External Antenna/ RDS-TMC Traffic Receiver

Jul 17, 2010
Does anybody know if it is possible to connect the External Antenna to the RDS-TMC Traffic Receiver USB together?
RDS-TMC reception has been something of a challenge since its introduction in the UK, its fair to say reception is now considerably better than it used to be, but in some areas it is still far from reliable. If your vehicle is fitted with heat reflective glass otherwise known as Athermic windscreen this can effectively block the FM signal making reception even more difficult even in areas with good signal coverage.

The best way around these issues is to connect the RDS-TMC receivers aerial ?Shoe String? to the car aerial, this provides a much better signal level and overcomes the Athermic windscreen problem as the cars aerial is outside of the cockpit. Another ?spin off? from doing this is the ability to hide all the wiring and getting rid of the dashboard spaghetti with cables strewn everywhere.

Some vehicles feed +12 Volts up the aerial wire to power an amplifier, if you connect the RDS-TMC receiver directly to such an installation you will fry the unit and probably damage the PND the receiver is connected to. By fitting a small ceramic capacitor in series between the cars aerial cable and the RDS-TMC unit you block the 12V from the RDS device but still let the important FM signal through.

This short write up details how to make an adaptor cable to achieve the above, the connector/ socket to your car stereo might differ from the ones needed for my vehicle as shown and listed within this guide, so check what?s fitted to your stereo before you purchase the connectors needed for the adaptor.

A quick word about safety, if you intend to bury the installation in the back of the dashboard take extra care when messing around due to the airbags fitted in modern cars. Set one of these off and not only will you end up in hospital it will cost you quite a bit to fix the damage to the car. If you are in any doubt consult a qualified auto electrician.

Below are the parts needed for this modification, a full parts index with links is supplied at the end of the article with UK links for the parts, as for tools:

Sharp Knife
Wire cutters/ Kitchen scissors (don?t let the wife catch you though)
Soldering Iron
Small screw driver
Small scissors

The short length of cable at the bottom of the picture is something I made earlier, if you need explanations as to how these plugs are fitted to the cable then the rest of this write up won?t be of much use.
The long coil of coax cable is used to route through the back of the dashboard to where the RDS-TMC receiver is located within the car, the length of cable feeding the RDS-TMC receiver will differ from car to car, this cable is cheap so purchase too much rather than too little.

Cut a small 1?/ 25MM ?window? in the outer insulation, take care not to cut the braided screen, and remove the outer insulation

Cut through the copper braid in the middle of the window and twist the strands to form two equal length ?tails? strip and remove ?? / 5MM of the inner plastic insulator (dielectric) to expose the centre core.

Bend the cable to form a loop, bind the two ends together using PVC tape as shown.

Strip around 1?/ 25MM of the cable going to the RDS-MC lead, form the braid in to a single core cable by twisting together the strands, strip 0,25?/ 5MM of dielectric to expose the inner core.

Position the two cables as shown below and bind together with PVC tape. Make sure the ends are level and the three lengths of screen braid are next to each other.

Twist all three screen braids together and solder the joint, do not touch the inner conductors at this stage.

Use the soldering iron to tin the inner core of both cables and use PVC tape over the braided screen joint, ensure no small copper strands are sticking out, if you find any trim them off as required. Cut the capacitor leads down so they are around ?? / 12MM long

Solder the capacitor across the two cable assemblies, don?t keep the soldering iron in place for too long or the dielectric will melt potentially shorting out the cable assembly.

Add PVC tape around both leads from the capacitor to insulate them from each other, fiddly to do and it doesn?t look too elegant but this is a very necessary step.

Put a 1?/ 25MM length of the outer insulation over the capacitor to add some physical protection to the assembled joint, strip this 1?/ 25MM length of outer jacket from the other end of the long cable going to the RDS-MC receiver.

Wrap the whole joint up with PVC tape and that?s the last step on this end of the cable, it can now be fitted to the car. The aerial lead is simply pulled out of the stereo in the car and inserted to the free socket, the free plug is then inserted in to the stereo, but first we need to terminate the other end of the feeder cable to the RDS-TMC device.

At the other end of the longer cable going to the RDS-TMC unit I fit a 50 ohm BNC bulkhead socket by soldering it to the cable. A corresponding plug is soldered to the end of the RDS-TMC aerial wire which makes it possible to remove the RDS-TMC unit and use it within another car from time to time without cutting cables.

Again PVC tape is used to wrap up the inner cable joint to the connector.

Solder the screen to the outer tag on the connectors body.

Tape the whole assembled connector up with yet more PVC tape

Now lets look at the actual work needed on the RDS-TMC unit, its quite simple to strip just ??/ 5MM from the end of the cable and attach a 50 ohm BNC plug. The wire is soldered straight to the centre pin, there is no termination for the screen/ connector body. The reason I don?t shorten the receivers cable is because I can still use the receiver in another car, whilst it won?t work as well as when its connected to the cars aerial if you use hire cars quite often then the ability to remove a receiver from your own vehicle can be quite handy.

Here we have the x40 active mount with built in RDS-TMC receiver, the end of the cable now sports a BNC plug:

Its not just TomTom units than can be improved using this technique, here the Garmin GTM12 receiver has a new connector fitted:

Back to the more widely used TomTom RDS-TMC units, there are two different types, the first being the slightly older unit with a round plug, often referred to as the part number ending .013

And finally the newer RDS-TMC unit with the mini USB style connector:

Parts List (All from Maplins) which is UK based, Radio Shack or similar will have the same kit but with different part numbers:

1 off Car Aerial Plug HH12N
1 off Car Aerial Skt FJ76H
1 off BNC Socket HH18U
1 off BNC Plug FJ72P
1 off 10,000 pico-fared ceramic capacitor WX77J
Coax cable as required XR29G
PVC Tape FT20W
Does anybody know if it is possible to connect the External Antenna to the RDS-TMC Traffic Receiver USB together?

Thanks for your response, I should point out, my arrangement relates to my house.
My computer setup is on my ground floor and I bought the External Antenna for use with my One 2nd Edition
to give me a better signal from the next floor up, I have now bought the XL model with IQ, but still use the One for my son, thus wanted to swap between the two machines, but use the length of the external antenna.
I've looked on the internet and seen that it's a MCX Female Cable end to use, which would connect to the external antenna and would have that fitted on the end of the RDS-TMC Receiver.
That's what I was trying to find out if they where compatible.
The GPS signal can be fed in to the device via an MCX socket but I have never seen a detachable RDS-TMC aerial with a socket on it - Mike
Thanks again Mike for your response, I was considering cut the end off and attaching the MCX female end. Cheers Donald

I have a TomTom XL 340TM with the integrated car charger/traffic receiver.


Does the 340TM's integrated traffic receiver/car charger have the same reception problem like described above and would need a similar mod?

Also, I want to get an additional traffic receiver and install it in another car. Would the mini USB external antenna/RDS-TMC traffic receiver described in this thread work fine with the 340TM -- and have lifetime traffic subscription?
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The combo rds-tmc antenna/car charger seems more robust concerning getting a signal that earlier antenna.

The earlier antennas do NOT come with lifetime traffic in North America.

I am a little confused then--I purchased a TomTom XL 340TM model -- meaning it comes with lifetime Map and Traffic subscription.

It should have lifetime traffic subscription -- regardless of what traffic receiver it is connected too, right?

The lifetime traffic subscription should follow the TomTom device--and not the antenna I suppose? The antenna is just an antenna. The traffic subscription should be associated with the *device* and not the antenna.

So this means if I have 4 vehicles, I can set up 4 antennas. All I need when I drive a different vehicle would be just to disconnect the TomTom XL 340TM and hook it up to the other antenna in the other vehicle--and all should be good to go.

I sure don't want to carry around the bulky antenna and have to connect/disconnect the antenna from the car charger port in my vehicle everytime I switch vehicle.

The TomTom system should be smart enough to recognize the TomTom device already has lifetime traffic subscription when I hook up the device to another antenna -- and therefore won't need to ask for 'another' lifetime subscription--true?

I would be surprised if I've already *paid* for lifetime traffic and not seeing lifetime traffic available when I hook it up to a compatible traffic receiver.

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Nope. The functionality for the combo antenna/charger is built into the antenna. And it works only for certain unit/applications.

For example, if I hook my combo antenna that I have for my 540 to my 720, the 720 doesn't even recognize the rds-antenna is attached and uses only the charger part. The 720 application precludes the traffic usage.
> And it works only for certain unit/applications.

Yes I totally agree the lifetime traffic feature would work for certain models (devices)/applications.

But now as the consumer, I have to pay for 3 more 'lifetime traffic' subscriptions for my TomTom 340TM--even if this device can have, and has the application that operates this feature--and I already have paid for a lifetime traffic feature?

TomTom should be *smart* enough--so it knows a device can operate with that feature, and that it *also* knows whether the device already has that lifetime subscription feature enabled--and so would not *re-require* that same subscription again.

If TomTom can fix this lacking feature--of smartly knowing the device already has the lifetime traffic feature--let me know. I am not going to spend $70 for each additional antenna to have an antenna on each of my vehicles.

I guess now I have to carry that bulky antenna/car charger around--from vehicle to vehicle.

I've already dropped that antenna one time while hastily carrying it to another vehicle. If I ever lose that antenna, I might as well purchase another TomTom device.

I think a much better way would be to have the antenna built-in to a TomTom device. That way there is no worry over losing that feature from the device if I lose that antenna. And TomTom doesn't need to charge outrageous price for each antenna. And I can also see traffic everywhere I carry the device. With the built-in antenna, for sure the TomTom system can tell the device has lifetime traffic subscription (should be fairly easy to do a basic check on the device).

As an example, Navigon 7200T has a built-in antenna, and the external USB/ car charger can also enhance the reception. Don't know why TomTom doesn't do it this way.
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