As per previous posts, I am not aware that the two antennae are different, apart from different stickers and obviously different prices.
I’ve upgraded my old Garmin GNS530 (non WAAS) to a new Avidyne IFD540, that works with the new GA35 WAAS antenna.
I also want to update all the old GPS antennas to the new GA35 WAAS type.
Will my remaining GNS430 (non WAAS) work with new WAAS antenna, or do I really need an attenuator? What’s the worst that can happen if I connect it to the new antenna with no attenuator?
I don’t know which specific antennae were involved in the Garmin exercise but the WAAS / NON WAAS antennae from Comant (see link posted above) are so close together in specs that it must be a marketing / product differentiation exercise.
And when you have boxed yourself into a corner by sticking “WAAS” stickers on and charging a lot more, there can be a comeback on you under a warranty 
On the plus side, the more pricey antenna isn’t costing you any more because, ahem, it is the same as the other one
It wasn’t marketing that Garmin swaps out the antenna at its cost for the WAAS upgrade. The pre-WAAS units had the antenna replaced as part of the original $1500 upgrade that was promised to owners of the GNS530. But even then, there was a limitation in the original AFMS issued in Nov 2006 that the upgraded units or early new units were not approved for sole source of IFR navigation in the US. The reason was both software and antenna related. Then Garmin upgraded the software a year later and if you had a compliant antenna, the restriction in the AFMS was removed and the system was approved for sole source of IFR navigation in the US NAS. Garmin supplied a replacement antenna at their cost. So if it was marketing, it was a very expensive ploy. The WAAS satellites are twice as high in their orbits than GPS satellites and to reliably obtain the correction information and also use the new geostationary satellites as stationary GPS satellites, the new antenna was required to achieve the desired reliability and performance.
Well, as you probably guessed my suspicion is that the “WAAS antenna” is no more than a load of marketing bollox used to create product differentiation so you can make 1000 antennae and flog 500 at $700 and 500 at $1400, with just different stickers 
This is done in every area of human activity, undoubtedly starting with the world’s oldest profession
I was trying to not make that assumption and dig myself in on it, in case I was wrong, but thus far I have not seen anything suggesting I was wrong (or the long-disappeared “tomjnx” in the thread I linked to; he used to design RF chips for a living and I am still sort of in contact with him).
The GA35 has a 27dB gain, and the antennas normally used along with the non WAAS unit have a 16dB gain
I wonder what the “antennas normally used” were – probably not anything from Garmin. That is a massive difference in output. They were prob99 comparing an active antenna with a passive antenna; the latter is not used in GA and AFAIK never was; never was compatible with any GA certified avionics. Actually I have one on my plane, which does Iridium / Thuraya / L1 GPS / L2 GPS and is purely passive, and after a decade of sitting there it was finally wired up to the Golze ADL150, but that’s a special case.
Peter wrote:
“Our IFD systems do not detect the antenna type”
They cannot, since the antenna is just a piece of analog circuitry, comprising of an L-band antenna and a pre-amp which is powered by 5V sent up the antenna cable from the GPS. There is no means of interrogating the antenna, like one does in “plug and play” PC products (e.g. the earliest CRT monitors with that feature had an I2C data channel via which the PC could interrogate it re its capabilities).
@Peter, it seems to me that you just answered the OP question yourself!
So if let’s say I incorrectly leave the old cable when installing the GA35 WAAS antenna my new G5 or GNS530W could never detect and alert letting me know there’s something wrong with the installation?
My understanding from multiple sources is that the only difference in WAAS and non-WAAS antennas is the antenna gain. The most definitive information I received was from Garmin in response to an enquiry about installing a GA35 WAAS antenna and low loss cable but feeding my non-WAAS GNS530’s.
Their very speedy response was:
“The GA35 can be used with a non-WAAS GNS530, but you will need to install a -11db attenuator on the line between the antenna an the unit. The GA35 has a 27dB gain, and the antennas normally used along with the non WAAS unit have a 16dB gain. That is why you need and a -11dB attenuator.”
So going back to Peter’s original question – I can’t see either how a WAAS receiver can actually detect if a non-WAAS antenna is connected.
But in that case the RF signal to noise ratio at the receiver may be too low for some features to work properly and it is conceivable that some features may be suppressed if the signal integrity was inadequate.
Many thanks Steve for your input.
This requirement does remain a puzzle in the light of this analysis, which was posted by someone who is a real RF design specialist (he’s still “out there” but seems to have dropped off social media).
Our IFD systems do not detect the antenna type
They cannot, since the antenna is just a piece of analog circuitry, comprising of an L-band antenna and a pre-amp which is powered by 5V sent up the antenna cable from the GPS. There is no means of interrogating the antenna, like one does in “plug and play” PC products (e.g. the earliest CRT monitors with that feature had an I2C data channel via which the PC could interrogate it re its capabilities).
Presumably this one would not be W compatible
Here is the wording from our installation manual. It is a case of having to use an approved WAAS antenna if you want the Gamma 3 approval for LPV. You can use other antennas but you will not have an approved installation via our STC. Our IFD systems do not detect the antenna type. they merely use the signal and if it does not give the right signal then we will get integrity problems or not even pick up a sat signal at all. The GA56 is an example, it is not compatible with our systems. Nor is it compliant for approval.
The IFD5XX/4XX can be interfaced to all TSO-C190 antennas and the approved antennas listed in the table below. If connected to an approved WAAS Antenna, the IFD5XX/4XX is approved for TSO-C146d Gamma 3 operation.
The IFD5XX can be interfaced to non-WAAS antennas but the system will not be approved for any type of WAAS operations. In this case, the Antenna Type selection as described in Section 7.5.11 of the installation manual must be selected as “Non-WAAS”. This will result in the FMS functionality of the IFD5XX/4XX inhibiting selection of any WAAS (SBAS) approach in the database.
Approved GPS Antennas
Model Number Description Supplier
CI-428-200 GPS WAAS Antenna Cobham (Comant)
CI-2580-200 VHF/GPS WAAS Antenna Cobham (Comant)
CI-2728-410 VHF/GPS/XM Antenna Cobham (Comant)
GA-35 GPS/WAAS Antenna Garmin
GA-36 GPS/WAAS Antenna Garmin
GA-37 GPS/WAAS Antenna Garmin
GA-56A GPS/WAAS Antenna Garmin
GA-56W GPS/WAAS Antenna Garmin
GA-57 WAAS/XM Antenna Garmin
A33 (AT575-9UW) GPS/WAAS Antenna Garmin /AeroAntenna
A34 GPS/WAAS Antenna Garmin /AeroAntenna
AV-801 GPS/WAAS Antenna RAMI
AT575-93AVW-TNCF-000-RG-27-NM GPS/WAAS Antenna AeroAntenna Technology
