mmarien
Murray M.
The question I would have then, does a small ground plane still work at all angles? I can see that a metal fuselage is a big target at any angle. But does a small 12 centimetre ground plane still work if it's viewed from the edge?
Approach "Not recieving Transponder recycle..."
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jakej
Well-Known Member
I don't have the time to go into antenna design now however here is a useful link http://www.wingsandwheels.com/antenna_vhf_airband_gps_mobile_e.htm FWIW I sometimes make my own, for composite aircraft, using 2" wide x .020" aluminum strips secured in a cross section as a ground plane, then bolt a Comant CI121 or CI122 to that.
Jake J
Jake J
jnmeade
Active Member
The ground plane should be perpendicular to the transponder antenna and it's radius should be at least as great as the length of the antenna. This if for a quarter wave monopole on a composite airplane.
For more than that, I'm over my head.
For more than that, I'm over my head.
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We actually have detailed instructions on ground planes for transponders in our install guide:
This thread is also required reading on the subject if you are making your own ground planes:
http://dynonavionics.com/cgi-bin/yabb2/YaBB.pl?num=1328697560/5
As you can see, if you haven't chosen the right size ground plane you may have a really inefficient antenna. Notice that since a transponder receives and transmits on different frequencies, it's possible to build an awesome antenna that lights up the reply light but very little of that reply energy makes it out of the antenna.
This thread is also required reading on the subject if you are making your own ground planes:
http://dynonavionics.com/cgi-bin/yabb2/YaBB.pl?num=1328697560/5
As you can see, if you haven't chosen the right size ground plane you may have a really inefficient antenna. Notice that since a transponder receives and transmits on different frequencies, it's possible to build an awesome antenna that lights up the reply light but very little of that reply energy makes it out of the antenna.
jnmeade
Active Member
I just had long talks with a guy who has been an avionics tech for 3-4 decades and a guy who retired from working with transponders at Rockwell Collins.
They both independently supported the Dynon post just above. Both stated that using radial wires (like I do on my HF ham radio antenna) does not work as well as a large mass of ground plane material. It can be copper window screen or other mesh, just not a bunch of wires because as is mentioned above the wires may be resonant at a frequency that is not helpful for operation at 1030/1090.
The Rockwell guy also said you need to keep the antenna away from wheels or other large metallic abjects that have the effect of decreasing the size of the lobe where the signal hits the wheels. I think my FD is pretty close to the wheels and that will cause a lot of apparent directivity - I don't want to be broadside to the ground station.
The Rockwell guy also agreed that the ground plane should be pretty large - he said at least 12" on a side and more is good.
They both independently supported the Dynon post just above. Both stated that using radial wires (like I do on my HF ham radio antenna) does not work as well as a large mass of ground plane material. It can be copper window screen or other mesh, just not a bunch of wires because as is mentioned above the wires may be resonant at a frequency that is not helpful for operation at 1030/1090.
The Rockwell guy also said you need to keep the antenna away from wheels or other large metallic abjects that have the effect of decreasing the size of the lobe where the signal hits the wheels. I think my FD is pretty close to the wheels and that will cause a lot of apparent directivity - I don't want to be broadside to the ground station.
The Rockwell guy also agreed that the ground plane should be pretty large - he said at least 12" on a side and more is good.
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Just a note, but the math actually says 12" is a bad ground plane. That's 300mm, and the wavelength of 1090MHz is 291 mm, so 300mm is a really bad ground plane for a transponder. You really do want to do a 120mm square (4.75") , or jump up to 700mm (27.75") or more. Between those dimensions is bad.
jnmeade
Active Member
I probably misquoted him and won't dispute the Dynon numbers. However, please educate me on why 300mm on a side is an issue? What we'd be talking about is a radius of about 220 mm on the diagonal (center to the point) and about 153 on the perpendicular side (center to the nearest side). Since we're talking about a quarter wave of 1090 or 1030, we're looking at 272.5 or 257.5, respectively and the above numbers (without some math) don't look like they are harmonics of the transponder quarter wave, do they?
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The full wave wavelength of 1030 is 291mm and 1090 is 275mm.
You want a ground plane that is quarter wavelength from the radiator all around. You really have two ground planes that are effective: quarter wave, or "infinite". Ground planes between a quarter wave and infinite are much worse than either of those choices.
So if a circle, the radius should be 1/4 wave, or 72.75/68.75mm.
Now, look at our recommendation of a square 120mm per side. This gives you between 60mm and 85mm from the antenna as you travel along one edge. This nicely brackets the 68 and 72mm optimal distances.
If you have a "12" on a side" ground plane, you have 305mm on each side. This means you vary from 152.5mm to 215.6 versus the antenna in the center. This is just over 1/2 wavelength to just under a full wavelength. It's exactly where you don't want to be. Half wavelengths are bad. You actually want a quarter wave, but the issue is that there isn't one quarter that works with both frequencies. So something that brackets the two quarter wave lengths you have is really a good choice.
Once you're back up to 700mm, you vary from 350 to 495mm, which is at least 1.25 wavelengths, which is approximately the same as 0.25 wavelengths. Anything above is still a good place to be as it starts along the increasing efficiency path towards "infinity".
You want a ground plane that is quarter wavelength from the radiator all around. You really have two ground planes that are effective: quarter wave, or "infinite". Ground planes between a quarter wave and infinite are much worse than either of those choices.
So if a circle, the radius should be 1/4 wave, or 72.75/68.75mm.
Now, look at our recommendation of a square 120mm per side. This gives you between 60mm and 85mm from the antenna as you travel along one edge. This nicely brackets the 68 and 72mm optimal distances.
If you have a "12" on a side" ground plane, you have 305mm on each side. This means you vary from 152.5mm to 215.6 versus the antenna in the center. This is just over 1/2 wavelength to just under a full wavelength. It's exactly where you don't want to be. Half wavelengths are bad. You actually want a quarter wave, but the issue is that there isn't one quarter that works with both frequencies. So something that brackets the two quarter wave lengths you have is really a good choice.
Once you're back up to 700mm, you vary from 350 to 495mm, which is at least 1.25 wavelengths, which is approximately the same as 0.25 wavelengths. Anything above is still a good place to be as it starts along the increasing efficiency path towards "infinity".
jnmeade
Active Member
What accommodation do you use when putting a quarter wave ground plane on the interior of a carbon fiber or fiberglass airplane? I understand it affects the properties such that a standard size is no longer exactly right. Do some of the odd shapes theoretically take care of this or do you have to resize the ground plane?
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The safest thing is to go with the 700mm or larger option, which takes care of almost any issues you could have. If you really are trying to stay near 120mm, then that should still be OK, as part of the reason for that size as a square is that it gives you some protection from the effects of the composite.
I am a retired Boeing technical fellow. My engineering specialty for 36+ years was electromagnetics with an emphasis on antenna design, analysis and measurement. Virtually all Boeing products possess multiple antenna systems that collectively occupy a large portion of the electromagnetic spectrum. I worked on systems from ELF up through millimeter waves and have performed extensive computational simulations of antenna performance specifically for aircraft and spacecraft applications. Recently, I have been providing technical consultation services regrading antenna systems to fellow aircraft owners. One close friend is in the process of evaluating various experimental aircraft designs for the purpose of potentially building an aircraft. As part of his evaluation process he is looking into various avionics suppliers and has asked me to comment upon antenna siting for both metallic and non-metallic aircraft structures. He provided me with the link to your install guide. While most of the technical information you provide is technically very good, your technical information relative to antenna ground plane size and shape is flat out wrong. A circular ground plane performs just as well as a square or rectangular ground plane and ground plane dimensions that are small multiples of the wavelength will in fact perform just fine. The idea that a ground plane of a certain size will "resonate" is also incorrect. If you are interested I can provide electromagnetic simulations (that use the method of moments) of these geometries that show there is neither a ground plane shape dependence nor a ground plane resonance effect. I suggest your install guides be updated to remove these technically incorrect statements.
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Gary,
Extremely well formed circular planes do work well, but they are hard to fabricate. Getting the size off by a small amount can cause significantly mis-tuned antennas. Additionally, the transponder has both 1090MHz and 1030MHz frequencies to deal with, so there is no perfect size. A square ground plane takes care of all of these issues, and is simple to fabricate, with very low tolerance requirements. We're not in the business of making our customers do a lot of work to make a perfect circle when snipping out a square of aluminum from scrap works just as well.
In fact, the thing we're most trying to get across is that the ground plane does not have critical dimensions, and that the installer should not put all their fabrication skills to task for this ground plane. It appears we're in agreement there.
Please make sure you read a bit more of the detail we have posted here:
dynonavionics.com/cgi-bin/yabb2/YaBB.pl?num=1328697560/4#4
The transponder we sell is TSO'd, and the RF section of that is done by our partner at Trig avionics. We can't change our installation manual without their approval as it is part of the TSO. If you'd like to discuss this further, you'll need to speak with them. They generally do their best to have installation guidance which is easy to understand and leads to a proper install, without a lot of hassle, testing, or risk of error. Remember that this is guidance to a transponder installed in an unknown aircraft, with no knowledge of what is around that, not a transport category aircraft which is tested and engineered to the nth degree, and their guidance is built for that market. The majority of our installers have never installed a transponder before and just need to know how to get it right, not all the technical details.
Thanks for your help in trying to make our products and manuals better, and we're happy to update our manuals if Trig agrees they can be worded better.
Extremely well formed circular planes do work well, but they are hard to fabricate. Getting the size off by a small amount can cause significantly mis-tuned antennas. Additionally, the transponder has both 1090MHz and 1030MHz frequencies to deal with, so there is no perfect size. A square ground plane takes care of all of these issues, and is simple to fabricate, with very low tolerance requirements. We're not in the business of making our customers do a lot of work to make a perfect circle when snipping out a square of aluminum from scrap works just as well.
In fact, the thing we're most trying to get across is that the ground plane does not have critical dimensions, and that the installer should not put all their fabrication skills to task for this ground plane. It appears we're in agreement there.
Please make sure you read a bit more of the detail we have posted here:
dynonavionics.com/cgi-bin/yabb2/YaBB.pl?num=1328697560/4#4
The transponder we sell is TSO'd, and the RF section of that is done by our partner at Trig avionics. We can't change our installation manual without their approval as it is part of the TSO. If you'd like to discuss this further, you'll need to speak with them. They generally do their best to have installation guidance which is easy to understand and leads to a proper install, without a lot of hassle, testing, or risk of error. Remember that this is guidance to a transponder installed in an unknown aircraft, with no knowledge of what is around that, not a transport category aircraft which is tested and engineered to the nth degree, and their guidance is built for that market. The majority of our installers have never installed a transponder before and just need to know how to get it right, not all the technical details.
Thanks for your help in trying to make our products and manuals better, and we're happy to update our manuals if Trig agrees they can be worded better.
I don't want to belabor this issue of the RF performance of antennas and associated ground planes, but it should be obvious that I have a passion for antenna technology. It is difficult for me to accept technical performance anecdotes (with neither accompanying measured data nor validated computer simulations) that I know to be incorrect.
So I performed 6 separate analyses of a quarter wave monopole antenna mounted in the center of 6 different ground planes. The antenna analyzed is a 69 mm (2.71 in) monopole with a diameter of 4 mm (0.16 in). This antenna is comparable in size to the Comant model CI-105-3, that is a commonly used transponder and DME antenna on general aviation aircraft. I chose 6 different ground planes to analyze - squares of 120, 240 and 700 mm (4.72, 9.45 and 27.56 in) and circles with diameters of 120, 240 and 700 mm. I used the valildated method of moments software called FEKO that is an recognized tool used by antenna engineers worldwide. I computed the currents induced on the ground plane, the antenna input reflection coefficient(referenced to 50 ohms) and the radiation patterns over a frequency band of 960-1160 MHz. The resulting data are shown in the attached pdf file.
Page 1 summarizes the geometry. Page 2 shows the computed ground plane currents for the 700 mm ground planes at 1060 MHz. Note that the RF currents are concentrated within about 100 mm of the center of the ground planes and that they are basically independent of ground plane shape. Page 3 shows the RF currents for the 240 mm ground planes. Note that in the center of the ground planes the currents are about the same but the square ground plane has hot and cold spots around the periphery of the ground plane while the circular ground plane currents are essentially rotationally symmetric. Page 4 shows the RF currents for the 120 mm ground planes. For these cases the RF currents are a significant magnitude out near the edges of the ground plane and there is variation around the periphery of the ground planes for both geometries. Examination of the RF currents indicates that the larger ground planes should provide good isolation of structure mounted under the ground planes while the smaller ground planes would be expected to provide less isolation and the geometry of the structure behind the ground planes will likely significantly modify the antenna performance. Also note the lack of a significant difference in the induced RF currents as a function of ground plane geometry. Only the size of the ground plane is important.
Pages 5-7 show the computed radiation pattern realized gain (over a range of +10 to -30 dBi) as a function of angle. We choose the antenna geometry to be such that the antennas are vertical and mounted on a horizontal ground plane. An angle of 0 deg is then directly overhead and 90 deg is along the horizon, while 180 deg is directly below the ground plane. Calculations were performed at both 1030 and 1090 MHz. A monopole on an infinite ground plane should have about 5 dBi of gain and page 5 shows the performance of the 700 mm ground planes. The peak gains are about 5 dBi, as expected, and the peak gain below the ground planes are about -5 dBi, indicating a radiated power level of about 1/10 of the peak level above the ground plane. Page 6 shows the patterns for the 240 mm ground plane geometries. The peak gain has dropped a small amount and the backlobe has come up about 5 dB, indicating less isolation is provided by the smaller ground plane. Page 7 shows the radiation patterns for the 120 mm ground planes. Note here that the pattern level below the ground plane is essentially the same as that above the ground plane, indicating that the ground plane is providing very little isolation. Also note that the radiation patterns for each of the ground plane sizes are essentially independent of ground plane shape (size, not shape controls the performance).
Finally, pages 8-10 show the voltage reflection coefficient linear magnitude as a function of frequency. Page 8 shows the performance for the 700 mm ground planes, page 9 shows the performance for the 240 mm ground planes and page 10 shows the performance for the 120 mm ground planes. Note that other than a slight shift in the location of the voltage reflection coefficient minimum as a function of frequency, the reflection coefficient performance is basically independent of either ground plane size or shape.
So, what conclusions can we draw from all this? I offer the following:
1) Getting the size of the ground plane off by small amount will NOT detune the antenna
2) A simple monopole antenna has inherent bandwidth to simultaneously cover both 1030 and 1090 MHz (indeed, it can cover the entire DME band of 960-1220 MHz)
3) A circular ground plane works just as well as a square ground plane - size not shape determines performance
4) For small ground planes, what is installed behind the ground plane (cabling, metallic structure, etc.) will have a significant effect upon antenna performance
5) Avionics equipment even for amateur-built aircraft ARE "engineered to the nth degree" (hence the TSO) and slapping an antenna on a ground plane and hoping it works is to be avoided. Some engineering has to occur to get it right. Unfortunately, this rarely happens. Even metallic, production aircraft (like my Mooney) have issues with antenna installations as I have evidence - both measured and simulated - that reveals significant antenna performance variations as a function of azimuth angle and frequency owing to a poorly sited antenna
6) An octagonal ground plane would be expected to have performance similar to square, rectangular or circular ground planes of similar area
7) Because the performance of a ground plane is dependent upon the size of the ground plane relative to a wavelength, a 240 mm ground plane will perform better than a 120 mm ground plane - the idea that there is some sort of "ground plane resonance" phenomenon is flat out wrong
In closing, I note that this is not the first time that a subcontractor (Trig) has provided incorrect technical information to a prime contractor (Dynon) that ultimately gets supplied to the customer - nor will it be the last time, unfortunately. The prime contractor needs to actively perform a "trust but verify" operation on technical data that will be transferred from the subcontractor to the customer. Not doing so violates the due diligence responsibility of the prime contractor.
If you are interested in discussing any of the analysis described above, I would be happy to stop by (I live between Auburn and Enumclaw) and discuss it.
So I performed 6 separate analyses of a quarter wave monopole antenna mounted in the center of 6 different ground planes. The antenna analyzed is a 69 mm (2.71 in) monopole with a diameter of 4 mm (0.16 in). This antenna is comparable in size to the Comant model CI-105-3, that is a commonly used transponder and DME antenna on general aviation aircraft. I chose 6 different ground planes to analyze - squares of 120, 240 and 700 mm (4.72, 9.45 and 27.56 in) and circles with diameters of 120, 240 and 700 mm. I used the valildated method of moments software called FEKO that is an recognized tool used by antenna engineers worldwide. I computed the currents induced on the ground plane, the antenna input reflection coefficient(referenced to 50 ohms) and the radiation patterns over a frequency band of 960-1160 MHz. The resulting data are shown in the attached pdf file.
Page 1 summarizes the geometry. Page 2 shows the computed ground plane currents for the 700 mm ground planes at 1060 MHz. Note that the RF currents are concentrated within about 100 mm of the center of the ground planes and that they are basically independent of ground plane shape. Page 3 shows the RF currents for the 240 mm ground planes. Note that in the center of the ground planes the currents are about the same but the square ground plane has hot and cold spots around the periphery of the ground plane while the circular ground plane currents are essentially rotationally symmetric. Page 4 shows the RF currents for the 120 mm ground planes. For these cases the RF currents are a significant magnitude out near the edges of the ground plane and there is variation around the periphery of the ground planes for both geometries. Examination of the RF currents indicates that the larger ground planes should provide good isolation of structure mounted under the ground planes while the smaller ground planes would be expected to provide less isolation and the geometry of the structure behind the ground planes will likely significantly modify the antenna performance. Also note the lack of a significant difference in the induced RF currents as a function of ground plane geometry. Only the size of the ground plane is important.
Pages 5-7 show the computed radiation pattern realized gain (over a range of +10 to -30 dBi) as a function of angle. We choose the antenna geometry to be such that the antennas are vertical and mounted on a horizontal ground plane. An angle of 0 deg is then directly overhead and 90 deg is along the horizon, while 180 deg is directly below the ground plane. Calculations were performed at both 1030 and 1090 MHz. A monopole on an infinite ground plane should have about 5 dBi of gain and page 5 shows the performance of the 700 mm ground planes. The peak gains are about 5 dBi, as expected, and the peak gain below the ground planes are about -5 dBi, indicating a radiated power level of about 1/10 of the peak level above the ground plane. Page 6 shows the patterns for the 240 mm ground plane geometries. The peak gain has dropped a small amount and the backlobe has come up about 5 dB, indicating less isolation is provided by the smaller ground plane. Page 7 shows the radiation patterns for the 120 mm ground planes. Note here that the pattern level below the ground plane is essentially the same as that above the ground plane, indicating that the ground plane is providing very little isolation. Also note that the radiation patterns for each of the ground plane sizes are essentially independent of ground plane shape (size, not shape controls the performance).
Finally, pages 8-10 show the voltage reflection coefficient linear magnitude as a function of frequency. Page 8 shows the performance for the 700 mm ground planes, page 9 shows the performance for the 240 mm ground planes and page 10 shows the performance for the 120 mm ground planes. Note that other than a slight shift in the location of the voltage reflection coefficient minimum as a function of frequency, the reflection coefficient performance is basically independent of either ground plane size or shape.
So, what conclusions can we draw from all this? I offer the following:
1) Getting the size of the ground plane off by small amount will NOT detune the antenna
2) A simple monopole antenna has inherent bandwidth to simultaneously cover both 1030 and 1090 MHz (indeed, it can cover the entire DME band of 960-1220 MHz)
3) A circular ground plane works just as well as a square ground plane - size not shape determines performance
4) For small ground planes, what is installed behind the ground plane (cabling, metallic structure, etc.) will have a significant effect upon antenna performance
5) Avionics equipment even for amateur-built aircraft ARE "engineered to the nth degree" (hence the TSO) and slapping an antenna on a ground plane and hoping it works is to be avoided. Some engineering has to occur to get it right. Unfortunately, this rarely happens. Even metallic, production aircraft (like my Mooney) have issues with antenna installations as I have evidence - both measured and simulated - that reveals significant antenna performance variations as a function of azimuth angle and frequency owing to a poorly sited antenna
6) An octagonal ground plane would be expected to have performance similar to square, rectangular or circular ground planes of similar area
7) Because the performance of a ground plane is dependent upon the size of the ground plane relative to a wavelength, a 240 mm ground plane will perform better than a 120 mm ground plane - the idea that there is some sort of "ground plane resonance" phenomenon is flat out wrong
In closing, I note that this is not the first time that a subcontractor (Trig) has provided incorrect technical information to a prime contractor (Dynon) that ultimately gets supplied to the customer - nor will it be the last time, unfortunately. The prime contractor needs to actively perform a "trust but verify" operation on technical data that will be transferred from the subcontractor to the customer. Not doing so violates the due diligence responsibility of the prime contractor.
If you are interested in discussing any of the analysis described above, I would be happy to stop by (I live between Auburn and Enumclaw) and discuss it.
dpbarnes
New Member
Gary,
I also appreciate you taking the time to give us good information regarding ground planes. I have always thought of antennas and ground planes as a little bit of black art. I wonder if you could comment on using 1 inch wide copper strips as a ground plane for the vhf com antennas. I used four at 90 degree apart 1/4 wavelenght long.
I also appreciate you taking the time to give us good information regarding ground planes. I have always thought of antennas and ground planes as a little bit of black art. I wonder if you could comment on using 1 inch wide copper strips as a ground plane for the vhf com antennas. I used four at 90 degree apart 1/4 wavelenght long.
I constructed the ground plane Dynon recomends which is basically a square with slightly rounded corners. If memory serves it is 120 mm on each side. It is in an all composite Glasair and works extremely well. Given the small size it is not difficult to mount. Mine is internal (inside the belly pan).
For the ground plane installed on the inside skin of a carbon fiber honeycomb the problem is that the carbon fiber composite is several thousand times less conductive than aluminum. So what you have is a highly conductive ground plane covered with a lossy (carbon composite) layer. RF energy will be dissipated in the carbon composite (to determine how much one would have to model it). The comm/nav RF links we use in general aviation have lots of margin so some loss in antenna performance may not be noticeable. We had Lancair build some turbine powered aircraft and to maximize antenna performance we applied a conductive paint to the outside surface and then mounted the antennas to the conductive surface. This concept worked out just fine. The 787 embeds a metallic surface under the in the skin and attaches the antennas to the metallic surface and this also works well.
As far as the radial ground screen goes, what happens is that the radials provide a poor amount of isolation compared to a metallic ground plane. The result is that the outer conductor of the coaxial cable will have RF currents on it and the interior of the aircraft will be "lit up." The interior structure will then scatter the electromagnetic energy in an undesirable manner. Again the comm /nav links we use are very robust and you may not notice much degradation in performance but a solid ground plane will be better (one would have to analyze the entire structure to quantify the performance loss).
As far as the radial ground screen goes, what happens is that the radials provide a poor amount of isolation compared to a metallic ground plane. The result is that the outer conductor of the coaxial cable will have RF currents on it and the interior of the aircraft will be "lit up." The interior structure will then scatter the electromagnetic energy in an undesirable manner. Again the comm /nav links we use are very robust and you may not notice much degradation in performance but a solid ground plane will be better (one would have to analyze the entire structure to quantify the performance loss).
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Gary,
Thanks for your input on this topic. We've started a conversation with Trig and we'll see what their opinion and data is, and we'll update our install manual if it is correct to do so.
As a note, a transponder does not need to be TSO'd in an experimental aircraft. In fact, nothing needs to be. The only thing that regulates EAB aircraft are the FARs and the FAR does not require a TSO. It's an amazing corner of aviation with a very limited set of regulations that doesn't look anything like your experience with transport category aircraft nor certified GA aircraft. We do our very best at Dynon to supply this industry with good advice, and we're happy for feedback if we can make things better.
Thanks for your input on this topic. We've started a conversation with Trig and we'll see what their opinion and data is, and we'll update our install manual if it is correct to do so.
As a note, a transponder does not need to be TSO'd in an experimental aircraft. In fact, nothing needs to be. The only thing that regulates EAB aircraft are the FARs and the FAR does not require a TSO. It's an amazing corner of aviation with a very limited set of regulations that doesn't look anything like your experience with transport category aircraft nor certified GA aircraft. We do our very best at Dynon to supply this industry with good advice, and we're happy for feedback if we can make things better.
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Gary,
In general, Trig agrees with you. The ground plane needs to be at least 120mm to be effective, but beyond that, small improvements can be had with larger ground planes. As a rule of thumb, they consider the improvement to have diminished to the point of no practical improvement at 700mm. So the language that appears to indicate that sizes between 120mm and 700mm are worse than 120mm can be improved, and we will update this in our manuals.
Trig does however disagree that a perfect circle with the antenna in the middle has no ill effects. They stated that this does create lobes in the vertical plane, assuming the antenna is perfectly centered and the circle is a specific size. This is the reason for their suggestion to create non-circular ground planes.
As I stated before, our goal is to create installation guidance that creates a good ground plane in non-metal aircraft installation, in an installation environment where the antenna installation cannot be evaluated or tested like it would be on production aircraft. This is just the reality of our market, and I don't think any of our customers want installation guidance that says an experimental aircraft cannot fly until a professional antenna engineer has evaluated their install.
Some of the reason for this language is to try and convey the fact that there is no specific size ground plane that is "correct". Many times a builder wants us to give them exact dimensions for a ground plane, with the expectation that there is some magic size and shape that can be achieved for optimal performance. That's the expectation after you are three years into your project and all the parts you have been working with up to that point have tolerances in the .005" ranges. It's sometimes hard to convince someone that it just doesn't matter and they can do whatever works right in their airframe within some minimal constraints.
A non-circular ground plane where the shortest dimension is 120 mm or greater does a good job of creating a good ground plane in all installations, and avoids the one case where performance could be harmed. I understand that you may disagree on the non-circular suggestion, but it appears that there is agreement that a non-circular ground plane does work, so I do not believe we are creating a problem by making this suggestion, we are just limiting the installer's options slightly.
Thanks for your input and helping us make our products better.
In general, Trig agrees with you. The ground plane needs to be at least 120mm to be effective, but beyond that, small improvements can be had with larger ground planes. As a rule of thumb, they consider the improvement to have diminished to the point of no practical improvement at 700mm. So the language that appears to indicate that sizes between 120mm and 700mm are worse than 120mm can be improved, and we will update this in our manuals.
Trig does however disagree that a perfect circle with the antenna in the middle has no ill effects. They stated that this does create lobes in the vertical plane, assuming the antenna is perfectly centered and the circle is a specific size. This is the reason for their suggestion to create non-circular ground planes.
As I stated before, our goal is to create installation guidance that creates a good ground plane in non-metal aircraft installation, in an installation environment where the antenna installation cannot be evaluated or tested like it would be on production aircraft. This is just the reality of our market, and I don't think any of our customers want installation guidance that says an experimental aircraft cannot fly until a professional antenna engineer has evaluated their install.
Some of the reason for this language is to try and convey the fact that there is no specific size ground plane that is "correct". Many times a builder wants us to give them exact dimensions for a ground plane, with the expectation that there is some magic size and shape that can be achieved for optimal performance. That's the expectation after you are three years into your project and all the parts you have been working with up to that point have tolerances in the .005" ranges. It's sometimes hard to convince someone that it just doesn't matter and they can do whatever works right in their airframe within some minimal constraints.
A non-circular ground plane where the shortest dimension is 120 mm or greater does a good job of creating a good ground plane in all installations, and avoids the one case where performance could be harmed. I understand that you may disagree on the non-circular suggestion, but it appears that there is agreement that a non-circular ground plane does work, so I do not believe we are creating a problem by making this suggestion, we are just limiting the installer's options slightly.
Thanks for your input and helping us make our products better.