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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 |
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