I thought I'd bore you, fellow B7ers,with the universal Audi/VW dilema of bad FM reception (affects many many contemporary other makes too -particularly German). Might serve as a reference for someone, even though most of us have well passed the need now. Although this has been covered extensively through the years, almost everyone is wrong about it, particularly the antenna adaptor makers and most fitters too. Although I'm not a pro mechanic I am a professional broadcast technician and have been involved in sound systems and recording likewise; and I'm OCD about ICE too and I like things to work as the should. This is literally years of idle discovery too, so I can refer back to it myself. 
Firstly, FM and Dab are bad and poor in the UK anyway - too crowded and too little bandwidth; dying too and internet radio is a 1000 times better (Radioplayer and BBC Sounds on Android Auto just about covers everything for me and mobile data reception is no longer an issue, almost everywhere) but if you really want it here goes. A little history: FM was always so so since whenever shark fins and window antennas did away with telescopic ones; if you've got good FM it's the area not the system - my Symphony II was pretty dire also. Audi's first diversity system up to about mid late 2006 was called a switch diversity system; later cars have what is known as a phase diversity system. The switch type is much harder to sort. It consists of two cables a blue HF FM/AM cable and a return IF (intermediate frequency; or ZF, Zwischenfrequenz in Audi's schematics) and power for the amp. Most double Fakras to single or ISO adaptors, with power, supply the power correctly but the ZF cable is useless, since it is a return. This ZF cable receives information, from the HU (Concert, Symphony and MK I RNS-E, all pre II+ phase) to tell the amp of the need to switch to another antenna. The amp has up to 3/4 antennas contained on the glass (rear for saloon, side windows Avant) and it chooses from these. These amps do fail, more on the saloons than the Avants but they also receive the signal from your remote fob, for the doors. Very poor range on your fob can be an indication of a failed amp (water ingress usually the cause, or cracked rubber pulling components off the board in the case of the saloon mostly).
I say 3/4 antennas, but there are only two Fakras, connected to the main amp: FM2 and 3 (at least on my Avant, in the picture) - there are further amps involved in the switching at other points. Theoretically, since I have not tried, you could by-pass the main amp, take FM 2 and 3 and directly connect them to the blue and curry IF/ZF cables but you would still need to provide amplification for the antennas, since most adaptors only provide power; it would still not be as good as true diversity because half of that function is missing; it would be a weak noisy signal mixed with a stronger one. You can then use the only known proper adaptor, I know of, for phase diversity systems: https://tinyurl.com/28jzsx7b.
Alternatively and preferably you could just use a new antenna altogether. If you want an OEM invisible installation and you have an existing shark fin one, then something like this, which I'm going to fit: https://www.ebay.co.uk/itm/265923946169?chn=ps&_ul=GB. Most every adaptor you can buy, from Connect2, all of 'em is wrong and will not work properly; and their info is almost always wrong too. You get some signal and power but it is poor. Many HUs (Chinese ones particularly) don't supply the full 12 volts either, from the amp cable; better to take that from somewhere else. A multi meter can tell you, if this is the case.
Audi in common with most other makes, never envisaged or wanted us to fit aftermarket systems. Most all HUs, including premium branded ones have just one FM antena plug; there were some makes that had true diversity duel attachments but they were for phase and they are no longer made, since these systems have long disappeared. I'll talk about the later phase diversity antenna in the next part, if anyone's interested.
Firstly, FM and Dab are bad and poor in the UK anyway - too crowded and too little bandwidth; dying too and internet radio is a 1000 times better (Radioplayer and BBC Sounds on Android Auto just about covers everything for me and mobile data reception is no longer an issue, almost everywhere) but if you really want it here goes. A little history: FM was always so so since whenever shark fins and window antennas did away with telescopic ones; if you've got good FM it's the area not the system - my Symphony II was pretty dire also. Audi's first diversity system up to about mid late 2006 was called a switch diversity system; later cars have what is known as a phase diversity system. The switch type is much harder to sort. It consists of two cables a blue HF FM/AM cable and a return IF (intermediate frequency; or ZF, Zwischenfrequenz in Audi's schematics) and power for the amp. Most double Fakras to single or ISO adaptors, with power, supply the power correctly but the ZF cable is useless, since it is a return. This ZF cable receives information, from the HU (Concert, Symphony and MK I RNS-E, all pre II+ phase) to tell the amp of the need to switch to another antenna. The amp has up to 3/4 antennas contained on the glass (rear for saloon, side windows Avant) and it chooses from these. These amps do fail, more on the saloons than the Avants but they also receive the signal from your remote fob, for the doors. Very poor range on your fob can be an indication of a failed amp (water ingress usually the cause, or cracked rubber pulling components off the board in the case of the saloon mostly).
I say 3/4 antennas, but there are only two Fakras, connected to the main amp: FM2 and 3 (at least on my Avant, in the picture) - there are further amps involved in the switching at other points. Theoretically, since I have not tried, you could by-pass the main amp, take FM 2 and 3 and directly connect them to the blue and curry IF/ZF cables but you would still need to provide amplification for the antennas, since most adaptors only provide power; it would still not be as good as true diversity because half of that function is missing; it would be a weak noisy signal mixed with a stronger one. You can then use the only known proper adaptor, I know of, for phase diversity systems: https://tinyurl.com/28jzsx7b.
Alternatively and preferably you could just use a new antenna altogether. If you want an OEM invisible installation and you have an existing shark fin one, then something like this, which I'm going to fit: https://www.ebay.co.uk/itm/265923946169?chn=ps&_ul=GB. Most every adaptor you can buy, from Connect2, all of 'em is wrong and will not work properly; and their info is almost always wrong too. You get some signal and power but it is poor. Many HUs (Chinese ones particularly) don't supply the full 12 volts either, from the amp cable; better to take that from somewhere else. A multi meter can tell you, if this is the case.
Audi in common with most other makes, never envisaged or wanted us to fit aftermarket systems. Most all HUs, including premium branded ones have just one FM antena plug; there were some makes that had true diversity duel attachments but they were for phase and they are no longer made, since these systems have long disappeared. I'll talk about the later phase diversity antenna in the next part, if anyone's interested.
