I just did the math and with all my planned upgrades, I will end up with 4 GPS WAAS antennas on top of my TB20, along with 2 other VHF antennas and an ELT one.
I am starting to worry this will change the aerodynamics and possible reduce the speed by a few more knots…
I saw there are some combo VHF/GPS antennas out there (e. g. COMANT CI 2580-200) that will help mitigate this issue, by reducing the total number of antennas installed.
Do you have any experience with these? Will the quality of the VHF and GPS signal degrade compared to a standard, single purpose, let’s say Garmin GA35 antenna? And more important, will the $800 difference in price make any sense over the presumed loss of speed?
I have one, shown here and there are others shown on a Cessna (?) further back up that thread.
They work fine. They contain a 1575MHz notch filter (implemented as a resonant cavity) which blocks the 11th and 13th harmonic of the VHF frequencies messing up the GPS signal (a common issue in aviation e.g. on 121.15 × 13).
They come in WAAS and non-WAAS versions, with a predictable price difference.
However I doubt a GA35 will have any impact whatsoever on speed. It is the long VHF ones which knock perhaps 0.3kt off. So the usefulness of these combined antennae is in saving roof space / making the required room for TAS/TCAS antennae.
Also on a TB20GT you have the composite / honeycomb roof which has defined (scalloped-out) antenna locations which is another issue; you can mount antennae elsewhere but it needs to be done just the right way and somebody might day day declare it “illegal”… When I went G to N, the FAA DAR dismantled the ceiling trim to check this, and the avionics guy from Air Touring was summoned to come down to Shoreham to generate some allegedly missing paperwork See here for some pics.
The GPS / VHF performance should not be an issue as the antenna has a notch filter that stops interference , I’m sure that Garmin would not have approved the unit in the GTN STC if it was not up to the job.
I have had to go for one of these antennas as I don’t have the real estate on the aircraft get both GPS antennas and VHF antennas far away from each other enough to meet the STC.
I was mostly concerned with the loss of speed due to multiple GA35s. On the other hand, they are very cheap now, about $300 vs $1,200 for the combo ones.
@Peter, when installing the TAS from Avidyne on your GT, what did they say about the location of the active traffic antenna? Is it better to place it in front of the aircraft, or at the back, behind the ‘curve’ of the roof?