SR20 Transponder Problem Finally Fixed - As reported earlier in this forum there have been various reports of intermittent transponder problems on the SR20 aircraft. The usual scenario is that ATC will temporally loose the “ID code” signal and request that the pilot recycle the transponder. After one or two recycling attempts ATC starts receiving the signal again. All is well for a time until the same thing happens again. This can be very disconcerting and dangerous, especially in densely populated arrears under instrument conditions. After experiencing this outage on several occasions I had the transponder checked and it appeared to be operating properly. Cirrus felt the problem was associated with the so called “ground plane”. This is especially critical in a composite aircraft where there is limited metal surface to reflect a transponder signal. Cirrus sent a new ground plane to be installed at a nearby service center. When the Mechanic tried to find the original ground plane, which is located under the co-pilots flooring, he found nothing. It turns out that in my plane, which was one of the first 50 delivered, no ground plane was ever installed. The new ground plane has been in the plane for over 5 weeks now, on numerous flights, totaling over 25 hours, without experiencing any loss of signal. Many of these flights were in areas where the signal was lost before. My recommendation, especially for the early delivery SR20s, is to check the status of the ground plane. You may be in for a surprise.
OK, enough! Could somebody please explain to us non-engineers, Valiant drivers, et al. what the hell a “ground plane” is? Thanks,
Glenn
SR20 Transponder Problem Finally Fixed - As reported earlier in this forum there have been various reports of intermittent transponder problems on the SR20 aircraft. The usual scenario is that ATC will temporally loose the “ID code” signal and request that the pilot recycle the transponder. After one or two recycling attempts ATC starts receiving the signal again. All is well for a time until the same thing happens again. This can be very disconcerting and dangerous, especially in densely populated arrears under instrument conditions. After experiencing this outage on several occasions I had the transponder checked and it appeared to be operating properly. Cirrus felt the problem was associated with the so called “ground plane”. This is especially critical in a composite aircraft where there is limited metal surface to reflect a transponder signal. Cirrus sent a new ground plane to be installed at a nearby service center. When the Mechanic tried to find the original ground plane, which is located under the co-pilots flooring, he found nothing. It turns out that in my plane, which was one of the first 50 delivered, no ground plane was ever installed. The new ground plane has been in the plane for over 5 weeks now, on numerous flights, totaling over 25 hours, without experiencing any loss of signal. Many of these flights were in areas where the signal was lost before. My recommendation, especially for the early delivery SR20s, is to check the status of the ground plane. You may be in for a surprise.
My SR20 was delivered on 04-17, serial # 1140. I have had a total of five “not reading transponder, please recycle” calls from ATC. Four were on an IFR X-C this past week. All five have been from approach control, never from center. My A/C surely is fitted with the latest ground plane. I am starting to wonder if it could be a directional issue or perhaps the distance to the receiving station is a factor. It seems to me that the antenna location is questionable. The exhaust pipes are located directly parallel to the antenna. I’m wondering if they could be acting as a directional shield. Recycling does seem to restore the signal, however with Birmingham (BHM) approach, I had to recycle twice before it came back up. During the apparent failures, I still see the “R” light on the transponder. If I understand correctly,that indicates the transponder is receiving an ATC inquiry. Perhaps if I had simply waited a few minutes for my position to change, the signal may have been restored without recycling. Cirrus told me to clean the antenna which I have done twice but it doesn’t seem to help. I will contact them again on Monday. Aside from this and a few pieces of loose Velcro, I have no other squawks. (Hobbs = 37 hours) Quite a contrast from the C206 we took delivery on last August with a list of over twenty items . This plane is a pleasure to own and fly. Well worth the wait. I’m sure they will get the transponder issue resolved, I’m just not convinced the ground plane is the final solution.
OK, enough! Could somebody please explain to us non-engineers, Valiant drivers, et al. what the hell a “ground plane” is? Thanks,
Glenn
“Ground plane” is what the FAA does when there’s an AD on your airplane.
There’s a nice description of a ground-plane antenna at the whatis.techtarget.com site (http://WhatIs.techtarget.com/definition/0,289893,sid9_gci214329,00.htmlclick here). The bottom line is that to function efficiently as either a transmitter or receiver, the antenna has to be located about 1/4 of a wavelength from an electrically conducting surface. Aviation transponders receive signals at a frequency of 1030 MHz and transmit at a frequency of 1090 MHz (see http://www.avweb.com/articles/transpon.htmlhttp://www.avweb.com/articles/transpon.html), corresponding to wavelengths of 29.1 cm and 27.5 cm respectively. So the transponder antenna should be to 7.3 to 6.9 cm (2.9 to 2.7 inches) from the conducting surface. That’s easily accomplished for an all-metal airplane, which is electrically conducting all over. For a composite airplane like a Cirrus (or, for that matter, a wooden airplane), a dedicated ground plane must be installed in the airframe.
A crude but super-simple way to think about ground planes: Imagine a vertical transmitting antenna, like those used for AM broadcasting. If you put a large, shiny piece of metal under the antenna, you’ll see the reflection of the entire antenna in the metal. It will look like there are two antennas — the real antenna and its upside-down reflection.
If this is the case for visible light, it will also be the case for radio waves. (Recall that radio waves are the same phenomenon as visible light but at a much lower frequency and longer wavelength.) So the antenna atop the piece of metal acts like two antennas acting in unison. It’s as though you had a single antenna with twice the length of the original.
The efficiency of an antenna as either a transmitter or receiver is proportional to the square of the antenna’s length. So if you double the length, you’ve quadrupled the antenna efficiency. For transponders, this can make the difference between showing up or not showing up on the controller’s radar.
That’s the best explanation I can give as a humble theoretical physicist. Any electrical engineers on the list who can clarify further?
Cheers,
Roger
That’s the best explanation I can give as a humble theoretical physicist. Any electrical engineers on the list who can clarify further?
Roger,
OK, I am an electrical engineer, although not one with a deep radio background. Antenna behavior has always struck me as weird. For the record, most electrical engineers I know (myself included, certainly) live in absolute awe of theoretical physicists, who actually understand this stuff.
Your explanation of what the ground plane does caused a pretty good “AHA!” in my head. If the ground plane does indeed cause a change in the apparent length of the antenna, then I know (from experience, not as much from understanding) that things will be pretty bad without one. An antenna of the wrong length works very badly.
I had always thought of a ground plane a little differently; but both explanations may be correct. To radiate a signal, an antenna must be presented with an oscillating electrical voltage, or Radio Frequency, which oscillating relative to something - the ground plane. In a sense, it’s a lot like the explosive charge which drives a canon ball out of the barrel - one end of the barrel must be sealed and relatively unyielding for the whole thing to work efficiently.
A slightly different view is expressed quite eloquently by Clem Wehner, who is a ham radio enthusiast and (I think) a Kitfox builder, in his online article http://www.sportflight.com/kfb/groundplane.htm Do You Need an Antenna Ground Plane?. I don’t know the gentleman - I found this by searching the web - but I thank him for this useful nugget.
Anyway, now all the non-physicist/non-engineering flyers have at least a few ways of looking at the whole “ground-plane” issue. Hopefully one of them makes sense for you.
- Mike.
These are great explanations of ground planes, both theoretical here and applied a little further along this thread. Very educational for a poor rock-doc (geologist) - Many thanks. We have also had intermittent problems with the transponder on N255DD, and as other people have described, ATC always seems to be happy after we recycle the transponder. I hope the ground plane is the problem, and we can fix it just as easily.
My question is – a completely missing ground plane is a ‘permanent’ situation degrading the transponder signal. Why would the signal improve with recycling, and then not degrade again for weeks or months? Its not as if recycling inserts a temporary ground plane, right? I need more help from the big-brains out there.
Phil
OK, enough! Could somebody please explain to us non-engineers, Valiant drivers, et al. what the hell a “ground plane” is? Thanks,
Glenn“Ground plane” is what the FAA does when there’s an AD on your airplane.
There’s a nice description of a ground-plane antenna at the whatis.techtarget.com site (http://WhatIs.techtarget.com/definition/0,289893,sid9_gci214329,00.htmlclick here). The bottom line is that to function efficiently as either a transmitter or receiver, the antenna has to be located about 1/4 of a wavelength from an electrically conducting surface. Aviation transponders receive signals at a frequency of 1030 MHz and transmit at a frequency of 1090 MHz (see http://www.avweb.com/articles/transpon.htmlhttp://www.avweb.com/articles/transpon.html), corresponding to wavelengths of 29.1 cm and 27.5 cm respectively. So the transponder antenna should be to 7.3 to 6.9 cm (2.9 to 2.7 inches) from the conducting surface. That’s easily accomplished for an all-metal airplane, which is electrically conducting all over. For a composite airplane like a Cirrus (or, for that matter, a wooden airplane), a dedicated ground plane must be installed in the airframe.
A crude but super-simple way to think about ground planes: Imagine a vertical transmitting antenna, like those used for AM broadcasting. If you put a large, shiny piece of metal under the antenna, you’ll see the reflection of the entire antenna in the metal. It will look like there are two antennas — the real antenna and its upside-down reflection.
If this is the case for visible light, it will also be the case for radio waves. (Recall that radio waves are the same phenomenon as visible light but at a much lower frequency and longer wavelength.) So the antenna atop the piece of metal acts like two antennas acting in unison. It’s as though you had a single antenna with twice the length of the original.
The efficiency of an antenna as either a transmitter or receiver is proportional to the square of the antenna’s length. So if you double the length, you’ve quadrupled the antenna efficiency. For transponders, this can make the difference between showing up or not showing up on the controller’s radar.
That’s the best explanation I can give as a humble theoretical physicist. Any electrical engineers on the list who can clarify further?
Cheers,
Roger
OK, I am an electrical engineer, although not one with a deep radio background.
- Mike.
WOW! - Is it ok if I think of it as a piece of metal that reflects the transponder signal?
Mick-NC UNC’63 B.S. Accounting
(who sometimes pulls up Mike’s schematic on how he wired his SR20 to directly power his headsets for yuks)
what about the energy of 200W , 1 GHz periodically transmitted quite near to the pilot/copilot ?
Is the ground plane not also serving as a kind of shield upward ?
How would this energy affect us ?
Mick,
Well, actually, it would seem to be a corollary to the well known principal that debits are on the window side of the sheet. One cannot have an antenna without a ground plane, and one similarly cannot have a debit without a credit. It’s elementary, my friend!
Pete - OSU d B.S. Accounting
OK, I am an electrical engineer, although not one with a deep radio background.
- Mike.
WOW! - Is it ok if I think of it as a piece of metal that reflects the transponder signal?
Mick-NC UNC’63 B.S. Accounting
(who sometimes pulls up Mike’s schematic on how he wired his SR20 to directly power his headsets for yuks)
Mick,
Well, actually, it would seem to be a corollary to the well known principal that debits are on the window side of the sheet. One cannot have an antenna without a ground plane, and one similarly cannot have a debit without a credit. It’s elementary, my friend!
Pete - OSU d B.S. Accounting
Pete,
Does OSU translate into OKLAHOMA STATE UNIVERSITY and if it does I have to say “Go Cowboys”. If it translates into Ohio State University I have to say I’m sorry ;). Just fooling.
Bob #509 N212FH
OSU BS Fire Protection and Safety Engineering
One cannot have an antenna without a ground plane, and one similarly cannot have a debit without a credit. It’s elementary, my friend!
Well, my ex-wife would point out to you that she never needed a credit to balance a debit, but as far as antennas go, some need a groundplane, some don’t. Have a look at the NAV antenna that sticks out either side of the vertical stab on lots of planes, including the SR20 - this is a dipole antenna and does not need a ground plane - it’s also horizontally polarized, i.e. the antenna elements are oriented horizontally.
The COM antenna sticking out the top and bottom of the SR20 is what’s called a 1/4 wave whip (it could be called a unipole, but never is) - it is basically half the dipole antenna, and does need a ground plane to provide the equivalent of the missing half, by reflection from the half that is there.
Vertical antennas are usually done this way simply because it’s inconvenient to mount a dipole antenna vertically, but there are some vertical whip antennas that do not need a ground plane, e.g. the 5/8 wave whip antennas used for UHF CB installations. But the transponder antenna is a 1/4 wave whip and does need the groundplane. Its absence will degrade the signal significantly.
Bob #509,
My OSU translates into the Mighty (sometimes) Buckeyes of Ohio State. Sadly, I must agree that they have been “sorry” a bit too often since the days of Woody Hayes, an outstanding fellow I had the very good fortune to know who was vilified by an unbelievably superficial press.
But with cheer, ans as you and I both have written, we “OSUers” (and others, I’m sure) admittedly share one well known and well earned trait known by its universal acronym, “BS”!
Pete
Pete,
Does OSU translate into OKLAHOMA STATE UNIVERSITY and if it does I have to say “Go Cowboys”. If it translates into Ohio State University I have to say I’m sorry ;). Just fooling.
Bob #509 N212FH
OSU BS Fire Protection and Safety Engineering
Sorry too 
that’s because they’ve been so mightily humbled by the awesome Wolverines to the north for such a looooong time now. Just poking fun at ya, I remember when Woody ran up the score back in 1968, my freshman year at U-M, that’s MICHIGAN!
Bob #509,
My OSU translates into the Mighty (sometimes) Buckeyes of Ohio State. Sadly, I must agree that they have been “sorry” a bit too often since the days of Woody Hayes, an outstanding fellow I had the very good fortune to know who was vilified by an unbelievably superficial press.
But with cheer, ans as you and I both have written, we “OSUers” (and others, I’m sure) admittedly share one well known and well earned trait known by its universal acronym, “BS”!
what about the energy of 200W , 1 GHz periodically transmitted quite near to the pilot/copilot ?
Is the ground plane not also serving as a kind of shield upward ?
How would this energy affect us ?
It distorts the brain
It makes one insane;
To the power I yield
With no ground-plane-shield;
Maybe that’s why I
Have the urge to fly;
And ignore the pain
Of my pockets’ drain;
Enough of this talk!
Can you read my squawk?
- (Anon - I won’t admit to anything).
Here’s a theory about the relationship of the ground plane to the observed failures and the successful method for recovery:
- The problem is the ground plane, the ground plan is poor.
- The result is the the impedance of the antenna, as seen by the transponder is not 50 Ohms (or 75, or whatever the transpond is designed to drive).
- Due to the impedance mismatch, there is reflected energy. In transistor-based high power designs, it is common to have a thermal-limit circuit to shut down the output stage in case of too high a temperate. The reflected energy is dissipated partly in the output stage (instead of being radiated in the air).
- This would more likely show up in dense airspaces, where a lot of radars (center, approach, and perhaps a few TCAS I/II’s in airliners) tigger the transponder often, thus causing higher average power in the output stage.
- The combination of 3 and 4 trips the thermal limiter, shutting down the output stage.
- Recycling resets the thermal limit and hopefully in the meantime the output stage has cooled a little.
- Improving the ground plane makes the antenna system’s impedance closer to what it is supposed to be, results in more radiated energy and less reflected-and-absorbed energy, thus solving the problem.
One could test this theory by tacking a thermocouple to the output transistors of the transponder and watching the correlation between failure and temperature. And then, if you are a really competent RF engineer, measure the impedance of the antenna system (but this is tricky to do correctly). Or you could measure the SWR of the antenna system coupled to the transponder (easier).
what about the energy of 200W , 1 GHz periodically transmitted quite near to the pilot/copilot ?
This is something the Europeans are more concerned with. Intuition tells many people that such energy must be harmful. But I think intuition is wrong here. There have been a lot of people working in megawatt non ionizing fields for 50 years (think high power transmitters and linear accelerators) and there’s been no difference measured in their health.
The cell phone companies are in hot water over this, but so far no epidemiological study has any convincing evidence that non ionizing radiation, other than that that heats water (2.65 GHz, I believe, the freq microwave ovens use) causes damage to tissue. And the only known mechanism that causes damage at that frequency is by tissue heating.
Is the ground plane not also serving as a kind of shield upward ?
In a way, yes, because it helps direct the radiation downwards, but not so much that if 1GHz/200W pulsed were bad for that you should feel good.
How would this energy affect us ?
It doesn’t, on the other hand all that cigarette smoke that y’all inhale over there does cause a lot of health problems. Worry about that, nuclear war, global warming, doing something dumb close to the ground, but I wouldn’t sweat the transponder :-).
Here’s a theory about the relationship of the ground plane to the observed failures and the successful method for recovery:
The problem is the ground plane, the ground plan is poor.
The result is the the impedance of the antenna, as seen by the transponder is not 50 Ohms (or 75, or whatever the transpond is designed to drive).
Due to the impedance mismatch, there is reflected energy. In transistor-based high power designs, it is common to have a thermal-limit circuit to shut down the output stage in case of too high a temperate. The reflected energy is dissipated partly in the output stage (instead of being radiated in the air).
This would more likely show up in dense airspaces, where a lot of radars (center, approach, and perhaps a few TCAS I/II’s in airliners) tigger the transponder often, thus causing higher average power in the output stage.
The combination of 3 and 4 trips the thermal limiter, shutting down the output stage.
Recycling resets the thermal limit and hopefully in the meantime the output stage has cooled a little.
Improving the ground plane makes the antenna system’s impedance closer to what it is supposed to be, results in more radiated energy and less reflected-and-absorbed energy, thus solving the problem.
One could test this theory by tacking a thermocouple to the output transistors of the transponder and watching the correlation between failure and temperature. And then, if you are a really competent RF engineer, measure the impedance of the antenna system (but this is tricky to do correctly). Or you could measure the SWR of the antenna system coupled to the transponder (easier).
The explanation above makes sense and is the most comprehensive so far.
what about the energy of 200W , 1 GHz periodically transmitted quite near to the pilot/copilot ?
This is something the Europeans are more concerned with. Intuition tells many people that such energy must be harmful. But I think intuition is wrong here. There have been a lot of people working in megawatt non ionizing fields for 50 years (think high power transmitters and linear accelerators) and there’s been no difference measured in their health.
I can’t quite believe this, though I have no figures.
There was a few month ago an issue with severe health affection of soldiers having served for years in radar stations of the german army.
The cell phone companies are in hot water over this, but so far no epidemiological study has any convincing evidence that non ionizing radiation, other than that that heats water (2.65 GHz, I believe, the freq microwave ovens use) causes damage to tissue. And the only known mechanism that causes damage at that frequency is by tissue heating.
Is the ground plane not also serving as a kind of shield upward ?
In a way, yes, because it helps direct the radiation downwards, but not so much that if 1GHz/200W pulsed were bad for that you should feel good.
How would this energy affect us ?
It doesn’t, on the other hand all that cigarette smoke that y’all inhale over there does cause a lot of health problems. Worry about that, nuclear war, global warming, doing something dumb close to the ground, but I wouldn’t sweat the transponder :-).
I don’t smoke, so my contribution to global warming is little ;-))
Thank you Robert,
Regards,
Wilfried
Here’s a theory about the relationship of the ground plane to the observed failures and the successful method for recovery…
Robert,
At last - a plausible theory. Thanks.
- Mike.
This is something the Europeans are more concerned with. Intuition tells many people that such energy must be harmful. But I think intuition is wrong here. There have been a lot of people working in megawatt non ionizing fields for 50 years (think high power transmitters and linear accelerators) and there’s been no difference measured in their health.
The cell phone companies are in hot water over this, but so far no epidemiological study has any convincing evidence that non ionizing radiation, other than that that heats water (2.65 GHz, I believe, the freq microwave ovens use) causes damage to tissue. And the only known mechanism that causes damage at that frequency is by tissue heating.
… are microwave ovens working in the states differently than in the rest of the world ? Except they are sometimes used for drying pets…
((-:
One word…Awesome. Great explantation. Thanks
Bob 509
My question is – a completely missing ground plane is a ‘permanent’ situation degrading the transponder signal. Why would the signal improve with recycling, and then not degrade again for weeks or months? Its not as if recycling inserts a temporary ground plane, right? I need more help from the big-brains out there.
Phil
Phil,
It’s a really good question, and one which has been nagging at the back of my mind, too. Unlike Stuart, who had no ground plane at all, I have been told that my ground plane “looks like it’s in good shape”. Of course, I haven’t seen it with my own two peepers… but, either way, it really doesn’t make sense that recycling would help.
I still don’t know the answer…
- Mike.