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Paul Burridge wrote: Hi all, I wanted to build an RF relative field strength meter, so set about searching on the Web for any existing designs. Those I turned up weren't particularly impressive, so I decided to start from scratch and design my own. I've just completed that this afternoon. I've allowed for 0.25mV input to give rise to FSD on the microammeter. Question being, however, is that going to be sensitive enough? Yes. If it's not, either move closer, or add an op amp with a gain control after the diode(s) & cap. Mine uses 2 gain controls - 1 on the input to the op amp, and one that sets the gain of the op amp. A further improvement is the use of a cheap DPM - no parallax and a wider "full scale" range with no loss of sensitivity. The downside of the cheap DPM was the need for 2 9 volt batteries. (I have since built a small DC-DC converter that occupies the volume of a single 9V battery. That may allow using a single rechargeable 9V to power the converter - but I haven't tried it yet to be sure that the converter doesn't produce noise that would be detected by the instrument. The converter does produce dual 9V fully isolated outputs at at least 10 ma per output, way more than the instrument needs.) Does anyone have any idea what the field strength in microvolts or millivolts is from a half Watt transmitter at about 6 feet away? I guess I should have posed this question *before* designing it, but who among us can honestly say they haven't designed something without knowing what the spec is? :-) Anyway, ballpark figures gentlemen, please. p. -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
Mike Andrews wrote:
The Netscape 4.6 browser on the same system _does_ put the black text directly on the dark-blue background, and it is decidedly unpleasant. Mozilla 1.4a doesn't have any problems. A newer browser might be nice, but it also would be good if web page designers built pages with older code in mind. Like trying to accommodate fighting parents, having Micro$oft constantly extending and subverting standards. It _definitely_ has some cool stuff. Thanks, Scott! Thanks. Any things you like more than others? When I make time, I plan on documenting my tinkering with windows html/hta app to md5 sign and test your pc files, laser-pointer velocimeter and interferometers, PIC hardware and software for alarm system, and micro-power radar sensors. I should update the bookmarks and clean all the dead-links too. -- Scott ********************************** DIY Piezo-Gyro, PCB Drill Bot & More Soon! http://home.comcast.net/~scottxs/ ********************************** |
Mike Andrews wrote:
The Netscape 4.6 browser on the same system _does_ put the black text directly on the dark-blue background, and it is decidedly unpleasant. Mozilla 1.4a doesn't have any problems. A newer browser might be nice, but it also would be good if web page designers built pages with older code in mind. Like trying to accommodate fighting parents, having Micro$oft constantly extending and subverting standards. It _definitely_ has some cool stuff. Thanks, Scott! Thanks. Any things you like more than others? When I make time, I plan on documenting my tinkering with windows html/hta app to md5 sign and test your pc files, laser-pointer velocimeter and interferometers, PIC hardware and software for alarm system, and micro-power radar sensors. I should update the bookmarks and clean all the dead-links too. -- Scott ********************************** DIY Piezo-Gyro, PCB Drill Bot & More Soon! http://home.comcast.net/~scottxs/ ********************************** |
Paul Burridge wrote:
I'll add that my Optoelectronics handi-counter's fsm bargraph can detect a few hundred milliwatts at several feet away. I visualize a radio-field power density being grid-lines painted on a balloon around a 1/2 wave dipole. As the balloon is blown up, the grid-line expand, given by 4 pi r^2. For some good app notes on designing electrically small antennas and radio link calculations: Goto - http://www.rfm.com/corp/apnotes.htm Checkout: http://www.rfm.com/corp/appdata/antenna.pdf http://www.rfm.com/products/tr_des24.pdf HP (now Agilent) has some app-notes on designing diode detectors, and has some utility programs that will help you design and calculate performance. Some I think, in context of RFID reading devices. But they point out you get a square-law for small signals, which gets nonlinear for larger signal levels. And what bias the detector diode has on it. -- Scott ********************************** DIY Piezo-Gyro, PCB Drill Bot & More Soon! http://home.comcast.net/~scottxs/ ********************************** |
Paul Burridge wrote:
I'll add that my Optoelectronics handi-counter's fsm bargraph can detect a few hundred milliwatts at several feet away. I visualize a radio-field power density being grid-lines painted on a balloon around a 1/2 wave dipole. As the balloon is blown up, the grid-line expand, given by 4 pi r^2. For some good app notes on designing electrically small antennas and radio link calculations: Goto - http://www.rfm.com/corp/apnotes.htm Checkout: http://www.rfm.com/corp/appdata/antenna.pdf http://www.rfm.com/products/tr_des24.pdf HP (now Agilent) has some app-notes on designing diode detectors, and has some utility programs that will help you design and calculate performance. Some I think, in context of RFID reading devices. But they point out you get a square-law for small signals, which gets nonlinear for larger signal levels. And what bias the detector diode has on it. -- Scott ********************************** DIY Piezo-Gyro, PCB Drill Bot & More Soon! http://home.comcast.net/~scottxs/ ********************************** |
Michael A. Terrell wrote:
Your web site is very hard to read with the dark blue background and black text. A lot of people have vision problems, and can not read this color combination. Sorry. The next time I update it I'll see if I can find a better color. I think I noticed that once while I was developing it - I think it is due to a low-color VGA mode. I spent too long fooling around trying to develop the fractal-smith chart background (thanks to da GIMP) to mess with it any further. You can always edit the HTML yourself and change my background color. Its not like I've got advertisers paying me. -- Scott ********************************** DIY Piezo-Gyro, PCB Drill Bot & More Soon! http://home.comcast.net/~scottxs/ ********************************** |
Michael A. Terrell wrote:
Your web site is very hard to read with the dark blue background and black text. A lot of people have vision problems, and can not read this color combination. Sorry. The next time I update it I'll see if I can find a better color. I think I noticed that once while I was developing it - I think it is due to a low-color VGA mode. I spent too long fooling around trying to develop the fractal-smith chart background (thanks to da GIMP) to mess with it any further. You can always edit the HTML yourself and change my background color. Its not like I've got advertisers paying me. -- Scott ********************************** DIY Piezo-Gyro, PCB Drill Bot & More Soon! http://home.comcast.net/~scottxs/ ********************************** |
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On Mon, 15 Dec 2003 07:16:35 GMT, Active8
,invalid wrote: Yes, but I thought the 40MHz project was *not* bot related. Nope. *All* the 40Mhz stuff I post about is 'bot related. That's the r/c band we're permitted to use. There's an alternative option to use something up in UHF but I'm giving that a miss for obvious reasons. :-) -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On Mon, 15 Dec 2003 07:16:35 GMT, Active8
,invalid wrote: Yes, but I thought the 40MHz project was *not* bot related. Nope. *All* the 40Mhz stuff I post about is 'bot related. That's the r/c band we're permitted to use. There's an alternative option to use something up in UHF but I'm giving that a miss for obvious reasons. :-) -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
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On Sun, 14 Dec 2003 06:31:14 GMT, "Michael A. Terrell"
wrote: Some websites are a royal pain. I recently ran into an electronics distributor who put their entire website in "Flash". There is no way I will wait five minutes or more per page to download and run stupid animation when I am looking for parts. I share your opinion completelly; I hate Flash made sites & banners, but not downlodable fun stuff in Flash ... /like jokes/ :-) -- Regards, SPAJKY & visit site - http://www.spajky.vze.com Celly-III OC-ed,"Tualatin on BX-Slot1-MoBo!" E-mail AntiSpam: remove ## |
On Sun, 14 Dec 2003 06:31:14 GMT, "Michael A. Terrell"
wrote: Some websites are a royal pain. I recently ran into an electronics distributor who put their entire website in "Flash". There is no way I will wait five minutes or more per page to download and run stupid animation when I am looking for parts. I share your opinion completelly; I hate Flash made sites & banners, but not downlodable fun stuff in Flash ... /like jokes/ :-) -- Regards, SPAJKY & visit site - http://www.spajky.vze.com Celly-III OC-ed,"Tualatin on BX-Slot1-MoBo!" E-mail AntiSpam: remove ## |
On Sun, 14 Dec 2003 03:18:35 GMT, Active8
,invalid wrote: Refresh my ram. | E(uV/m) | V (dBmV) = 20log | --------- / 1000 | | 0.021f(MHz) | plus correction for distance (regulations for limits are for specific measuring distances), etc. I'll mull the above eq over. Gotta figure out where the .021 came from, but not now. I use simplified formula for practical life with also some antenna mismatchings & soft obstacles in-calculated: dBm = ~90dB x 1Mhz x 1km or better dBm = 100dB x 1Mhz x 1km for having better Rx SINAD since voltage on Rx antenna drops linearly with rising a frequency & distance & vice-versa. From dBm´s got I just recalculate RX voltage on RX input simple & effective calculation IMHO ... -- Regards, SPAJKY & visit site - http://www.spajky.vze.com Celly-III OC-ed,"Tualatin on BX-Slot1-MoBo!" E-mail AntiSpam: remove ## |
On Sun, 14 Dec 2003 03:18:35 GMT, Active8
,invalid wrote: Refresh my ram. | E(uV/m) | V (dBmV) = 20log | --------- / 1000 | | 0.021f(MHz) | plus correction for distance (regulations for limits are for specific measuring distances), etc. I'll mull the above eq over. Gotta figure out where the .021 came from, but not now. I use simplified formula for practical life with also some antenna mismatchings & soft obstacles in-calculated: dBm = ~90dB x 1Mhz x 1km or better dBm = 100dB x 1Mhz x 1km for having better Rx SINAD since voltage on Rx antenna drops linearly with rising a frequency & distance & vice-versa. From dBm´s got I just recalculate RX voltage on RX input simple & effective calculation IMHO ... -- Regards, SPAJKY & visit site - http://www.spajky.vze.com Celly-III OC-ed,"Tualatin on BX-Slot1-MoBo!" E-mail AntiSpam: remove ## |
On Mon, 15 Dec 2003 06:59:30 GMT, said...
Paul Burridge wrote: I'll add that my Optoelectronics handi-counter's fsm bargraph can detect a few hundred milliwatts at several feet away. I visualize a radio-field power density being grid-lines painted on a balloon around a 1/2 wave dipole. As the balloon is blown up, the grid-line expand, given by 4 pi r^2. For some good app notes on designing electrically small antennas and radio link calculations: Goto - http://www.rfm.com/corp/apnotes.htm Checkout: http://www.rfm.com/corp/appdata/antenna.pdf http://www.rfm.com/products/tr_des24.pdf HP (now Agilent) has some app-notes on designing diode detectors, and has some utility programs that will help you design and calculate performance. Some I think, in context of RFID reading devices. But they point out you get a square-law for small signals, which gets nonlinear for larger signal levels. And what bias the detector diode has on it. Any particular search string to use for that Agilent stuff? They're usually good for info on stuff like that. Mike |
Active8 wrote:
On Mon, 15 Dec 2003 06:59:30 GMT, said... HP (now Agilent) has some app-notes on designing diode detectors Any particular search string to use for that Agilent stuff? They're usually good for info on stuff like that. I though Agilent made you answer a bunch of questions before they would let you at their stuff. Doing a web search on one of the app notes I downloaded, I get: http://rf.rfglobalnet.com/library/Ap...s/1/An1089.pdf RF Globalnet's got lots of good stuff. Probably where I got the design apps from too. HP app notes I collected: pin diode pi atten an1048 pin diode switch an1049 x-band mixer an1052 pin diode freq mult. an1054 (comb gen) pin diode t/r switch an1067 schottkey diode apps an1069 schot. diode det - RFID an1089 atf-36163 10GHz LNA an1091 pin diode switch an957-1,2,3 z-mtchng for mix & dets an963 0-bias schotky diode an969 broadbnd mixers an976 square-law detectors an986 diode bias an987 zero-bias an988 harmonic mixing an991 shttky diode mixer an995 choke network design ana001 osc design ana008 vco design anm024 5ghz active mixer ans010 transistors primer1 noise and s-parms primer2 thermal primer3 thermal primer3a gasfets primer4 HP product line quickguide "Designing Detectors for RF/ID Tags Application Note 1089" -- Scott ********************************** DIY Piezo-Gyro, PCB Drill Bot & More Soon! http://home.comcast.net/~scottxs/ ********************************** |
Active8 wrote:
On Mon, 15 Dec 2003 06:59:30 GMT, said... HP (now Agilent) has some app-notes on designing diode detectors Any particular search string to use for that Agilent stuff? They're usually good for info on stuff like that. I though Agilent made you answer a bunch of questions before they would let you at their stuff. Doing a web search on one of the app notes I downloaded, I get: http://rf.rfglobalnet.com/library/Ap...s/1/An1089.pdf RF Globalnet's got lots of good stuff. Probably where I got the design apps from too. HP app notes I collected: pin diode pi atten an1048 pin diode switch an1049 x-band mixer an1052 pin diode freq mult. an1054 (comb gen) pin diode t/r switch an1067 schottkey diode apps an1069 schot. diode det - RFID an1089 atf-36163 10GHz LNA an1091 pin diode switch an957-1,2,3 z-mtchng for mix & dets an963 0-bias schotky diode an969 broadbnd mixers an976 square-law detectors an986 diode bias an987 zero-bias an988 harmonic mixing an991 shttky diode mixer an995 choke network design ana001 osc design ana008 vco design anm024 5ghz active mixer ans010 transistors primer1 noise and s-parms primer2 thermal primer3 thermal primer3a gasfets primer4 HP product line quickguide "Designing Detectors for RF/ID Tags Application Note 1089" -- Scott ********************************** DIY Piezo-Gyro, PCB Drill Bot & More Soon! http://home.comcast.net/~scottxs/ ********************************** |
On Tue, 16 Dec 2003 23:56:14 GMT, said...
Active8 wrote: On Mon, 15 Dec 2003 06:59:30 GMT, said... HP (now Agilent) has some app-notes on designing diode detectors Any particular search string to use for that Agilent stuff? They're usually good for info on stuff like that. I though Agilent made you answer a bunch of questions before they would let you at their stuff. Doing a web search on one of the app notes I downloaded, I get: I don't remember having to do that. I better get a sneakemail (.com) addys ready for the next registration proceeses, though :) http://rf.rfglobalnet.com/library/Ap...s/1/An1089.pdf Ah. RFID. Big market in the works. RF Globalnet's got lots of good stuff. Probably where I got the design apps from too. Thanks. You're right. I haven't had time to hit RF Globalnet in a while. I used to get the news letter. Tnx for the reminder. I wish I had more time. Maybe someone has the time to collect all our app notes and get permission to organize them with comprehensive synopses and mirror them on a free site. Maybe qsl.net or something. HP app notes I collected: That's all? ;) Oh, just from HP, ok. Mike pin diode pi atten an1048 pin diode switch an1049 x-band mixer an1052 pin diode freq mult. an1054 (comb gen) pin diode t/r switch an1067 schottkey diode apps an1069 schot. diode det - RFID an1089 atf-36163 10GHz LNA an1091 pin diode switch an957-1,2,3 z-mtchng for mix & dets an963 0-bias schotky diode an969 broadbnd mixers an976 square-law detectors an986 diode bias an987 zero-bias an988 harmonic mixing an991 shttky diode mixer an995 choke network design ana001 osc design ana008 vco design anm024 5ghz active mixer ans010 transistors primer1 noise and s-parms primer2 thermal primer3 thermal primer3a gasfets primer4 HP product line quickguide "Designing Detectors for RF/ID Tags Application Note 1089" |
On Tue, 16 Dec 2003 23:56:14 GMT, said...
Active8 wrote: On Mon, 15 Dec 2003 06:59:30 GMT, said... HP (now Agilent) has some app-notes on designing diode detectors Any particular search string to use for that Agilent stuff? They're usually good for info on stuff like that. I though Agilent made you answer a bunch of questions before they would let you at their stuff. Doing a web search on one of the app notes I downloaded, I get: I don't remember having to do that. I better get a sneakemail (.com) addys ready for the next registration proceeses, though :) http://rf.rfglobalnet.com/library/Ap...s/1/An1089.pdf Ah. RFID. Big market in the works. RF Globalnet's got lots of good stuff. Probably where I got the design apps from too. Thanks. You're right. I haven't had time to hit RF Globalnet in a while. I used to get the news letter. Tnx for the reminder. I wish I had more time. Maybe someone has the time to collect all our app notes and get permission to organize them with comprehensive synopses and mirror them on a free site. Maybe qsl.net or something. HP app notes I collected: That's all? ;) Oh, just from HP, ok. Mike pin diode pi atten an1048 pin diode switch an1049 x-band mixer an1052 pin diode freq mult. an1054 (comb gen) pin diode t/r switch an1067 schottkey diode apps an1069 schot. diode det - RFID an1089 atf-36163 10GHz LNA an1091 pin diode switch an957-1,2,3 z-mtchng for mix & dets an963 0-bias schotky diode an969 broadbnd mixers an976 square-law detectors an986 diode bias an987 zero-bias an988 harmonic mixing an991 shttky diode mixer an995 choke network design ana001 osc design ana008 vco design anm024 5ghz active mixer ans010 transistors primer1 noise and s-parms primer2 thermal primer3 thermal primer3a gasfets primer4 HP product line quickguide "Designing Detectors for RF/ID Tags Application Note 1089" |
Sorry for the slow response.
The voltage at the terminals of a short, lossless dipole of length l which is perfectly terminated with the complex conjugate of its feedpoint impedance is E * l / 2 where E is the impinging field in volts per meter. Any mismatch lowers this value -- it's one of those valid and useful applications of the concept of "mismatch loss", which can be used to quantify the reduction when mismatched. It's one of the contributions to the extensive variation with frequency of the "antenna factor"(*) of test dipoles used in EMI work. (And careful measurement of this factor is a large fraction of what you're paying for when you purchase one.) Doesn't matter how wonderful your field strength meter is unless you have a very good handle on the "antenna factor" of the test antenna. In simple terms, the factor of 1/2 is the ratio of the average current to the feedpoint current. The average current along a short dipole is about 1/2 the current at the feedpoint. You could get a factor of 1 if you used a short dipole with large end hats to make the current distribution uniform. Kraus addresses this case in detail in _Antennas_. And no, I don't mean "antenna to free space" match. I mean the match between the antenna feedpoint as a source and the detector as a load. (*) Ratio of output voltage when terminated in 50 ohms to the impinging field in volts/meter. Roy Lewallen, W7EL Active8 wrote: On Sat, 13 Dec 2003 17:39:27 -0800, said... It's too bad it isn't that simple. A 1V/m field doesn't result in one volt at the feedpoint of a perfectly matched one meter dipole or monopole, and the value it does induce depends on the quality of the impedance match you mean antenna to free space, right? as well as the fraction of a wavelength the one meter antenna length represents. amplify, very please. por favor. Refresh my ram. | E(uV/m) | V (dBmV) = 20log | --------- / 1000 | | 0.021f(MHz) | plus correction for distance (regulations for limits are for specific measuring distances), etc. I'll mull the above eq over. Gotta figure out where the .021 came from, but not now. My eyes are getting fatigued from this 'puter. And, if one volt does appear at the feedpoint, it's very unlikely that a simple circuit will measure it as one volt. It would have to be calibrated to compensate for the circuit. Maybe that's why it's called a "relative" field strength meter. Relative to another signal or no signal ;) Probably best to stick with your $1.5 kilobuck meter if you really want to measure field strength. I don't. He does :) At least not tonight. But my SLM *will* measure field strenth using a cheap ass dipole cut to the frequency of interest with or without an external preamp and do it to the satisfaction of the FCC, assuming it's calibrated. I even have a near-field probe, not so cheap. It beats guess work. I wouldn't expect his sniffer to be real accurate but he did ask for guesstimates. Started off as "around 4 feet" for a half watter now we're at 100mW - prob his reference Tx. BRs, Mike Roy Lewallen, W7EL Active8 wrote: On 13 Dec 2003 03:50:41 -0800, said... What you are descrbing is a "signal sniffer", not a signal strength meter. Who are you replying to? Paul did not say Signal Stength Meter, but the guy you replied to kinda hints at it when he mentions power level. I would have to say that my 1GHz Signal Level Meter, which cost $1500 would be the better than a relative field strength meter, but if he measures say, 1V with his Tx off and 2V with it on, then that's 1V and if his Rx antenna is a 1m dipole, that's 1V/m. Mike |
Sorry for the slow response.
The voltage at the terminals of a short, lossless dipole of length l which is perfectly terminated with the complex conjugate of its feedpoint impedance is E * l / 2 where E is the impinging field in volts per meter. Any mismatch lowers this value -- it's one of those valid and useful applications of the concept of "mismatch loss", which can be used to quantify the reduction when mismatched. It's one of the contributions to the extensive variation with frequency of the "antenna factor"(*) of test dipoles used in EMI work. (And careful measurement of this factor is a large fraction of what you're paying for when you purchase one.) Doesn't matter how wonderful your field strength meter is unless you have a very good handle on the "antenna factor" of the test antenna. In simple terms, the factor of 1/2 is the ratio of the average current to the feedpoint current. The average current along a short dipole is about 1/2 the current at the feedpoint. You could get a factor of 1 if you used a short dipole with large end hats to make the current distribution uniform. Kraus addresses this case in detail in _Antennas_. And no, I don't mean "antenna to free space" match. I mean the match between the antenna feedpoint as a source and the detector as a load. (*) Ratio of output voltage when terminated in 50 ohms to the impinging field in volts/meter. Roy Lewallen, W7EL Active8 wrote: On Sat, 13 Dec 2003 17:39:27 -0800, said... It's too bad it isn't that simple. A 1V/m field doesn't result in one volt at the feedpoint of a perfectly matched one meter dipole or monopole, and the value it does induce depends on the quality of the impedance match you mean antenna to free space, right? as well as the fraction of a wavelength the one meter antenna length represents. amplify, very please. por favor. Refresh my ram. | E(uV/m) | V (dBmV) = 20log | --------- / 1000 | | 0.021f(MHz) | plus correction for distance (regulations for limits are for specific measuring distances), etc. I'll mull the above eq over. Gotta figure out where the .021 came from, but not now. My eyes are getting fatigued from this 'puter. And, if one volt does appear at the feedpoint, it's very unlikely that a simple circuit will measure it as one volt. It would have to be calibrated to compensate for the circuit. Maybe that's why it's called a "relative" field strength meter. Relative to another signal or no signal ;) Probably best to stick with your $1.5 kilobuck meter if you really want to measure field strength. I don't. He does :) At least not tonight. But my SLM *will* measure field strenth using a cheap ass dipole cut to the frequency of interest with or without an external preamp and do it to the satisfaction of the FCC, assuming it's calibrated. I even have a near-field probe, not so cheap. It beats guess work. I wouldn't expect his sniffer to be real accurate but he did ask for guesstimates. Started off as "around 4 feet" for a half watter now we're at 100mW - prob his reference Tx. BRs, Mike Roy Lewallen, W7EL Active8 wrote: On 13 Dec 2003 03:50:41 -0800, said... What you are descrbing is a "signal sniffer", not a signal strength meter. Who are you replying to? Paul did not say Signal Stength Meter, but the guy you replied to kinda hints at it when he mentions power level. I would have to say that my 1GHz Signal Level Meter, which cost $1500 would be the better than a relative field strength meter, but if he measures say, 1V with his Tx off and 2V with it on, then that's 1V and if his Rx antenna is a 1m dipole, that's 1V/m. Mike |
On Tue, 16 Dec 2003 22:38:50 -0800, said...
Sorry for the slow response. s'ok. but i don't think my stinkin' reader alerted me to the response. or I missed it. The voltage at the terminals of a short, lossless dipole of length l which is perfectly terminated with the complex conjugate of its feedpoint impedance is E * l / 2 where E is the impinging field in volts per meter. Any mismatch lowers this value -- it's one of those valid and useful applications of the concept of "mismatch loss", which can be used to quantify the reduction when mismatched. It's one of the contributions to the extensive variation with frequency of the "antenna factor"(*) of test dipoles used in EMI work. (And careful measurement of this factor is a large fraction of what you're paying for when you purchase one.) Doesn't matter how wonderful your field strength meter is unless you have a very good handle on the "antenna factor" of the test antenna. I'll dig into the Antenna Engrs Handbook. I guess we've been lucky. So far no FCC gigs on leakage. Haven't heard about anyone failing a flyover either. Maybe the systems consider the FCC spec and set their spec to account for cheap-assed dipoles. If so, that's smart and I'd be surprised. Some day I'll check that out. In simple terms, the factor of 1/2 is the ratio of the average current to the feedpoint current. The average current along a short dipole is about 1/2 the current at the feedpoint. You could get a factor of 1 if you used a short dipole with large end hats to make the current distribution uniform. Kraus addresses this case in detail in _Antennas_. And no, I don't mean "antenna to free space" match. I mean the match between the antenna feedpoint as a source and the detector as a load. That's a given. I figure Paul already knows that. BRs, Mike (*) Ratio of output voltage when terminated in 50 ohms to the impinging field in volts/meter. Roy Lewallen, W7EL Active8 wrote: On Sat, 13 Dec 2003 17:39:27 -0800, said... It's too bad it isn't that simple. A 1V/m field doesn't result in one volt at the feedpoint of a perfectly matched one meter dipole or monopole, and the value it does induce depends on the quality of the impedance match you mean antenna to free space, right? as well as the fraction of a wavelength the one meter antenna length represents. amplify, very please. por favor. Refresh my ram. | E(uV/m) | V (dBmV) = 20log | --------- / 1000 | | 0.021f(MHz) | plus correction for distance (regulations for limits are for specific measuring distances), etc. I'll mull the above eq over. Gotta figure out where the .021 came from, but not now. My eyes are getting fatigued from this 'puter. And, if one volt does appear at the feedpoint, it's very unlikely that a simple circuit will measure it as one volt. It would have to be calibrated to compensate for the circuit. Maybe that's why it's called a "relative" field strength meter. Relative to another signal or no signal ;) Probably best to stick with your $1.5 kilobuck meter if you really want to measure field strength. I don't. He does :) At least not tonight. But my SLM *will* measure field strenth using a cheap ass dipole cut to the frequency of interest with or without an external preamp and do it to the satisfaction of the FCC, assuming it's calibrated. I even have a near-field probe, not so cheap. It beats guess work. I wouldn't expect his sniffer to be real accurate but he did ask for guesstimates. Started off as "around 4 feet" for a half watter now we're at 100mW - prob his reference Tx. BRs, Mike Roy Lewallen, W7EL Active8 wrote: On 13 Dec 2003 03:50:41 -0800, said... What you are descrbing is a "signal sniffer", not a signal strength meter. Who are you replying to? Paul did not say Signal Stength Meter, but the guy you replied to kinda hints at it when he mentions power level. I would have to say that my 1GHz Signal Level Meter, which cost $1500 would be the better than a relative field strength meter, but if he measures say, 1V with his Tx off and 2V with it on, then that's 1V and if his Rx antenna is a 1m dipole, that's 1V/m. Mike |
On Tue, 16 Dec 2003 22:38:50 -0800, said...
Sorry for the slow response. s'ok. but i don't think my stinkin' reader alerted me to the response. or I missed it. The voltage at the terminals of a short, lossless dipole of length l which is perfectly terminated with the complex conjugate of its feedpoint impedance is E * l / 2 where E is the impinging field in volts per meter. Any mismatch lowers this value -- it's one of those valid and useful applications of the concept of "mismatch loss", which can be used to quantify the reduction when mismatched. It's one of the contributions to the extensive variation with frequency of the "antenna factor"(*) of test dipoles used in EMI work. (And careful measurement of this factor is a large fraction of what you're paying for when you purchase one.) Doesn't matter how wonderful your field strength meter is unless you have a very good handle on the "antenna factor" of the test antenna. I'll dig into the Antenna Engrs Handbook. I guess we've been lucky. So far no FCC gigs on leakage. Haven't heard about anyone failing a flyover either. Maybe the systems consider the FCC spec and set their spec to account for cheap-assed dipoles. If so, that's smart and I'd be surprised. Some day I'll check that out. In simple terms, the factor of 1/2 is the ratio of the average current to the feedpoint current. The average current along a short dipole is about 1/2 the current at the feedpoint. You could get a factor of 1 if you used a short dipole with large end hats to make the current distribution uniform. Kraus addresses this case in detail in _Antennas_. And no, I don't mean "antenna to free space" match. I mean the match between the antenna feedpoint as a source and the detector as a load. That's a given. I figure Paul already knows that. BRs, Mike (*) Ratio of output voltage when terminated in 50 ohms to the impinging field in volts/meter. Roy Lewallen, W7EL Active8 wrote: On Sat, 13 Dec 2003 17:39:27 -0800, said... It's too bad it isn't that simple. A 1V/m field doesn't result in one volt at the feedpoint of a perfectly matched one meter dipole or monopole, and the value it does induce depends on the quality of the impedance match you mean antenna to free space, right? as well as the fraction of a wavelength the one meter antenna length represents. amplify, very please. por favor. Refresh my ram. | E(uV/m) | V (dBmV) = 20log | --------- / 1000 | | 0.021f(MHz) | plus correction for distance (regulations for limits are for specific measuring distances), etc. I'll mull the above eq over. Gotta figure out where the .021 came from, but not now. My eyes are getting fatigued from this 'puter. And, if one volt does appear at the feedpoint, it's very unlikely that a simple circuit will measure it as one volt. It would have to be calibrated to compensate for the circuit. Maybe that's why it's called a "relative" field strength meter. Relative to another signal or no signal ;) Probably best to stick with your $1.5 kilobuck meter if you really want to measure field strength. I don't. He does :) At least not tonight. But my SLM *will* measure field strenth using a cheap ass dipole cut to the frequency of interest with or without an external preamp and do it to the satisfaction of the FCC, assuming it's calibrated. I even have a near-field probe, not so cheap. It beats guess work. I wouldn't expect his sniffer to be real accurate but he did ask for guesstimates. Started off as "around 4 feet" for a half watter now we're at 100mW - prob his reference Tx. BRs, Mike Roy Lewallen, W7EL Active8 wrote: On 13 Dec 2003 03:50:41 -0800, said... What you are descrbing is a "signal sniffer", not a signal strength meter. Who are you replying to? Paul did not say Signal Stength Meter, but the guy you replied to kinda hints at it when he mentions power level. I would have to say that my 1GHz Signal Level Meter, which cost $1500 would be the better than a relative field strength meter, but if he measures say, 1V with his Tx off and 2V with it on, then that's 1V and if his Rx antenna is a 1m dipole, that's 1V/m. Mike |
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