Understanding the Next Generation of Satellite Radio FM Transmitters
Tags: 2, XM
With the FCC's crack down on FM modulators exceeding emission levels a new alternative needed to be created that satisfies two major criteria: 1) the FM signal can't "bleed" to other vehicles and 2) it needs to provide the best user experience possible. None of these factors are negociable.
Enter the next-generation of FM Transmitters for satellite radio. Both XM and SIRIUS have very similar approaches: localize the transmission as close as possible to the car's pre-existing antenna.
The next obvious approach is to hard-wire the satellite radio directly to the headunit, but that's asking a bit much from the consumer. That's not to say that running wires all over your car should be considered an easy task for most consumers, but this is a good compromise. The Big Box Retailers with installation services are going to be loving this by the way, because these extra FM wires are going to entice the consumer to go with a professional installation (which provides an upsell opportunity to go with the hard-wire approach).
But enough with this palaver, on with the modulators...
Here you'll see the most common locations of a vehicle's FM antenna. External locations include numbers 1-4 (front fender, rear fender, front roof-top and rear roof-top - these can be either the retractable or fixed "aerial" antennas, or those fancy "shark-fin" ones you see). Internal locations include numbers 5-7 (which are the inside-glass versions that look like a window defroster - #7 specifically shows how some SUVs/Mini-vans will be located in on the side-windows).
The separation between Internal and External are important, because this is where XM's and SIRIUS' approaches with their FM Coupler and FM Extender differ. See a whole lot more on this after the jump...
SIRIUS FM Extender
First let's look at SIRIUS' FM Extender. This was discovered as part of the new FCC certification for the SIRIUS Sportster4, and I assume this will be used across the board as the FM transmitter of choice for the rest of Sirius' car-based receivers.
The parts involved are simple. The main FM Extender Cable, as well as 2 suction cups and 3 self-adhesive cable guides.
The suction cups are meant to allow for temporary placement of the FM Extender wire so you can test how it sounds.
Then using the self-adhesive pasties, you can permanently attach the FM Extender to the inside of your glass.
Depending on the orientation of your car's antenna, you have several different configurations that give you the best performance for FM transmission.
In the above diagram you see a front and rear fender ("aerial") antenna, and the SIRIUS FM Extender is located vertically. Makes sense. Got a vertical antenna? Then mount it vertically. (Note: some states don't like you putting things on your windows, so you'll need to attach the FM Extender to the pillar closest to the antenna - shown in the first diagram).
In cases where you have a rear roof-top FM antenna, you should mount the FM Extender horizontally on the edge of the glass:
The same orientation applies for antennas that are either front roof-top, those fancy shark-fin style antennas, or inside-glass FM antennas (not pictured).
If you happen to have one of those SUVs or Mini-Vans with the in-glass FM antennas located on the side, your installation would look like this:
And that's it for SIRIUS' solution.
Now let's compare this to XM's FM Coupler (which I believe is being called "Sure Connect").
Again, the concept is similar - bring the FM transmitter and antenna closer to each other - but XM is directly attaching the two with the coupler.
To the right you'll see the hookup to the radio is the same, but that there's a "coupling module" in-line where the FM transmitter cable and the satellite radio antenna split off (previously the FM signal was transmitted through the antenna cable itself).
At the end of the FM transmitter cable (labeled "output cable" on the right) is the coupler-clip with two different configurations for internal or external antennas.
For external antenna, the coupling clip attaches directly to the base of the antenna. Whereas for the internal antenna the coupler is attached directly to the on-glass antenna.
Here you can see different configurations of the FM Coupler and the XM antenna locations.
Simply attach the coupling clip, and then use the boot to secure the clip to the antenna.
For the in-glass antenna installation, you first should adhere the contact bracket directly onto the car's in-glass antenna, then you attach the coupler to this bracket. It's important to note that you should make sure it's the car's antenna you're attaching to and not the window's rear defogger... because, afterall, that's not the antenna.
And that's the XM method.
Now let's argue over which is better. Obviously the answer is "neither" because, as with anything in engineering, it's all a compromise. SIRIUS' solution is easier to install because you don't need to go outside of the vehicle if you have an external antenna. That said, SIRIUS' installation is a bit bulkier for internal antennas when compared to XM's. XM's external boot loses points for the ugly-factor. That said, XM doesn't need to transmit nearly as much emissions as SIRIUS does with their solution because the coupler is directly attached to the antenna rather than just closer. I said it was all a compromise didn't I?