This post will hopefully clear up some confusion that exists about a Sirius acessory known as the FM Direct Adapter, (Also known as a Wired FM Modulator Relay).
Despite being called a Modulator Relay, the device doesn't modulate anything. This accessory takes the FM signal produced by the FM transmitter that is built inside your Sirius receiver and connects it DIRECTLY to the antenna input of your car FM radio. At the same time, it disconnects your car's FM/AM antenna. The adapter is nothing more than a simple switch.
It works like this:
When your Sirius radio is OFF, the FM Direct adapter switch is set so your car antenna is connected directly to the antenna input of your car radio. In this position you can listen to local AM and FM stations as you always have.
When your Sirius radio is ON, 5 volts is sent down the cable to the Direct FM adapter. When this happens, the switch changes positions. Your car antenna is disconnected
from your radio and the FM-OUT from the modulator built into your Sirius receiver is connected directly to the antenna input of your car radio.
The adapter provides two big advantages.
- First: When you disconnect your car antenna from your radio, signals from local radio stations are either eliminated or greatly reduced. This prevents local FM stations from interfering with the FM signal put out by the transmitter built into your Sirius receiver.
- Second: FM radio signals travel much better through wire than they do through the air. Connecting the transmitter built inside your Sirius unit directly to your FM radio antenna plug by wire will deliver a far
more powerful signal to your FM radio. This will make sure the signal is loud and free from hiss.
The image below shows pictorially how the FM direct adapter connects between your Sirius Receiver and your car FM radio.
The PDF version of the FM Direct Adapter installation manual