Transmit Antenna on M20J for a GNX375

[zerotact]

Hello,

    I have gotten Conflicting stories Via Quotes on a GNX375 (ADSB transponder/GPS)  install on an M20J,  it stems around if I will have to swap output Antennas on the install. I have been told both ways. Can someone give me an educated explanation?

[PT20J]

The STC installation manual should have a list of approved antennas. 

[Aerodon]

Depends what you are starting with.  If you have no GPS you will need a GPS antenna and RG400 cable to go with it.  If you have an older GPS you may need a new GPS antenna and maybe cable.  It's not hard to find a list of approved antennas and then in your logbook to see what you have already.

You probably  already have a reasonable transponder antenna, but more than likely have old cable.  

In the overall scheme of things, I would budget for new cables and new antennas. 

 

Aerodon

 

 

 

 

 

 

 

 

[zerotact]

The GPS Antenna   and both the cables was never in question,  It was all about conflicting stories of re using the transponder antenna. After Digging deeper, the Main thing is...  An M20J with original Avionics, Has BNC Connections at the antennae,  therefore its not recommended to rely on a crusty BNC connector from 1981 to transmit critical Transponder data through.

[PT20J]

11 hours ago, zerotact said:

The GPS Antenna   and both the cables was never in question,  It was all about conflicting stories of re using the transponder antenna. After Digging deeper, the Main thing is...  An M20J with original Avionics, Has BNC Connections at the antennae,  therefore its not recommended to rely on a crusty BNC connector from 1981 to transmit critical Transponder data through.

If you believe that, you might also want to replace all your nav/com antennas as they also have BNC connectors. But if your BNC connector is truly "crusty" with visible corrosion, then that's a different matter. 

[N201MKTurbo]

I installed one in my Cessna. I replaced the transponder antenna with a RAMI antenna. It was reasonable. 
 

The problem isn’t old and crusty, it is the old antenna doesn’t have the bandwidth to handle both the transponder and ADS-B signals.

https://www.rami.com/product/av-74_transponder/

 

[Yetti]

21 minutes ago, N201MKTurbo said:

I installed one in my Cessna. I replaced the transponder antenna with a RAMI antenna. It was reasonable. 
 

The problem isn’t old and crusty, it is the old antenna doesn’t have the bandwidth to handle both the transponder and ADS-B signals.

https://www.rami.com/product/av-74_transponder/

 

Not even how antennas work.  Nor is it how Transponder and ADS-B protocols work. 

[Aerodon]

12 hours ago, zerotact said:

The GPS Antenna   and both the cables was never in question,  It was all about conflicting stories of re using the transponder antenna. After Digging deeper, the Main thing is...  An M20J with original Avionics, Has BNC Connections at the antennae,  therefore its not recommended to rely on a crusty BNC connector from 1981 to transmit critical Transponder data through.

Now you are overthinking this.  Just rely on the shop to make sure you have a good antenna and cable and move on.  I would not rely on a 'whisker pole' antenna, and the issue you need to deal with is whether you have a 2 screw or 4 screw mount already.  Replace with the same if yours is bad, but the 4 hole antennas are quite expensive.

 

Aerodon

[PT20J]

The install manual for my GTX 345 states that the antenna is considered part of existing equipment and that any antenna approved toTSO-C66() or C74() is acceptable. We reused the original King antenna from the KT 76A and it works fine.

[PT20J]

I agree with Don. The avionics shop probably wants to start with all new parts because it reduces any chance of issues with existing components. The antennas are not very expensive and once you select a shop it's best not to tell them how to run their business. But, the OP asked if replacing the antenna was necessary and the answer is no, if it is working correctly.

[EricJ]

12 hours ago, zerotact said:

The GPS Antenna   and both the cables was never in question,  It was all about conflicting stories of re using the transponder antenna. After Digging deeper, the Main thing is...  An M20J with original Avionics, Has BNC Connections at the antennae,  therefore its not recommended to rely on a crusty BNC connector from 1981 to transmit critical Transponder data through.

If it isn't visibly corroded or damaged there shouldn't be an issue.   If the shop has a Vector Nework Analyzer (VNA, they're not expensive these days), the antenna can be electrically tested to see if there are any issues.  Many shops don't have those or don't want to test such things since they can sell you a new antenna otherwise.

[N201MKTurbo]

1 hour ago, Yetti said:

Not even how antennas work.  Nor is it how Transponder and ADS-B protocols work. 

Huh?

[Yetti]

49 minutes ago, N201MKTurbo said:

Huh?

Antennas are all about being the correct length for what frequency they are to be used to transmit or receive on.   The little wire with a ball antennas are a  1/4 wave of the frequency for ADSB and Mode S  1090 and 978Mhz.  1090 and 978 are pretty close so the same quarter wave can be tuned for both frequencies. Mode S and ADSB share 1090Mhz Antennas have nothing to do with data bandwidth as the Mode S and ADSB  transmit in serial protocol their data sentences. Data Bandwidth issues would be inside the transceiver unit.

[N201MKTurbo]

34 minutes ago, Yetti said:

Antennas are all about being the correct length for what frequency they are to be used to transmit or receive on.   The little wire with a ball antennas are a  1/4 wave of the frequency for ADSB and Mode S  1090 and 978Mhz.  1090 and 978 are pretty close so the same quarter wave can be tuned for both frequencies. Mode S and ADSB share 1090Mhz Antennas have nothing to do with data bandwidth as the Mode S and ADSB  transmit in serial protocol their data sentences. Data Bandwidth issues would be inside the transceiver unit.

All antennas have a bandwidth. It is usually defined by VSWR it is usually the difference between lowest frequency for 2:1 VSWR and the highest frequency for 2:1 VSWR. The VSWR end Points vary by requirements and may be lower than 2:1, 1.5:1 is often used. 

The older antennas were optimized for 1030 and 1090 MHz operation. The ADS-B compatible antennas are optimized to also have adequate performance at 978 MHz.
 

http://wireless.ictp.it/handbook/C4.pdf

 

[EricJ]

2 hours ago, N201MKTurbo said:

All antennas have a bandwidth. It is usually defined by VSWR it is usually the difference between lowest frequency for 2:1 VSWR and the highest frequency for 2:1 VSWR. The VSWR end Points vary by requirements and may be lower than 2:1, 1.5:1 is often used. 

The older antennas were optimized for 1030 and 1090 MHz operation. The ADS-B compatible antennas are optimized to also have adequate performance at 978 MHz.
 

http://wireless.ictp.it/handbook/C4.pdf

 

Antenna frequency responses drop off fairly slowly, i.e., they're not very good filters generally, and 978 MHz or 1090 MHz for ADS-B wind up being well within the bandwidth of an antenna made for a transponder, or even a DME, so those antennas all work fine for ADS-B.    My in-panel ADS-B-in is hooked up to the former DME antenna, which is otherwise unused.   Really the main issue is just that the connector and antenna still function decently, which they should barring corrosion or damage.

Edit:   I should add that if there's something that transmits on an antenna, like a transponder or DME, there generally should not be any other independent receivers connected to it.    This is to prevent the entire transmit power of whatever is transmitting from going into the receiver, which at minimum will saturate the amplifiers and knock it out for a bit, and more likely would damage the receive amplifiers.    Any transmitting device generally gets its own antenna for this reason.

 

[N201MKTurbo]

Everybody on here is always trying to get that last 1% of performance out of every aspect of their plane, and now they are espousing using a sub optimal antenna when a shiny new one is $150 bucks and two minutes to change it out.I don’t get it….

You are going to spend that much for the new coax and connectors.

[MikeOH]

17 minutes ago, N201MKTurbo said:

Everybody on here is always trying to get that last 1% of performance out of every aspect of their plane, and now they are espousing using a sub optimal antenna when a shiny new one is $150 bucks and two minutes to change it out.I don’t get it….

You are going to spend that much for the new coax and connectors.

Keep up with posts like that and you're going to lose your CB card!

[Yetti]

15 minutes ago, N201MKTurbo said:

Everybody on here is always trying to get that last 1% of performance out of every aspect of their plane, and now they are espousing using a sub optimal antenna when a shiny new one is $150 bucks and two minutes to change it out.I don’t get it….

You are going to spend that much for the new coax and connectors.

Bandwidth describes the range of frequencies over which the antenna can properly radiate or receive

Which is what I was describing, yes I used to tune 5/8 wave antennas using a SWR meter.   I was pretty good at it. The frequencies targeted here are Mode S/ADSB 1090 and ADSB UAT 978 are much closer together than the Antenna  you are suggesting that does DME which is down in the 960Mhz up to 1215MHZ.   So your suggested new antenna is not really that better.    There is actually a formula to calculate the length of the Antenna and in the case of the 1/4 wave just take .25 of the calculated length.   The best thing to do would be to hook up an SWR meter and tune the antenna to the lowest SWR. 

For us mortals everyone should strive for a good antenna install is make sure the ground plane is good.   So the ground of the antenna needs to have really good contact with the aluminum skin of the airplane that forms the ground plane.  For cloth planes an aluminum disc should be used.  This would include removing paint before installing the antenna or cleaning corrosion after it's been on for awhile. For a time there was a dielectric compound that was used to insure a good ground plane contact with the antenna.    The Motorola antenna hole saw had a lip to clean the metal around the outside of the hole.   

[skykrawler]

The transponder and ADS-B frequencies are fixed, so the required 'bandwith' is not that wide.  If you were talking about VHF comm or NAV then talking about bandwidth makes more sense and typically those antenna perform best around the middle of the frequency range so you get decent performance at each end of the range.

My airplane is still using what I think is the original King transponder antenna.   The UAT has the 1/4 wave stick antenna, but I've been considering replacing that with a transponder blade antenna.  There is no reason to replace an antenna if its working properly.

 

[201Mooniac]

I guess I’m a bit confused why people are referring to the carrier frequency when discussing the bandwidth.  The modulation around the carrier defines the bandwidth so I could easily define a system that used the same carrier frequency (1090MHz, 978MHz, etc.) but used very different bandwidths based on the amount of information I need to transmit, the modulation scheme, the encoding scheme, etc.  I’m not saying that old antennas aren’t good for new transponder plus ADS-B systems but discussing the center frequency instead of the bandwidth confused me.  What am I missing?

[MikeOH]

1 hour ago, 201Mooniac said:

What am I missing?

Not a thing, IMHO as an EE.

The reality here is that these antennae we are discussing are NOT high-Q; i.e., they are 'broadband' enough and easily handle the bandwidth required by the modulation schemes used for both the transponder and ADS-B.  As @skykrawler said, 'no reason to replace an antenna if it's working properly'

I doubt most antennas 'fail'.  Rather, the coax, connectors, and connection to the ground plane deteriorate over time.

[EricJ]

1 hour ago, 201Mooniac said:

I guess I’m a bit confused why people are referring to the carrier frequency when discussing the bandwidth.  The modulation around the carrier defines the bandwidth so I could easily define a system that used the same carrier frequency (1090MHz, 978MHz, etc.) but used very different bandwidths based on the amount of information I need to transmit, the modulation scheme, the encoding scheme, etc.  I’m not saying that old antennas aren’t good for new transponder plus ADS-B systems but discussing the center frequency instead of the bandwidth confused me.  What am I missing?

Since the discussion is about antenna suitability for an application, it's the BW of the antenna that matters, not the signal, and whether the carrier frequency of the signals of interest fall within the usable BW of the antenna.   We know that all of the relevant signals are relatively narrowband compared to the carrier frequency and antenna passband, so the signal bandwidths aren't really relevant other than they're narrow enough to fit in their assigned channels and not interfere with each other and therefore fit in the antenna bandwidth.

e.g., transponders listen on 1030 MHz and transmit on 1090 MHz.   ADS-B uses 978 MHz (only about 8% off the center of the 1030-1090 pair, and only about 5% from the 1030 MHz interrogating signal).   ADS-B also uses 1090 MHz, which is right in the band of interest for the transponder antenna.   The antenna would have to have a fairly (unexpectedly) high Q to attenuate the 978MHz signal very much, so it should work pretty well for listening to either ADS-B frequency.    The bandwidths of any of those signals don't really matter, just their carrier frequencies.

DME uses 960-1215 MHz, which covers all of the ADS-B and transponder frequencies, so a DME antenna should be suitable for listening to any of it.   Many suitable antennas with TSOs are shown as being interchangable as either transponder or DME antennas.   Again, the signal BW doesn't really matter, just that the antenna response BW is wide enough to cover the desired applications.

[skykrawler]

antenna length = wavelength = speed of light / frequency.

So a UAT antenna:   299792000 / 978000000 = .3065 meters or 306.5 mm or 12.068 inches.

quarter wave is then 3.017 inches.   That's why those transponder antennas are so short.

Apparently quarter wave antennas perform well omnidirectionally and are easy to make.