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Measuring radiation resistance
Hi guys, How does one go about measuring (with a reasonable degree of accuracy) the radiation resistance of antennas? And when I say "antennas" I mean any radiator from a balanced dipole through to a random length of wet string with a damp matchbox for a ground plane. Must it be done with a noise bridge or is there another way that requires no special test equipment (aside from a scope/sig.gen etc.). I'm primarily interested in checking out highly *non*-ideal antennas for use in non-ideal situations/locations. -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On Mon, 08 Dec 2003 00:55:46 +0000, Paul Burridge
wrote: | |Hi guys, | |How does one go about measuring (with a reasonable degree of |accuracy) the radiation resistance of antennas? And when I say |"antennas" I mean any radiator from a balanced dipole through to a |random length of wet string with a damp matchbox for a ground plane. |Must it be done with a noise bridge or is there another way that |requires no special test equipment (aside from a scope/sig.gen etc.). |I'm primarily interested in checking out highly *non*-ideal antennas |for use in non-ideal situations/locations. This was best posted to rec.radio.amateur.antennas but... For ordinary wire antennas operated independent of ground and without loading coils, the radiation resistance is very close to the resistive part of the feedpoint impedance. Any number of instruments can measure this with reasonable accuracy. A noise bridge or some of the popular impedance bridges (AEA CIA, MFJ, etc) can do it. If the antenna has loading coils, or is operated against ground, then the losses in those (sometimes greatly) affect the measurement. In either case, "special" equipment in the sense that it's more than a scope and signal generator *is* required. Very often transmission lines are a part of the equation and their effects need to be accounted for as well. I suspect that the more non-ideal the antenna, the more difficult the measurement. Wes N7WS |
On Mon, 08 Dec 2003 00:55:46 +0000, Paul Burridge
wrote: | |Hi guys, | |How does one go about measuring (with a reasonable degree of |accuracy) the radiation resistance of antennas? And when I say |"antennas" I mean any radiator from a balanced dipole through to a |random length of wet string with a damp matchbox for a ground plane. |Must it be done with a noise bridge or is there another way that |requires no special test equipment (aside from a scope/sig.gen etc.). |I'm primarily interested in checking out highly *non*-ideal antennas |for use in non-ideal situations/locations. This was best posted to rec.radio.amateur.antennas but... For ordinary wire antennas operated independent of ground and without loading coils, the radiation resistance is very close to the resistive part of the feedpoint impedance. Any number of instruments can measure this with reasonable accuracy. A noise bridge or some of the popular impedance bridges (AEA CIA, MFJ, etc) can do it. If the antenna has loading coils, or is operated against ground, then the losses in those (sometimes greatly) affect the measurement. In either case, "special" equipment in the sense that it's more than a scope and signal generator *is* required. Very often transmission lines are a part of the equation and their effects need to be accounted for as well. I suspect that the more non-ideal the antenna, the more difficult the measurement. Wes N7WS |
On Mon, 08 Dec 2003 00:55:46 +0000, Paul Burridge
wrote: Hi guys, How does one go about measuring (with a reasonable degree of accuracy) the radiation resistance of antennas? And when I say "antennas" I mean any radiator from a balanced dipole through to a random length of wet string with a damp matchbox for a ground plane. Are you really interested in the radiation resistance or are you actually trying to figure out the efficiency or total radiated power of the antenna ? One way would be to first measure the total radiated power. To do this, you would have to measure the EiRP to all directions and integrate it. If the measurement is done in the far field, a calibrated magnetic probe would be enough, but at closer distances, additionally the electric field would have to be measured with a reference dipole. At short wavelengths the antenna can be put on a rotating table but for instance to measure the radiated power for an LF system, you would have to fly around in a plane with DGPS navigation and make the measurements from all directions. If the antenna has a symmetrical or otherwise well known radiation pattern, the number of measurements could be greatly reduced. When you have the total radiated power and then measure the input power, you can calculate the efficiency and radiating resistance. Assuming that the radiation resistance is in series with the loss resistance, it would be enough to measure just the antenna current, which also flows through the radiation resistance, producing the already measured total radiated field. Paul OH3LWR |
On Mon, 08 Dec 2003 00:55:46 +0000, Paul Burridge
wrote: Hi guys, How does one go about measuring (with a reasonable degree of accuracy) the radiation resistance of antennas? And when I say "antennas" I mean any radiator from a balanced dipole through to a random length of wet string with a damp matchbox for a ground plane. Are you really interested in the radiation resistance or are you actually trying to figure out the efficiency or total radiated power of the antenna ? One way would be to first measure the total radiated power. To do this, you would have to measure the EiRP to all directions and integrate it. If the measurement is done in the far field, a calibrated magnetic probe would be enough, but at closer distances, additionally the electric field would have to be measured with a reference dipole. At short wavelengths the antenna can be put on a rotating table but for instance to measure the radiated power for an LF system, you would have to fly around in a plane with DGPS navigation and make the measurements from all directions. If the antenna has a symmetrical or otherwise well known radiation pattern, the number of measurements could be greatly reduced. When you have the total radiated power and then measure the input power, you can calculate the efficiency and radiating resistance. Assuming that the radiation resistance is in series with the loss resistance, it would be enough to measure just the antenna current, which also flows through the radiation resistance, producing the already measured total radiated field. Paul OH3LWR |
Paul Burridge wrote:
Hi guys, How does one go about measuring (with a reasonable degree of accuracy) the radiation resistance of antennas? And when I say "antennas" I mean any radiator from a balanced dipole through to a random length of wet string with a damp matchbox for a ground plane. Must it be done with a noise bridge or is there another way that requires no special test equipment (aside from a scope/sig.gen etc.). I'm primarily interested in checking out highly *non*-ideal antennas for use in non-ideal situations/locations. You can't directly measure radiation resistance, because it's embedded in loss resistances and usually reactance too. Also there's more than one definition of radiation resistance: some would say that the radiation resistance of a half-wave dipole depends on where and how it is fed (centre, end, off-centre); while others would say that the radiation resistance stays the same, and it's only the feedpoint impedance that depends on the method of feed. Since you can't measure it and there isn't even a universally agreed definition, it's best to tiptoe quietly away from "radiation resistance" before the Holy Wars begin... What you can measure directly is impedance at the feedpoint. That is of much more practical interest. For that job, an R-X noise bridge is probably the minimum entry-level instrument. In principle you could measure impedance using a sig gen and a two-channel scope, measuring the amplitude and phase difference across a series resistor feeding the unknown load. But in practice you wouldn't get very accurate results above MF, and it's a heap of equipment to carry outside where you'd want to use it. 'Antenna analysers' such as the MFJ-259B are deservedly popular because they will do R-X measurements with acceptable accuracy, and the whole thing can be battery-powered and held in one hand. I'm looking forward to making some outdoor antenna measurements using the new N2PK vector network analyser... but not today brrrr! -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) Editor, 'The VHF/UHF DX Book' http://www.ifwtech.co.uk/g3sek |
Paul Burridge wrote:
Hi guys, How does one go about measuring (with a reasonable degree of accuracy) the radiation resistance of antennas? And when I say "antennas" I mean any radiator from a balanced dipole through to a random length of wet string with a damp matchbox for a ground plane. Must it be done with a noise bridge or is there another way that requires no special test equipment (aside from a scope/sig.gen etc.). I'm primarily interested in checking out highly *non*-ideal antennas for use in non-ideal situations/locations. You can't directly measure radiation resistance, because it's embedded in loss resistances and usually reactance too. Also there's more than one definition of radiation resistance: some would say that the radiation resistance of a half-wave dipole depends on where and how it is fed (centre, end, off-centre); while others would say that the radiation resistance stays the same, and it's only the feedpoint impedance that depends on the method of feed. Since you can't measure it and there isn't even a universally agreed definition, it's best to tiptoe quietly away from "radiation resistance" before the Holy Wars begin... What you can measure directly is impedance at the feedpoint. That is of much more practical interest. For that job, an R-X noise bridge is probably the minimum entry-level instrument. In principle you could measure impedance using a sig gen and a two-channel scope, measuring the amplitude and phase difference across a series resistor feeding the unknown load. But in practice you wouldn't get very accurate results above MF, and it's a heap of equipment to carry outside where you'd want to use it. 'Antenna analysers' such as the MFJ-259B are deservedly popular because they will do R-X measurements with acceptable accuracy, and the whole thing can be battery-powered and held in one hand. I'm looking forward to making some outdoor antenna measurements using the new N2PK vector network analyser... but not today brrrr! -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) Editor, 'The VHF/UHF DX Book' http://www.ifwtech.co.uk/g3sek |
On Mon, 08 Dec 2003 09:48:02 +0200, Paul Keinanen
wrote: On Mon, 08 Dec 2003 00:55:46 +0000, Paul Burridge wrote: Hi guys, How does one go about measuring (with a reasonable degree of accuracy) the radiation resistance of antennas? And when I say "antennas" I mean any radiator from a balanced dipole through to a random length of wet string with a damp matchbox for a ground plane. Are you really interested in the radiation resistance or are you actually trying to figure out the efficiency or total radiated power of the antenna ? I'm simply trying to establish the radiation resistance of a non-ideal antenna so I can reasonably match it to the output impedance of the transmitter PA stage. The figure is likely to be very low, given that this antenna is highly non-ideal. I'm not interested in plotting polar patterns of the radiation distribution! -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On Mon, 08 Dec 2003 09:48:02 +0200, Paul Keinanen
wrote: On Mon, 08 Dec 2003 00:55:46 +0000, Paul Burridge wrote: Hi guys, How does one go about measuring (with a reasonable degree of accuracy) the radiation resistance of antennas? And when I say "antennas" I mean any radiator from a balanced dipole through to a random length of wet string with a damp matchbox for a ground plane. Are you really interested in the radiation resistance or are you actually trying to figure out the efficiency or total radiated power of the antenna ? I'm simply trying to establish the radiation resistance of a non-ideal antenna so I can reasonably match it to the output impedance of the transmitter PA stage. The figure is likely to be very low, given that this antenna is highly non-ideal. I'm not interested in plotting polar patterns of the radiation distribution! -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
Radiation resistance should always be referred to a particular point in an
aerial. It is not of much use unless used to calculate radiating efficiency in conjunction with conductor and other loss resistances. The problem is not how to measure it but how to distinguish it from the aerial loss resistance in series with it. It is that fictional resistance which, if inserted in the aerial at that point, dissipates the same power as is radiated when the same aerial current flows. Radiation resistance can also be considered to be uniformly distributed along an aerial wire. It can then be directly compared with wire loss resistance. It so happens the uniformly distributed radiation resistance is exactly twice the radiation resistance of a 1/2-wave dipole when concentrated at its centre. So the uniformly distributed radiation resistance along a 1/2-wave dipole is about 140 ohms. It cannot be measured. It can be calculated from aerial dimensions. But best just to remember the approximate number 140. It does depend to small extent on wire diameter and 'end-effect'. If the wire end-to-end resistance of a 40m, 14-gauge dipole is, say, 2.76 ohms then - Aerial efficiency = 100 * 140 / ( 140 + 2.76 ) = 98.0 percent. Which is very good, isn't it? It's equivalent to 1/68th of an S-unit which cannot be detected even by using a magnifying glass and the bloody needle stands still for long enough. Which also serves to illustrate how VERY uncrtical are aerial impedance measurements. ---- Reg, G4FGQ |
Radiation resistance should always be referred to a particular point in an
aerial. It is not of much use unless used to calculate radiating efficiency in conjunction with conductor and other loss resistances. The problem is not how to measure it but how to distinguish it from the aerial loss resistance in series with it. It is that fictional resistance which, if inserted in the aerial at that point, dissipates the same power as is radiated when the same aerial current flows. Radiation resistance can also be considered to be uniformly distributed along an aerial wire. It can then be directly compared with wire loss resistance. It so happens the uniformly distributed radiation resistance is exactly twice the radiation resistance of a 1/2-wave dipole when concentrated at its centre. So the uniformly distributed radiation resistance along a 1/2-wave dipole is about 140 ohms. It cannot be measured. It can be calculated from aerial dimensions. But best just to remember the approximate number 140. It does depend to small extent on wire diameter and 'end-effect'. If the wire end-to-end resistance of a 40m, 14-gauge dipole is, say, 2.76 ohms then - Aerial efficiency = 100 * 140 / ( 140 + 2.76 ) = 98.0 percent. Which is very good, isn't it? It's equivalent to 1/68th of an S-unit which cannot be detected even by using a magnifying glass and the bloody needle stands still for long enough. Which also serves to illustrate how VERY uncrtical are aerial impedance measurements. ---- Reg, G4FGQ |
I'm simply trying to establish the radiation resistance of a non-ideal antenna so I can reasonably match it to the output impedance of the transmitter PA stage. ========================= The input impedance of a 1/2-wave resonant dipole is about 70 ohms. But this may be at a considerable distance from the transmitter output terminals. What do you have in mind to put in between? Then all you have to do is find the purely resistive load the transmitter would be most happy with. Almost certainly it will not correspond to an impedance match. ---- Reg G4FGQ |
I'm simply trying to establish the radiation resistance of a non-ideal antenna so I can reasonably match it to the output impedance of the transmitter PA stage. ========================= The input impedance of a 1/2-wave resonant dipole is about 70 ohms. But this may be at a considerable distance from the transmitter output terminals. What do you have in mind to put in between? Then all you have to do is find the purely resistive load the transmitter would be most happy with. Almost certainly it will not correspond to an impedance match. ---- Reg G4FGQ |
I read in sci.electronics.design that Reg Edwards
wrote (in et.com) about 'Measuring radiation resistance', on Mon, 8 Dec 2003: If the wire end-to-end resistance of a 40m, 14-gauge dipole is, say, 2.76 ohms then - Aerial efficiency = 100 * 140 / ( 140 + 2.76 ) = 98.0 percent. Which is very good, isn't it? It's equivalent to 1/68th of an S-unit which cannot be detected even by using a magnifying glass and the bloody needle stands still for long enough. Which also serves to illustrate how VERY uncrtical are aerial impedance measurements. Reg, The OP is working with very non-ideal antennas, for which the radiation resistance is probably only an ohm or two, and he wants to know if it's 0.5 ohms or 5 ohms, for obvious reasons. I don't suppose there is any realistic way of measuring it, and modelling may be extremely difficult if the antenna shape is not simple. I've seen antenna evaluation does on 1:10 and 1:20 scale models, but we don't know what frequencies the OP is using, so even that may not be practicable, but if it is, one could work back from field strength measurements to radiation resistance, with a big pad between the transmitter and the antenna to 'soak up' the mismatch. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! |
I read in sci.electronics.design that Reg Edwards
wrote (in et.com) about 'Measuring radiation resistance', on Mon, 8 Dec 2003: If the wire end-to-end resistance of a 40m, 14-gauge dipole is, say, 2.76 ohms then - Aerial efficiency = 100 * 140 / ( 140 + 2.76 ) = 98.0 percent. Which is very good, isn't it? It's equivalent to 1/68th of an S-unit which cannot be detected even by using a magnifying glass and the bloody needle stands still for long enough. Which also serves to illustrate how VERY uncrtical are aerial impedance measurements. Reg, The OP is working with very non-ideal antennas, for which the radiation resistance is probably only an ohm or two, and he wants to know if it's 0.5 ohms or 5 ohms, for obvious reasons. I don't suppose there is any realistic way of measuring it, and modelling may be extremely difficult if the antenna shape is not simple. I've seen antenna evaluation does on 1:10 and 1:20 scale models, but we don't know what frequencies the OP is using, so even that may not be practicable, but if it is, one could work back from field strength measurements to radiation resistance, with a big pad between the transmitter and the antenna to 'soak up' the mismatch. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! |
On Mon, 8 Dec 2003 15:29:59 +0000 (UTC), "Reg Edwards"
wrote: [snip] Hi Reg, It so happens the uniformly distributed radiation resistance is exactly twice the radiation resistance of a 1/2-wave dipole when concentrated at its centre. So the uniformly distributed radiation resistance along a 1/2-wave dipole is about 140 ohms. It cannot be measured. It can be calculated from aerial dimensions. But best just to remember the approximate number 140. It does depend to small extent on wire diameter and 'end-effect'. It's annoying, because the tx output Z I'm trying to match is (by a strange coincidence) 140 ohms! So a folded dipole would be ideal, I guess. However - and it's a big *however* - I can't use one. I'm stuck with a telescopic whip and a ground plane the size of a box of Swan Vestas. I imagine the radiation resistance of such a non-ideal antenna is pretty low, but until someone can gimme a ballpark figure for it, I can't even begin to think about how to go about matching it. :-( -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On Mon, 8 Dec 2003 15:29:59 +0000 (UTC), "Reg Edwards"
wrote: [snip] Hi Reg, It so happens the uniformly distributed radiation resistance is exactly twice the radiation resistance of a 1/2-wave dipole when concentrated at its centre. So the uniformly distributed radiation resistance along a 1/2-wave dipole is about 140 ohms. It cannot be measured. It can be calculated from aerial dimensions. But best just to remember the approximate number 140. It does depend to small extent on wire diameter and 'end-effect'. It's annoying, because the tx output Z I'm trying to match is (by a strange coincidence) 140 ohms! So a folded dipole would be ideal, I guess. However - and it's a big *however* - I can't use one. I'm stuck with a telescopic whip and a ground plane the size of a box of Swan Vestas. I imagine the radiation resistance of such a non-ideal antenna is pretty low, but until someone can gimme a ballpark figure for it, I can't even begin to think about how to go about matching it. :-( -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
It's annoying, because the tx output Z I'm trying to match is (by a strange coincidence) 140 ohms! So a folded dipole would be ideal, I guess. However - and it's a big *however* - I can't use one. I'm stuck with a telescopic whip and a ground plane the size of a box of Swan Vestas. I imagine the radiation resistance of such a non-ideal antenna is pretty low, but until someone can gimme a ballpark figure for it, I can't even begin to think about how to go about matching it. :-( Depending on frequency in use, could you do it experimentally? The way I tune up antennas for ham bands is to hook up a mfj 249 and the tuner to the antenna, get a best fit with the 249 and then replace the 249 with the radio and fine tune from there. There always seems to be some small differance between the result from the mfj and the meter built into the tuner, and at full power I would rather trust the meter built into the tuner (a Millen transmatch jr). You could build an L match with a tapped inductor and variable cap, then experiment with values until you get somewhere in the ballpark of being matched. From there it is is just small adjustments to get perfect matching. thanks, John. KC5DWD |
It's annoying, because the tx output Z I'm trying to match is (by a strange coincidence) 140 ohms! So a folded dipole would be ideal, I guess. However - and it's a big *however* - I can't use one. I'm stuck with a telescopic whip and a ground plane the size of a box of Swan Vestas. I imagine the radiation resistance of such a non-ideal antenna is pretty low, but until someone can gimme a ballpark figure for it, I can't even begin to think about how to go about matching it. :-( Depending on frequency in use, could you do it experimentally? The way I tune up antennas for ham bands is to hook up a mfj 249 and the tuner to the antenna, get a best fit with the 249 and then replace the 249 with the radio and fine tune from there. There always seems to be some small differance between the result from the mfj and the meter built into the tuner, and at full power I would rather trust the meter built into the tuner (a Millen transmatch jr). You could build an L match with a tapped inductor and variable cap, then experiment with values until you get somewhere in the ballpark of being matched. From there it is is just small adjustments to get perfect matching. thanks, John. KC5DWD |
Hi,
For a whip, much shorter than a quarter wave against a poor ground - who knows? However, you want a number? So here's a number; 2 - j500 and it won't be too far wrong. The name of the game in such a situation is "Suck-it and see." Make an intelligent guess at what the impedance is likely to be, rig up a far-field meter and adjust the tap/link/network until it peaks. Then go out for a curry and maybe a drink or two or... Alternatively, buy an antenna book and RTFM :-) Cheers - Joe |
Hi,
For a whip, much shorter than a quarter wave against a poor ground - who knows? However, you want a number? So here's a number; 2 - j500 and it won't be too far wrong. The name of the game in such a situation is "Suck-it and see." Make an intelligent guess at what the impedance is likely to be, rig up a far-field meter and adjust the tap/link/network until it peaks. Then go out for a curry and maybe a drink or two or... Alternatively, buy an antenna book and RTFM :-) Cheers - Joe |
Paul Burridge wrote in message . ..
.... I'm simply trying to establish the radiation resistance of a non-ideal antenna so I can reasonably match it to the output impedance of the transmitter PA stage. The figure is likely to be very low, given that this antenna is highly non-ideal. I'm not interested in plotting polar patterns of the radiation distribution! Then you want to know the feedpoint impedance, not the radiation resistance. You can measure that with any of a variety of impedance-measuring devices. You need to be careful to not disturb the antenna with your measurement, and to properly couple the measurement device to the feedpoint, taking into account the effect of the coupling on the measurement. For example, like Wes said, if you measure it through a length of feedline, you need to accout for the impedance transformation performed by that line, or calibrate your measurements at the end of that line. You can generally get a fairly accurate number through a simulation of the antenna, too. Cheers, Tom |
Paul Burridge wrote in message . ..
.... I'm simply trying to establish the radiation resistance of a non-ideal antenna so I can reasonably match it to the output impedance of the transmitter PA stage. The figure is likely to be very low, given that this antenna is highly non-ideal. I'm not interested in plotting polar patterns of the radiation distribution! Then you want to know the feedpoint impedance, not the radiation resistance. You can measure that with any of a variety of impedance-measuring devices. You need to be careful to not disturb the antenna with your measurement, and to properly couple the measurement device to the feedpoint, taking into account the effect of the coupling on the measurement. For example, like Wes said, if you measure it through a length of feedline, you need to accout for the impedance transformation performed by that line, or calibrate your measurements at the end of that line. You can generally get a fairly accurate number through a simulation of the antenna, too. Cheers, Tom |
I can see from the way you have made your enquiry you havn't the foggiest
idea about what you are trying to accomplish. Do you know the frequency? What transmitter power output do you have in mind - 10 milliwatts or 1KW? For starters forget all about folding anything - you've been reading the wrong books. However you now mention a short whip above a groundplane of unknown construction. The input radiation resistance at the base of a very short vertical antenna, say less than 1/10th of a wavelength, is given by - Rrad = Squareroot( 20 * Height in metres / Wavelength in metres ) ohms. Rrad will be in the order of a few tenths of an ohm at 2 MHz but increases fast as the square of frequency. In series with this radiation resistance is a high value of capacitative reactance which has to be tuned out somewhere by a lot of micro-henries. Best located at or near the antenna base. For a very crude guess the input reactance will be in the order of - Xin = -300 * Cotangent( Angle ) ohms. where Angle = 360 * Height / Wavelength degrees. You will then have the task of winding the correct number of turns on a coil former, of your chosen length and diameter, to provide an inductance of similar value of reactance as presented by the whip. Download program SOLNOID3 for coil design. In series with Rrad and Xin there will be a loss resistance due to the connection to the ground plane. If the ground plane is a vehicle then you can expect a loss resistance between 3 and 15 ohms. If the ground plane consists of a cigarette-pack size metal plate buried in your back yard then expect a ground loss resistance between 500 and 5,000 ohms. Overall antenna input resistance is then Rin = Rrad + Rcoil + Rground. If it is your intention to connect the antenna directly to the transmitter, or via a very, very, short length of coaxial line, then Rin is the resistance which has to be matched to your 150-ohm transmitter by using an L and C impedance matching network. Frankly, it may be easier to redesign the transmitter to match the antenna ;o) But you won't get very far without an impedance measuring device such as a borrowed, begged or stolen antenna analyser. As I have no idea of the purpose of the transmitter + antenna I suggest you ask around for sombody who has already solved the problem and copy his. It may be that a very short miniature centre-loaded dipole would do the job. It doesn't need a groundplane and can be driven via a 150-ohm balanced, twisted-pair line and, if needed at the transmitter end, a simple 150-ohm, 1-to-1 balun. Download program MIDLOAD and amuse yourself. It also designs the loading coil. I KNOW it works. Been there - done that! ---- .................................................. .......... Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.btinternet.com/~g4fgq.regp .................................................. ......... "Paul Burridge" wrote - . "Reg Edwards" [snip] Hi Reg, It so happens the uniformly distributed radiation resistance is exactly twice the radiation resistance of a 1/2-wave dipole when concentrated at its centre. So the uniformly distributed radiation resistance along a 1/2-wave dipole is about 140 ohms. It cannot be measured. It can be calculated from aerial dimensions. But best just to remember the approximate number 140. It does depend to small extent on wire diameter and 'end-effect'. It's annoying, because the tx output Z I'm trying to match is (by a strange coincidence) 140 ohms! So a folded dipole would be ideal, I guess. However - and it's a big *however* - I can't use one. I'm stuck with a telescopic whip and a ground plane the size of a box of Swan Vestas. I imagine the radiation resistance of such a non-ideal antenna is pretty low, but until someone can gimme a ballpark figure for it, I can't even begin to think about how to go about matching it. :-( -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
I can see from the way you have made your enquiry you havn't the foggiest
idea about what you are trying to accomplish. Do you know the frequency? What transmitter power output do you have in mind - 10 milliwatts or 1KW? For starters forget all about folding anything - you've been reading the wrong books. However you now mention a short whip above a groundplane of unknown construction. The input radiation resistance at the base of a very short vertical antenna, say less than 1/10th of a wavelength, is given by - Rrad = Squareroot( 20 * Height in metres / Wavelength in metres ) ohms. Rrad will be in the order of a few tenths of an ohm at 2 MHz but increases fast as the square of frequency. In series with this radiation resistance is a high value of capacitative reactance which has to be tuned out somewhere by a lot of micro-henries. Best located at or near the antenna base. For a very crude guess the input reactance will be in the order of - Xin = -300 * Cotangent( Angle ) ohms. where Angle = 360 * Height / Wavelength degrees. You will then have the task of winding the correct number of turns on a coil former, of your chosen length and diameter, to provide an inductance of similar value of reactance as presented by the whip. Download program SOLNOID3 for coil design. In series with Rrad and Xin there will be a loss resistance due to the connection to the ground plane. If the ground plane is a vehicle then you can expect a loss resistance between 3 and 15 ohms. If the ground plane consists of a cigarette-pack size metal plate buried in your back yard then expect a ground loss resistance between 500 and 5,000 ohms. Overall antenna input resistance is then Rin = Rrad + Rcoil + Rground. If it is your intention to connect the antenna directly to the transmitter, or via a very, very, short length of coaxial line, then Rin is the resistance which has to be matched to your 150-ohm transmitter by using an L and C impedance matching network. Frankly, it may be easier to redesign the transmitter to match the antenna ;o) But you won't get very far without an impedance measuring device such as a borrowed, begged or stolen antenna analyser. As I have no idea of the purpose of the transmitter + antenna I suggest you ask around for sombody who has already solved the problem and copy his. It may be that a very short miniature centre-loaded dipole would do the job. It doesn't need a groundplane and can be driven via a 150-ohm balanced, twisted-pair line and, if needed at the transmitter end, a simple 150-ohm, 1-to-1 balun. Download program MIDLOAD and amuse yourself. It also designs the loading coil. I KNOW it works. Been there - done that! ---- .................................................. .......... Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.btinternet.com/~g4fgq.regp .................................................. ......... "Paul Burridge" wrote - . "Reg Edwards" [snip] Hi Reg, It so happens the uniformly distributed radiation resistance is exactly twice the radiation resistance of a 1/2-wave dipole when concentrated at its centre. So the uniformly distributed radiation resistance along a 1/2-wave dipole is about 140 ohms. It cannot be measured. It can be calculated from aerial dimensions. But best just to remember the approximate number 140. It does depend to small extent on wire diameter and 'end-effect'. It's annoying, because the tx output Z I'm trying to match is (by a strange coincidence) 140 ohms! So a folded dipole would be ideal, I guess. However - and it's a big *however* - I can't use one. I'm stuck with a telescopic whip and a ground plane the size of a box of Swan Vestas. I imagine the radiation resistance of such a non-ideal antenna is pretty low, but until someone can gimme a ballpark figure for it, I can't even begin to think about how to go about matching it. :-( -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
Paul Burridge wrote in message . ..
.... It's annoying, because the tx output Z I'm trying to match is (by a strange coincidence) 140 ohms! So a folded dipole would be ideal, I guess. However - and it's a big *however* - I can't use one. I'm stuck with a telescopic whip and a ground plane the size of a box of Swan Vestas. I imagine the radiation resistance of such a non-ideal antenna is pretty low, but until someone can gimme a ballpark figure for it, I can't even begin to think about how to go about matching it. :-( Paul, it would be reeeeally helpful if you'd include enough info so we could give you a ballpark figure. What wavelength (or frequency)? How long is the antenna? How big is that box? (Will someone be holding it during operation?) Yeah, someone did offer a ballpark figure, but that depends a whole lot on what fraction of a wavelength you have for your antenna and your ground plane. (Or perhaps the posting in which you explained all that hasn't made it to this corner of the universe.) Cheers, Tom |
Paul Burridge wrote in message . ..
.... It's annoying, because the tx output Z I'm trying to match is (by a strange coincidence) 140 ohms! So a folded dipole would be ideal, I guess. However - and it's a big *however* - I can't use one. I'm stuck with a telescopic whip and a ground plane the size of a box of Swan Vestas. I imagine the radiation resistance of such a non-ideal antenna is pretty low, but until someone can gimme a ballpark figure for it, I can't even begin to think about how to go about matching it. :-( Paul, it would be reeeeally helpful if you'd include enough info so we could give you a ballpark figure. What wavelength (or frequency)? How long is the antenna? How big is that box? (Will someone be holding it during operation?) Yeah, someone did offer a ballpark figure, but that depends a whole lot on what fraction of a wavelength you have for your antenna and your ground plane. (Or perhaps the posting in which you explained all that hasn't made it to this corner of the universe.) Cheers, Tom |
On Mon, 8 Dec 2003 21:32:52 +0000 (UTC), "Reg Edwards"
wrote: I can see from the way you have made your enquiry you havn't the foggiest idea about what you are trying to accomplish. Do you know the frequency? What transmitter power output do you have in mind - 10 milliwatts or 1KW? For starters forget all about folding anything - you've been reading the wrong books. However you now mention a short whip above a groundplane of unknown construction. Perhaps you didn't see the earlier posts on the subject. The frequency is 40Mhz (radio control band) and the tx output stage as it stands puts out maximum power of 475mW with a 140 ohm resistor as load. I only *need* 50mW ERP., however, so can stand to see quite a bit of loss from an inefficient antenna. The antenna I will be using is a telescopic whip - exactly the same set-up as you see with model vehicle radio control transmitters- which is what it is, in fact. It ain't rocket science. Someone out there must know the radiation resistance of such a telescopic whip (which has a ground plane of just around 16 square inches contained within the remote control handset) and the best way to couple it to a PA with a 140 ohm output impedance? -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
On Mon, 8 Dec 2003 21:32:52 +0000 (UTC), "Reg Edwards"
wrote: I can see from the way you have made your enquiry you havn't the foggiest idea about what you are trying to accomplish. Do you know the frequency? What transmitter power output do you have in mind - 10 milliwatts or 1KW? For starters forget all about folding anything - you've been reading the wrong books. However you now mention a short whip above a groundplane of unknown construction. Perhaps you didn't see the earlier posts on the subject. The frequency is 40Mhz (radio control band) and the tx output stage as it stands puts out maximum power of 475mW with a 140 ohm resistor as load. I only *need* 50mW ERP., however, so can stand to see quite a bit of loss from an inefficient antenna. The antenna I will be using is a telescopic whip - exactly the same set-up as you see with model vehicle radio control transmitters- which is what it is, in fact. It ain't rocket science. Someone out there must know the radiation resistance of such a telescopic whip (which has a ground plane of just around 16 square inches contained within the remote control handset) and the best way to couple it to a PA with a 140 ohm output impedance? -- "I expect history will be kind to me, since I intend to write it." - Winston Churchill |
Its not a simple matter of matching the transmitter to the radiation
resistance. First a short antenna is going to have a capacitive reactance. You must add an equal amount of inductive reactance (a loading coil) in order to cancel the capacitive reactance and make the antenna resonant at the operating frequency. Then what the transmitter needs to match is the total load impedance of the antenna "system". The load impedance includes the radiation resistance plus the resistance of the loading inductance plus the ground losses. The hardest thing to get a handle on will be the ground losses. The physical size of the transmitter housing is a small portion of a wavelength and losses will change as you handle the unit. |
Its not a simple matter of matching the transmitter to the radiation
resistance. First a short antenna is going to have a capacitive reactance. You must add an equal amount of inductive reactance (a loading coil) in order to cancel the capacitive reactance and make the antenna resonant at the operating frequency. Then what the transmitter needs to match is the total load impedance of the antenna "system". The load impedance includes the radiation resistance plus the resistance of the loading inductance plus the ground losses. The hardest thing to get a handle on will be the ground losses. The physical size of the transmitter housing is a small portion of a wavelength and losses will change as you handle the unit. |
Paul Burridge wrote:
... the best way to couple it to a PA with a 140 ohm output impedance? Use a loading coil to resonate the antenna to 40MHz. Use a transformer or tank circuit to transform the impedance. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
Paul Burridge wrote:
... the best way to couple it to a PA with a 140 ohm output impedance? Use a loading coil to resonate the antenna to 40MHz. Use a transformer or tank circuit to transform the impedance. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
On Tue, 09 Dec 2003 00:04:00 +0000, Paul Burridge wrote:
On Mon, 8 Dec 2003 21:32:52 +0000 (UTC), "Reg Edwards" wrote: I can see from the way you have made your enquiry you havn't the foggiest idea about what you are trying to accomplish. Do you know the frequency? What transmitter power output do you have in mind - 10 milliwatts or 1KW? For starters forget all about folding anything - you've been reading the wrong books. However you now mention a short whip above a groundplane of unknown construction. Perhaps you didn't see the earlier posts on the subject. The frequency is 40Mhz (radio control band) and the tx output stage as it stands puts out maximum power of 475mW with a 140 ohm resistor as load. I only *need* 50mW ERP., however, so can stand to see quite a bit of loss from an inefficient antenna. The antenna I will be using is a telescopic whip - exactly the same set-up as you see with model vehicle radio control transmitters- which is what it is, in fact. It ain't rocket science. Someone out there must know the radiation resistance of such a telescopic whip (which has a ground plane of just around 16 square inches contained within the remote control handset) and the best way to couple it to a PA with a 140 ohm output impedance? You have had quite a few answers which seem pretty good to me. I have a few comments. Stop saying "radiation resistance" and start saying "input impedance" or "feed impedance." Once you know the input impedance, getting maximum power transfer is a straightforward matching problem that can be solved in a variety of ways. True, you won't know for sure how much power is "getting out," but some of the posts in this thread have given you some estimates. If that isn't good enough, then you will have to measure by putting a receive antenna in the far field. Anyway, if you want to measure the input impedance, you should be able to do it with a two-channel oscilloscope and a 40 MHz function generator. Drive the antenna with the function generator and simultaneously measure the input voltage and current, including the phase relationship between them. The input impedance is V/I. To measure the current, put a suitably sized resistor in series with the antenna and measure the voltage across it. The voltage across the resistor should be small compared to the voltage into the antenna. If it is too small to see, use a bigger resistor until you can see it. Make sure both oscilloscope probes are grounded in the same place. Unless you have a differential probe (in which case, use it to measure the voltage across the current sensing resistor). From what others have already said, it sounds like the input will be extremely capacitive with a small series resistance. Once you know the details, however, it will be easy to tell you how to match this to your amplifier. That's my $0.02. Mac |
On Tue, 09 Dec 2003 00:04:00 +0000, Paul Burridge wrote:
On Mon, 8 Dec 2003 21:32:52 +0000 (UTC), "Reg Edwards" wrote: I can see from the way you have made your enquiry you havn't the foggiest idea about what you are trying to accomplish. Do you know the frequency? What transmitter power output do you have in mind - 10 milliwatts or 1KW? For starters forget all about folding anything - you've been reading the wrong books. However you now mention a short whip above a groundplane of unknown construction. Perhaps you didn't see the earlier posts on the subject. The frequency is 40Mhz (radio control band) and the tx output stage as it stands puts out maximum power of 475mW with a 140 ohm resistor as load. I only *need* 50mW ERP., however, so can stand to see quite a bit of loss from an inefficient antenna. The antenna I will be using is a telescopic whip - exactly the same set-up as you see with model vehicle radio control transmitters- which is what it is, in fact. It ain't rocket science. Someone out there must know the radiation resistance of such a telescopic whip (which has a ground plane of just around 16 square inches contained within the remote control handset) and the best way to couple it to a PA with a 140 ohm output impedance? You have had quite a few answers which seem pretty good to me. I have a few comments. Stop saying "radiation resistance" and start saying "input impedance" or "feed impedance." Once you know the input impedance, getting maximum power transfer is a straightforward matching problem that can be solved in a variety of ways. True, you won't know for sure how much power is "getting out," but some of the posts in this thread have given you some estimates. If that isn't good enough, then you will have to measure by putting a receive antenna in the far field. Anyway, if you want to measure the input impedance, you should be able to do it with a two-channel oscilloscope and a 40 MHz function generator. Drive the antenna with the function generator and simultaneously measure the input voltage and current, including the phase relationship between them. The input impedance is V/I. To measure the current, put a suitably sized resistor in series with the antenna and measure the voltage across it. The voltage across the resistor should be small compared to the voltage into the antenna. If it is too small to see, use a bigger resistor until you can see it. Make sure both oscilloscope probes are grounded in the same place. Unless you have a differential probe (in which case, use it to measure the voltage across the current sensing resistor). From what others have already said, it sounds like the input will be extremely capacitive with a small series resistance. Once you know the details, however, it will be easy to tell you how to match this to your amplifier. That's my $0.02. Mac |
In article ,
Paul Burridge wrote: It ain't rocket science. Someone out there must know the radiation resistance of such a telescopic whip (which has a ground plane of just around 16 square inches contained within the remote control handset) and the best way to couple it to a PA with a 140 ohm output impedance? Paul, standard CB antennas are loaded whips, for operation at around 30MHz. Available in the CPC catalogue for about £10 to £20, Perhaps you could start off with one of those and modify to suit. Note also that there is a 433MHz licence-free band for short distance (100m) remote control. Pre-built Tx and Rx modules are cheap, and a 433MHz antenna would be a breeze. www.cpc.co.uk or 08701 202530. -- Tony Williams. |
In article ,
Paul Burridge wrote: It ain't rocket science. Someone out there must know the radiation resistance of such a telescopic whip (which has a ground plane of just around 16 square inches contained within the remote control handset) and the best way to couple it to a PA with a 140 ohm output impedance? Paul, standard CB antennas are loaded whips, for operation at around 30MHz. Available in the CPC catalogue for about £10 to £20, Perhaps you could start off with one of those and modify to suit. Note also that there is a 433MHz licence-free band for short distance (100m) remote control. Pre-built Tx and Rx modules are cheap, and a 433MHz antenna would be a breeze. www.cpc.co.uk or 08701 202530. -- Tony Williams. |
I'm looking forward
to making some outdoor antenna measurements using the new N2PK vector network analyser... but not today brrrr! Gee, Ian, you only will have to go out and do the short-open-50R calibration at the end of the coax. The software can move the reference plane from the VNA out to the antenna feedpoint, and you can do the rest from the comfort of your shack! Yes, I'm on Harold's list for a kit. 73, John - K6QQ, who thought that after retirement he'd never have the use of a VNA again! |
I'm looking forward
to making some outdoor antenna measurements using the new N2PK vector network analyser... but not today brrrr! Gee, Ian, you only will have to go out and do the short-open-50R calibration at the end of the coax. The software can move the reference plane from the VNA out to the antenna feedpoint, and you can do the rest from the comfort of your shack! Yes, I'm on Harold's list for a kit. 73, John - K6QQ, who thought that after retirement he'd never have the use of a VNA again! |
I read in sci.electronics.design that Ian White, G3SEK
wrote (in ) about 'Measuring radiation resistance', on Mon, 8 Dec 2003: I'm looking forward to making some outdoor antenna measurements using the new N2PK vector network analyser Is N2PK a call-sign? Are there any details of this analyser on the Internet? -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! |
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