dpansier
Well Known Member
Many of the ADS-B / UAT builders have published information that their device will work with any transponder antenna, I believe that may be the case but they are giving up some system performance with that broad statement.
The GDL-39 on the other hand has published antenna performance specifications for use with the device. Looking at it from an engineering point of view, I'm sure the GDL-39 design team was tasked with achieving a certain level of product performance and to insure that level, they had to have some expectation of what the customer may connect to the device. The following from the manual:
The GDL 39 requires a UHF antenna that meets the following specifications:
? Standard 50 ohm vertically polarized antenna with a VSWR < 1.7:1 at 978 MHz and < 1.5:1 at 1090 MHz.
? TSO-C66, TSO-C74, or TSO-C112 antennas that also meet the VSWR specification.
I'm aware that many are planning to use the stubby monopole transponder antenna with the new ADS-B / UAT devices on the market, with that in mind, I set out to gather antenna performance data when used on frequencies out of the design parameters.
Since the antenna was designed to operate on transponder frequencies, (1030 and 1090 MHZ) the data indicates it does well with the 1090 requirement but exceeds the limits on the 978 MHZ side. 2.6 with a requirement of <1.7.
The VSWR data points may be difficult to see on the graph and are as follows:
M1 2.623 @ 976.74 MHZ
M2 1.762 @ 1029.17 MHZ
M3 1.195 @ 1087.21 MHZ
In summary, use an antenna that is designed for the task along with good quality coax if you are expecting maximum system performance.
The GDL-39 on the other hand has published antenna performance specifications for use with the device. Looking at it from an engineering point of view, I'm sure the GDL-39 design team was tasked with achieving a certain level of product performance and to insure that level, they had to have some expectation of what the customer may connect to the device. The following from the manual:
The GDL 39 requires a UHF antenna that meets the following specifications:
? Standard 50 ohm vertically polarized antenna with a VSWR < 1.7:1 at 978 MHz and < 1.5:1 at 1090 MHz.
? TSO-C66, TSO-C74, or TSO-C112 antennas that also meet the VSWR specification.
I'm aware that many are planning to use the stubby monopole transponder antenna with the new ADS-B / UAT devices on the market, with that in mind, I set out to gather antenna performance data when used on frequencies out of the design parameters.
Since the antenna was designed to operate on transponder frequencies, (1030 and 1090 MHZ) the data indicates it does well with the 1090 requirement but exceeds the limits on the 978 MHZ side. 2.6 with a requirement of <1.7.
The VSWR data points may be difficult to see on the graph and are as follows:
M1 2.623 @ 976.74 MHZ
M2 1.762 @ 1029.17 MHZ
M3 1.195 @ 1087.21 MHZ
In summary, use an antenna that is designed for the task along with good quality coax if you are expecting maximum system performance.