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Ho-made non-inductive resitor WAS: Folded Dipole Antenna
In message , Richard Clark
writes On Thu, 9 Dec 2010 19:21:48 -0800 (PST), JIMMIE wrote: I built a wattmeter of sorts around a circuit like that. After tuning up the transmitter to light bulb dummy load I would measure the resistnace of a photo resistor exposed to the light then see how much 60Hz AC voltage and current took to light the lamps to the same brillance. Hi Jimmie, An excellent example of the concept of a transfer standard. This method is one of the oldest and most robust methods of accurate measurement. It removes a lot of variables (by, ironically, not changing them) that would be exceedingly difficult to determine their impact upon an accurate measurement. Do a Google on "Grease Spot Photometer" (back to school physics, over 50 years ago!). Very simple to make, and pretty accurate. -- Ian |
Ho-made non-inductive resitor WAS: Folded Dipole Antenna
On Fri, 10 Dec 2010 19:52:55 +0000, Ian Jackson
wrote: Do a Google on "Grease Spot Photometer" (back to school physics, over 50 years ago!). Very simple to make, and pretty accurate. Hi Ian, A very good suggestion which immediately led me to: http://www.phy6.org/outreach/edu/greaspot.htm A variation of this that I calibrated in the lab is an Optical Pyrometer: http://www.pyrometer.com/Pyro_Optical.html Comparison measurements can be very accurate iff what you are comparing to (aka standard) is known to sufficient accuracy. Both references provide more than enough to inform the reader with the essential details. Slightly more elaborate methods of measuring RF power fall into the Calorimetric method (actual heat). Hewlett Packard made the ultimate heat transfer standard - the HP 434A power meter DC to 12GHz! 73's Richard Clark, KB7QHC |
Ho-made non-inductive resitor WAS: Folded Dipole Antenna
Richard Clark wrote:
On Fri, 10 Dec 2010 19:52:55 +0000, Ian Jackson wrote: Do a Google on "Grease Spot Photometer" (back to school physics, over 50 years ago!). Very simple to make, and pretty accurate. Hi Ian, A very good suggestion which immediately led me to: http://www.phy6.org/outreach/edu/greaspot.htm A variation of this that I calibrated in the lab is an Optical Pyrometer: http://www.pyrometer.com/Pyro_Optical.html Comparison measurements can be very accurate iff what you are comparing to (aka standard) is known to sufficient accuracy. Both references provide more than enough to inform the reader with the essential details. Slightly more elaborate methods of measuring RF power fall into the Calorimetric method (actual heat). Hewlett Packard made the ultimate heat transfer standard - the HP 434A power meter DC to 12GHz! I think HP made a number of DC substitution/transfer sorts of power meters The 434 was unique in using flowing oil and it could directly measure watts, without needing attenuators (which have their own calibration issues). Didn't the 432 used the idea of DC power substituting for RF power to bring the sum to a fixed temperature? The difference between the 432 and the 434 is that the thing measuring the temperature is also the RF load in the 432. The 434 just uses the RF as a heater, and relies on the DC powered heater and RF powered heater being matched. A NIST Type IV power meter (like those from Arbiter) definitely does DC power substitution, and uses the HP/Agilent thermistor heads. Once you get away from "replacement heat" sorts of schemes, you'd be into the classic calorimeter.. measure the temperature change over time, and then turn that into energy. You'd calibrate it by putting DC on the same sensor, essentially measuring the thermal capacity. The thermocouple heads (8481A, 8482A for instance) work pretty much the same way as the thermistor mounts.. measuring the heat dissipated by the RF power coming in. They're not a substitution measurement though.. The actual sensor changes voltage in response to temperature (with a clever compensation scheme so that overall temperature doesn't affect it) But these are all basically thermal sensors (as opposed to, say, RF voltmeters, like a diode detector as in a 8481D or 8484A head) and they dissipate the RF power being measured. 73's Richard Clark, KB7QHC |
Ho-made non-inductive resitor WAS: Folded Dipole Antenna
On Fri, 10 Dec 2010 17:09:25 -0800, Jim Lux
wrote: Didn't the 432 used the idea of DC power substituting for RF power to bring the sum to a fixed temperature? The difference between the 432 and the 434 is that the thing measuring the temperature is also the RF load in the 432. The 434 just uses the RF as a heater, and relies on the DC powered heater and RF powered heater being matched. Hi Jim, I worked with a world of different methods of measuring RF power. Thermistors, Thermocouples (the 8481A, 8482 you identify - refer to HP Application Note 64-1, "Fundamentals of RF and Microwave Power Measurements"), and the older technologies of Thermopile, Wollaston wire, Bolometers (which encompass the same things as those already mentioned), and Barreters (a variation upon, or exactly the same as the Wollaston wire - I've seen 10mA fuses used for the same purpose), diodes certainly (generally for peak power). I have had the occasion to burn out more than a couple of these. Those in a bridge configurations (many in fact as the 434 is an example) are temperature tracking. An excellent description can be found on page 14 of: http://www.hpmemory.org/an/pdf/an_64-1a.pdf for a thermistor bridge that compensates for ambient heat. "The fundamental premise in using a thermistor for power measurements is that the RF power absorbed by the thermistor has the same heating effect on the thermistor as the DC power." This from the HP432A which has a dual thermistor, dual bridge design. Pages 18 through 20 describe how heats are separated in a thermocouple bridge to compensate for ambient. From page 26 is discussion of diodes. As for accuracies: "All thermocouple and diode power sensors require a power reference to absolute power, traceable to the manufacturer or national standards." A good remainder of the application note goes into the issues of accurate determination. Pages 51 and 60 each has a table of all the various sources of error (rarely considered outside of the Metrology Lab, but ever present nonetheless). This was standard consideration and the examples the author offers yields roughly 5% accurate measurements from the best of instrumentation - an accuracy figure that I frequently read here as commonly available from the Bird RF meter (and I have the experience to the matter having calibrated these meters to know that is a fantasy of the first order). 73's Richard Clark, KB7QHC |
Quote:
i'm wanna design multiband antenna, not sure about impedance.. |
Folded Dipole Antenna
On Sat, 11 Dec 2010 16:56:34 +0000, frankieNrosie
wrote: freq=5.8GHz i'm wanna design multiband antenna, not sure about impedance.. If you want one frequency, then it would imply one band. If you want multiband, then you need to give us the other frequencies. 73's Richard Clark, KB7QHC |
Quote:
i use the diode as the switch to reconfigure the freq..later on i will upload the figure of proposed antenna |
Folded Dipole Antenna
On Sun, 12 Dec 2010 12:18:11 +0000, frankieNrosie
wrote: If you want one frequency, then it would imply one band. If you want multiband, then you need to give us the other frequencies. the other freq, not specific yet.. Then how do you know it is multiband? Your most specific request is for a folded dipole at one frequency. From end-to-end it will be slightly less than 1/2 wavelength long at that frequency. Now, if you add another frequency, your folded dipole will need to be 1/2 wavelength long at that frequency. Obviously to be "multiband" your folded dipole cannot be both lengths at the same time. The requirment for the fold of the folded dipole is obscure. A fan dipole would work for multiband. i use the diode as the switch to reconfigure the freq..later on i will upload the figure of proposed antenna I have not the vaguest idea what that is about. "The diode"? 73's Richard Clark, KB7QHC |
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so, if i got many length of the driven element, thus i'll have many freq.. do u understand me? |
Folded Dipole Antenna
On Tue, 14 Dec 2010 06:19:35 +0000, frankieNrosie
wrote: i use the diode as the switch to reconfigure the freq..later on i will upload the figure of proposed antenna- I have not the vaguest idea what that is about. "The diode"? diode is use as a switch to vary the length of the driven element.. so, if i got many length of the driven element, thus i'll have many freq.. do u understand me? I understand what you write, but not why you choose this arrangement. I can guess many configurations, but your goal is obscure. Inventive to say the least. Conventional antenna design already has passive solutions for your active element requirement and they stand the chance of reducing the risks inherent in putting a diode into an antenna structure for switching its length. 73's Richard Clark, KB7QHC |
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