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Losses in shunt-fed towers
For 75 and 80 meters, I use a shunt-fed tower (58 feet high, with a fairly big
HF yagi on its top). The home-made tuner, placed at the tower base, has a motorized variable vacuum capacitor in series and a (properly selected) fixed capacitor in parallel. I can so remotely tune the variable capacitor across the 3500 - 3800 kHz range obtaining a perfect match to 50-ohm everywhere in the band. The antenna works fine and I receive good reports. The only thing which worries me a bit is the very high voltage that develops at the antenna end (about 6,900 Vrms at 3.500 MHz with 2kW applied, growing to about 8,600 Vrms at 3.800 MHz). I am not sure whether such high voltage could be a source of significant losses. To solve my doubt, I started by precisely measuring the capacitance of the tuner capacitors, and I could then easily calculate the antenna impedance at 3.500 MHz, which resulted to be (18.2 + j 656) ohm. ABOUT CURRENT With 2 kW applied, one can easily determine that the RF current flowing through the antenna is about 10.5A, quite higher than the 6.3A figure one would get should the antenna resistance be 50 ohm instead of 18.2 ohm. So, I must expect some more loss in the conductors due to the fairly high current. ABOUT VOLTAGE The 10.5 A current flowing through the big 656 ohm antenna reactance causes the antenna RF voltage to get up to 6,900 Vrms at 3.500 MHz. Touching the wire with a (well insulated) screwdriver, you would see a nice Tesla-like arc. I am wondering whether such a high voltage could constitute, by itself, a source of extra loss. Please note that the wire coming down from the tower is connected directly to the vacuum capacitor terminal, with no stand-off insulator. So, I do not see a place where power can get dissipated due to the high RFvoltage, other than perhaps in the humid air (?). I could probably avoid this situation by changing the tap height on the tower, but I would run the risk of not being any longer able to tune across the whole 3500 - 3800 kHz band adjusting one of the two capacitors, and not both. Any idea on whether the high RF voltage present on the antenna could cause some significant loss? Thanks and 73 Tony I0JX Rome, Italy |
Losses in shunt-fed towers
On Fri, 9 Sep 2011 21:10:08 +0200, "Antonio Vernucci"
wrote: For 75 and 80 meters, I use a shunt-fed tower (58 feet high, with a fairly big HF yagi on its top). The home-made tuner, placed at the tower base, has a motorized variable vacuum capacitor in series and a (properly selected) fixed capacitor in parallel. I can so remotely tune the variable capacitor across the 3500 - 3800 kHz range obtaining a perfect match to 50-ohm everywhere in the band. Hi Antonio, Describe the "shunt." (I presume it is a long wire from the tuner, standing off 10cm or so, up maybe 5 - 10 meters where it attaches to the tower? Can I also presume that the tower goes to ground at the base?) The antenna works fine and I receive good reports. And so it would seem this would be the end of the post. But no..... The only thing which worries me a bit is the very high voltage that develops at the antenna end End? There are at least two, and perhaps you mean the end of the shunt at the tuner? If by "end" you mean up at the very top, then high voltage would be expected (but worry about it would not be expected). By this point, there are a lot of gaps in the description (at least for me). (about 6,900 Vrms at 3.500 MHz with 2kW applied, growing to about 8,600 Vrms at 3.800 MHz). I am not sure whether such high voltage could be a source of significant losses. You worry about loss, when you receive good reports? High voltage reveals loss in heat. Corona would be an example, but you don't describe corona. You also don't describe heat. To solve my doubt, I started by precisely measuring the capacitance of the tuner capacitors, and I could then easily calculate the antenna impedance at 3.500 MHz, which resulted to be (18.2 + j 656) ohm. I think you have a sign problem. You are describing a short antenna with inductive reactance (given the height and wavelength, very improbable). However, this might be the feedpoint impedance at the lower end of the shunt - maybe, maybe not. ABOUT CURRENT With 2 kW applied, one can easily determine that the RF current flowing through the antenna is about 10.5A, quite higher than the 6.3A figure one would get should the antenna resistance be 50 ohm instead of 18.2 ohm. So, I must expect some more loss in the conductors due to the fairly high current. Only in the shunt wire. Only if you are using wirewrap wire. Measure the RESISTANCE. Multiply by CURRENT squared. You get WATTs. How many? That is your loss. Now, if you can measure the resistance of your tower, stand very far back because it is about to fall over from being rusted out. ABOUT VOLTAGE The 10.5 A current flowing through the big 656 ohm antenna reactance causes the antenna RF voltage to get up to 6,900 Vrms at 3.500 MHz. Touching the wire with a (well insulated) screwdriver, you would see a nice Tesla-like arc. With this statement, it is obvious that you are not touching the screwdriver to the "end" of the antenna (some 15-20 meters above your head). Current and voltage are not uniformly equal along the length of the antenna. Your beam at the top acting as a top hat might yield some uniformity, but that doesn't come from simple math analysis. Instead, what you describe is a feedpoint impedance calculation. Any idea on whether the high RF voltage present on the antenna could cause some significant loss? You are working very hard to find a problem that isn't there. 73's Richard Clark, KB7QHC |
Losses in shunt-fed towers
Antonio Vernucci wrote:
For 75 and 80 meters, I use a shunt-fed tower (58 feet high, with a fairly big HF yagi on its top). The home-made tuner, placed at the tower base, has a motorized variable vacuum capacitor in series and a (properly selected) fixed capacitor in parallel. I can so remotely tune the variable capacitor across the 3500 - 3800 kHz range obtaining a perfect match to 50-ohm everywhere in the band. The antenna works fine and I receive good reports. The only thing which worries me a bit is the very high voltage that develops at the antenna end (about 6,900 Vrms at 3.500 MHz with 2kW applied, growing to about 8,600 Vrms at 3.800 MHz). I am not sure whether such high voltage could be a source of significant losses. To solve my doubt, I started by precisely measuring the capacitance of the tuner capacitors, and I could then easily calculate the antenna impedance at 3.500 MHz, which resulted to be (18.2 + j 656) ohm. ABOUT CURRENT With 2 kW applied, one can easily determine that the RF current flowing through the antenna is about 10.5A, quite higher than the 6.3A figure one would get should the antenna resistance be 50 ohm instead of 18.2 ohm. So, I must expect some more loss in the conductors due to the fairly high current. ABOUT VOLTAGE The 10.5 A current flowing through the big 656 ohm antenna reactance causes the antenna RF voltage to get up to 6,900 Vrms at 3.500 MHz. Touching the wire with a (well insulated) screwdriver, you would see a nice Tesla-like arc. I am wondering whether such a high voltage could constitute, by itself, a source of extra loss. Please note that the wire coming down from the tower is connected directly to the vacuum capacitor terminal, with no stand-off insulator. So, I do not see a place where power can get dissipated due to the high RFvoltage, other than perhaps in the humid air (?). I could probably avoid this situation by changing the tap height on the tower, but I would run the risk of not being any longer able to tune across the whole 3500 - 3800 kHz band adjusting one of the two capacitors, and not both. Any idea on whether the high RF voltage present on the antenna could cause some significant loss? Thanks and 73 Tony I0JX Rome, Italy You did not describe your RF ground. |
Losses in shunt-fed towers
I did not describe the RF ground because I consider it not relevant to my
question. Anyway, the ground system is formed by 64 quarter-wave radials. 73 Tony I0JX Rome, Italy "dave" ha scritto nel messaggio . .. You did not describe your RF ground. Antonio Vernucci wrote: For 75 and 80 meters, I use a shunt-fed tower (58 feet high, with a fairly big HF yagi on its top). The home-made tuner, placed at the tower base, has a motorized variable vacuum capacitor in series and a (properly selected) fixed capacitor in parallel. I can so remotely tune the variable capacitor across the 3500 - 3800 kHz range obtaining a perfect match to 50-ohm everywhere in the band. The antenna works fine and I receive good reports. The only thing which worries me a bit is the very high voltage that develops at the antenna end (about 6,900 Vrms at 3.500 MHz with 2kW applied, growing to about 8,600 Vrms at 3.800 MHz). I am not sure whether such high voltage could be a source of significant losses. To solve my doubt, I started by precisely measuring the capacitance of the tuner capacitors, and I could then easily calculate the antenna impedance at 3.500 MHz, which resulted to be (18.2 + j 656) ohm. ABOUT CURRENT With 2 kW applied, one can easily determine that the RF current flowing through the antenna is about 10.5A, quite higher than the 6.3A figure one would get should the antenna resistance be 50 ohm instead of 18.2 ohm. So, I must expect some more loss in the conductors due to the fairly high current. ABOUT VOLTAGE The 10.5 A current flowing through the big 656 ohm antenna reactance causes the antenna RF voltage to get up to 6,900 Vrms at 3.500 MHz. Touching the wire with a (well insulated) screwdriver, you would see a nice Tesla-like arc. I am wondering whether such a high voltage could constitute, by itself, a source of extra loss. Please note that the wire coming down from the tower is connected directly to the vacuum capacitor terminal, with no stand-off insulator. So, I do not see a place where power can get dissipated due to the high RFvoltage, other than perhaps in the humid air (?). I could probably avoid this situation by changing the tap height on the tower, but I would run the risk of not being any longer able to tune across the whole 3500 - 3800 kHz band adjusting one of the two capacitors, and not both. Any idea on whether the high RF voltage present on the antenna could cause some significant loss? Thanks and 73 Tony I0JX Rome, Italy |
Losses in shunt-fed towers
In article ,
"Antonio Vernucci" wrote: I did not describe the RF ground because I consider it not relevant to my question. Anyway, the ground system is formed by 64 quarter-wave radials. 73 Tony I0JX Rome, Italy Until the RF Ground is described, You can't make any definitive decisions, about any kind of Marconi Antenna, no matter how it is feed. This is common problem with many, less than educated, Hams, that try and design Antennas Systems for HF. Most of the Losses in these designs are in the RF Ground System, and not in the Radiators. Just Say'en.... YMMV... |
Losses in shunt-fed towers
Until the RF Ground is described, You can't make any definitive
decisions, about any kind of Marconi Antenna, no matter how it is feed. This is common problem with many, less than educated, Hams, that try and design Antennas Systems for HF. Most of the Losses in these designs are in the RF Ground System, and not in the Radiators. Just Say'en.... YMMV... I am not interested in knowing the overall losses and making decisions on my antenna . I am just interested to know whether someone has experience on the effect of very high voltage on the antenna terminal. And for that very purpose the ground system is not relevant. No useful answer received so far. The problem of this newsgroup is that people not knowing how to answer a specific question, reply to other questions, just to write something. 73 Tony I0JX Rome, Italy |
Losses in shunt-fed towers
On Mon, 12 Sep 2011 19:59:50 +0200, "Antonio Vernucci"
wrote: No useful answer received so far. Hi Antonio, Then, it would appear, after your having discarded technical points curiously you want to be validated for thinking your antenna is lossy. It will be hard to do that without your evidence (back of the envelope computation does not qualify) that is contrary to everyone's experience. 73's Richard Clark, KB7QHC |
Losses in shunt-fed towers
On 9/9/2011 12:10 PM, Antonio Vernucci wrote:
For 75 and 80 meters, I use a shunt-fed tower (58 feet high, with a fairly big HF yagi on its top). The home-made tuner, placed at the tower base, has a motorized variable vacuum capacitor in series and a (properly selected) fixed capacitor in parallel. I can so remotely tune the variable capacitor across the 3500 - 3800 kHz range obtaining a perfect match to 50-ohm everywhere in the band. The antenna works fine and I receive good reports. The only thing which worries me a bit is the very high voltage that develops at the antenna end (about 6,900 Vrms at 3.500 MHz with 2kW applied, growing to about 8,600 Vrms at 3.800 MHz). I am not sure whether such high voltage could be a source of significant losses. Generally, your losses are going to be IR losses. High voltage implies low current, and low loss. However, when you start talking kilovolts, you might have significant loss from corona discharge. To solve my doubt, I started by precisely measuring the capacitance of the tuner capacitors, and I could then easily calculate the antenna impedance at 3.500 MHz, which resulted to be (18.2 + j 656) ohm. That's the feed point Z of the basic radiator? or of the whole assembly, including the shunt feed wire/tube? ABOUT CURRENT With 2 kW applied, one can easily determine that the RF current flowing through the antenna is about 10.5A, quite higher than the 6.3A figure one would get should the antenna resistance be 50 ohm instead of 18.2 ohm. So, I must expect some more loss in the conductors due to the fairly high current. Yes, that seems reasonable, but what conductor is that 10 amps flowing in? What is its AC resistance at 3 MHz? ABOUT VOLTAGE The 10.5 A current flowing through the big 656 ohm antenna reactance causes the antenna RF voltage to get up to 6,900 Vrms at 3.500 MHz. Touching the wire with a (well insulated) screwdriver, you would see a nice Tesla-like arc. I am wondering whether such a high voltage could constitute, by itself, a source of extra loss. Please note that the wire coming down from the tower is connected directly to the vacuum capacitor terminal, with no stand-off insulator. So, I do not see a place where power can get dissipated due to the high RFvoltage, other than perhaps in the humid air (?). Corona is definitely a source of loss. Humidity doesn't change corona much, but does change leakage across insulators (i.e. the crackling one hears under HV power lines on damp mornings). However, is it a *significant* source of loss? How much stuff is at the HV? What's the actual surface field? (Running a screwdriver near it perturbs the field) I could probably avoid this situation by changing the tap height on the tower, but I would run the risk of not being any longer able to tune across the whole 3500 - 3800 kHz band adjusting one of the two capacitors, and not both. Any idea on whether the high RF voltage present on the antenna could cause some significant loss? HV loss is hard to estimate. You could make your shunt wire a shunt tube or rod and basically eliminate corona (A rule of thumb is that 10-15 kV/cm radius will have virtually no corona.. so for your 6kV, a 1cm diameter tube is in the right ballpark) Skin depth at 3.5 MHz in Aluminum is .043mm, and the usual rule of thumb is to make the tubing wall thickness 3-5 skin depths. 0.12-0.20 mm seems about right. Copper could be thinner wall (skin depth is less) I'd worry more about loss across dirty insulators. And even more about IR losses. Thanks and 73 Tony I0JX Rome, Italy |
Losses in shunt-fed towers
El 09-09-11 21:10, Antonio Vernucci escribió:
For 75 and 80 meters, I use a shunt-fed tower (58 feet high, with a fairly big HF yagi on its top). The home-made tuner, placed at the tower base, has a motorized variable vacuum capacitor in series and a (properly selected) fixed capacitor in parallel. I can so remotely tune the variable capacitor across the 3500 - 3800 kHz range obtaining a perfect match to 50-ohm everywhere in the band. The antenna works fine and I receive good reports. The only thing which worries me a bit is the very high voltage that develops at the antenna end (about 6,900 Vrms at 3.500 MHz with 2kW applied, growing to about 8,600 Vrms at 3.800 MHz). I am not sure whether such high voltage could be a source of significant losses. To solve my doubt, I started by precisely measuring the capacitance of the tuner capacitors, and I could then easily calculate the antenna impedance at 3.500 MHz, which resulted to be (18.2 + j 656) ohm. ABOUT CURRENT With 2 kW applied, one can easily determine that the RF current flowing through the antenna is about 10.5A, quite higher than the 6.3A figure one would get should the antenna resistance be 50 ohm instead of 18.2 ohm. So, I must expect some more loss in the conductors due to the fairly high current. ABOUT VOLTAGE The 10.5 A current flowing through the big 656 ohm antenna reactance causes the antenna RF voltage to get up to 6,900 Vrms at 3.500 MHz. Touching the wire with a (well insulated) screwdriver, you would see a nice Tesla-like arc. I am wondering whether such a high voltage could constitute, by itself, a source of extra loss. Please note that the wire coming down from the tower is connected directly to the vacuum capacitor terminal, with no stand-off insulator. So, I do not see a place where power can get dissipated due to the high RFvoltage, other than perhaps in the humid air (?). I could probably avoid this situation by changing the tap height on the tower, but I would run the risk of not being any longer able to tune across the whole 3500 - 3800 kHz band adjusting one of the two capacitors, and not both. Any idea on whether the high RF voltage present on the antenna could cause some significant loss? Thanks and 73 Tony I0JX Rome, Italy Hello Tony, It is not the voltage that will introduce loss (as the field lines will go into air and frequency is low), but it is (corona) discharge. Whether this occurs, depends on presence of sharp edges and especially sharp (double curved) surfaces with small radius, think of bolts, threaded rods, etc that point away from the tower into the air. A thin end of an antenne element can also cause breakdown as it points far into the air (away from other solid structures) and the voltage maximum will be at the elements of your HF antenna. In case of your 8 kVrms (that is 12 kVp), an antenna element with 10mm diameter and spherical (smooth) end may result in 2.4kVp/mm at the end. If the end of the element is just straight (just cut aluminium), you will exceed 3kVp/mm and corona will occur. If (corona) discharge occurs, you will notice a sudden change in VSWR with increasing power. -- Wim PA3DJS www.tetech.nl Please remove abc first in case of PM |
Losses in shunt-fed towers
HV loss is hard to estimate. You could make your shunt wire a shunt tube or
rod and basically eliminate corona (A rule of thumb is that 10-15 kV/cm radius will have virtually no corona.. so for your 6kV, a 1cm diameter tube is in the right ballpark) Skin depth at 3.5 MHz in Aluminum is .043mm, and the usual rule of thumb is to make the tubing wall thickness 3-5 skin depths. 0.12-0.20 mm seems about right. Copper could be thinner wall (skin depth is less) I presume that the corona effect should be visible at dark. So far I have seen none, but we had not a single day of rain since I mounted this antenna (incredible summer season...). So, I must verify when rain will come, in a few days from now they say. Or do you think that corona may not be visible? I am using a 4-mm diameter wire so, if I will really have corona problems, I could insert the (vertical) wire into a 1-cm aluminum tube connected at the very bottom of the wire (i.e. at the antenna feed point). Probably a 2-meter long tube could be sufficient (RF voltage gradually diminishes getting away from the antenna feed point). 73 Tony I0JX |
Losses in shunt-fed towers
Hi Wim,
despite my wire is only 4mm in diameter, SWR does not vary at all applying a steady full-power carrier (not even some small oscillations of the reflected power meter needle). I then presume that no big corona effect takes place (also because I see no corona light at dark). I am not sure whether a light corona effect (i.e neither visible nor influencing SWR) could take place, but if so I presume loss would anyway be very low. I will check again under rain conditions (test not done so far), but I am not sure whether rain increases susceptibility to corona, with respect to hot summer days (with umidity close to 80% sometimes). Any idea at that regard? 73 Tony I0JX Hello Tony, It is not the voltage that will introduce loss (as the field lines will go into air and frequency is low), but it is (corona) discharge. Whether this occurs, depends on presence of sharp edges and especially sharp (double curved) surfaces with small radius, think of bolts, threaded rods, etc that point away from the tower into the air. A thin end of an antenne element can also cause breakdown as it points far into the air (away from other solid structures) and the voltage maximum will be at the elements of your HF antenna. In case of your 8 kVrms (that is 12 kVp), an antenna element with 10mm diameter and spherical (smooth) end may result in 2.4kVp/mm at the end. If the end of the element is just straight (just cut aluminium), you will exceed 3kVp/mm and corona will occur. If (corona) discharge occurs, you will notice a sudden change in VSWR with increasing power. -- Wim PA3DJS www.tetech.nl Please remove abc first in case of PM |
Losses in shunt-fed towers
On 9/13/2011 8:06 AM, Antonio Vernucci wrote:
HV loss is hard to estimate. You could make your shunt wire a shunt tube or rod and basically eliminate corona (A rule of thumb is that 10-15 kV/cm radius will have virtually no corona.. so for your 6kV, a 1cm diameter tube is in the right ballpark) Skin depth at 3.5 MHz in Aluminum is .043mm, and the usual rule of thumb is to make the tubing wall thickness 3-5 skin depths. 0.12-0.20 mm seems about right. Copper could be thinner wall (skin depth is less) I presume that the corona effect should be visible at dark. So far I have seen none, but we had not a single day of rain since I mounted this antenna (incredible summer season...). So, I must verify when rain will come, in a few days from now they say. Or do you think that corona may not be visible? Nope.. you can't usually see the corona discharge. The professionals use a special camera with a narrow band solar blind filter. I am using a 4-mm diameter wire so, if I will really have corona problems, I could insert the (vertical) wire into a 1-cm aluminum tube connected at the very bottom of the wire (i.e. at the antenna feed point). Probably a 2-meter long tube could be sufficient (RF voltage gradually diminishes getting away from the antenna feed point). Sure.. |
Losses in shunt-fed towers
Antonio Vernucci wrote:
Hi Wim, despite my wire is only 4mm in diameter, SWR does not vary at all applying a steady full-power carrier (not even some small oscillations of the reflected power meter needle). I then presume that no big corona effect takes place (also because I see no corona light at dark). I am not sure whether a light corona effect (i.e neither visible nor influencing SWR) could take place, but if so I presume loss would anyway be very low. I will check again under rain conditions (test not done so far), but I am not sure whether rain increases susceptibility to corona, with respect to hot summer days (with umidity close to 80% sometimes). Any idea at that regard? What happens if you move the slant-wire up or down on the tower? |
Losses in shunt-fed towers
On Sep 9, 3:10*pm, "Antonio Vernucci" wrote:
For 75 and 80 meters, I use a shunt-fed tower (58 feet high, with a fairly big HF yagi on its top). The home-made tuner, placed at the tower base, has a motorized variable vacuum capacitor in series and a (properly selected) fixed capacitor in parallel. I can so remotely tune the variable capacitor across the 3500 - 3800 kHz range obtaining a perfect match to 50-ohm everywhere in the band. The antenna works fine and I receive good reports. The only thing which worries me a bit is the very high voltage that develops at the antenna end (about 6,900 Vrms at 3.500 MHz with 2kW applied, growing to about 8,600 Vrms at 3.800 MHz). I am not sure whether such high voltage could be a source of significant losses. To solve my doubt, I started by precisely measuring the capacitance of the tuner capacitors, and I could then easily calculate the antenna impedance at 3.500 MHz, which resulted to be (18.2 + j 656) ohm. ABOUT CURRENT With 2 kW applied, one can easily determine that the RF current flowing through the antenna is about 10.5A, quite higher than the 6.3A figure one would get should the antenna resistance be 50 ohm instead of 18.2 ohm. So, I must expect some more loss in the conductors due to the fairly high current. ABOUT VOLTAGE The 10.5 A current flowing through the big 656 ohm antenna reactance causes the antenna RF voltage to get up to 6,900 Vrms at 3.500 MHz. Touching the wire with a (well insulated) screwdriver, you would see a nice Tesla-like arc. I am wondering whether such a high voltage could constitute, by itself, a source of extra loss. Please note that the wire coming down from the tower is connected directly to the vacuum capacitor terminal, with no stand-off insulator. So, I do not see a place where power can get dissipated due to the high RFvoltage, other than perhaps in the humid air (?). I could probably avoid this situation by changing the tap height on the tower, but I would run the risk of not being any longer able to tune across the whole 3500 - 3800 kHz band adjusting one of the two capacitors, and not both. Any idea on whether the high RF voltage present on the antenna could cause some significant loss? Thanks and 73 Tony I0JX Rome, Italy I would think the losses as a result of 10 amperes current at the feed point not to be excessive unless your shunt feed is made of small wire. This is usually not the case if only designed for sound mechanical integrity. Jimmie |
Losses in shunt-fed towers
On Sep 9, 3:10*pm, "Antonio Vernucci" wrote:
For 75 and 80 meters, I use a shunt-fed tower (58 feet high, with a fairly big HF yagi on its top). The home-made tuner, placed at the tower base, has a motorized variable vacuum capacitor in series and a (properly selected) fixed capacitor in parallel. I can so remotely tune the variable capacitor across the 3500 - 3800 kHz range obtaining a perfect match to 50-ohm everywhere in the band. The antenna works fine and I receive good reports. The only thing which worries me a bit is the very high voltage that develops at the antenna end (about 6,900 Vrms at 3.500 MHz with 2kW applied, growing to about 8,600 Vrms at 3.800 MHz). I am not sure whether such high voltage could be a source of significant losses. To solve my doubt, I started by precisely measuring the capacitance of the tuner capacitors, and I could then easily calculate the antenna impedance at 3.500 MHz, which resulted to be (18.2 + j 656) ohm. ABOUT CURRENT With 2 kW applied, one can easily determine that the RF current flowing through the antenna is about 10.5A, quite higher than the 6.3A figure one would get should the antenna resistance be 50 ohm instead of 18.2 ohm. So, I must expect some more loss in the conductors due to the fairly high current. ABOUT VOLTAGE The 10.5 A current flowing through the big 656 ohm antenna reactance causes the antenna RF voltage to get up to 6,900 Vrms at 3.500 MHz. Touching the wire with a (well insulated) screwdriver, you would see a nice Tesla-like arc. I am wondering whether such a high voltage could constitute, by itself, a source of extra loss. Please note that the wire coming down from the tower is connected directly to the vacuum capacitor terminal, with no stand-off insulator. So, I do not see a place where power can get dissipated due to the high RFvoltage, other than perhaps in the humid air (?). I could probably avoid this situation by changing the tap height on the tower, but I would run the risk of not being any longer able to tune across the whole 3500 - 3800 kHz band adjusting one of the two capacitors, and not both. Any idea on whether the high RF voltage present on the antenna could cause some significant loss? Thanks and 73 Tony I0JX Rome, Italy Please forgive me but it seems a little odd that someone who can make the calculations you have made can not compute the IR losses in the feed. You are almost there. Jimmie Jimmie |
Losses in shunt-fed towers
Hi Jimmie,
I am not sure of what you mean to say. With regard to the IR loss (by the way, I presume that "IR" means I squared times R), there is little difficulty to estimate them once you know the RF current. But my point was to estimate any extra losses caused by the very high voltage that develops at the antenna terminal. I would not know how to estimate those. 73 Tony I0JX Please forgive me but it seems a little odd that someone who can make the calculations you have made can not compute the IR losses in the feed. You are almost there. Jimmie Jimmie |
Losses in shunt-fed towers
Nope.. you can't usually see the corona discharge. The professionals use a
special camera with a narrow band solar blind filter. That is interesting. But I presume that, increasing voltage, at a certain point the corona effect becomes visible. 73 Tony I0JX |
Losses in shunt-fed towers
El 13-09-11 17:30, Antonio Vernucci escribió:
Hi Wim, despite my wire is only 4mm in diameter, SWR does not vary at all applying a steady full-power carrier (not even some small oscillations of the reflected power meter needle). I then presume that no big corona effect takes place (also because I see no corona light at dark). I am not sure whether a light corona effect (i.e neither visible nor influencing SWR) could take place, but if so I presume loss would anyway be very low. I will check again under rain conditions (test not done so far), but I am not sure whether rain increases susceptibility to corona, with respect to hot summer days (with umidity close to 80% sometimes). Any idea at that regard? 73 Tony I0JX Hello Tony, It is not the voltage that will introduce loss (as the field lines will go into air and frequency is low), but it is (corona) discharge. Whether this occurs, depends on presence of sharp edges and especially sharp (double curved) surfaces with small radius, think of bolts, threaded rods, etc that point away from the tower into the air. A thin end of an antenne element can also cause breakdown as it points far into the air (away from other solid structures) and the voltage maximum will be at the elements of your HF antenna. In case of your 8 kVrms (that is 12 kVp), an antenna element with 10mm diameter and spherical (smooth) end may result in 2.4kVp/mm at the end. If the end of the element is just straight (just cut aluminium), you will exceed 3kVp/mm and corona will occur. If (corona) discharge occurs, you will notice a sudden change in VSWR with increasing power. -- Wim PA3DJS www.tetech.nl Please remove abc first in case of PM Hello Tony, As long as your wire with D=4mm is not close to other structures (say 0.25...0.5m), you will not exceed 1 kVp/mm when the wire itself is carrying 12kVp. You still have some margin before getting close to 3 kV/mm. At ground level, influence of HF frequency on dielectric breakdown voltage of air is negligible. If the wire is coated, the diameter increases, hence reducing the E-field at the plastic-air interface. When VSWR doesn't, change (with increasing power), the loss due to corona discharge will be minimal. Of course when you lose 10W from 2kW, you will not notice this probably. However if you get a discharge between metal and plastic insulation, the plastic will fail in the end (due to spark erosion). conducting Droplets (rain) do reduce the breakdown voltage as it distorts the field. This introduces 'hot spots'. Unfortunately I don't have data on this. I could imagine that moisture/water layer on the wire would introduces some loss (as you get a current through the moisture layer). But this is not due to corona effect. It can be calculated, as you know the voltage and you can calculate the capacitance to ground (so you know the capacitive current that "leaves" the wire. -- Wim PA3DJS www.tetech.nl Please remove abc first in case of PM |
Losses in shunt-fed towers
Hello Tony,
As long as your wire with D=4mm is not close to other structures (say 0.25...0.5m), you will not exceed 1 kVp/mm when the wire itself is carrying 12kVp. You still have some margin before getting close to 3 kV/mm. At ground level, influence of HF frequency on dielectric breakdown voltage of air is negligible. If the wire is coated, the diameter increases, hence reducing the E-field at the plastic-air interface. When VSWR doesn't, change (with increasing power), the loss due to corona discharge will be minimal. Of course when you lose 10W from 2kW, you will not notice this probably. However if you get a discharge between metal and plastic insulation, the plastic will fail in the end (due to spark erosion). conducting Droplets (rain) do reduce the breakdown voltage as it distorts the field. This introduces 'hot spots'. Unfortunately I don't have data on this. I could imagine that moisture/water layer on the wire would introduces some loss (as you get a current through the moisture layer). But this is not due to corona effect. It can be calculated, as you know the voltage and you can calculate the capacitance to ground (so you know the capacitive current that "leaves" the wire. -- Wim PA3DJS Thanks for info. When things go down to the physics level, it is not so easy to understand phaenomena, unless one has specific knowledge on them. 73 Tony I0JX |
Losses in shunt-fed towers
On 9/13/2011 10:55 AM, Antonio Vernucci wrote:
Nope.. you can't usually see the corona discharge. The professionals use a special camera with a narrow band solar blind filter. That is interesting. But I presume that, increasing voltage, at a certain point the corona effect becomes visible. Yes.. but that's usually a LONG way from where there's detectable RFI or noise for instance. When you can see visible corona from some meters away, you're probably in the 10s or maybe low 100s of kV range. For instance, I can run my small 14" Van deGraaff generator with no visible corona in a semi-darkened room, but, judging from the copious ozone production, there's quite a bit of corona (as well as the characteristic crackle sound). Maybe people have seen noticeable corona at 2-3kV, but that's usually indoors in a dark room, and you're fairly close and well dark adapted. Corona from aged ignition wires (around 15-20 kV) also often visible, in the dark. |
Losses in shunt-fed towers
Yes.. but that's usually a LONG way from where there's detectable RFI or noise
for instance. When you can see visible corona from some meters away, you're probably in the 10s or maybe low 100s of kV range. My antenna voltage gets up to about 12 kV peak. I will try listening at a short wave receiver placed very close to the antenna and tuned to a vacant frequency slot. Increasing the transmitted carrier power, at the power level at which the corona effect has instated, I should note an abrupt received noise increase. . 73 Tony I0JX |
Losses in shunt-fed towers
El 13-09-11 20:32, Wimpie escribió:
El 13-09-11 17:30, Antonio Vernucci escribió: Hi Wim, despite my wire is only 4mm in diameter, SWR does not vary at all applying a steady full-power carrier (not even some small oscillations of the reflected power meter needle). I then presume that no big corona effect takes place (also because I see no corona light at dark). I am not sure whether a light corona effect (i.e neither visible nor influencing SWR) could take place, but if so I presume loss would anyway be very low. I will check again under rain conditions (test not done so far), but I am not sure whether rain increases susceptibility to corona, with respect to hot summer days (with umidity close to 80% sometimes). Any idea at that regard? 73 Tony I0JX Hello Tony, It is not the voltage that will introduce loss (as the field lines will go into air and frequency is low), but it is (corona) discharge. Whether this occurs, depends on presence of sharp edges and especially sharp (double curved) surfaces with small radius, think of bolts, threaded rods, etc that point away from the tower into the air. A thin end of an antenne element can also cause breakdown as it points far into the air (away from other solid structures) and the voltage maximum will be at the elements of your HF antenna. In case of your 8 kVrms (that is 12 kVp), an antenna element with 10mm diameter and spherical (smooth) end may result in 2.4kVp/mm at the end. If the end of the element is just straight (just cut aluminium), you will exceed 3kVp/mm and corona will occur. If (corona) discharge occurs, you will notice a sudden change in VSWR with increasing power. -- Wim PA3DJS www.tetech.nl Please remove abc first in case of PM Hello Tony, As long as your wire with D=4mm is not close to other structures (say 0.25...0.5m), you will not exceed 1 kVp/mm when the wire itself is carrying 12kVp. You still have some margin before getting close to 3 kV/mm. At ground level, influence of HF frequency on dielectric breakdown voltage of air is negligible. If the wire is coated, the diameter increases, hence reducing the E-field at the plastic-air interface. When VSWR doesn't, change (with increasing power), the loss due to corona discharge will be minimal. Of course when you lose 10W from 2kW, you will not notice this probably. However if you get a discharge between metal and plastic insulation, the plastic will fail in the end (due to spark erosion). conducting Droplets (rain) do reduce the breakdown voltage as it distorts the field. This introduces 'hot spots'. Unfortunately I don't have data on this. I could imagine that moisture/water layer on the wire would introduces some loss (as you get a current through the moisture layer). But this is not due to corona effect. It can be calculated, as you know the voltage and you can calculate the capacitance to ground (so you know the capacitive current that "leaves" the wire. Short note on change in VSWR due to corona. Many reflectometers use diode rectifiers with relative heavy load. If there is no bias (or the bias is wrong), you will not notice small variations in VSWR, when VSWR = (almost) 1. If you have a directional coupler you may use your oscilloscope or spectrum analyser as detector device. The oscilloscope (50 Ohms input impedance) you can use to verify if the residual VSWR is because of higher harmonics or real reflection. Of course the spectrum analyzer, oscilloscope with FFT or a receiver (with heavy attenuation) will show you the desired signal directly. If you match to very low return signal, even a few Watts loss due to corona you can detect at full power. you may need to do this during very quiet weather as small mechanical changes due to elasticity of the construction materials will lead to small changes in reflection coefficient. -- Wim PA3DJS www.tetech.nl Please remove abc first in case of PM |
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