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Discussion Starter · #1 · (Edited)
Hi All,
As many other owners I was annoyed by the weak signal of FM radio signal in the 4C

After investigating this issue, the problem is clear, the FM amplifier mounted in the front of the 4C is not supplied with 12Volt by Alfa Romeo.
It seems they have completely forgotten this.
In the front (removing the bonnet) you will find the Fakra cable connected to the amplifier (black block), which is connected to ground (bolted onto the chassis), connected to the adhesive antenna inside the front bumper

When removing the footrest from the passenger floor, the same Fakra connector is found, connected to the antenna cable which leads to the back of the radio

The following conversion steps I made;
  • Disconnect the Fakra connector and the Fakra Radion antenna under the footrest
  • An RF Bias Tee board is to be connected in between these two Fakra connectors
  • The Bias Tee board has SMA screw connectors, so 2 extra cables are needed
  • Bias Tee direction DC+RF towards the amplifier in front of the 4C
  • Fakra A Male to SMA male, RG174 coax cable 15cm
  • Fakra A Female to SMA male, RG174 coax cable 15cm
  • The SMA male connectors are connected tothe Bias Tee board
  • The Bias Tee board is supplied with switched +12V en Ground, these can be found close by
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The result of this Bias Tee is that the amplifier in front of the 4C gets +12V
so now it is actually amplifying the FM reception

When i started my Joying radio FM app, I was greatly surprised as many FM channels were now strong
Next test was when I disconnected the FM antenna glued behind the bumper, it made no difference in the FM channels found on my radio...
So I decided to create a new antenna, a small brass rod with an exact length of 70cm (quarter lambda). I mounted this in the middle on top of the plastic housing of the radiator.
The top of the antenna bent so it would not touch the bonnet
Fixed in place with a few tiewraps
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The antenna bottom is soldered to a wire with an AMP connector, connected to the original amplifier (black wire) of the 4C
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This new simple antenna doubled the amount of FM stations easily, so decided to keep this antenna instead of the original 4C bumper version.

Costs involved :
Fakra A male to SMA male 15cm RG174; $5
Fakra A male to SMA male 15cm RG174; $5
Bias Tee; $5


As maybe some of you noticed a second Fakra coax connector (Fakra F brown color) is also present under the bonnet and under the passenger footrest (Fakra G grey color).
I start to belive this was intended for DAB+ radio in the 4C
So next I will do is make a connection of this antenna to the glued bumper antenna, connect this to my Joying DAB+ USB dongle (will post this when I have the hardware and results)
 

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this is an incredible post because because the 4c radio reception is bad. Can you post pics showing Bias Tee SMA screw connectors with the extra cables? Where are you getting the power from?
 

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Discussion Starter · #3 ·
Hi Steven,
I made a few pictures for you to better understand how I connected the Bias Tee
Under the footrest, there is one position on the fuse box with is not used, this you can use for switched +12V
The bolt next to it you can use as Ground
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I needed extra connections for +12V, so I made a small PCB board with connectors on top
Needed this for connection of Side view cams and GPS tracking device;)
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Next you see the Bias Tee connected to the 2 Fakra A connectors
Make sure you follow the arrow on the Bias Tee board for the +12V to go towards to amplifier under the 4C bonnet in the front
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Hope this makes it all clear for you
 

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If the 12v power goes towards the amplifier, how does the FM signal get back to the radio?
 

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Discussion Starter · #5 ·
Hi,
The BiasTee board is connected to 12V
This 12V is together with RF going to the Amplifier (through the coax cable already present) in the front of the 4C and supplying the amplifier with 12V

The amplifier has only a Coax cable connection and is mounted itself to Ground with a bolt
In general all amplifiers need power to work
So my huge increase in FM signal strength is proving the Amplifier to work via this Bias Tee board

Unfortunately AlfaRomeo never have seemed to finish this and left the FM for what is was..
 

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Excellent sleuthing! This is a great find and I certainly plan to do this. Already sourcing the parts.

Now the total lack of change in FM signal strength no matter if I turn Joying ant power on or off makes sense.

The lack of decent FM has been really annoying - bad enough not having RDS.

Thanks @Henkjan !
 

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Appreciated this post, was fascinating to read as an RF engineer. Hadn't really paid much attention to the radio, because much rather listen to the engine. Bottom line, it seems AR really focused on the machine-driver experience, leaving radio system design a distant afterthought!

It's kind of amazing they thought they needed to put an RF pre-amp on their FM radio - and then, they didn't even give the amp a power supply! Of course, to give the amp power they should have installed a bias-T too. Which the OP did for them.

That bumper-mounted antenna seems to be pretty poorly performing to begin with, on top of it being mounted so low, so maybe they thought the amp would make up for it - but that usually doesn't work very well. A pre-amp's more useful for overcoming losses in a long cable or for extremely weak signals.

In addition, the unpowered pre-amp acts like an overly expensive, large attenuator, further burying radio signals! That can only work at all because radio receivers are so sensitive, and FM signals so strong (around cities).

The bumper antenna likely is just a simple horizontal dipole, which would mean it has even worse reception in the directions off the two ends of the bumper, and also needs a balun to match coax line. The OP's vertical monopole is a very good idea since it gives omnidirectional performance, and matches more easily (with care). It also appears that the front aluminum subframe makes a decent ground counterpoise for the monopole.

I'm curious to know, did the OP see the bumper antenna to describe it, and what was the make & model number of the pre-amp? Thanks!
 

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Discussion Starter · #11 ·
Hi thanks for your reply, you are correct!
When I have the 4C opened again at the front the part numbers can be found of AR
I think they are also in the technical manual which is available..
 

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That bumper-mounted antenna seems to be pretty poorly performing to begin with, on top of it being mounted so low, so maybe they thought the amp would make up for it - but that usually doesn't work very well. A pre-amp's more useful for overcoming losses in a long cable or for extremely weak signals.
When I removed the right side A pillar, there were 2 antennas behind it. Does anybody know what these are for, if they are not the radio antenna?
 

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Discussion Starter · #14 ·
Yes you are correct a White Fakra A
Which is used for FM and connected
Then there is a second coax with a Brown Fakra F wich is not attached

what i want to try is to connect that Brown Fakra, with a second amplifier and a Bias Tee to the dipole antenna glued to the bumper
This i like to test as DAB antenna and attach to my Joying DAB dongle
 

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When I removed the right side A pillar, there were 2 antennas behind it. Does anybody know what these are for, if they are not the radio antenna?
What, they DID put a vertical antenna (or two) in one of the 4C's A-pillars ??? (I assume you mean you just removed the trim from the A-pillar.)

Well, that's about the best place available on the 4C for an antenna, particularly if there is little to no metal in the windshield frame, so why they connected the FM radio to the front bumper antenna then and not to the A-pillar antenna is kind of beyond me.

Unless they had something "more important" to do with those cables. Or maybe, particularly if those cables are coax, and not just a simple wire, the cable serves the built-in microphone.

However, if those cables both are antennas, then they will load each other down and possibly even seriously interfere with each other.
 

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OK, now I've got to get into the soft-copy manuals, even though they are sketchy, unclear & a pain, and see if I can find any info on this. There is even a separate wiring manual, which is kind of like looking at multiple plates of spaghetti.
 

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But I love spaghetti.
Anyhow, looked through the MOPAR wiring manual and found just a couple of very minimal descriptions. They do indicate that the FM radio uses an antenna on the front fascia, together with a "block" (the amp, I guess). The vehicle body wiring depiction is only broadly indicative. For example, it doesn't show that the cable is a separate, individual (unbundled) coax, which is what @Henkjan found. The included schematic diagram does show a separate radio input line but doesn't show more than that (doesn't indicate whether it's coax or what type and doesn't specify the connector), and it also doesn't show the amp, that I can see.

However, the vehicle wiring drawing does indicate that the two cables running up the right-hand A-pillar are wiring (coaxs probably) for the microphones that are positioned over the rear-view mirror.

The horizontal metal bumper behind the bumper antenna with also have an effect on the antenna, probably worsening its performance. I sincerely hope they stuck the adhesive antenna onto the plastic bumper cover and not directly onto the metal bumper, unless that antenna is a special design.
 
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