Micro-Air or other Tiny Transponder on a battery

[Windknot]

I'm looking at Champs - 85hp and up - some of them have C-XX-12 engines that will accept the accessories for a generator and starter but many others do not, but the HP is there and the plane is affordable. I like having the generator on board to power a Transceiver and a Transpoder though...

Will I be using the transponder all the time? NO, but it sure is nice to have one for when I need it.

My question is (I know nothing of batteries, amp draws etc) can a Micro-Air or ANY small transponder be installed permanently under the dash or in the bulkhead in front of the wing etc and powered ONLY by a battery if I were to buy a Champ without an electrical system?

Suggestions?

WK.

[Daidalos]

This is simply math. Determine the amp hours your battery is capable of with a full charge, Then look at the power draw from the transponder and radio. That should give you a rough idea. Keep in mind that the comm wil specify a different current draw for receive versus transmit.

You need to know how busy the airspace is and how much you will be key down (talking). Typically you can assume 90/10, 90% RX and 10% TX. But TX could be higher, The physical size doesn't matter it's the transmitter power for the radio and transponder that matter.

The same is true for transponders. Each time it blinks it is responding to a radar hit. Sometime it's rhythmic and steady, and other times it may seem crazy. That is when it is being interrogated by multiple sites. Modern systems present controllers with a composite view of multiple radar sites.

If you every get a chance make sure to take an Operation Sunshine or Operation Rain-check tour (IFR Pilots), see http://faasafety.gov/. You get to see the inside of a control facility. If you fly VFR a TRACON is a better choice to visit than a CENTER.

[Jim Stewart]

My Garmin GTX320A consumes 12 watts, which would be 1 amp at 12 volts. I don't think you'll find a cheaper or lower powered transponder. Since it draws 1 ampere, just pick a 12 volt battery with an ampere/hour rating equal to the number of hours you want it to work plus a little more to be on the safe side. Keep in mind that the transponder installation itself should be considered permanent because if you go moving it in or out, you are supposed to have the calibration rechecked. You'll also need an altitude encoder.

[Windknot]

A blind encoder should be OK for a light sport application shouldn't it?

[Daidalos]

My Garmin GTX320A consumes 12 watts, which would be 1 amp at 12 volts. I don't think you'll find a cheaper or lower powered transponder. Since it draws 1 ampere, just pick a 12 volt battery with an ampere/hour rating equal to the number of hours you want it to work plus a little more to be on the safe side. Keep in mind that the transponder installation itself should be considered permanent because if you go moving it in or out, you are supposed to have the calibration rechecked. You'll also need an altitude encoder.

I wonder if that 12 watts is in standby or receive mode only? The specs call for 200 watts nominal power on transmit. That is 16.66 amps at 12 volts. The good news is that is only for short bursts. It is not 16.66 amp hours. It really depends upon the duty cycle of the transmitter (radar hits).

Your estimate of 1 amp at 12 volts is equal to 12 watts or a duty cycle of only 6% on a 200 watt transponder. See https://buy.garmin.com/shop/store/asset ... 0_spec.pdf

As I said it's about the math.

[Daidalos]

Duplicate post - deleted

[Windknot]

My Garmin GTX320A consumes 12 watts, which would be 1 amp at 12 volts. I don't think you'll find a cheaper or lower powered transponder. Since it draws 1 ampere, just pick a 12 volt battery with an ampere/hour rating equal to the number of hours you want it to work plus a little more to be on the safe side. Keep in mind that the transponder installation itself should be considered permanent because if you go moving it in or out, you are supposed to have the calibration rechecked. You'll also need an altitude encoder.

I wonder if that 12 watts is in standby or receive mode only? The specs call for 200 watts nominal power on transmit. That is 16.66 amps at 12 volts. The good news is that is only for short bursts. It is not 16.66 amp hours. It really depends upon the duty cycle of the transmitter (radar hits).

Your estimate of 1 amp at 12 volts is equal to 12 watts or a duty cycle of only 6% on a 200 watt transponder.

Like I said.....HUH?

So the question as to whether you can install a small avionics package - transciever and transponder, permanently with a battery power source is "YES"....what size battery....I'll discuss it with the guy that does the install!!!

[Jim Stewart]

My Garmin GTX320A consumes 12 watts, which would be 1 amp at 12 volts. I don't think you'll find a cheaper or lower powered transponder. Since it draws 1 ampere, just pick a 12 volt battery with an ampere/hour rating equal to the number of hours you want it to work plus a little more to be on the safe side. Keep in mind that the transponder installation itself should be considered permanent because if you go moving it in or out, you are supposed to have the calibration rechecked. You'll also need an altitude encoder.

I wonder if that 12 watts is in standby or receive mode only? The specs call for 200 watts nominal power on transmit. That is 16.66 amps at 12 volts. The good news is that is only for short bursts. It is not 16.66 amp hours. It really depends upon the duty cycle of the transmitter (radar hits).

Your estimate of 1 amp at 12 volts is equal to 12 watts or a duty cycle of only 6% on a 200 watt transponder. See https://buy.garmin.com/shop/store/asset ... 0_spec.pdf

As I said it's about the math.

My "estimate" wasn't an estimate. It was off the spec sheet you referenced. I see no contradiction between a specification of 12 watts max power input and a 200 watt pulse output. You just need a couple of capacitors and a low duty cycle. As I said, one ampere at 12 volts should be fine.

A little bit of googling shows that the pulse transmission from the transponder takes about 20 microseconds. Assuming 10 interrogations a second, that would be 200 microseconds of transmitting per second or a duty cycle of 0.02%. As you said, all in the math.

[Windknot]

Assuming someone else may have the same question some day.......could one of you explain what all that means.........are we talking about 12V car battery or are we talking about a light weight lantern sized lithium battery smaller than a half-gallon of milk?

Maybe a link to an example battery?

[nbjeeptj]

LiPo batteries like the ones used in the RC hobby world may be a good choice fast charge ( can in some cases be charged in half an hour or less ). They are designed to be charged and disharged a lot of times ( a lead acid battery is not as good at this unless they are deep cycle and heavy) and they are really light for the amp hour rating they have. They come in voltages of 3.7 volts per cell so a 3 cell battery would be 11.1 volts however that is a discharged voltage rating a fully charged one is closer to the 12.6 volts equal to that of a lead acid battery. I have used the 11.1v ones to power 12v equipment with no problem, however some people prefer to go with a higher voltage battery and use a voltage regulator to get the voltage they need. The latter may in this case with the high dollar equipment being powered may be the better option. I use a lipo in my airplane to power my Noise Cancellation head set due to the fact that my plane does not have a built in power plug for them. There are some concerns about fire with a lipo battery but with many years using them in my RC helicopter flying habit I have found that the fire potental seems to only exist when charging. There is a small learning curve to get over with the lipo batterys since you have to use ballance charging equipment but it is really no big deal if you stay with one brand battery and charger. I have always used the thunder power batteries and charger and have been very happy with them but I am sure there are others. Here is a link to thunder power http://www.thunderpowerrc.com/

[Jim Stewart]

Assuming someone else may have the same question some day.......could one of you explain what all that means.........are we talking about 12V car battery or are we talking about a light weight lantern sized lithium battery smaller than a half-gallon of milk?

Maybe a link to an example battery?

This battery:

http://search.digikey.com/us/en/product ... -ND/653331

Claims to deliver 1.2 amperes for 3 hours. Weighs 4 pounds. This:

http://search.digikey.com/us/en/product ... ND/1245111

Looks like a reasonable charger.

Lithium batteries would be much lighter, but finding the right voltage and current and an acceptable charger is harder.

[Daidalos]

My "estimate" wasn't an estimate. It was off the spec sheet you referenced. I see no contradiction between a specification of 12 watts max power input and a 200 watt pulse output. You just need a couple of capacitors and a low duty cycle. As I said, one ampere at 12 volts should be fine.

A little bit of googling shows that the pulse transmission from the transponder takes about 20 microseconds. Assuming 10 interrogations a second, that would be 200 microseconds of transmitting per second or a duty cycle of 0.02%. As you said, all in the math.

Thank you, I did google it and found the same 20 microseconds. I get it with capacitors storing energy to provide for the peaks, I work with electronics also.. The question is the number of interrogations. Do you disagree that in some areas the interrogations will be much higher?

[Jim Stewart]

My "estimate" wasn't an estimate. It was off the spec sheet you referenced. I see no contradiction between a specification of 12 watts max power input and a 200 watt pulse output. You just need a couple of capacitors and a low duty cycle. As I said, one ampere at 12 volts should be fine.

A little bit of googling shows that the pulse transmission from the transponder takes about 20 microseconds. Assuming 10 interrogations a second, that would be 200 microseconds of transmitting per second or a duty cycle of 0.02%. As you said, all in the math.

Thank you, I did google it and found the same 20 microseconds. I get it with capacitors storing energy to provide for the peaks, I work with electronics also.. The question is the number of interrogations. Do you disagree that in some areas the interrogations will be much higher?

I don't think it matters. Garmin specs a power consumption of 12 watts. If the average power consumption were higher, I believe that they would have spec'ed a higher number.

If we really want to get to the bottom of this, I can call Jim Weir. He teaches avionics and has designed at least one transponder.

[drseti]

A little bit of googling shows that the pulse transmission from the transponder takes about 20 microseconds. Assuming 10 interrogations a second, that would be 200 microseconds of transmitting per second or a duty cycle of 0.02%.

Not quite, Jim. That figure would be true if one interrogation produced a single-pulse reply. But, to respond to an interrogation, a transponder needs to send its discrete code (four octal words, at three bits per word), plus a Mode C burst (ten bits or so to transmit altitude), plus checksum data for error detection and correction bits, plus clock timing pulses. Let's say a reply burst is about 40 bits long. Each bit is indeed 20 microseconds long, but transmitted at about a 1% duty cycle, if memory serves. So, that's 20 microseconds for a bit, and 1.98 milliseconds of resting time before the next bit is transmitted. Which means, even if continually interrogated, a continuous barrage of bits at 1% duty cycle, and 200 watts peak RF power, will average 2 watts of RF output. That's about as much as the typical hand-held comm produces when transmitting, and is worst case.

Of course, it takes quite a bit more than 2 watts of DC to produce that 2 watts average of RF. Let's say the transmitter's output circuitry is 50% efficient. So, it needs 4 watts average of DC available, at maximum output duty cycle.

But that's just for the microwave transmitter circuitry. There's also a microwave receiver to be powered up inside the transponder, and digital encoders and decoders, plus various overhead circuits (voltage regulators, ident circuitry, etc.) Conservatively, let's assume all this additional circuitry consumes about as much power as the transmitter's output stage. We're up to 8 watts average now (about one-eighth of a standard residential light bulb).

Clearly, a twelve volt system, with a 1 amp current capacity (that is, 12 watts of DC) will be about right, even using my worst-case assumptions.

[Jim Stewart]

Here's my source,

"Figure 1 shows the original 64 code format, and today's 4096 code format under it. Each reply consisted of a framing pulse, some combination of the six possible data pulses, and another framing pulse. It all took place in 20.75 microseconds."

http://www.airsport-corp.com/modec.htm

What I'm reading out of this is that the whole interrogation reply is 20.75 microseconds. Of course I could be misinterpreting the whole thing or he may be wrong, but I get that a single reply is a pulse-coded burst 20.75 microseconds, and being modulated, the burst itself probably isn't 200 watts continuous.

In any case, the real issue is whether the 12 watt spec from Garmin is believable. I think it is.