DD unit with diversity aerial amp

[petergw60]

Hi Guys,

Has anyone managed to get a decent radio signal when fitting a DD unit in the older cars with the diversity amplifier. As there are 3 FM aerials and an AM aerial all fed into the diversity unit, without the IF feed from the radio unit - there just isn't the same amount of signal available to the radio in the DD. I am using a separate amplifier for the TMC and this is working fine but radio reception is variable to poor. (It is no better with TMC aerial disconnected).

Apart from this - the unit performs well (a few problems with the nav software seizing up, usually sorted eventually!).

Peter

[HarryM1BYT]

Mine is a Mk 2, so I don't have this issue. Not expecting it to function very well - When I asked those who do have the diversity type, I was told by someone that it works fine with the DD.

The extra connection between radio and diversity unit, simply provides feedback to the unit as to which antenna offers the best reception at that instant, for the actual station you are tuned in to. Rather than an IF signal it will more likely be a voltage relating to received signal strength.

None of which is really that much help to you. What might be of some help, is that if you used the supplied DIN to ISO adaptor to adapt the car antenna plug to fit the DD, then I found that was the cause of my poor reception. Snip the plug off the lead coming down from the amp, fit a proper plug instead and chuck the adaptor in the bin.

[petergw60]

Hi Harry,
According to the description of operation, the extra connection is an IF output from the radio - this tells the amplifier which aerial is giving the best results for the station being listened to.
Already cut off the old plug and replaced as I thought that may have been the problem - no change!
Did consider making separate amplifiers for each aerial and then combining them - not sure if that will work though, need to think more on it.

Peter

[HarryM1BYT]

Hi Harry,
According to the description of operation, the extra connection is an IF output from the radio - this tells the amplifier which aerial is giving the best results for the station being listened to.
Already cut off the old plug and replaced as I thought that may have been the problem - no change!
Did consider making separate amplifiers for each aerial and then combining them - not sure if that will work though, need to think more on it.

Peter

I don't think it will help at all to combine the antennas, better might be to just use one via a normal amp.

I think they have over simplified the diversity system operation. Feeding what is normally called the IF back to the amp would not have the desired effect. What they were likely refering to is feeding the AGC voltage back to the anttena diversity system. The AGC (Automatic Gain Control) voltage is used to adjust the gain of the IF, so it can boost the weaker stations and decrease the IF gain on the stronger ones. The voltage can be then also used to tell the antenna system which antenna is providing the best signal at any instant, by rapidly switching between them and comparing the AGC voltage.

I'm really not sure what to suggest as a solution. Your new DD will definately have an AGC system inside it, and it could be tapped into to recreate the missing signal - but it would be quite an involved task even for a specialist. Not the sort of problem that your average ICE installer would even begin to understand.

FYFI - The signal strength meters as often fitted to specialist communications receivers and the magic eye display for the tuning of some of the old domestic valve radios - utilise the AGC voltage as a measure of the signal strength.

One thing you might try, is to firmly short together the end of the wire which Rover describe as the 'IF'. That might force the discrimination system to maybe latch firmly onto one antenna system, rather than constantly scanning if the 'IF' cable is constantly floating about in voltage.