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Poor transponder performance - thinking of relocating the antenna

He might be equipped, but I wouldn't see him unless I'm getting the TIS-B re-broadcast of ATC radar data. He would be transmitting on 1070, mode S and I'm listening on 978 UAT. I only have single frequency ADS-B in.With no reception, I only see those with UAT 978 ADS-B out. The dual band receiver might be a good idea.

A friend of mine who works at a major jet avionics retrofitting outfit in Van Nuys tells me that a whole lot of jets are not fitting ADS-B because they don't want the downtime and lost revenue. When they wait til the end, they'll probably find out that the downtime will be a lot longer while everybody tries to get legal all at once and they are grounded until they can comply.

Yes, I commonly see traffic of all types, including jets on the ILS, when I have TIS-B coverage, which has been pretty much all the time. I've gotten used to it. That's why I was a bit surprised to physically see the aircraft, but not see him on my EFIS. It is easy to become dependent on this technology and complacent. My scan will now include frequent checks to see if I have contact with ADS-B ground stations and will still take what I see with a grain of salt. Just because it isn't displayed, doesn't mean it isn't there.

Ed Holyoke

A Gulfstream will need to have a mode S-ES transponder, so maybe they?re waiting for a scheduled transponder check before upgrading?
Have you seen other ADSB traffic on 1090 MHz (e.g., any jets)?
 
Q: Are you sure your SA-005 antenna is mounted correctly? The printed circuit from the coax connector must be horizontal, and the "bow-ties" at the end need to be vertical. e.g., the 6" x 7" PC board needs the 6" side vertical.
 
Q: Are you sure your SA-005 antenna is mounted correctly? The printed circuit from the coax connector must be horizontal, and the "bow-ties" at the end need to be vertical. e.g., the 6" x 7" PC board needs the 6" side vertical.
That's the way I did it, and the coax connector is facing forward. One advantage to a half-wave dipole antenna in the ADS-B world is that it receives signals equally from above and below. Mine is 5.4" tip to tip.
 
...a whole lot of jets are not fitting ADS-B because they don't want the downtime and lost revenue.
One would think that they'd be eager to show themselves to us "little guys", but then they mostly fly IFR and are used to ATC providing separation. I suppose this will all sort itself out in 2020...
 
Thread drift...

One would think that they'd be eager to show themselves to us "little guys", but then they mostly fly IFR and are used to ATC providing separation. I suppose this will all sort itself out in 2020...

Except in class A and B airspace, ATC does not promise separation between an ifr airplane and vfr ones, though they usually try.
 
Google Earth has a legend section on the left to switch on/off coverage areas based on height.
I just figured that out. Duh... On the map above, the darker areas are Radar Coverage and the light green are ADS-B coverage, both at 500' AGL. Even at 1.500' AGL where I was mostly flying, I was out of Radar coverage for a good portion of the way. Maybe that's been the problem all along - flying too low. As I posted earlier, I was a "TIS-B Client" for only 41% of the time. My buddy who was following me 1,000' higher got 61%. So the problem may not be my antenna at all, but rather my lack of understanding that I had to be in radar coverage in order to get TIS-B traffic from a ground station. I still don't understand the rationale for that, but it is what it is.
 
You gotta ping them (on ATC radar) before they will target your approximate area with a broadcast of TIS-B. They don't broadcast it all the time, everywhere - just where they detect somebody broadcasting ADS-B out. That's why all the folks with ADS-B in only (Stratus, etc.) don't always see traffic. They have to have somebody in their vicinity with ADS-B out to light it up.

Ed Holyoke

So the problem may not be my antenna at all, but rather my lack of understanding that I had to be in radar coverage in order to get TIS-B traffic from a ground station. I still don't understand the rationale for that, but it is what it is.
 
You gotta ping them (on ATC radar) before they will target your approximate area with a broadcast of TIS-B.
I'm still having trouble with that. Wasn't the whole idea behind this ADS-B system "free-flight" so that we no longer needed ATC radar for traffic separation? Maybe there's a technical reason for the radar requirement, but it sure seems like the FAA just won't let go of their radar-centric system. /rant

Anyway, my next flight will be AVL==>RUQ at a higher altitude. We'll see how my Client Percentage does at 7,500'.
 
but rather my lack of understanding that I had to be in radar coverage in order to get TIS-B traffic from a ground station. I still don't understand the rationale for that, but it is what it is.

I?m not sure that?s true. I thought you only had to have a ground station receive your adsb-out message, to get a reply, including TIS-B data. However, if it is true that you need to be in radar coverage to get TIS-B data, there is some logic: if they can?t see you, then they can?t see nearby traffic either, so there?s nothing to send. Remember the ground does not send up all traffic - just nearby traffic. And for you, just nearby non-ADSB radar traffic, since you told them you can see all adsb traffic directly.
 
...if they can?t see you, then they can?t see nearby traffic either...
That makes a lot of sense. That's probably the explanation for this wacky system. Also, like you said, in 2020 the ground stations will only be needed for weather.
 
That makes a lot of sense. That's probably the explanation for this wacky system. Also, like you said, in 2020 the ground stations will only be needed for weather.

I didn?t say that. In 2020 you must have ADSB-out in class A,B,C, and E above 10,000? (and inside class B mode C veil). But there will be tons of guys below 10,000? who avoid those (B,C) areas, and who won?t upgrade to ADSB-out. You won?t see them without radar and the ground stations.

And of course, the faa wants the ground stations so they can see you.
 
OK, I got my transponder/UAT recertified today. Signal strength was fine front to back, but lower (but acceptable) side to side. That's due to the figure eight radiation pattern of my half-wave dipole transponder antenna. The tech said I was broadcasting on 1090 MHz so I'm scratching my head over that. I'm guessing his equipment just wasn't set up to measure the UAT frequency. I had intended to do an A-B comparison with a standard spike antenna, but the darn thing didn't arrive in the mail until I got home. Maybe in another two years :D

The Performance Report for the trip had my client percentage up to 61% - probably due to the fact that a ground station was located right on my destination airport [KGYH].

I feel like I've wasted enough of everybody's time trying to figure all this out, so I'm just going to keep flying with what I've got. Thanks for all the education and suggestions.
 
OK, I got my transponder/UAT recertified today. Signal strength was fine front to back, but lower (but acceptable) side to side. That's due to the figure eight radiation pattern of my half-wave dipole transponder antenna. The tech said I was broadcasting on 1090 MHz so I'm scratching my head over that. I'm guessing his equipment just wasn't set up to measure the UAT frequency. I had intended to do an A-B comparison with a standard spike antenna, but the darn thing didn't arrive in the mail until I got home. Maybe in another two years :D

The Performance Report for the trip had my client percentage up to 61% - probably due to the fact that a ground station was located right on my destination airport [KGYH].

I feel like I've wasted enough of everybody's time trying to figure all this out, so I'm just going to keep flying with what I've got. Thanks for all the education and suggestions.
If your antenna is mounted properly, as you said it was earlier, left, right, front, back should all be the same.
The tech checks the transponder on 1090.
 
... left, right, front, back should all be the same.
I really didn't state it correctly. The figure eight radiation pattern has nulls off the ends of the antenna - up and down in my installation. The problem I have is that the antenna is printed on a double-sided pc board. The elements are 2" wide in the front-to-back direction and .020" side-to-side. The wideness increases the bandwidth (lowers the "Q"...but only in the front to back direction in my installation. It's entirely possible that in the side-to-side direction, my antenna is resonant at the wrong frequency. Without the proper test equipment to investigate further, I'm just going to live with what I have.
 
For the record, I measured some transponder antennas today. From the face of the ground plane to the tip of the ball was:

1090 MHz quarter wave spike antenna = 2.43" (4.86" for half wave)
978 UAT quarter wave spike antenna = 2.66" (5.32" for half-wave)
My Archer SA-005 half wave dipole = 5.4"

The common formula for length vs. frequency is f = 468/length, where
f is in MHz
length of a half-wave dipole is in feet

I'm not sure the 468 works at these frequencies. A factor of 440 seems to work better. Any antenna gurus out there?
 
I really didn't state it correctly. The figure eight radiation pattern has nulls off the ends of the antenna - up and down in my installation. The problem I have is that the antenna is printed on a double-sided pc board. The elements are 2" wide in the front-to-back direction and .020" side-to-side. The wideness increases the bandwidth (lowers the "Q"...but only in the front to back direction in my installation. It's entirely possible that in the side-to-side direction, my antenna is resonant at the wrong frequency. Without the proper test equipment to investigate further, I'm just going to live with what I have.

Having rolled over under my Tiger and broken a standard Narco blade transponder antenna :)

I found that the metal radiating element inside is actually a flat sheet and not a rod encased in plastic. It would be similar to the PCB version picture posted earlier, just a 1/4 wave not a 1/2 wave.
 
Thanks for that. I think those are called "blade" antennas and are supposed to be broadband. I wish I had the equipment to play around with these various antennas. I made up a ground plane for my UAT antenna today. I was going to have the guy as the avionics shop check it out, but I was too late. I was going to mount it (Spike down, of course) in the floor of my baggage compartment.

MrxBl2.jpg
 
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For the record, I measured some transponder antennas today. From the face of the ground plane to the tip of the ball was:

1090 MHz quarter wave spike antenna = 2.43" (4.86" for half wave)
978 UAT quarter wave spike antenna = 2.66" (5.32" for half-wave)
My Archer SA-005 half wave dipole = 5.4"

The common formula for length vs. frequency is f = 468/length, where
f is in MHz
length of a half-wave dipole is in feet

I'm not sure the 468 works at these frequencies. A factor of 440 seems to work better. Any antenna gurus out there?

The little ball at the end adds some capacitance, so the antenna will resonate at a bit shorter length than given by the formula for a given frequency.
 
Thanks, Bob. There's a lot of black magic involved here. If I only had an Anritsu antenna analyzer, but the $4,000 price is prohibitive.
 
I finally found some more detailed information on my Archer SA-005 transponder antenna. Source
The SA-005 Transponder antenna is designed for reception of vertically
polarized energy in the frequency range of 960 to 1215 MHz. The antenna is constructed of etched
copper clad glass epoxy circuit board material etched to form a vertical dipole. The voltage standing
wave ratio (VSWR) is less than 1.5:1 over the frequency range of 1030 to 1090 MHz. and is less than
2.0:1 over the range of 960 to 1215 MHz. There may be variations in VSWR due to installation
variations. These frequency bands cover the DME band of 960 to 1215 and the two transponder
operating frequencies of 1030 and 1090 Mhz. The antenna has been optimized for the 1030 to 1090
band and in this band the VSWR is normally less than 1.2:1.
Since Garmin specifies "VSWR < 1.7:1 at 978 MHz and < 1.5:1 at 1090 MHz.", my antenna is probably marginal at 978 MHz.

So I guess what I need for best performance is a blade-type DME/Transponder antenna like the $167 COMANT DME / TRANSPONDER CI -105-16 or maybe the $70 non-TSOd 978 MHZ UAT Antenna from DeltaPop Aviation.

Gee, Walt was right all along...:eek:
 
I finally found some more detailed information on my Archer SA-005 transponder antenna. SourceSince Garmin specifies "VSWR < 1.7:1 at 978 MHz and < 1.5:1 at 1090 MHz.", my antenna is probably marginal at 978 MHz.

So I guess what I need for best performance is a blade-type DME/Transponder antenna like the $167 COMANT DME / TRANSPONDER CI -105-16 or maybe the $70 non-TSOd 978 MHZ UAT Antenna from DeltaPop Aviation.

Gee, Walt was right all along...:eek:

I had similar issues with my GTX-327 and my new GDL-82 with my existing Rami AV-22 transponder antenna. It's range is 1030-1090 MHz. I was told by Rami that it can still work down to the 978 MHz UAT freq but could be marginal. "Marginal" was not what I was hoping for nor what I wanted when it came to my ADS-B signals.

I upgraded my antenna to the Rami AV-74 blade antenna which has a range of 960-1220 MHz and my transponder and ADS-B have worked flawlessly since then. It's not "marginal" anymore.

The AV-74 from Spruce is $127.95 and worth every penny. I know the Deltapop is less expensive but my COMM antenna is also a Rami AV-17 belly mount and they match and look great together on the belly. Sorry, I'm rather anal like that.:D

Shawn
 
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