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Resonant condition
Hello,
my mobile setup is composed by a 2 meter vertical whip feeded immediately close to it by an automatic antenna tuner. My poor knowledge of antenna systems think that the ATU, choosing the appropriate LC value, bring the entire system in a resonant condition: true or false ? And , second question, because a friend of mine own a MFJ 269 analyzer, how i can *approximately* have an idea of the efficiency on a frequency F for my whip ? Thanks in advance, -.-. --.- |
Resonant condition
"-.-. --.-" ha scritto nel messaggio ... Hello, my mobile setup is composed by a 2 meter vertical whip feeded immediately close to it by an automatic antenna tuner. Missed that the expected frequency of the system is between 14 and 30 MHz, but just curious if i had any chance to work 40 meters :) -.-. --.- |
Resonant condition
On Thu, 03 Jun 2010 10:08:15 +0200, -.-. --.- rearranged some electrons to
say: Hello, my mobile setup is composed by a 2 meter vertical whip feeded immediately close to it by an automatic antenna tuner. My poor knowledge of antenna systems think that the ATU, choosing the appropriate LC value, bring the entire system in a resonant condition: true or false ? And , second question, because a friend of mine own a MFJ 269 analyzer, how i can *approximately* have an idea of the efficiency on a frequency F for my whip ? Thanks in advance, -.-. --.- The tuner will provide an impedance match to your transceiver (50 ohms). It won't make your antenna resonant on 20m (or any other band, other than 2m). |
Resonant condition
On 06/03/2010 04:08 AM, -.-. --.- wrote:
Hello, my mobile setup is composed by a 2 meter vertical whip feeded immediately close to it by an automatic antenna tuner. My poor knowledge of antenna systems think that the ATU, choosing the appropriate LC value, bring the entire system in a resonant condition: true or false ? Hello, and that's sort of the ultimate goal. The tuner is designed to match the antenna type(s) to the source (transmitter) and transmission line design (characteristic) impedance. And , second question, because a friend of mine own a MFJ 269 analyzer, how i can *approximately* have an idea of the efficiency on a frequency F for my whip ? The problem here is an impedance analyzer can't distinguish between resistive losses (antenna, ground, tuner (if considered part of the antenna)) and the radiation resistance of the antenna. To determine efficiency you'd have to make some field strength measurements (usually performed with a calibrated field strength meter) in order to determine how much of the power going into the antenna terminals is being radiated into free space. The only "approximation" would be to measure the resistive part of the antenna feedpoint impedance and then subtract from this the radiation resistance obtained from calculation. Sincerely, and 73s from N4GGO, -- John Wood (Code 5520) e-mail: Naval Research Laboratory 4555 Overlook Avenue, SW Washington, DC 20375-5337 |
Resonant condition
On Jun 3, 3:08*am, "-.-. --.-" wrote:
My poor knowledge of antenna systems think that the ATU, choosing the appropriate LC value, bring the entire system in a resonant condition: true or false ? If you perform a frequency sweep with an antenna analyzer connected to the tuner input and the impedance goes from R-jX ohms to 50+j0 ohms to R+jX ohms, the purely resistive frequency meets the definition of the *system* resonant frequency. Unfortunately, under those extremely lossy conditions, "system resonance" means almost nothing. -- 73, Cecil, w5dxp.com |
Resonant condition
On Jun 3, 5:15*am, david wrote:
On Thu, 03 Jun 2010 10:08:15 +0200, -.-. --.- rearranged some electrons to say: Hello, my mobile setup is composed by a 2 meter vertical whip feeded immediately close to it by an automatic antenna tuner. My poor knowledge of antenna systems think that the ATU, choosing the appropriate LC value, bring the entire system in a resonant condition: true or false ? And , second question, because a friend of mine own a MFJ 269 analyzer, how i can *approximately* have an idea of the efficiency on a frequency F for my whip ? Thanks in advance, -.-. --.- The tuner will provide an impedance match to your transceiver (50 ohms). * It won't make your antenna resonant on 20m (or any other band, other than 2m).- Hide quoted text - - Show quoted text - I think "CQ" meant the whip was 2 meters long. That would be 1/4 wave on 38 MHZ if I did the math right. This is the 8 M band? The tuner might get it on 20 M, but 40 M will be a stretch. Gary N4AST |
Resonant condition
-.-. --.- wrote:
"-.-. --.-" ha scritto nel messaggio ... Hello, my mobile setup is composed by a 2 meter vertical whip feeded immediately close to it by an automatic antenna tuner. Missed that the expected frequency of the system is between 14 and 30 MHz, but just curious if i had any chance to work 40 meters :) -.-. --.- presuming you mean your whip is 2 meters long (not tuned for the 2 meter band, and only 50 cm long) On 40m? Maybe, maybe not. people have used a light bulb as an antenna. It will almost certainly be inefficient. Here's a short description (filled with technical detail errors, which will no doubt provoke a long discussion).. All antennas have a property called "radiation resistance" (smaller antennas have smaller radiation resistances) All antennas also have resistive losses. There are also losses "after the wave has left the antenna" (e.g. ground reflections, etc. why salt water marshes are prized for vertical pol). The latter are not considered here. To a first order (back, all you nit pickers.. first order), the power from the transmitter gets distributed between the radiation resistance and the loss resistance. So, the "efficiency" can be thought of as that fraction of power that winds up in the radiation resistance, as opposed to in the loss resistance. Important he if the radiation resistance is low, that doesn't mean poor efficiency: as long as you keep the resistive losses low too.. which can be a challenge (assuming you're not carrying a vat of liquid helium, for instance). Part of the problem is that the resistive losses aren't just in the antenna, but also in any substance which is immersed in the antenna's electric and magnetic fields (like the steel of your car, and the not particularly good conductivity, but not a perfect insulator either, soil under the car). And, then, there are losses in how you get from Tx to Antenna. If your antenna presents an impedance that is not what the transmitter is providing, you've got to transform it somehow, typically using Ls and Cs, etc. (in your autotuner). Those components also have some amount of loss, although I'll bet it's less than 10% in most situations (otherwise, the tuner/matching network would melt, and they don't) Moral of story.. radiation resistance doesn't drop much from a full size antenna until you get around 1/10th wavelength, then it starts to drop real fast (as length squared) For a dipole: L/lambda = 1/2, R= 73 ohms L/lambda = 1/5, R = 8 1/10, 2 1/20, 0.5 1/50, 0.08 1/100, 0.02 On 40m, your 2m long whip is like a 4m long dipole: 1/10 (in the above table) for 2 ohms. Since it's a monopole, it's actually half.. 1 ohm.. That's pretty low.. A bunch of people have measured loss resistances for typical mobile installations and they get numbers in the 10-20 ohms range, so you're looking at an efficiency of about 10% of what you'd get with a full sized 10m tall vertical. (this isn't far off the "mobile antenna is a 6dB hit" empirical observation) I'm assuming here that somehow you'll be able to match the 50 ohms in the feed line to the 10-20 ohms with a lot of reactance at the feedpoint without too much loss (a reasonable assumption) |
Resonant condition
J.B. Wood wrote:
The problem here is an impedance analyzer can't distinguish between resistive losses (antenna, ground, tuner (if considered part of the antenna)) and the radiation resistance of the antenna. To determine efficiency you'd have to make some field strength measurements (usually performed with a calibrated field strength meter) in order to determine how much of the power going into the antenna terminals is being radiated into free space. The only "approximation" would be to measure the resistive part of the antenna feedpoint impedance and then subtract from this the radiation resistance obtained from calculation. Sincerely, and 73s from N4GGO, A good deal of, if not the majority of, the loss in a short antenna system is in the matching system components. So even if this method allowed you to get a reasonable estimate of the efficiency of the antenna itself(*), you still wouldn't know what fraction of the transmitter power is getting radiated, since you can't tell how much is lost in the tuner. (*)My limited experience in doing careful antenna measurements leads me to be very skeptical of the ability to determine antenna efficiency even very roughly by a single impedance measurement. I think comparison of measured bandwidth to lossy model results is probably the best indicator. The bottom line is that the impedance meter won't tell you much about the efficiency of the antenna or system. About the only practical way available to most amateurs is comparison of received signal strengths between the antenna and a known reference antenna, using a step attenuator to measure the difference. Roy Lewallen, W7EL |
Resonant condition
On Jun 3, 7:41*am, Gary wrote:
On Jun 3, 5:15*am, david wrote: On Thu, 03 Jun 2010 10:08:15 +0200, -.-. --.- rearranged some electrons to say: Hello, my mobile setup is composed by a 2 meter vertical whip feeded immediately close to it by an automatic antenna tuner. My poor knowledge of antenna systems think that the ATU, choosing the appropriate LC value, bring the entire system in a resonant condition: true or false ? And , second question, because a friend of mine own a MFJ 269 analyzer, how i can *approximately* have an idea of the efficiency on a frequency F for my whip ? Thanks in advance, -.-. --.- The tuner will provide an impedance match to your transceiver (50 ohms).. * It won't make your antenna resonant on 20m (or any other band, other than 2m).- Hide quoted text - - Show quoted text - I think "CQ" meant the whip was 2 meters long. *That would be 1/4 wave on 38 MHZ if I did the math right. *This is the 8 M band? *The tuner might get it on 20 M, but 40 M will be a stretch. Gary N4AST Gary I hope your interpretation is correct. I used to operate a homebrew 2m 5/8 WL on 6 10 and 15 by shorting out the coil and using a tuner. Worked fairly decently for local rag chewing. Jimmie |
Resonant condition
On Jun 3, 1:21*pm, Jim Lux wrote:
-.-. --.- wrote: "-.-. --.-" ha scritto nel messaggio ... Hello, my mobile setup is composed by a 2 meter vertical whip feeded immediately close to it by an automatic antenna tuner. Missed that the expected frequency of the system is between 14 and 30 MHz, but just curious if i had any chance to work 40 meters :) -.-. --.- presuming you mean your whip is 2 meters long (not tuned for the 2 meter band, and only 50 cm long) On 40m? *Maybe, maybe not. * people have used a light bulb as an antenna. It will almost certainly be inefficient. *Here's a short description (filled with technical detail errors, which will no doubt provoke a long discussion).. All antennas have a property called "radiation resistance" *(smaller antennas have smaller radiation resistances) All antennas also have resistive losses. There are also losses "after the wave has left the antenna" (e.g. ground reflections, etc. *why salt water marshes are prized for vertical pol). * The latter are not considered here. To a first order (back, all you nit pickers.. first order), the power from the transmitter gets distributed between the radiation resistance and the loss resistance. *So, the "efficiency" can be thought of as that fraction of power that winds up in the radiation resistance, as opposed to in the loss resistance. Important he if the radiation resistance is low, that doesn't mean poor efficiency: as long as you keep the resistive losses low too.. which can be a challenge (assuming you're not carrying a vat of liquid helium, for instance). Part of the problem is that the resistive losses aren't just in the antenna, but also in any substance which is immersed in the antenna's electric and magnetic fields (like the steel of your car, and the not particularly good conductivity, but not a perfect insulator either, soil under the car). And, then, there are losses in how you get from Tx to Antenna. *If your antenna presents an impedance that is not what the transmitter is providing, you've got to transform it somehow, typically using Ls and Cs, etc. (in your autotuner). *Those components also have some amount of loss, although I'll bet it's less than 10% in most situations (otherwise, the tuner/matching network would melt, and they don't) Moral of story.. radiation resistance doesn't drop much from a full size antenna until you get around 1/10th wavelength, then it starts to drop real fast (as length squared) For a dipole: L/lambda = 1/2, R= 73 ohms L/lambda = 1/5, R = 8 1/10, 2 1/20, 0.5 1/50, 0.08 1/100, 0.02 I checked this out one time and my memory is a bit hazy. Using my computer program which ofcourse could have a glitch in it, I found that the gain hit a max at the very low resistance value and then backed off as the resistance became close to zero. This blip occurred during the last fraction of an ohm just prior to closure of supplying gain figures. At the time I put this down as the point where the skin depth penetration was minimal and thus we had reached a point where the applied current was totally to provide gain after which it dropped dramatically. I looked at this as a progression for the current flow in the surface of the element where it progressed to the surface and the flow progressed above the surface but below the particle encapsulation. Further progression created losses and thus the gain blip dropped. Resistance never dropped to zero thus ohms law was not declared invalid. If somebody could duplicate the above via another program it may prove interesting. Keep- dropping the applied frequency until you reach about 0.7 ohms and then start recording. |
Resonant condition
On Jun 3, 3:41*am, "-.-. --.-" wrote:
"-.-. --.-" ha scritto nel ... Hello, my mobile setup is composed by a 2 meter vertical whip feeded immediately close to it by an automatic antenna tuner. Missed that the expected frequency of the system is between 14 and 30 MHz, but just curious if i had any chance to work 40 meters :) -.-. --.- It's possible.. But feeding a whip with a tuner usually does not make for an efficient mobile antenna. Not only are many/most tuners more lossy than say using a loading coil on the whip, but current distribution suffers. Maximum current will be at the tuner which is not desirable. The location of the loading coil has a large effect on the current distribution and efficiency of the antenna. Where you have it is about the worst possible place. :( I have lots of people ask me about running whips matched with tuners.. I pretty much have a standard reply.. No! Not on my watch! Chortle.. My mobile antenna is center loaded in the driving config.. Even higher if I add the 3 foot lower mast, but that's only when parked. In the parked config, my loading coil is 8 ft above the base of the whip. "14 ft tall whip" And yes, you can tell a pretty good difference from the normal driving config, with the coil at 5 ft above the base. "11 foot tall whip" |
Resonant condition
"-.-. --.-" ha scritto nel messaggio ... Hello, my mobile setup is composed by a 2 meter vertical whip feeded immediately close to it by an automatic antenna tuner. Thanks to all. Now is a clear statement that my problem is measuring the "inefficiency" of the entire system :) Only to mention that due to a sort of "quiet life way" with the neighbors, my home setup is a ATU CG-3000 feeding 9,5 meters height monopole with about 40 random lenght radials (from 5 to 20 meters of lenght) and electrically connectet to 200 square meters of chicken fence, with real chickens inside :) perform quite well on 80, from 40 to 12 meters is probably the best antenna i ever had compared to old dipoles, in 10 maybe it is too close to 1 WL... but work almost all , even in furiouses pile ups. Now that I annoyed with this info, the conclusion of the OP or the third question if you want: how it is possible that mobile setups with the "motorized" antennas can have a minimum of efficiency in 40 meters ?? What the difference from a variabile motoryzed L and an ATU at the feed point ?? Or i miss something important about the 2 feeding methods ?? TIA, -.-. --.- |
Resonant condition
On Jun 3, 11:03*pm, wrote:
But feeding a whip with a tuner usually does not make for an efficient mobile antenna. A 11.5 foot (~3.5m) whip driven by an SG-230 autotuner was measured to be 12 dB down from the top-rated bugcatchers and screwdrivers at one of the CA 75m mobile shootouts back in the 1980's. -- 73, Cecil, w5dxp.com |
Resonant condition
*To determine efficiency you'd have to make some field strength measurements
(usually performed with a calibrated field strength meter) in order to determine how much of the power going into the antenna terminals is being radiated into free space. The radiation resistance present at the base of an electrically short, linear, monopole (whip) antenna of various ODs can be calculated rather accurately using equations found in various antenna engineering textbooks So if the base current entering the whip itself can be measured, and whether or not a loading coil was used at the base to resonate* the antenna system, and regardless of the loss in the r-f ground used by the whip -- then for practical purposes the power radiated by the antenna system will be the product of the square of that base current, and the radiation resistance of the whip. * However "resonated" antenna systems using a short whip do not have the same radiation performance as those using a naturally resonant 1/4- wave monopole. Resonance just means that the capacitive reactance at the feedpoint of the short whip has been exactly offset by the inductive reactance of a loading coil, which can allow for the most efficient power transfer into the radiator. However the radiation resistance of the whip is not changed by this process -- it is still very low compared to a naturally resonant 1/4- wave monopole. Therefore the radiation efficiency of such a short, loaded, whip antenna system cannot approach that of a naturally resonant 1/4-wave monopole unless the losses in the loading coil and r- f ground are nearly zero. Loading coils do not replace the "missing degrees" of an electrically short radiator, as far as its radiation resistance is concerned. The radiation resistance of a whip depends only on the electrical length/ OD of the whip, itself. RF |
Resonant condition
On Jun 4, 6:19*am, "-.-. --.-" wrote:
... how it is possible that mobile setups with the "motorized" antennas can have a minimum of efficiency in 40 meters ?? What the difference from a variabile motoryzed L and an ATU at the feed point ?? Most screwdrivers and bugcatchers are more center-loaded than base loaded. The section of the antenna that supplies a good part of the radiation is the straight section between the feedpoint and the bottom of the loading coil. An ATU driven whip doesn't possess that high- efficiency, high-current section. The highest current sections in an ATU system are inside the ATU - not good for radiation. Everything else being equal, a center-loaded antenna will beat a base-loaded antenna by ~3-5 dB according to mobile shootout results. The radiation resistance for a center-loaded 75m mobile antenna is approximately double that for a base-loaded 75m mobile antenna, i.e. close to double the efficiency. According to 75m mobile shootout results, an ATU driven whip is ~8 dB down from a base-loaded bugcatcher because the bugcatcher coil radiates and an ATU is usually shielded and often uses powdered iron toroids for the coils. -- 73, Cecil, w5dxp.com |
Resonant condition
Richard Fry wrote:
To determine efficiency you'd have to make some field strength measurements (usually performed with a calibrated field strength meter) in order to determine how much of the power going into the antenna terminals is being radiated into free space. The radiation resistance present at the base of an electrically short, linear, monopole (whip) antenna of various ODs can be calculated rather accurately using equations found in various antenna engineering textbooks . . . This is true only if you don't confuse the idealized textbook models with real antennas. But most of us are unfortunately stuck with using the latter. In general, the impedance you calculate with the idealized models doesn't match that of real world antennas. It works pretty well for AM broadcast installations, where the length and large number of radials make the impedance relatively independent of ground characteristics. But this doesn't describe the typical amateur monopole antenna, either ground or mobile mounted. An approximation to input resistance can be made by adjusting for an abbreviated radial system, but this gets increasingly unreliable as the number of radials decreases. The best readily available modeling program allowing the inclusion of a buried ground system, which uses the same well-established equations as textbooks, is NEC-4. It, however, suffers from a serious shortcoming in doing this calculation -- it assumes that the ground is homogeneous to an infinite depth. Real ground is typically stratified, and skin depth at HF is as much as several tens of feet, so the representation of real ground is very poor. There are many cases where a single "equivalent" value of homogeneous ground doesn't exist which gives the same results as actual measurement. I've made very careful measurements of a simple vertical monopole with various numbers of buried radials whose impedance couldn't be matched with NEC-4 using any ground parameters, and I believe this to be a common occurrence. In no case would I depend on a computer model, let alone an even more simplified textbook model, to predict the resistance of a real monopole having an abbreviated ground system with enough accuracy to reasonably estimate the efficiency. As a side note, Brown, Lewis, and Epstein's sparse radial results can be matched reasonably well with NEC-4, but it does require a fair amount of ground constant adjustment for various numbers and lengths of radials. Mobile mounted whip antennas fare even worse relative to simple textbook models. I don't have any experience with comparison of computer models with actual measurement. Those results should depend on the care with which the model is constructed and the amount of influence the ground has on the impedance. Roy Lewallen, W7EL |
Resonant condition
"Cecil Moore" ha scritto nel messaggio ... According to 75m mobile shootout results, an ATU driven whip is ~8 dB down from a base-loaded bugcatcher because the bugcatcher coil radiates and an ATU is usually shielded and often uses powdered iron toroids for the coils. Yes, understand *perfectly*. My energy go somewhere, and this "somewhere" is heating some toroid into a shield case or radiate on the air. Conservation law. Center load i think mean also lower ground loss, IIRC. And as i understand, a RX shootout of the mobile whip compared with my full size 1/4 wl vertical antenna with the same transceiver, to have a suitable relative dB comparison, can give the best test measure i can do with my actual equipement. Thanks Cecil an other posters, other suggestions or links to learn more are very appreciated. -.-. --.- |
Resonant condition
On Jun 4, 9:53*am, Roy Lewallen wrote:
Richard Fry wrote: *To determine efficiency you'd have to make some field strength measurements (usually performed with a calibrated field strength meter) in order to determine how much of the power going into the antenna terminals is being radiated into free space. The radiation resistance present at the base of an electrically short, linear, monopole (whip) antenna of various ODs can be calculated rather accurately using equations found in various antenna engineering textbooks . . . This is true only if you don't confuse the idealized textbook models with real antennas. ... For the sake of discussion, below are two pastes from the same NEC model using the demo version of EZNEC v. 5.0 -- which rather well support my earlier post that the radiation resistance (NOT the impedance) of an electrically short monopole is a function of its electrical length, and not the loss resistance of the r-f ground and/ or the loading coil. CASE 1 = Zero loss resistance and reactance in the r-f ground, and zero loss resistance in the loading coil: EZNEC Demo ver. 5.0 1650 kHz 3 meter monopole 6/4/2010 10:50:57 AM --------------- SOURCE DATA --------------- Frequency = 1.65 MHz Source 1 Voltage = 0.08578 V at 35.09 deg. Current = 0.4986 A at 0.0 deg. Impedance = 0.1408 + J 0.09888 ohms Power = 0.035 watts SWR (50 ohm system) 100 (25.17 ohm system) 100 CASE 2 = Same model as above, except with a total of 25 ohms loss in a loading coil and r-f ground, and no reactance in the r-f ground: EZNEC Demo ver. 5.0 1650 kHz 3 meter monopole 6/4/2010 10:49:40 AM --------------- SOURCE DATA --------------- Frequency = 1.65 MHz Source 1 Voltage = 0.9386 V at 0.22 deg. Current = 0.03729 A at 0.0 deg. Impedance = 25.17 + J 0.09579 ohms Power = 0.035 watts SWR (50 ohm system) = 1.987 (25.17 ohm system) = 1.004 EZNEC calculated the radiation resistances of these two cases to be 0.14 ohms and 0.17 ohms, respectively -- fairly close, but not exact. Perhaps Roy could comment on the reason why their agreement using NEC/ EZNEC is not better. Those wanting a good resource for the measured results for monopoles of less than 1/8 electrical wavelength might try to locate the paper by Carl E. Smith and Earl M. Johnson titled PERFORMANCE OF SHORT ANTENNAS, published in the October, 1947 edition of the Proceedings of the I.R.E. The equation for the radiation resistance of short antennas given in that paper is independent of the resistive losses in any loading coil or r-f ground system. RF |
Resonant condition
On Jun 4, 12:21*pm, "-.-. --.-" wrote:
"Cecil Moore" ha scritto nel ... According to 75m mobile shootout results, an ATU driven whip is ~8 dB down from a base-loaded bugcatcher because the bugcatcher coil radiates and an ATU is usually shielded and often uses powdered iron toroids for the coils. Yes, understand *perfectly*. My energy go somewhere, and this "somewhere" is heating some toroid into a shield case or radiate on the air. Conservation law. Center load i think mean also lower ground loss, IIRC. And as i understand, a RX shootout of the mobile whip compared with my full size 1/4 wl vertical antenna with the same transceiver, to have a suitable relative dB comparison, can give the best test measure i can do with my actual equipement. Thanks Cecil an other posters, other suggestions or links to learn more are very appreciated. -.-. --.- While that is true your exiting installation may be better than you think. Best Antenna can be subject to to any number of parameters from greatest gain to practical operations and installation to significant others opinion of aesthetic appeal. Jimmie |
Resonant condition
Roy Lewallen wrote:
As a side note, Brown, Lewis, and Epstein's sparse radial results can be matched reasonably well with NEC-4, but it does require a fair amount of ground constant adjustment for various numbers and lengths of radials. Mobile mounted whip antennas fare even worse relative to simple textbook models. I don't have any experience with comparison of computer models with actual measurement. Those results should depend on the care with which the model is constructed and the amount of influence the ground has on the impedance. I would figure that getting an accurate ground influence in an antenna design program would have to be a daunting project, indeed. While installing my bugcatcher, I did a lot of it in stages, noting the positive influence on the results. My thinking at this point is that the make and model, and the size of the vehicle would be critical for the model, and small changes go a long way. What's more, the more efficient and narrow the antenna, the more effect the changes have. Given that a good setup always involves a lot of custom work like bonding and turning potential radiators like the exhaust system into more bonded area, it is a really tough exercise. - 73 de Mike N3LI - |
Resonant condition
Cecil Moore wrote:
On Jun 4, 6:19 am, "-.-. --.-" wrote: ... how it is possible that mobile setups with the "motorized" antennas can have a minimum of efficiency in 40 meters ?? What the difference from a variabile motoryzed L and an ATU at the feed point ?? Most screwdrivers and bugcatchers are more center-loaded than base loaded. The section of the antenna that supplies a good part of the radiation is the straight section between the feedpoint and the bottom of the loading coil. An ATU driven whip doesn't possess that high- efficiency, high-current section. The highest current sections in an ATU system are inside the ATU - not good for radiation. Everything else being equal, a center-loaded antenna will beat a base-loaded antenna by ~3-5 dB according to mobile shootout results. The radiation resistance for a center-loaded 75m mobile antenna is approximately double that for a base-loaded 75m mobile antenna, i.e. close to double the efficiency. According to 75m mobile shootout results, an ATU driven whip is ~8 dB down from a base-loaded bugcatcher because the bugcatcher coil radiates and an ATU is usually shielded and often uses powdered iron toroids for the coils. As a point of clarification, Cecil, the bottom loaded bug catcher you refer to - is it the matching coil or the loading coil? I only knew of mid-loaded bugcatchers. - 73 de Mike N3LI - |
Resonant condition
On Jun 4, 12:55*pm, Michael Coslo wrote:
As a point of clarification, Cecil, the bottom loaded bug catcher you refer to - is it the matching coil or the loading coil? I only knew of mid-loaded bugcatchers. Jim, k7jeb, once used a standard 75m Texas Bugcatcher coil as a base- loaded whip (no top hat) and entered one of the CA 75m mobile shootouts. He was "only" 3 dB down from similar center-loaded Texas Bugcatchers (no top hat). This fits well with the radiation resistance estimate for the center-loaded bugcatcher being double that of the base-loaded configuration. -- 73, Cecil, w5dxp.com |
Resonant condition
On Jun 4, 12:55*pm, Michael Coslo wrote:
As a point of clarification, Cecil, the bottom loaded bug catcher you refer to - is it the matching coil or the loading coil? I only knew of mid-loaded bugcatchers. A short whip can be fed at any point on the radiator. In Cecil's case, I assume the coil was a true loading coil, and not the matching coil. As per his numbers, the base loaded was quite a bit better than the "tuner" loaded whip, which was 12 db down from the center loaded bugcatcher. In general, appx 3/4 the length of the whip from the base will be the appx best location for the coil. The higher the coil is, the better the current distribution. But.. The higher the coil is, the more turns of wire you need to tune. So there is a trade off of current distribution vs coil loss due to the extra turns. You could have the coil at 95% high, and have great current distribution, but the losses of all the turns required would eat you for lunch. So... usually around 3/4 of the way up will be about the optimum location. 1/2 way up is good, and a good compromise between current distribution and coil losses. For a given length whip, Reg Edwards "vertload" program can be used to calculate the best location for the coil, and having played with it, and using the real antennas to compare, I think it is very close. Also, it jives with the info and graphs used in the ARRL antenna handbook on that subject. |
Resonant condition
On Jun 4, 3:32*pm, wrote:
On Jun 4, 12:55*pm, Michael Coslo wrote: As a point of clarification, Cecil, the bottom loaded bug catcher you refer to - is it the matching coil or the loading coil? I only knew of mid-loaded bugcatchers. A short whip can be fed at any point on the radiator. er.. I meant to say the coil can be placed at any point on the radiator.. |
Resonant condition
Richard Fry wrote:
For the sake of discussion, below are two pastes from the same NEC model using the demo version of EZNEC v. 5.0 -- which rather well support my earlier post that the radiation resistance (NOT the impedance) of an electrically short monopole is a function of its electrical length, and not the loss resistance of the r-f ground and/ or the loading coil. . . . EZNEC calculated the radiation resistances of these two cases to be 0.14 ohms and 0.17 ohms, respectively -- fairly close, but not exact. Perhaps Roy could comment on the reason why their agreement using NEC/ EZNEC is not better. Sorry, I can't tell without seeing the EZNEC description file. If you'll attach the .EZ file to an email message to me, I'll be glad to answer your question. I wasn't able to get a radiation resistance that high at that frequency for a 3 meter vertical of any diameter, so there's something in the model which isn't immediately apparent. Those wanting a good resource for the measured results for monopoles of less than 1/8 electrical wavelength might try to locate the paper by Carl E. Smith and Earl M. Johnson titled PERFORMANCE OF SHORT ANTENNAS, published in the October, 1947 edition of the Proceedings of the I.R.E. The equation for the radiation resistance of short antennas given in that paper is independent of the resistive losses in any loading coil or r-f ground system. And the same fundamental equations are used by modeling programs. The problem is that interaction between the antenna, an abbreviated ground system, and the Earth can modify the radiation resistance as well as adding loss resistance. You might try modeling a few short verticals with a few radials just above ground, and looking at the gain with various radial systems. You'll find that the gain change doesn't exactly correlate with the feedpoint resistance change when you assume a constant radiation resistance. This isn't a shortcoming of the modeling program, but a real effect. I doubt you'll find much about it in pre-computer age texts, though, because it's probably a very tough, or maybe impossible, manual calculation. Roy Lewallen, W7EL |
Resonant condition
On Jun 4, 4:40*pm, Roy Lewallen wrote:
Richard Fry wrote: The equation for the radiation resistance of short antennas given in that paper is independent of the resistive losses in any loading coil or r-f ground system. And the same fundamental equations are used by modeling programs. The problem is that interaction between the antenna, an abbreviated ground system, and the Earth can modify the radiation resistance as well as adding loss resistance. Could you please explain why, if the same fundamental equations given in antenna engineering textbooks and I.R.E. papers are used by modeling programs, the results of their use do not always support each other very well? If it is accepted that the radiation resistance of a short monopole is independent of the loss resistance in the loading coil and r-f ground either alone or together, then what is the basis for the variation in radiation resistance that you report? BTW, the equations in the Carl Smith paper I referred to earlier in this thread produce a radiation resistance of 0.113 ohms for a 1.65 MHz, 9.84' (3-m) x 0.25" OD, base driven monopole -- which is not _hugely_ different than the values calculated by EZNEC. RF |
Resonant condition
Richard Fry wrote:
Could you please explain why, if the same fundamental equations given in antenna engineering textbooks and I.R.E. papers are used by modeling programs, the results of their use do not always support each other very well? I'm not aware of any cases where engineering textbooks and papers disagree with modeling programs. NEC, for example, has been very extensively tested against both theory and measurement. If there are cases where the programs seem to disagree with theory, it's very likely due to careless modeling resulting in a model which isn't the same as the textbook model. Can you cite an example of disagreement between computer model and textbook theory? If it is accepted that the radiation resistance of a short monopole is independent of the loss resistance in the loading coil and r-f ground either alone or together, then what is the basis for the variation in radiation resistance that you report? It is indeed accepted that the radiation resistance of a monopole over a perfect ground of infinite extent has the characteristics you ascribe, and computer models show this independence as they should. (I haven't yet received your model which you feel seems to show differently.) But it's neither true nor "accepted" when the ground system is much less than perfect. The variation is due to interaction between the vertical and ground system, just as the radiation resistance of a VHF ground plane antenna changes as you bend the radials downward. Altering the number, length, depth, and orientation of radials has more of an effect than simply adding loss. BTW, the equations in the Carl Smith paper I referred to earlier in this thread produce a radiation resistance of 0.113 ohms for a 1.65 MHz, 9.84' (3-m) x 0.25" OD, base driven monopole -- which is not _hugely_ different than the values calculated by EZNEC. EZNEC gives a result of 0.1095 ohm with 20 segments, converging to around 0.103 ohms with many more segments. Keep in mind that the model source position moves closer to the base as the number of segments increases. The author's result is good. If you examine the paper carefully, I'm sure you'll find that the author had to make some assumptions and approximations to arrive at his equations -- the most fundamental equations can't be solved in closed form, and many, many papers and several books were written describing various approximations to calculate something as basic as the input impedance of an arbitrary length dipole. If you do some research, you'll find that the many different approximating methods all give slightly different results. The small disagreement in the cited paper is really a measure of how good his approximations were. Modeling programs have to use numerical methods which are limited by quantization, but they have the advantage of not needing the various approximation methods required for calculation by other means. Roy Lewallen, W7EL |
Resonant condition
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Resonant condition
Cecil Moore wrote:
On Jun 3, 11:03 pm, wrote: But feeding a whip with a tuner usually does not make for an efficient mobile antenna. A 11.5 foot (~3.5m) whip driven by an SG-230 autotuner was measured to be 12 dB down from the top-rated bugcatchers and screwdrivers at one of the CA 75m mobile shootouts back in the 1980's. that's a pretty big difference.. (12 dB implies a factor of 16.. that's like most of the Tx power being dissipated somewhere, and that sounds like "component melting" levels) Have you a link to the data and test methodology? |
Resonant condition
On Jun 6, 4:58*pm, Roy Lewallen wrote:
This is why, for example, a center loading coil must have more inductance than a base loading coil to effect the same change in reactance at the base. The following is based on a fixed length antenna. The phase shift at the top of each coil is associated with the abrupt shift in characteristic impedances at the coil-stinger junction (according to W8JI). When a base section is added to a base-loaded antenna, there is an opposite abrupt shift in characteristic impedance at the base-coil junction. That bottom (negative) phase shift subtracts from the (positive) phase shift at the coil-stinger junction so more phase shift must be added through the coil to compensate for the phase shift lost at the base-coil junction. Increasing the coil length provides the necessary additional phase shift. Assume a loading coil has a characteristic impedance of 4000 ohms and the stinger has a characteristic impedance of 600 ohms at the coil- stinger junction. Given the impedance looking into the stinger, it is easy to calculate the phase shift at the coil-stinger junction. Let's (for instance) say the stinger's input impedance is 0.25 - j2500 ohms. If we normalize that impedance to the assumed Z0=600 ohms of the stinger, we get very close to -j4.167. The impedance at the very top of the coil is the same and if we normalize to the assumed Z0=4000 ohms of the coil, we get -j0.625 ohms. If we subtract the arctangent of those two values, we get the phase shift: 76.5 - 32 degrees = 44.5 degrees at the top of the loading coil. We can also read that same value from a Smith Chart. When we go to a center-loaded coil, the calculations are complicated by the resistive portion of the impedance, but we will find a negative phase shift at the bottom of the coil that subtracts from the positive phase shift at the top of the coil. Since we have reduced the total system phase shift by moving the coil to the center of the antenna, we need to add more length to the coil to increase the phase shift through the coil in order to compensate for the negative phase shift lost at the bottom of the coil. One can emulate the loading coil problem using pieces of transmission line with different Z0s. The basics of shortened dual-Z0 stubs are covered he http://www.w5dxp.com/shrtstub.htm For instance, the following shortened stub has a resonant frequency at which it is electrically 1/4WL long even though it is only 1/8WL long physically because of the 45 degree phase shift between the two sections. -----22.5 deg 300 ohm-----+-----22.5 deg 50 ohms----- What happens to the resonant frequency if we move half of the 50 ohm line to the bottom? ----11.25 deg 50 ohm---+---22.5 deg 300 ohm---+---11.25 deg 50 ohm How many degrees do we need to add to the 300 ohm line to achieve the same resonant frequency as before? Can anyone out there solve this problem? -- 73, Cecil, w5dxp.com |
Resonant condition
In the admittedly very few looks I've had at mobile "shootout" results,
there seems to be more of a correlation between vehicle size and field strength than antenna and field strength. This comes as no surprise, since the vehicle is usually a comparable or even greater part of the radiating system than the titular antenna, and its coupling to ground has a large impact on the efficiency. Roy Lewallen, W7EL |
Resonant condition
On Jun 7, 11:36*am, Jim Lux wrote:
* Have you a link to the data and test methodology? I summarized the data from three CA 75m mobile shootouts at: http://www.w5dxp.com/shootout.htm I don't recall a test methodology being published. The test receiver consisted of a ferrite loop antenna in the far field feeding a lab- grade RF voltmeter. The power incident upon the 75m mobile antenna system was assumed to be forward power minus reflected power on the coax to the antenna system, measured using two Birds. The receive results were normalized accordingly. I may have left out a detail or two. The SG-230 plus 11.5 whip at -12 dB was equal to a 75m hamstick. I entered both the top-rated (0 dB reference) antenna and the (-12 dB) autotuner+whip on the same vehicle. When I "superposed" all of the three results, I assumed 0 dB for each top-rated antenna and let the rest fall where they might. That may or may not have been a reasonable assumption. I suspect the SG-230 is designed to dissipate 100 watts (using large #2 material powdered-iron toroids). During one shootout episode, I forgot to attach the antenna to the mobile mount. The SG-230 faithfully tuned to close to a 1:1 match on the input - with a near- infinite SWR on the output. It was a damp foggy day and the mobile mount arced. That taught me not to mount the SG-230 unobserved in the attic. :-) -- 73, Cecil, w5dxp.com |
Resonant condition
On Jun 7, 1:23*pm, Roy Lewallen wrote:
In the admittedly very few looks I've had at mobile "shootout" results, there seems to be more of a correlation between vehicle size and field strength than antenna and field strength. This comes as no surprise, since the vehicle is usually a comparable or even greater part of the radiating system than the titular antenna, and its coupling to ground has a large impact on the efficiency. Which is why, in this case, it is well to note that the 0 dB top-rated antenna and the -12 dB antenna were mounted on the same vehicle (mine). -- 73, Cecil, w5dxp.com |
Resonant condition
Roy Lewallen wrote:
In the admittedly very few looks I've had at mobile "shootout" results, there seems to be more of a correlation between vehicle size and field strength than antenna and field strength. This comes as no surprise, since the vehicle is usually a comparable or even greater part of the radiating system than the titular antenna, and its coupling to ground has a large impact on the efficiency. Roy Lewallen, W7EL that seems quite plausible. A bigger vehicle essentially means a physically larger antenna (think of the whole system as a dipole fed off center, and a fan on one side but not the other. |
Resonant condition
Cecil Moore wrote:
On Jun 7, 11:36 am, Jim Lux wrote: Have you a link to the data and test methodology? I summarized the data from three CA 75m mobile shootouts at: http://www.w5dxp.com/shootout.htm I don't recall a test methodology being published. The test receiver consisted of a ferrite loop antenna in the far field feeding a lab- grade RF voltmeter. The power incident upon the 75m mobile antenna system was assumed to be forward power minus reflected power on the coax to the antenna system, measured using two Birds. The receive results were normalized accordingly. I may have left out a detail or two. Were those all mounted in the same place on the same vehicle, e.g. the license plate bracket? |
Resonant condition
On Jun 7, 11:48*am, Cecil Moore wrote:
For instance, the following shortened stub has a resonant frequency at which it is electrically 1/4WL long even though it is only 1/8WL long physically because of the 45 degree phase shift between the two sections. ... Are you stating that such a radiator with a physical length of 1/8- lambda has ALL of the electrical characteristics of a self-resonant, 1/4-wave radiator -- including its radiation resistance, radiation pattern, and peak gain in dBi? Zero reactance at the input terminals of an electrically short radiator does not mean that such a radiator is the electrical equal of every other radiator with zero reactance at its input terminals. Zero input reactance for short radiators can be attained by various means, but the intrinsic, real radiation resistance/pattern/gain of an antenna is a function of the electrical length, configuration, and installation environment of the conductor(s) exposed to free space, regardless of the reactance at the feedpoint -- whether that reactance is zero or not. RF |
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