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Piano Wire Antenna for Experimental Rocket
I am looking for some help developing a properly tuned piano wire
antenna for an all metal experimental rocket. This particular vehicle currently uses a Digi International 9Xtream 100 mw transmitter which operates using Frequency Hopping Spread Spectrum from 910 to 918 MHz. The first flight used a .062" diameter length of piano wire cut so that the exposed length was about 1/2 wavelength. The wire antenna was isolated from the metal airfame with a small nylon penetrator and connected to the transmitter via about a 12" length of RG-178 coax. The coax was terminated at the antenna via two small jumpers (soldered to the center conductor and shield). The shield was grounded on the metal airframe transforming the entire rocket into an artificial ground plane (the antenna was also swept back at about a 45 degree angle to reduce drag. During this first flight we observed a range of about 10 miles, about half of the published range of this transmitter. The RF system is used as a downlink for GPS data and we did not get a good radio link after ignition until the rocket had descended to about 50,000 feet. Maximum velocity was about Mach 3 which is why a conventional antenna cannot be used as it will be melted by friction. For the second version, we are thinking of using a slightly smaller diameter piano wire for a 1/4 wave length antenna as I think this may be more suitable for a system using an artificial ground plane. In addition, I plan to terminate the RG-178 coax directly at the antenna and seal it with potting compound. This new antenna will be mounted totally on the exterior of the airframe and angled back only about 10 degrees to give a better radiation pattern. However, I would really like to properly tune this new antenna and was wondering if anyone in this group has any ideas as to how to do this. Doing a tuned RCL circuit at this frequency is proving to be difficult using discrete components, to say the least. I do not have an SWR, but do have an RF millivolt meter as well as the means to measure the capacitance between the antenna and metal airframe. Any help will be appreciated as I would hate to fly this thing again and not get it back because our RF downlink had insufficient range. C. Newport |
Piano Wire Antenna for Experimental Rocket
On Sat, 23 May 2009 16:40:22 -0700 (PDT), mr1956
wrote: Maximum velocity was about Mach 3 which is why a conventional antenna cannot be used as it will be melted by friction. Would seem to be at odds with: This new antenna will be mounted totally on the exterior of the airframe and angled back only about 10 degrees to give a better radiation pattern. At Mach 3, I seriously doubt you could hold it as erect (80 degrees to the fuselage) as you might try with smaller wire (in fact, I would say it would be plastered down). If your telemetry is directly below its ascendance, you might try a full-wave or longer antenna trailing below (if it can withstand the propulsion heat). If that is not possible, look into an inverted F: http://www.qsl.net/kb7qhc/antenna/In...%20F/index.htm (it doesn't have to be a thick antenna). 73's Richard Clark, KB7QHC |
Piano Wire Antenna for Experimental Rocket
"mr1956" wrote in message ... I am looking for some help developing a properly tuned piano wire antenna for an all metal experimental rocket. This particular vehicle currently uses a Digi International 9Xtream 100 mw transmitter which operates using Frequency Hopping Spread Spectrum from 910 to 918 MHz. The first flight used a .062" diameter length of piano wire cut so that the exposed length was about 1/2 wavelength. The wire antenna was isolated from the metal airfame with a small nylon penetrator and connected to the transmitter via about a 12" length of RG-178 coax. The coax was terminated at the antenna via two small jumpers (soldered to the center conductor and shield). The shield was grounded on the metal airframe transforming the entire rocket into an artificial ground plane (the antenna was also swept back at about a 45 degree angle to reduce drag. During this first flight we observed a range of about 10 miles, about half of the published range of this transmitter. The RF system is used as a downlink for GPS data and we did not get a good radio link after ignition until the rocket had descended to about 50,000 feet. Maximum velocity was about Mach 3 which is why a conventional antenna cannot be used as it will be melted by friction. For the second version, we are thinking of using a slightly smaller diameter piano wire for a 1/4 wave length antenna as I think this may be more suitable for a system using an artificial ground plane. In addition, I plan to terminate the RG-178 coax directly at the antenna and seal it with potting compound. This new antenna will be mounted totally on the exterior of the airframe and angled back only about 10 degrees to give a better radiation pattern. However, I would really like to properly tune this new antenna and was wondering if anyone in this group has any ideas as to how to do this. Doing a tuned RCL circuit at this frequency is proving to be difficult using discrete components, to say the least. I do not have an SWR, but do have an RF millivolt meter as well as the means to measure the capacitance between the antenna and metal airframe. Any help will be appreciated as I would hate to fly this thing again and not get it back because our RF downlink had insufficient range. C. Newport Hi C Have you considered flush antennas, like slots? Is is practical to use antenna directivity at the ground station to increase the "range"? It might be advantageous to use Linear Polarization on the rocket and Circular Polarization on the ground. For impedance measurement at L band, it is possible to build a slotted line using plumbing supplies. That presumes that you already have access to a signal generator and a detector to record the signal generator's output. A home built slotted line will have difficulty regestaring low VSWRs. But, once it is known that the load impedance is close to the line impedance (low VSWR), a directional coupler can be used to match more precisely. I have built a "plumber's delight" slotted line that works well at 130 MHZ. The high quality directional couplers are affordable on eBay. Note - If I can do it, it cant be too complicated! Jerry KD6JDJ |
Piano Wire Antenna for Experimental Rocket
On Sat, 23 May 2009 16:40:22 -0700 (PDT), mr1956
wrote: I am looking for some help developing a properly tuned piano wire antenna for an all metal experimental rocket. This particular vehicle currently uses a Digi International 9Xtream 100 mw transmitter which operates using Frequency Hopping Spread Spectrum from 910 to 918 MHz. Would that perhaps be one of these models? If so, which one? http://www.digi.com/products/wireless/point-multipoint/xstream-module.jsp http://www.digi.com/register/procregistrationnothankyou.jsp?urlredirect=/pdf/ds_xstreammodule.pdf During this first flight we observed a range of about 10 miles, about half of the published range of this transmitter. The RF system is used as a downlink for GPS data and we did not get a good radio link after ignition until the rocket had descended to about 50,000 feet. Maximum velocity was about Mach 3 which is why a conventional antenna cannot be used as it will be melted by friction. Ummm... let's do some path loss calcs first. I'll assume that you're using a matching XStream radio on the ground, with a moderate size yagi antenna pointed in the general direction of the sky. Rocket end: xmitter +20dBm tx coax loss 0dB tx ant gain 0dBi (monopole 1/4 wave) path loss (to be calculated) rx ant gain +12dBi rx coax loss 0db rx sens -110dBm (at 9600 baud) I'll assume a minimum fade margin of 20dB. Adding up all the gains, losses, fade margin, and guesswork, I end up with a maximum path loss of 132dB. Plugging into a handy online calculator at: http://www.terabeam.com/support/calculations/free-space-loss.php I get a maximum range of 25 miles. It should work. However, the weak link here is that I'm doing quite a bit of guesswork. For example, you really do need a +12dBi (tracking?) yagi antenna to make this work. If you use a simple dipole, with a gain of perhaps 2dBi in place of the yagi, your maximum path loss just dropped to 122dB which yields a maximum range of only 8 miles, which is roughly what you're getting. You may also be getting a major interference at altitude, which does not appear on the ground. From 5 miles up, the rocket is going to hear plenty of 900MHz interference from all over the country. Each signal will be fairly low level due to the distance, but there will be lots and lots of signals up there. Try your rocket radio from a local mountain top before launching to see if the receiver can handle the interference. Since it's a frequency hopper and not a direct sequence spread spectrum radio, it probably will, but it doesn't hurt to test. Incidentally, the previous back of the envelope path calcs are the BEST case model. There are always additional losses and reductions. For example, few manufacturers seem to meet their published specifications. The antenna gains really require a full NEC2 simulation which includes the metal rocket body. Things only get worse. For the second version, we are thinking of using a slightly smaller diameter piano wire for a 1/4 wave length antenna as I think this may be more suitable for a system using an artificial ground plane. In addition, I plan to terminate the RG-178 coax directly at the antenna and seal it with potting compound. This new antenna will be mounted totally on the exterior of the airframe and angled back only about 10 degrees to give a better radiation pattern. You're reinventing the wheel. Look at photos of the original Explorer I satellite (with Von Braun and others holding it over their heads). There are 4 wires radiating from the tubular rocket casing forming a circularly polarized double dipole "turnstile" antenna. Since it's since one antenna is always visible, there's no blocking by the missile body. Since it's circularly polarized, there's no positional nulls and peaks. http://history.nasa.gov/sputnik/expinfo.html However, I would really like to properly tune this new antenna and was wondering if anyone in this group has any ideas as to how to do this. Cut it for 1/4 wavelength at about 921MHz. Be careful when cutting the coax phasing sections to get the velocity factor calcs correct. Doing a tuned RCL circuit at this frequency is proving to be difficult using discrete components, to say the least. I do not have an SWR, but do have an RF millivolt meter as well as the means to measure the capacitance between the antenna and metal airframe. 900MHz VSWR meters that work at 100mw are not common. You would have problems using one anyway because the xmitter does not generate a CW signal suitable for easy testing. Methinks you'll do better dragging the rocket somewhere that has a network analyzer or suitable pile of test equipment for characterizing the antenna. Any help will be appreciated as I would hate to fly this thing again and not get it back because our RF downlink had insufficient range. C. Newport I think you'll find that the tracking 900MHz yagi antenna on the ground is the key part of the puzzle. Did you try driving the rocket 5 to 10 miles away and testing if it can be heard from the ground station? If you run this test, it's easiest to run between mountain tops to insure line of sight. Good luck. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Piano Wire Antenna for Experimental Rocket
On Sat, 23 May 2009 21:45:15 -0700, Jeff Liebermann
wrote: You're reinventing the wheel. Look at photos of the original Explorer I satellite (with Von Braun and others holding it over their heads). There are 4 wires radiating from the tubular rocket casing forming a circularly polarized double dipole "turnstile" antenna. Since it's since one antenna is always visible, there's no blocking by the missile body. Since it's circularly polarized, there's no positional nulls and peaks. http://history.nasa.gov/sputnik/expinfo.html More photos: http://www.redstone.army.mil/history/explorer/EXPLORER.html http://www.redstone.army.mil/history/explorer/Explorer%20One%20Satellite%20Diagram.jpg -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Piano Wire Antenna for Experimental Rocket
On Sun, 24 May 2009 04:04:52 GMT, "Jerry"
wrote: Have you considered flush antennas, like slots? Slot antennas have the maximum lobe perpendicular to the rocket axis. There's very little signal below the rocket. This is why tracking stations are far away from the launch site. If the rocket were overhead, and going straight up, there's no signal. Slot antennas are also a power waste. You need 4 slots, run by a power splitter, in order to insure that at least one antenna is oriented in the direction of the receiver. Meanwhile, the other 3 slot antennas are radiating power to nobody in particular. Say goodby to about 3/4th of your tx power. Receive sensitivity is not affected. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Piano Wire Antenna for Experimental Rocket
On Sat, 23 May 2009 16:40:22 -0700 (PDT), mr1956
wrote: I am looking for some help developing a properly tuned piano wire antenna for an all metal experimental rocket. This particular vehicle currently uses a Digi International 9Xtream 100 mw transmitter which operates using Frequency Hopping Spread Spectrum from 910 to 918 MHz. The first flight used a .062" diameter length of piano wire cut so that the exposed length was about 1/2 wavelength. The wire antenna was isolated from the metal airfame with a small nylon penetrator and connected to the transmitter via about a 12" length of RG-178 coax. The coax was terminated at the antenna via two small jumpers (soldered to the center conductor and shield). The shield was grounded on the metal airframe transforming the entire rocket into an artificial ground plane (the antenna was also swept back at about a 45 degree angle to reduce drag. During this first flight we observed a range of about 10 miles, about half of the published range of this transmitter. The RF system is used as a downlink for GPS data and we did not get a good radio link after ignition until the rocket had descended to about 50,000 feet. Maximum velocity was about Mach 3 which is why a conventional antenna cannot be used as it will be melted by friction. For the second version, we are thinking of using a slightly smaller diameter piano wire for a 1/4 wave length antenna as I think this may be more suitable for a system using an artificial ground plane. In addition, I plan to terminate the RG-178 coax directly at the antenna and seal it with potting compound. This new antenna will be mounted totally on the exterior of the airframe and angled back only about 10 degrees to give a better radiation pattern. However, I would really like to properly tune this new antenna and was wondering if anyone in this group has any ideas as to how to do this. Doing a tuned RCL circuit at this frequency is proving to be difficult using discrete components, to say the least. I do not have an SWR, but do have an RF millivolt meter as well as the means to measure the capacitance between the antenna and metal airframe. Any help will be appreciated as I would hate to fly this thing again and not get it back because our RF downlink had insufficient range. C. Newport Better length is about lamda/4 or lamda 5/8. Lamda/2 is way off. Do not use too thin wires, the thicker the better. For reception on the ground station use a helix antenna, because that antenna is polarization independent. The antenna must be pointed in the direction of the object, helixes are very directional. http://hamwaves.com/antennas/inducta...al_antenna.jpg http://www.rac.ca/tca/2006-01%20Heli...alculator.html http://jcoppens.com/ant/helix/calc.en.php If direction is a problem, use a quadrifilar as ground station antenna. http://homepages.ipact.nl/~pa1are/QHA.html w. |
Piano Wire Antenna for Experimental Rocket
"Jeff Liebermann" wrote in message ... On Sun, 24 May 2009 04:04:52 GMT, "Jerry" wrote: Have you considered flush antennas, like slots? Slot antennas have the maximum lobe perpendicular to the rocket axis. There's very little signal below the rocket. This is why tracking stations are far away from the launch site. If the rocket were overhead, and going straight up, there's no signal. Slot antennas are also a power waste. You need 4 slots, run by a power splitter, in order to insure that at least one antenna is oriented in the direction of the receiver. Meanwhile, the other 3 slot antennas are radiating power to nobody in particular. Say goodby to about 3/4th of your tx power. Receive sensitivity is not affected. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 Hi Jeff Why are you so negative?? I have designed and built lots of slot antennas that were widely used on military aircraft. The efficiency is quite good. I am sure a smart guy like you could design a slot antenna and locate it for those guys with the 3G rocket. There hasnt been any text that restricts where the slot could be located. As I read the original post, they werent receiving signal from the accending rocket. Maybe they only want to receive data that was recorded after the rocket reached it's peak. There are lots of information that you and I dont yet know. But, my major question for you is "Why are you so negative?". Jerry KD6JDJ |
Piano Wire Antenna for Experimental Rocket
"Helmut Wabnig" hwabnig@ .- --- -. dotat wrote in message ... On Sat, 23 May 2009 16:40:22 -0700 (PDT), mr1956 wrote: I am looking for some help developing a properly tuned piano wire antenna for an all metal experimental rocket. This particular vehicle currently uses a Digi International 9Xtream 100 mw transmitter which operates using Frequency Hopping Spread Spectrum from 910 to 918 MHz. The first flight used a .062" diameter length of piano wire cut so that the exposed length was about 1/2 wavelength. The wire antenna was isolated from the metal airfame with a small nylon penetrator and connected to the transmitter via about a 12" length of RG-178 coax. The coax was terminated at the antenna via two small jumpers (soldered to the center conductor and shield). The shield was grounded on the metal airframe transforming the entire rocket into an artificial ground plane (the antenna was also swept back at about a 45 degree angle to reduce drag. During this first flight we observed a range of about 10 miles, about half of the published range of this transmitter. The RF system is used as a downlink for GPS data and we did not get a good radio link after ignition until the rocket had descended to about 50,000 feet. Maximum velocity was about Mach 3 which is why a conventional antenna cannot be used as it will be melted by friction. For the second version, we are thinking of using a slightly smaller diameter piano wire for a 1/4 wave length antenna as I think this may be more suitable for a system using an artificial ground plane. In addition, I plan to terminate the RG-178 coax directly at the antenna and seal it with potting compound. This new antenna will be mounted totally on the exterior of the airframe and angled back only about 10 degrees to give a better radiation pattern. However, I would really like to properly tune this new antenna and was wondering if anyone in this group has any ideas as to how to do this. Doing a tuned RCL circuit at this frequency is proving to be difficult using discrete components, to say the least. I do not have an SWR, but do have an RF millivolt meter as well as the means to measure the capacitance between the antenna and metal airframe. Any help will be appreciated as I would hate to fly this thing again and not get it back because our RF downlink had insufficient range. C. Newport Better length is about lamda/4 or lamda 5/8. Lamda/2 is way off. Do not use too thin wires, the thicker the better. For reception on the ground station use a helix antenna, because that antenna is polarization independent. The antenna must be pointed in the direction of the object, helixes are very directional. http://hamwaves.com/antennas/inducta...al_antenna.jpg http://www.rac.ca/tca/2006-01%20Heli...alculator.html http://jcoppens.com/ant/helix/calc.en.php If direction is a problem, use a quadrifilar as ground station antenna. http://homepages.ipact.nl/~pa1are/QHA.html w. Hi Helmut It may be that you are not familiar with a ground based antenna with similar caracteristics to a QHA, but muich easier to build at home. The DCA is much less critical to make work properly than a QHA. Jerry KD6JDJ |
Piano Wire Antenna for Experimental Rocket
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Piano Wire Antenna for Experimental Rocket
On May 23, 11:08*pm, Richard Clark wrote:
On Sat, 23 May 2009 16:40:22 -0700 (PDT), mr1956 wrote: Maximum velocity was about Mach 3 which is why a conventional antenna cannot be used as it will be melted by friction. Would seem to be at odds with: This new antenna will be mounted totally on the exterior of the airframe and angled back only about 10 degrees to give a better radiation pattern. At Mach 3, I seriously doubt you could hold it as erect (80 degrees to the fuselage) as you might try with smaller wire (in fact, I would say it would be plastered down). If your telemetry is directly below its ascendance, you might try a full-wave or longer antenna trailing below (if it can withstand the propulsion heat). *If that is not possible, look into an inverted F:http://www.qsl.net/kb7qhc/antenna/In...%20F/index.htm (it doesn't have to be a thick antenna). 73's Richard Clark, KB7QHC Yes the antenna will be pressed back against the airframe during flight but should be erect at apogee and during descent. The previous one survived just fine. C. Newport |
Piano Wire Antenna for Experimental Rocket
On May 24, 12:04*am, "Jerry" wrote:
"mr1956" wrote in message ... I am looking for some help developing a properly tuned piano wire antenna for an all metal experimental rocket. This particular vehicle currently uses a Digi International 9Xtream 100 mw transmitter which operates using Frequency Hopping Spread Spectrum from 910 to 918 MHz. The first flight used a .062" diameter length of piano wire cut so that the exposed length was about 1/2 wavelength. The wire antenna was isolated from the metal airfame with a small nylon penetrator and connected to the transmitter via about a 12" length of RG-178 coax. The coax was terminated at the antenna via two small jumpers (soldered to the center conductor and shield). *The shield was grounded on the metal airframe transforming the entire rocket into an artificial ground plane (the antenna was also swept back at about a 45 degree angle to reduce drag. During this first flight we observed a range of about 10 miles, about half of the published range of this transmitter. The RF system is used as a downlink for GPS data and we did not get a good radio link after ignition until the rocket had descended to about 50,000 feet. *Maximum velocity was about Mach 3 which is why a conventional antenna cannot be used as it will be melted by friction. For the second version, we are thinking of using a slightly smaller diameter piano wire for a 1/4 wave length antenna as I think this may be more suitable for a system using an artificial ground plane. *In addition, I plan to terminate the RG-178 coax directly at the antenna and seal it with potting compound. This new antenna will be mounted totally on the exterior of the airframe and angled back only about 10 degrees to give a better radiation pattern. However, I would really like to properly tune this new antenna and was wondering if anyone in this group has any ideas as to how to do this. Doing a tuned RCL circuit at this frequency is proving to be difficult using discrete components, to say the least. I do not have an SWR, but do have an RF millivolt meter as well as the means to measure the capacitance between the antenna and metal airframe. Any help will be appreciated as I would hate to fly this thing again and not get it back because our RF downlink had insufficient range. C. Newport * Hi C * Have you considered flush antennas, like slots? * Is is practical to use antenna directivity at the ground station to increase the "range"? * It might be advantageous to use Linear Polarization on the rocket and Circular Polarization on the ground. * For impedance measurement at L band, it is possible to build a slotted line using plumbing supplies. * That presumes that you already have access to a signal generator and a detector to record the signal generator's output. * A home built slotted line will have difficulty regestaring low VSWRs. But, once it is known that the load impedance is close to the line impedance (low VSWR), a directional coupler can be used to match more precisely. * I have built a "plumber's delight" slotted line that works well at 130 MHZ. * The high quality directional couplers are affordable on eBay. * Note - If I can do it, it cant be too complicated! * * * * * * * * * * * * Jerry * KD6JDJ I have looked at slotted or flush wrap around antennas but integrating one into the existing design would be difficult due to the wall thickness of the metal. Something like that would have to be mounted flush and I would be worried about the wind blast peeling the thing off. Certainly, there is much that can be done but I am unfortunately not Wallops Island and have limited resources like anyone else. |
Piano Wire Antenna for Experimental Rocket
On May 24, 4:26*am, Owen Duffy wrote:
mr1956 wrote in news:943a9bbd-214b-43b2-ac31- : I am looking for some help developing a properly tuned piano wire antenna for an all metal experimental rocket. This particular vehicle currently uses a Digi International 9Xtream 100 mw transmitter which operates using Frequency Hopping Spread Spectrum from 910 to 918 MHz. The first flight used a .062" diameter length of piano wire cut so that the exposed length was about 1/2 wavelength. The wire antenna was isolated from the metal airfame with a small nylon penetrator and connected to the transmitter via about a 12" length of RG-178 coax. The coax was terminated at the antenna via two small jumpers (soldered to the center conductor and shield). *The shield was grounded on the metal airframe transforming the entire rocket into an artificial ground plane (the antenna was also swept back at about a 45 degree angle to reduce drag. So you designed for a half wave antenna fed with 12" of RG178. Lets suppose for a moment that the antenna has a feedpoint Z of say, 2000 +j0 ohms. The line will transform that to 5+j14 at the tx end, and with a loss of 4.8dB (ie 33% efficiency). The tx is unlikely to develop is rated output power into such a load, so there will be some further reduction. Yes, an antenna of half the size (ie a quarter wave fed against the metal rocket body) might well work ten times as good. Owen I have been reading the ARRL antenna book and while there is much information, a lot of it is over my head. But one point I think a chapter makes is that a 1/4 wave monopole will work better than a 1/2 wave using an artificial ground plane because of the way the voltage peaks at the end of the antenna; or that is how it seems. |
Piano Wire Antenna for Experimental Rocket
On Sun, 24 May 2009 07:35:03 GMT, "Jerry"
wrote: "Helmut Wabnig" hwabnig@ .- --- -. dotat wrote in message .. . On Sat, 23 May 2009 16:40:22 -0700 (PDT), mr1956 wrote: I am looking for some help developing a properly tuned piano wire antenna for an all metal experimental rocket. This particular vehicle currently uses a Digi International 9Xtream 100 mw transmitter which operates using Frequency Hopping Spread Spectrum from 910 to 918 MHz. The first flight used a .062" diameter length of piano wire cut so that the exposed length was about 1/2 wavelength. The wire antenna was isolated from the metal airfame with a small nylon penetrator and connected to the transmitter via about a 12" length of RG-178 coax. The coax was terminated at the antenna via two small jumpers (soldered to the center conductor and shield). The shield was grounded on the metal airframe transforming the entire rocket into an artificial ground plane (the antenna was also swept back at about a 45 degree angle to reduce drag. During this first flight we observed a range of about 10 miles, about half of the published range of this transmitter. The RF system is used as a downlink for GPS data and we did not get a good radio link after ignition until the rocket had descended to about 50,000 feet. Maximum velocity was about Mach 3 which is why a conventional antenna cannot be used as it will be melted by friction. For the second version, we are thinking of using a slightly smaller diameter piano wire for a 1/4 wave length antenna as I think this may be more suitable for a system using an artificial ground plane. In addition, I plan to terminate the RG-178 coax directly at the antenna and seal it with potting compound. This new antenna will be mounted totally on the exterior of the airframe and angled back only about 10 degrees to give a better radiation pattern. However, I would really like to properly tune this new antenna and was wondering if anyone in this group has any ideas as to how to do this. Doing a tuned RCL circuit at this frequency is proving to be difficult using discrete components, to say the least. I do not have an SWR, but do have an RF millivolt meter as well as the means to measure the capacitance between the antenna and metal airframe. Any help will be appreciated as I would hate to fly this thing again and not get it back because our RF downlink had insufficient range. C. Newport Better length is about lamda/4 or lamda 5/8. Lamda/2 is way off. Do not use too thin wires, the thicker the better. For reception on the ground station use a helix antenna, because that antenna is polarization independent. The antenna must be pointed in the direction of the object, helixes are very directional. http://hamwaves.com/antennas/inducta...al_antenna.jpg http://www.rac.ca/tca/2006-01%20Heli...alculator.html http://jcoppens.com/ant/helix/calc.en.php If direction is a problem, use a quadrifilar as ground station antenna. http://homepages.ipact.nl/~pa1are/QHA.html w. Hi Helmut It may be that you are not familiar with a ground based antenna with similar caracteristics to a QHA, but muich easier to build at home. The DCA is much less critical to make work properly than a QHA. Jerry KD6JDJ This is correct, yes. w. |
Piano Wire Antenna for Experimental Rocket
"mr1956" wrote in message ... On May 24, 12:04 am, "Jerry" wrote: "mr1956" wrote in message ... I am looking for some help developing a properly tuned piano wire antenna for an all metal experimental rocket. This particular vehicle currently uses a Digi International 9Xtream 100 mw transmitter which operates using Frequency Hopping Spread Spectrum from 910 to 918 MHz. The first flight used a .062" diameter length of piano wire cut so that the exposed length was about 1/2 wavelength. The wire antenna was isolated from the metal airfame with a small nylon penetrator and connected to the transmitter via about a 12" length of RG-178 coax. The coax was terminated at the antenna via two small jumpers (soldered to the center conductor and shield). The shield was grounded on the metal airframe transforming the entire rocket into an artificial ground plane (the antenna was also swept back at about a 45 degree angle to reduce drag. During this first flight we observed a range of about 10 miles, about half of the published range of this transmitter. The RF system is used as a downlink for GPS data and we did not get a good radio link after ignition until the rocket had descended to about 50,000 feet. Maximum velocity was about Mach 3 which is why a conventional antenna cannot be used as it will be melted by friction. For the second version, we are thinking of using a slightly smaller diameter piano wire for a 1/4 wave length antenna as I think this may be more suitable for a system using an artificial ground plane. In addition, I plan to terminate the RG-178 coax directly at the antenna and seal it with potting compound. This new antenna will be mounted totally on the exterior of the airframe and angled back only about 10 degrees to give a better radiation pattern. However, I would really like to properly tune this new antenna and was wondering if anyone in this group has any ideas as to how to do this. Doing a tuned RCL circuit at this frequency is proving to be difficult using discrete components, to say the least. I do not have an SWR, but do have an RF millivolt meter as well as the means to measure the capacitance between the antenna and metal airframe. Any help will be appreciated as I would hate to fly this thing again and not get it back because our RF downlink had insufficient range. C. Newport Hi C Have you considered flush antennas, like slots? Is is practical to use antenna directivity at the ground station to increase the "range"? It might be advantageous to use Linear Polarization on the rocket and Circular Polarization on the ground. For impedance measurement at L band, it is possible to build a slotted line using plumbing supplies. That presumes that you already have access to a signal generator and a detector to record the signal generator's output. A home built slotted line will have difficulty regestaring low VSWRs. But, once it is known that the load impedance is close to the line impedance (low VSWR), a directional coupler can be used to match more precisely. I have built a "plumber's delight" slotted line that works well at 130 MHZ. The high quality directional couplers are affordable on eBay. Note - If I can do it, it cant be too complicated! Jerry KD6JDJ I have looked at slotted or flush wrap around antennas but integrating one into the existing design would be difficult due to the wall thickness of the metal. Something like that would have to be mounted flush and I would be worried about the wind blast peeling the thing off. Certainly, there is much that can be done but I am unfortunately not Wallops Island and have limited resources like anyone else. Hi C It looks like your "wire" antenna might actually be OK for your project. You have demonstrated that it works when the rocket is nearly 10 miles away when you hoped for 20 miles. Evidently, the antenna doesnt need to function while the rocket is accending. Are you able to improve the ground based antenna The equipment you now have would need only 6 dB improvement to provide the 20 mile range. I'd like to know more about your ground based antenna system. Jerry KD6JDJ (who has lots of small diameterTeflon dielectric coax to donate to a real project) |
Piano Wire Antenna for Experimental Rocket
"mr1956" wrote ... On May 24, 4:26 am, Owen Duffy wrote: mr1956 wrote in news:943a9bbd-214b-43b2-ac31- : I am looking for some help developing a properly tuned piano wire antenna for an all metal experimental rocket. This particular vehicle currently uses a Digi International 9Xtream 100 mw transmitter which operates using Frequency Hopping Spread Spectrum from 910 to 918 MHz. The first flight used a .062" diameter length of piano wire cut so that the exposed length was about 1/2 wavelength. The wire antenna was isolated from the metal airfame with a small nylon penetrator and connected to the transmitter via about a 12" length of RG-178 coax. The coax was terminated at the antenna via two small jumpers (soldered to the center conductor and shield). The shield was grounded on the metal airframe transforming the entire rocket into an artificial ground plane (the antenna was also swept back at about a 45 degree angle to reduce drag. So you designed for a half wave antenna fed with 12" of RG178. Lets suppose for a moment that the antenna has a feedpoint Z of say, 2000 +j0 ohms. The line will transform that to 5+j14 at the tx end, and with a loss of 4.8dB (ie 33% efficiency). The tx is unlikely to develop is rated output power into such a load, so there will be some further reduction. Yes, an antenna of half the size (ie a quarter wave fed against the metal rocket body) might well work ten times as good. Owen I have been reading the ARRL antenna book and while there is much information, a lot of it is over my head. But one point I think a chapter makes is that a 1/4 wave monopole will work better than a 1/2 wave using an artificial ground plane because of the way the voltage peaks at the end of the antenna; or that is how it seems. You are right. Antenna radiate from the end. So it should be thick as possible. How the length should be I do not know. See: http://www.arrl.org/news/features/2007/09/28/03/?nc=1 " The antennas are described as matched pairs, one 2.4 meters and the other 2.9 meters in length. The available literature gives no information on the electrical characteristics of the antenna system but it is probably safe to assume that they were some variation of a center-fed dipole with the longer pair radiating the 20 MHz (14 meter) signal and the shorter pair set up for 40 MHz or 7.5 meters. Figure 6 shows technicians attaching two of these elements to their angled connectors during assembly. Based on our assumption about the antenna configuration, we can use modern modeling software to analyze a dipole with of 5.8 meters in total length, angled to 70 degrees at the center where it is fed by a single 20 MHz source." Is possible to determine the length of the antennas in wave fractions? S* |
Piano Wire Antenna for Experimental Rocket
"Szczepan Bialek" wrote in message ... "mr1956" wrote ... On May 24, 4:26 am, Owen Duffy wrote: mr1956 wrote in news:943a9bbd-214b-43b2-ac31- : I am looking for some help developing a properly tuned piano wire antenna for an all metal experimental rocket. This particular vehicle currently uses a Digi International 9Xtream 100 mw transmitter which operates using Frequency Hopping Spread Spectrum from 910 to 918 MHz. The first flight used a .062" diameter length of piano wire cut so that the exposed length was about 1/2 wavelength. The wire antenna was isolated from the metal airfame with a small nylon penetrator and connected to the transmitter via about a 12" length of RG-178 coax. The coax was terminated at the antenna via two small jumpers (soldered to the center conductor and shield). The shield was grounded on the metal airframe transforming the entire rocket into an artificial ground plane (the antenna was also swept back at about a 45 degree angle to reduce drag. So you designed for a half wave antenna fed with 12" of RG178. Lets suppose for a moment that the antenna has a feedpoint Z of say, 2000 +j0 ohms. The line will transform that to 5+j14 at the tx end, and with a loss of 4.8dB (ie 33% efficiency). The tx is unlikely to develop is rated output power into such a load, so there will be some further reduction. Yes, an antenna of half the size (ie a quarter wave fed against the metal rocket body) might well work ten times as good. Owen I have been reading the ARRL antenna book and while there is much information, a lot of it is over my head. But one point I think a chapter makes is that a 1/4 wave monopole will work better than a 1/2 wave using an artificial ground plane because of the way the voltage peaks at the end of the antenna; or that is how it seems. You are right. Antenna radiate from the end. look, if you want to discuss your harebrained ideas please keep them in your own threads. don't try to confuse someone who has a real world problem with your idiotic theories. |
Piano Wire Antenna for Experimental Rocket
On Sun, 24 May 2009 19:22:24 +0200, Szczepan Bia?ek
wrote: So it should be thick as possible. Absolute nonsense, confirmed by the utter lack of experience: How the length should be I do not know. 73's Richard Clark, KB7QHC |
Piano Wire Antenna for Experimental Rocket
On May 24, 9:48*am, "Jerry" wrote:
"mr1956" wrote in message ... On May 24, 12:04 am, "Jerry" wrote: "mr1956" wrote in message .... I am looking for some help developing a properly tuned piano wire antenna for an all metal experimental rocket. This particular vehicle currently uses a Digi International 9Xtream 100 mw transmitter which operates using Frequency Hopping Spread Spectrum from 910 to 918 MHz. The first flight used a .062" diameter length of piano wire cut so that the exposed length was about 1/2 wavelength. The wire antenna was isolated from the metal airfame with a small nylon penetrator and connected to the transmitter via about a 12" length of RG-178 coax. The coax was terminated at the antenna via two small jumpers (soldered to the center conductor and shield). The shield was grounded on the metal airframe transforming the entire rocket into an artificial ground plane (the antenna was also swept back at about a 45 degree angle to reduce drag. During this first flight we observed a range of about 10 miles, about half of the published range of this transmitter. The RF system is used as a downlink for GPS data and we did not get a good radio link after ignition until the rocket had descended to about 50,000 feet. Maximum velocity was about Mach 3 which is why a conventional antenna cannot be used as it will be melted by friction. For the second version, we are thinking of using a slightly smaller diameter piano wire for a 1/4 wave length antenna as I think this may be more suitable for a system using an artificial ground plane. In addition, I plan to terminate the RG-178 coax directly at the antenna and seal it with potting compound. This new antenna will be mounted totally on the exterior of the airframe and angled back only about 10 degrees to give a better radiation pattern. However, I would really like to properly tune this new antenna and was wondering if anyone in this group has any ideas as to how to do this. Doing a tuned RCL circuit at this frequency is proving to be difficult using discrete components, to say the least. I do not have an SWR, but do have an RF millivolt meter as well as the means to measure the capacitance between the antenna and metal airframe. Any help will be appreciated as I would hate to fly this thing again and not get it back because our RF downlink had insufficient range. C. Newport Hi C Have you considered flush antennas, like slots? Is is practical to use antenna directivity at the ground station to increase the "range"? It might be advantageous to use Linear Polarization on the rocket and Circular Polarization on the ground. For impedance measurement at L band, it is possible to build a slotted line using plumbing supplies. That presumes that you already have access to a signal generator and a detector to record the signal generator's output. A home built slotted line will have difficulty regestaring low VSWRs. But, once it is known that the load impedance is close to the line impedance (low VSWR), a directional coupler can be used to match more precisely. I have built a "plumber's delight" slotted line that works well at 130 MHZ. The high quality directional couplers are affordable on eBay. Note - If I can do it, it cant be too complicated! Jerry KD6JDJ I have looked at slotted or flush wrap around antennas but integrating one into the existing design would be difficult due to the wall thickness of the metal. Something like that would have to be mounted flush and I would be worried about the wind blast peeling the thing off. *Certainly, there is much that can be done but I am unfortunately not Wallops Island and have limited resources like anyone else. * Hi C * It looks like your "wire" antenna might actually be OK for your project. You have demonstrated that it works when the rocket is nearly 10 miles away when you hoped for 20 miles. * Evidently, the antenna doesnt need to function while the rocket is accending. * Are you able to improve the ground based antenna * The equipment you now have would need only 6 dB improvement to provide the 20 mile range. * I'd like to know more about your ground based antenna system. * * * * * * * * * * * * *Jerry * *KD6JDJ * * *(who has lots of small diameterTeflon dielectric coax *to donate to a real project) Jerry: The "ground station" is not much; just a small receiver with a standard 1/2 wave dipole attached using an sma connector. The receiver is battery powered and connected to a laptop using a standard serial cable. We had thought about using a Yagi during the previous flight but it just got left by the wayside with everything else that had to be done. The basic telemetry and downlink system is manufactured by Ozark Aerospace (Erik Hall) but uses a Digi Int. 9Xstream radio modem (www.ozarkaerospace.com and I use the ARTS TX-900G and RX-900). I actually built my own transmitter and GPS system using a 5 watt data radio and Garmin GPS with a small Li-Ion battery pack. But that system was "lost" during a not so successful flight in Texas a few years ago. Hence, for this design I opted to buy something off the shelf that already worked. Most people using this system have fiberglass airframes so for them it is a simple matter to use a commercially available antenna mounted inside of the airfame. But this is an all metal design so I had the telemetry board made so I could connect my own antenna. FYI, the rocket is 15 feet long, 6" in diameter and weighs over 100 lbs empty and close to 200 lbs. on the pad. It is all aluminum, except for the nose cone which is laminated birch. I'm using metal for the airframe because it just easier to work with and the first 8 feet of the rocket is metal anyway as that is the motor case. The published range for this Digi Int. transmitter is 20 miles, though people doing amateur high-altitude balloon flights have reportedly observed even better reception from high altitudes (i.e., over 80 K feet). This rocket did about 85,000 feet last September at Black Rock, Nevada so it is doubtful that we will get GPS data at apogee anyway due to the COTS limitations. But I'd like to get as much as possible and will probably eventually convert to the 9Xtend modem which goes up to one watt xmit power. Curt |
Piano Wire Antenna for Experimental Rocket
Jeff Liebermann wrote:
On Sun, 24 May 2009 04:04:52 GMT, "Jerry" wrote: Have you considered flush antennas, like slots? Slot antennas have the maximum lobe perpendicular to the rocket axis. There's very little signal below the rocket. This is why tracking stations are far away from the launch site. If the rocket were overhead, and going straight up, there's no signal. That was my first thought on side mounted antennas. The best solution would be something on the fins if that were possible with the airframe design. -- Jim Pennino Remove .spam.sux to reply. |
Piano Wire Antenna for Experimental Rocket
Hi C It looks like your "wire" antenna might actually be OK for your project. You have demonstrated that it works when the rocket is nearly 10 miles away when you hoped for 20 miles. Evidently, the antenna doesnt need to function while the rocket is accending. Are you able to improve the ground based antenna The equipment you now have would need only 6 dB improvement to provide the 20 mile range. I'd like to know more about your ground based antenna system. Jerry KD6JDJ (who has lots of small diameterTeflon dielectric coax to donate to a real project) Jerry: The "ground station" is not much; just a small receiver with a standard 1/2 wave dipole attached using an sma connector. The receiver is battery powered and connected to a laptop using a standard serial cable. We had thought about using a Yagi during the previous flight but it just got left by the wayside with everything else that had to be done. The basic telemetry and downlink system is manufactured by Ozark Aerospace (Erik Hall) but uses a Digi Int. 9Xstream radio modem (www.ozarkaerospace.com and I use the ARTS TX-900G and RX-900). I actually built my own transmitter and GPS system using a 5 watt data radio and Garmin GPS with a small Li-Ion battery pack. But that system was "lost" during a not so successful flight in Texas a few years ago. Hence, for this design I opted to buy something off the shelf that already worked. Most people using this system have fiberglass airframes so for them it is a simple matter to use a commercially available antenna mounted inside of the airfame. But this is an all metal design so I had the telemetry board made so I could connect my own antenna. FYI, the rocket is 15 feet long, 6" in diameter and weighs over 100 lbs empty and close to 200 lbs. on the pad. It is all aluminum, except for the nose cone which is laminated birch. I'm using metal for the airframe because it just easier to work with and the first 8 feet of the rocket is metal anyway as that is the motor case. The published range for this Digi Int. transmitter is 20 miles, though people doing amateur high-altitude balloon flights have reportedly observed even better reception from high altitudes (i.e., over 80 K feet). This rocket did about 85,000 feet last September at Black Rock, Nevada so it is doubtful that we will get GPS data at apogee anyway due to the COTS limitations. But I'd like to get as much as possible and will probably eventually convert to the 9Xtend modem which goes up to one watt xmit power. Curt Hi Curt I strongly suspect that you can use the antenna you now have on the rocket. There is a possibility that you hadnt optimized the pointing of the stub on the base station. You might be able to get enough system gain by using a simple Turnstile Antenna at the ground station ( Wikipedia ). If you decide you want more signal than you get with a Turnstile, it would be easy to build a 900 MHz Helix or Crossed Yagii. It seems very important to use a Circularly Polarized antenna at the ground station. Jerry |
Piano Wire Antenna for Experimental Rocket
On Sun, 24 May 2009 07:30:03 GMT, "Jerry"
wrote: "Jeff Liebermann" wrote in message .. . On Sun, 24 May 2009 04:04:52 GMT, "Jerry" wrote: Have you considered flush antennas, like slots? Slot antennas have the maximum lobe perpendicular to the rocket axis. There's very little signal below the rocket. This is why tracking stations are far away from the launch site. If the rocket were overhead, and going straight up, there's no signal. Slot antennas are also a power waste. You need 4 slots, run by a power splitter, in order to insure that at least one antenna is oriented in the direction of the receiver. Meanwhile, the other 3 slot antennas are radiating power to nobody in particular. Say goodby to about 3/4th of your tx power. Receive sensitivity is not affected. Hi Jeff Why are you so negative?? I have designed and built lots of slot antennas that were widely used on military aircraft. The efficiency is quite good. I am sure a smart guy like you could design a slot antenna and locate it for those guys with the 3G rocket. There hasnt been any text that restricts where the slot could be located. As I read the original post, they werent receiving signal from the accending rocket. Maybe they only want to receive data that was recorded after the rocket reached it's peak. There are lots of information that you and I dont yet know. But, my major question for you is "Why are you so negative?". Jerry KD6JDJ I don't think I'm being particularly negative or offensive. I offered what I consider to be a more reasonable alternative (CP turnstile antenna) and ran the path loss calculations to insure that it would work. I also itemized why a slot antenna would be an inferior solution. My main point is that with a vertically ascending rocket, the antenna pattern should be primarily ahead and behind the rocket, not perpendicular. I've seen one paper design that used insulated fins for the 4 turnstile antenna elements. However, I haven't seen the actual rocket. I also built a 2.4GHz antenna system for a model airplane for a video downlink. The initial design was a slot antenna system in the fuselage, which worked great when the aircraft was overhead, but did badly when near the horizon and out of the antenna pattern. Two vertical monopoles and a power splitter did much better, especially since it worked well in inverted flight. I must admit that I haven't designed a slot antenna system for a high power Mach 3 rocket. I did build a small "G" motor rocket without telemetry but that doesn't count for much. Anyway, we're both guessing. Without a clue as to the size, geometry, and construction of the rocket, the configuration of the ground station antenna, the location of the ground station, and the approximate trajectory (probably straight up), we can only speculate as to the optimal design. My guess is that this a level 1 rocket with an "H" or "I" motor which could barely tolerate the slight increase in diameter required for multiple slot antnnas. Wire or fin antennas don't add anything. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Piano Wire Antenna for Experimental Rocket
On May 24, 2:59*pm, "Jerry" wrote:
Hi C It looks like your "wire" antenna might actually be OK for your project.. You have demonstrated that it works when the rocket is nearly 10 miles away when you hoped for 20 miles. Evidently, the antenna doesnt need to function while the rocket is accending. Are you able to improve the ground based antenna The equipment you now have would need only 6 dB improvement to provide the 20 mile range. I'd like to know more about your ground based antenna system. Jerry KD6JDJ (who has lots of small diameterTeflon dielectric coax to donate to a real project) Jerry: The "ground station" is not much; just a small receiver with a standard 1/2 wave dipole attached using an sma connector. *The receiver is battery powered and connected to a laptop using a standard serial cable. We had thought about using a Yagi during the previous flight but it just got left by the wayside with everything else that had to be done. The basic telemetry and downlink system is manufactured by Ozark Aerospace (Erik Hall) but uses a Digi Int. 9Xstream radio modem (www.ozarkaerospace.comand I use the ARTS TX-900G and RX-900). *I actually built my own transmitter and GPS system using a 5 watt data radio and Garmin GPS with a small Li-Ion battery pack. But that system was "lost" during a not so successful flight in Texas a few years ago. Hence, for this design I opted to buy something off the shelf that already worked. Most people using this system have fiberglass airframes so for them it is a simple matter to use a commercially available antenna mounted inside of the airfame. But this is an all metal design so I had the telemetry board made so I could connect my own antenna. FYI, the rocket is 15 feet long, 6" in diameter and weighs over 100 lbs empty and close to 200 lbs. on the pad. It is all aluminum, except for the nose cone which is laminated birch. I'm using metal for the airframe because it just easier to work with and the first 8 feet of the rocket is metal anyway as that is the motor case. The published range for this Digi Int. transmitter is 20 miles, though people doing amateur high-altitude balloon flights have reportedly observed even better reception from high altitudes (i.e., over 80 K feet). This rocket did about 85,000 feet last September at Black Rock, Nevada so it is doubtful that we will get GPS data at apogee anyway due to the COTS limitations. But I'd like to get as much as possible and will probably eventually convert to the 9Xtend modem which goes up to one watt xmit power. Curt * Hi Curt * I strongly suspect that you can use the antenna you now have on the rocket. *There is a possibility that you hadnt optimized the pointing of the stub on the base station. *You might be able to get enough system gain by using a simple Turnstile Antenna at the ground station ( Wikipedia ). * If you decide you want more signal than you get with a Turnstile, it would be easy to build a 900 MHz Helix or Crossed Yagii. * It seems very important to use a Circularly Polarized antenna at the ground station. * * * * * * * * * * * * * Jerry I am familiar with the Helix antenna and the Yagi, but not a "crossed yagi." Am I correct in assuming that the crossed yagi is simply two yagis with the reflectors at right angles to each other (so that it looks like a cross when viewed from the end)? Directionality on the ground station is an issue though. Once the rocket goes out of sight (it was only visible for about three seconds of a nine minute flight), pointing the receiving antenna in the right direction and elevation would be guess work. I know that the yagi must be much more directional than the 1/2 wave dipole used before but how much I do not know. I would imagine it is determined by the number and spacing of the elements. I also think that having the transmitting antenna more perpendicular to the airfame will help during apogee and descent. Curt |
Piano Wire Antenna for Experimental Rocket
"mr1956" wrote in message ... On May 24, 2:59 pm, "Jerry" wrote: Hi C It looks like your "wire" antenna might actually be OK for your project. You have demonstrated that it works when the rocket is nearly 10 miles away when you hoped for 20 miles. Evidently, the antenna doesnt need to function while the rocket is accending. Are you able to improve the ground based antenna The equipment you now have would need only 6 dB improvement to provide the 20 mile range. I'd like to know more about your ground based antenna system. Jerry KD6JDJ (who has lots of small diameterTeflon dielectric coax to donate to a real project) Jerry: The "ground station" is not much; just a small receiver with a standard 1/2 wave dipole attached using an sma connector. The receiver is battery powered and connected to a laptop using a standard serial cable. We had thought about using a Yagi during the previous flight but it just got left by the wayside with everything else that had to be done. The basic telemetry and downlink system is manufactured by Ozark Aerospace (Erik Hall) but uses a Digi Int. 9Xstream radio modem (www.ozarkaerospace.comand I use the ARTS TX-900G and RX-900). I actually built my own transmitter and GPS system using a 5 watt data radio and Garmin GPS with a small Li-Ion battery pack. But that system was "lost" during a not so successful flight in Texas a few years ago. Hence, for this design I opted to buy something off the shelf that already worked. Most people using this system have fiberglass airframes so for them it is a simple matter to use a commercially available antenna mounted inside of the airfame. But this is an all metal design so I had the telemetry board made so I could connect my own antenna. FYI, the rocket is 15 feet long, 6" in diameter and weighs over 100 lbs empty and close to 200 lbs. on the pad. It is all aluminum, except for the nose cone which is laminated birch. I'm using metal for the airframe because it just easier to work with and the first 8 feet of the rocket is metal anyway as that is the motor case. The published range for this Digi Int. transmitter is 20 miles, though people doing amateur high-altitude balloon flights have reportedly observed even better reception from high altitudes (i.e., over 80 K feet). This rocket did about 85,000 feet last September at Black Rock, Nevada so it is doubtful that we will get GPS data at apogee anyway due to the COTS limitations. But I'd like to get as much as possible and will probably eventually convert to the 9Xtend modem which goes up to one watt xmit power. Curt Hi Curt I strongly suspect that you can use the antenna you now have on the rocket. There is a possibility that you hadnt optimized the pointing of the stub on the base station. You might be able to get enough system gain by using a simple Turnstile Antenna at the ground station ( Wikipedia ). If you decide you want more signal than you get with a Turnstile, it would be easy to build a 900 MHz Helix or Crossed Yagii. It seems very important to use a Circularly Polarized antenna at the ground station. Jerry I am familiar with the Helix antenna and the Yagi, but not a "crossed yagi." Am I correct in assuming that the crossed yagi is simply two yagis with the reflectors at right angles to each other (so that it looks like a cross when viewed from the end)? Directionality on the ground station is an issue though. Once the rocket goes out of sight (it was only visible for about three seconds of a nine minute flight), pointing the receiving antenna in the right direction and elevation would be guess work. I know that the yagi must be much more directional than the 1/2 wave dipole used before but how much I do not know. I would imagine it is determined by the number and spacing of the elements. I also think that having the transmitting antenna more perpendicular to the airfame will help during apogee and descent. Curt Hi Curt I figured that your current telemetry system is demonstrated to be nearly good enough. From the info I now have, it seems that you need only 6 db additional gain to be able to double the distance between the rocket and the ground station. Since you used linear polarization on both the rocket and the ground station, it is possible that you will get adequate signal by making the base station antenna "Circular". If the rocket accends to 20 miles and the ground station is close to the place where the rocket started, the apogee point could be 2 miles from "straight up" and still be less than 5.7 degrees from straight up. I'd suggest using a base antenna with a pattern much wider than 11 1/2 degrees so the antenna needs to be pointed up, not aimed. I'd expect there are other rocket amateurs using simple Turnstile Antennas for their base station antenna. Jerry KD6JDJ |
Piano Wire Antenna for Experimental Rocket
On May 24, 3:41*pm, "Jerry" wrote:
"mr1956" wrote in message ... On May 24, 2:59 pm, "Jerry" wrote: Hi C It looks like your "wire" antenna might actually be OK for your project. You have demonstrated that it works when the rocket is nearly 10 miles away when you hoped for 20 miles. Evidently, the antenna doesnt need to function while the rocket is accending. Are you able to improve the ground based antenna The equipment you now have would need only 6 dB improvement to provide the 20 mile range. I'd like to know more about your ground based antenna system. Jerry KD6JDJ (who has lots of small diameterTeflon dielectric coax to donate to a real project) Jerry: The "ground station" is not much; just a small receiver with a standard 1/2 wave dipole attached using an sma connector. The receiver is battery powered and connected to a laptop using a standard serial cable. We had thought about using a Yagi during the previous flight but it just got left by the wayside with everything else that had to be done. The basic telemetry and downlink system is manufactured by Ozark Aerospace (Erik Hall) but uses a Digi Int. 9Xstream radio modem (www.ozarkaerospace.comandI use the ARTS TX-900G and RX-900). I actually built my own transmitter and GPS system using a 5 watt data radio and Garmin GPS with a small Li-Ion battery pack. But that system was "lost" during a not so successful flight in Texas a few years ago. Hence, for this design I opted to buy something off the shelf that already worked. Most people using this system have fiberglass airframes so for them it is a simple matter to use a commercially available antenna mounted inside of the airfame. But this is an all metal design so I had the telemetry board made so I could connect my own antenna. FYI, the rocket is 15 feet long, 6" in diameter and weighs over 100 lbs empty and close to 200 lbs. on the pad. It is all aluminum, except for the nose cone which is laminated birch. I'm using metal for the airframe because it just easier to work with and the first 8 feet of the rocket is metal anyway as that is the motor case. The published range for this Digi Int. transmitter is 20 miles, though people doing amateur high-altitude balloon flights have reportedly observed even better reception from high altitudes (i.e., over 80 K feet). This rocket did about 85,000 feet last September at Black Rock, Nevada so it is doubtful that we will get GPS data at apogee anyway due to the COTS limitations. But I'd like to get as much as possible and will probably eventually convert to the 9Xtend modem which goes up to one watt xmit power. Curt Hi Curt I strongly suspect that you can use the antenna you now have on the rocket. There is a possibility that you hadnt optimized the pointing of the stub on the base station. You might be able to get enough system gain by using a simple Turnstile Antenna at the ground station ( Wikipedia ). If you decide you want more signal than you get with a Turnstile, it would be easy to build a 900 MHz Helix or Crossed Yagii. It seems very important to use a Circularly Polarized antenna at the ground station. Jerry I am familiar with the Helix antenna and the Yagi, but not a "crossed yagi." Am I correct in assuming that the crossed yagi is simply two yagis with the reflectors at right angles to each other (so that it looks like a cross when viewed from the end)? Directionality on the ground station is an issue though. Once the rocket goes out of sight (it was only visible for about three seconds of a nine minute flight), pointing the receiving antenna in the right direction and elevation would be guess work. I know that the yagi must be much more directional than the 1/2 wave dipole used before but how much I do not know. I would imagine it is determined by the number and spacing of the elements. I also think that having the transmitting antenna more perpendicular to the airfame will help during apogee and descent. Curt * Hi Curt * I figured that your current telemetry system is demonstrated to be nearly good enough. * From the info I now have, it seems that you need only 6 db additional gain to be able to double the distance between the rocket and the ground station. * Since you used linear polarization on both the rocket and the ground station, it is possible that you will get adequate signal by making the base station antenna "Circular". * If the rocket accends to 20 miles and the ground station is close to the place where the rocket started, the apogee point could be 2 miles from "straight up" and still be less than 5.7 degrees from straight up. *I'd suggest using a base antenna with a pattern much wider than 11 1/2 degrees so the antenna needs to be pointed up, not aimed. * I'd expect there are other rocket amateurs using simple Turnstile Antennas for their base station antenna. * * * * * * * * * * * * * * Jerry * KD6JDJ Jerry: By "circular" I assume you mean either a helix or crossed yagi so that the receiving angle is uniform around the axis of the antenna? Your assumptions are pretty close. The launch pad was about 1.25 miles from the ground station (to the north I think) and the rocket landed about 2.5 miles east of the receiving station. If I remember correctly, we got a good link down to about 4,000 feet during the descent on parachute. So if I fabricate or purchase an antenna with a sufficient receiving pattern, point it generally up but maybe slightly towards the pad, it should work. We were lucky that day as the winds aloft were not too bad. In addition, the rocket comes down fast, at about 75 feet/second so as to minimize drift. Everything was built for strength because I knew it would hit pretty hard. Let em play with some of the online calculators and see if I can come up with a helix design and do some searching for a crossed yagi. Curt |
Piano Wire Antenna for Experimental Rocket
mr1956 wrote in
: .... I have been reading the ARRL antenna book and while there is much information, a lot of it is over my head. But one point I think a chapter makes is that a 1/4 wave monopole will work better than a 1/2 wave using an artificial ground plane because of the way the voltage peaks at the end of the antenna; or that is how it seems. Your transmitter is designed for a particular load impedance, almost certainly 50 ohms... check the specs. The load presented to the transmitter terminals depends on the feed point impedance of the antenna, and impedance transformation that may occur on the coaxial feedline. In the case you describe, the *antenna system* design was most unsuited to a 50 ohm transmitter, hence the considerable loss predicted. Your newer proposal for a quarter wave wire perpedicular to the rocket body with the coax shield connected to the metallic rocket body is not exactly 50 ohms, but probably close enough that antenna system efficiency will be fairly good and the transmitter should deliver most of its rated power. Owen |
Piano Wire Antenna for Experimental Rocket
On May 24, 5:49*pm, Owen Duffy wrote:
mr1956 wrote : ... I have been reading the ARRL antenna book and while there is much information, a lot of it is over my head. *But one point I think a chapter makes is that a 1/4 wave monopole will work better than a 1/2 wave using an artificial ground plane because of the way the voltage peaks at the end of the antenna; or that is how it seems. Your transmitter is designed for a particular load impedance, almost certainly 50 ohms... check the specs. The load presented to the transmitter terminals depends on the feed point impedance of the antenna, and impedance transformation that may occur on the coaxial feedline. In the case you describe, the *antenna system* design was most unsuited to a 50 ohm transmitter, hence the considerable loss predicted. Your newer proposal for a quarter wave wire perpedicular to the rocket body with the coax shield connected to the metallic rocket body is not exactly 50 ohms, but probably close enough that antenna system efficiency will be fairly good and the transmitter should deliver most of its rated power. Owen What I have been finding out is developing a matching circuit for the output of the transmitter is not easy, at least for me and at that frequency. It could be done if I was a cell phone manufacturer. I think much will be gained by using the 1/4 wave as opposed to the 1/2 wave on the airframe and a yagi or helix antenna on the ground station. I will post a link to some images of the project when I have time. Information on the flight can be found at www.lokiresearch.com and look for information about the Proteus 6 flight at the Balls 17 event, Black Rock, NV September 2008. I'll also add some thoughts about how I intend to proceed based on everyone's great input from this forum. One quick question: Am I correct in assuming that the ground lead of the rocket mounted antenna should be as short as possible? i.e., the connection between the shield of the RG-178 and the metal airframe. Thanks, Curt |
Piano Wire Antenna for Experimental Rocket
On Mon, 25 May 2009 09:48:23 -0700 (PDT), mr1956
wrote: One quick question: Am I correct in assuming that the ground lead of the rocket mounted antenna should be as short as possible? i.e., the connection between the shield of the RG-178 and the metal airframe. Very much so - mm instead of inches. 73's Richard Clark, KB7QHC |
Piano Wire Antenna for Experimental Rocket
mr1956 wrote in
: .... What I have been finding out is developing a matching circuit for the output of the transmitter is not easy, at least for me and at that frequency. It could be done if I was a cell phone manufacturer. I think much will be gained by using the 1/4 wave as opposed to the 1/2 wave on the airframe and a yagi or helix antenna on the ground station. The easy solution is the one I suggested. I will post a link to some images of the project when I have time. Information on the flight can be found at www.lokiresearch.com and look for information about the Proteus 6 flight at the Balls 17 event, Black Rock, NV September 2008. I'll also add some thoughts about how I intend to proceed based on everyone's great input from this forum. One quick question: Am I correct in assuming that the ground lead of the rocket mounted antenna should be as short as possible? i.e., the connection between the shield of the RG-178 and the metal airframe. Loss on the coax can be much worse when it is mismatched. I gave you figures for a 2000 ohm load (representing your end fed half wave antenna). The loss for 0.3m with a 30 ohm load is 0.5dB, it is 90% efficient. If you shorten it, you will reduce the loss, but I wouldn't obsess over it. The problem you have right now is due principally to high standing waves on the line and unsuitable transmitter load, and you solve much of that using a quarter wave antenna. Without a diagram, we are at great risk of misunderstanding. If you cannot affort the time to invest in making a drawing, you take the risk of misinterpretation of your words. The connection from the shield to the metal air frame and presumably metal skin should be short. Regard that the effective length of the antenna in that case is the distance from the shield to the tip of the whip. Owen |
Piano Wire Antenna for Experimental Rocket
On May 25, 5:26*pm, Owen Duffy wrote:
mr1956 wrote : ... What I have been finding out is developing a matching circuit for the output of the transmitter is not easy, at least for me and at that frequency. It could be done if I was a cell phone manufacturer. *I think much will be gained by using the 1/4 wave as opposed to the 1/2 wave on the airframe and a yagi or helix antenna on the ground station. The easy solution is the one I suggested. I will post a link to some images of the project when I have time. Information on the flight can be found atwww.lokiresearch.comand look for information about the Proteus 6 flight at the Balls 17 event, Black Rock, NV September 2008. I'll also add some thoughts about how I intend to proceed based on everyone's great input from this forum. *One quick question: Am I correct in assuming that the ground lead of the rocket mounted antenna should be as short as possible? i.e., the connection between the shield of the RG-178 and the metal airframe. Loss on the coax can be much worse when it is mismatched. I gave you figures for a 2000 ohm load (representing your end fed half wave antenna).. The loss for 0.3m with a 30 ohm load is 0.5dB, it is 90% efficient. If you shorten it, you will reduce the loss, but I wouldn't obsess over it. The problem you have right now is due principally to high standing waves on the line and unsuitable transmitter load, and you solve much of that using a quarter wave antenna. Without a diagram, we are at great risk of misunderstanding. If you cannot affort the time to invest in making a drawing, you take the risk of misinterpretation of your words. The connection from the shield to the metal air frame and presumably metal skin should be short. Regard that the effective length of the antenna in that case is the distance from the shield to the tip of the whip. Owen OK, I think I know generally how to proceed from here. FYI, some images of the previous design can be found at the following URLs: http://i647.photobucket.com/albums/u...9/Curtjeff.jpg http://i647.photobucket.com/albums/u...09/liftoff.jpg http://i647.photobucket.com/albums/u.../Coupler02.jpg http://i647.photobucket.com/albums/u...009/Elec05.jpg http://i647.photobucket.com/albums/u...ery04small.jpg This one is an image of the type of gear I normally work with: http://i647.photobucket.com/albums/u.../Guam02004.jpg So the key points are as follows: 1. Fabricate a 1/4 wave antenna and mount that is totally exterior to the metal airfame and terminate the coax as close as possible to the antenna. 2. I imagine that I could start off with a longer than needed whip antenna, then trim it back while taking measurements with the RF millivolt meter until it looks like I've gotten maximum signal strength. 3. Use either a helix or yagi receiving antenna on the ground station to boost the signal strength on the receiving end. I accept that words alone are not always sufficient to describe such things but I think the important issues have been well described by everyone here. I can only make the RG-178 whip connecting the transmitter to the antenna so short, as I need sufficient length to remove the electronics from inside of the avionics compartment to change batteries,etc. But I think the critical issues are having a direct and solid electrical connection between the antenna and coax and using a higher db antenna on the receiving end; the ground based antenna may make the most difference in the end. I'll have to go back and look at the posts to make sure I've gotten all of the information right and if anyone has any more questions after reviewing the images please let me know and I will respond as needed. Also, there should be an article coming out in Smithsonian Air & Space magazine in the "Above and Beyond" section sometime in the future describing this particular flight and more information is online at www.lokiresearch.com (the Proteus 6 flight). Curt |
Piano Wire Antenna for Experimental Rocket
"mr1956" wrote ... On May 25, 5:26 pm, Owen Duffy wrote: mr1956 wrote : What I have been finding out is developing a matching circuit for the output of the transmitter is not easy, at least for me and at that frequency. It could be done if I was a cell phone manufacturer. I think much will be gained by using the 1/4 wave as opposed to the 1/2 wave on the airframe and a yagi or helix antenna on the ground station. The easy solution is the one I suggested. I will post a link to some images of the project when I have time. Information on the flight can be found atwww.lokiresearch.comand look for information about the Proteus 6 flight at the Balls 17 event, Black Rock, NV September 2008. I'll also add some thoughts about how I intend to proceed based on everyone's great input from this forum. One quick question: Am I correct in assuming that the ground lead of the rocket mounted antenna should be as short as possible? i.e., the connection between the shield of the RG-178 and the metal airframe. Loss on the coax can be much worse when it is mismatched. I gave you figures for a 2000 ohm load (representing your end fed half wave antenna). The loss for 0.3m with a 30 ohm load is 0.5dB, it is 90% efficient. If you shorten it, you will reduce the loss, but I wouldn't obsess over it. The problem you have right now is due principally to high standing waves on the line and unsuitable transmitter load, and you solve much of that using a quarter wave antenna. Without a diagram, we are at great risk of misunderstanding. If you cannot affort the time to invest in making a drawing, you take the risk of misinterpretation of your words. The connection from the shield to the metal air frame and presumably metal skin should be short. Regard that the effective length of the antenna in that case is the distance from the shield to the tip of the whip. Owen OK, I think I know generally how to proceed from here. FYI, some images of the previous design can be found at the following URLs: http://i647.photobucket.com/albums/u...9/Curtjeff.jpg http://i647.photobucket.com/albums/u...09/liftoff.jpg http://i647.photobucket.com/albums/u.../Coupler02.jpg http://i647.photobucket.com/albums/u...009/Elec05.jpg http://i647.photobucket.com/albums/u...ery04small.jpg This one is an image of the type of gear I normally work with: http://i647.photobucket.com/albums/u.../Guam02004.jpg So the key points are as follows: 1. Fabricate a 1/4 wave antenna and mount that is totally exterior to the metal airfame and terminate the coax as close as possible to the antenna. 2. I imagine that I could start off with a longer than needed whip antenna, then trim it back while taking measurements with the RF millivolt meter until it looks like I've gotten maximum signal strength. 3. Use either a helix or yagi receiving antenna on the ground station to boost the signal strength on the receiving end. I accept that words alone are not always sufficient to describe such things but I think the important issues have been well described by everyone here. I can only make the RG-178 whip connecting the transmitter to the antenna so short, as I need sufficient length to remove the electronics from inside of the avionics compartment to change batteries,etc. But I think the critical issues are having a direct and solid electrical connection between the antenna and coax and using a higher db antenna on the receiving end; the ground based antenna may make the most difference in the end. I'll have to go back and look at the posts to make sure I've gotten all of the information right You miss the most important. Helmut wrote "Do not use too thin wires, the thicker the better." and if anyone has any more questions after reviewing the images please let me know and I will respond as needed. Also, there should be an article coming out in Smithsonian Air & Space magazine in the "Above and Beyond" section sometime in the future describing this particular flight and more information is online at www.lokiresearch.com (the Proteus 6 flight). S* |
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