Lewallen, W7EL Nail on the head. A step attenunator in line will be the fastest way to determine the linearity of your scale if you can't afford a calibrated generator. FM rigs are all over the map on this because some radios have better limiting than others, and it might be actual limiter reading or some other derived reading. You'll need an adequately-screened signal generator to do this test. If it isn't, the signals which leak out can bypass the attenuator and enter a not-too-well-screened receiver. If this happens, the results you get will be false. -- Ian Good point. If the signal is off the air it is less likely to be an issue. You need to seriously work on your setup to get better than 60 db of resolution. I homebrewed the 3, 6, 10, 20 step attenuator from one of the ARRL publications and find it quite useful. Output cable is double shielded. But, with homebrew attenuators, beware of the RF 'jumping over' the switches at the higher frequencies. This can limit the maximum attenuation attainable to something which is a lot less than the switches indicate. [Well, mine did, anyway!!] -- Ian http://www.arrl.org/notes/hbk-templates/stepatt.pdf This is the one I based mine on. Fewer sections. The Slide switches are superior because of lower inductance and the bulkheads help with isolation. I found there was another version in the TIS section: http://www.arrl.org/tis/info/pdf/9506033.pdf As the pictures show, there is screening between the switches (even though it's a bit skimpy on the second unit). Even in the first, for the ultimate in isolation, it would be even better if there was an RF gasket on the inside of the top cover which, when screwed down, makes contact with the tops of all the screens, and makes each switch compartment virtually watertight. I looked again at mine and it is actually 10, 10 and 20 built on G10 scraps. The blue foam Color TV tuner cleaner helped with insertion loss performance (if you can find it anymore). It seems to work well and in testing with several FM service monitors, seems to be within 1db to UHF. I had built it with T-hunting in mind. For more attenuation I use fixed BNC attenuators. -- Ian |
dB relation TX/RX
Owen Duffy wrote in
: .... In a typical receiver with a sensitivity of around -135dBm for 10dB S/N, the AGC will be delayed until the signal is about 20dB higher than that, or about -115dBm... so an AGC derived solution cannot indicate less than S2 on that scale. Nevertheless they do. That should read: In a typical receiver with a sensitivity of around -135dBm for 0dB S/N, the AGC will be delayed until the signal is about 20dB higher than that, or about -115dBm... so an AGC derived solution cannot indicate less than S2 on that scale. Nevertheless they do. Owen |
dB relation TX/RX
On Sun, 19 Jul 2009 07:31:01 -0700, Jitt wrote:
In article 753fee20-bfec-4b9a-81b2- , says... Hi to all, Hope that my question has some sense...: If we double the power radiated by an antenna (+3 dB), how does it translate on the S-meter at a receiver 'far' away.. i.e. is the difference in dB on that meter has some mathematical relation to the 3 dB change at the transmitter? 73 de Pierre Only in theory. It is possible to write an expression which relates an increase in Tx power to Rx antenna power at the remote site, but I doubt if it would agree with observed values in the field. If they did not - any linear mode such as SSB would have a pretty hard time working! AE5NE |
dB relation TX/RX
In message , AE5NE
writes On Sun, 19 Jul 2009 07:31:01 -0700, Jitt wrote: In article 753fee20-bfec-4b9a-81b2- , says... Hi to all, Hope that my question has some sense...: If we double the power radiated by an antenna (+3 dB), how does it translate on the S-meter at a receiver 'far' away.. i.e. is the difference in dB on that meter has some mathematical relation to the 3 dB change at the transmitter? 73 de Pierre Only in theory. It is possible to write an expression which relates an increase in Tx power to Rx antenna power at the remote site, but I doubt if it would agree with observed values in the field. If they did not - any linear mode such as SSB would have a pretty hard time working! Aye, Captain. Ye canna change the laws of physics. -- Ian |
dB relation TX/RX
http://www.arrl.org/notes/hbk-templates/stepatt.pdf
This is the one I based mine on. Fewer sections. The Slide switches are superior because of lower inductance and the bulkheads help with isolation. I found there was another version in the TIS section: http://www.arrl.org/tis/info/pdf/9506033.pdf As the pictures show, there is screening between the switches (even though it's a bit skimpy on the second unit). Even in the first, for the ultimate in isolation, it would be even better if there was an RF gasket on the inside of the top cover which, when screwed down, makes contact with the tops of all the screens, and makes each switch compartment virtually watertight. Tin plated solder wick |
dB relation TX/RX
You figure out these things when you have more time than money or sense. I
might not make it to mars before someone with a budget though. Actually it turns out the second version might have better isolation, because it turns out coupling between the resistors and the switch terminals is more unwanted than between the attenuator sections themselves. But it is wise to remember that probably 20db at each switch is the most to be expected because of the coupling within the switch. It is probably a good thing to compartmentalize also, to realize a maximum combined attenuation with all sections engaged. His worry seemed to be trying to keep 50 ohms between sections when all switches are bypassed, but I doubt that there is 50 ohms through the switches themselves. Of course if any section is engaged, the return loss would make it a moot point. I guess in the end, the switch itself becomes the limiting factor in the performance of the thing, as with any RF switching project. Hard to find non-inductive resistors too but surface mount chips might be a good answer to that. |
dB relation TX/RX
In message , JB
writes http://www.arrl.org/notes/hbk-templates/stepatt.pdf This is the one I based mine on. Fewer sections. The Slide switches are superior because of lower inductance and the bulkheads help with isolation. I found there was another version in the TIS section: http://www.arrl.org/tis/info/pdf/9506033.pdf As the pictures show, there is screening between the switches (even though it's a bit skimpy on the second unit). Even in the first, for the ultimate in isolation, it would be even better if there was an RF gasket on the inside of the top cover which, when screwed down, makes contact with the tops of all the screens, and makes each switch compartment virtually watertight. Tin plated solder wick Many CATV modules use a gasket made from the equivalent of a sheet of spongy foam rubber/plastic backed with BacoFoil. -- Ian |
dB relation TX/RX
No one is disagreeing. S-meters are renowned liars. Mind you, I have a CB set
(which has an AGC line) which is converted to 10m, and this has an S-meter which has remarkably consistent 6dB S-points between S2 and S9 +20dB. Even considering an ideal S-meter having a perfect one-S point-per-6 dB response, the answer to the original question is that only in theory doubling the transmit power turns into a 3-dB increase in S-meter reading. In practice what the S-meter "measures" is the sum of the "wanted" signal power, plus the background noise power (which on e.g. 160 meters could be quite high), plus possibly the power other signals falling in the receiver bandwidth. If the wanted signal is not strong enough to overwhelm al other contributions, doubling the transmit power will not turn into a 3-db S-meter reading increase. 73 Tony I0JX, Rome Italy : If we double the power radiated by an antenna (+3 dB), how does it translate on the S-meter at a receiver 'far' away.. |
dB relation TX/RX
On Wed, 22 Jul 2009 18:34:46 +0200, "Antonio Vernucci"
wrote: Even considering an ideal S-meter ... If the wanted signal is not strong enough to overwhelm al other contributions, doubling the transmit power will not turn into a 3-db S-meter reading increase. It would have to. Any additional power to the general spectrum is going to raise the power in the spectrum - even if you cannot distinguish it. That is, afterall, how the spectrum gets to become so noisy. If you are tuned to the radiator that has gone through a power boost, then your S-Meter should indicate it. 73's Richard Clark, KB7QHC |
dB relation TX/RX
Even considering an ideal S-meter ... If the
wanted signal is not strong enough to overwhelm al other contributions, doubling the transmit power will not turn into a 3-db S-meter reading increase. It would have to. Any additional power to the general spectrum is going to raise the power in the spectrum - even if you cannot distinguish it. That is, afterall, how the spectrum gets to become so noisy. If you are tuned to the radiator that has gone through a power boost, then your S-Meter should indicate it. 73's Richard Clark, KB7QHC Yes, but not quite 3dB when transmit power is doubled. A 3-dB increase would only occur if the "wanted" signal would be alone in the receiver bandwidth (viz no background noise, no interfering signal). 73 Tony I0JX |
dB relation TX/RX
On Wed, 22 Jul 2009 19:35:40 +0200, "Antonio Vernucci"
wrote: Yes, but not quite 3dB when transmit power is doubled. It would be dependent upon S+N/N, yes. 73's Richard Clark, KB7QHC |
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