I go back to my original theory that the principle is the same as the FM radio use in a mobile phone which uses the earphones cable as an aerial/antenna to receive the FM signal [actually, they all use the shielding on the cable i think, not the actual audio wire inside]. There seems to me to be no good reason why it would be any different with on-board traffic receivers in Garmins, other than they are using the shielding of the power cable. Alternatively, they may be using one power wire somehow with some kind of RF filter to separate the FM radio signal, maybe via dedicated choke coils in the 'TA' designated cables. I really don't know nor pretend to fully understand how any of this works at a deep technical level, so much of this is supposition but it does make sense to me.
So, if it uses the wire's shield [screen] as an aerial that would be the simplest because it's already floating free.
If it's using one of the power wires then FM signals need to be separated via a filter of some kind [my car uses the rear window's inbuilt heater to receive it's radio signals via an RF filter. Another less comparative example is ADSL signals over plain copper telephone wires with a filter at our end to separate the ADSL from the voice transmissions. Either way it's using a wire for two purposes and then separating them].
Regardless of the method used, there is no 'dedicated' antenna/aerial as such if my hypothesis is correct. It's the secondary function of an existing part of the cable. Also the aerial needs to be within certain lengths to properly receive the target frequency. This is why Garmin state that the lead needs to be uncoiled. Try bundling up the aerial wire on your FM radio, you'll lose or markedly degrade the reception. If the shielding is used, provided the cable is within the required unwound length then any shielded cable should work and then as reported in the other thread it is as Garmin states, i.e. older cables can do the job.
Certainly there could also be an internal function in the device itself which when it is sensed that the external power is missing no battery power is fed to the traffic receiver. That power sensor's already present in all battery powered Garmin units so it wouldn't be difficult for them to use it to isolate the traffic module from the battery power as Angelo's mused.
Further testing is probably needed as Angelo said, and if anyone is interested it should be in the other thread [Only registered and activated users can see links. ], or even a new one, but a bit OT here now.
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