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Initial problems with GTM 12
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Joined: Mar 27, 2006
Posts: 73

PostPosted: Wed Oct 10, 2007 9:53 am    Post subject: Reply with quote

simoh wrote:
As you come from London, at the Stokenchurch pass (J6) you get a great view of the Thames valley and also a direct line of sight to Beckley mast transmitting Trafficmaster TMC on FoxFM. While in view of the aerial you'll get traffic info, but heading the other way, to London, once the Chiltern hills are behind you, nada.

I've been up to Oxford a few times now and generally I get very good information. I've left the Unit on the GTM status page a couple of times and I've noticed something a couple of times now. On the way out of London I sometimes won't get any traffic until Exit 8 when I get off the M40, but Heading into London I'll get reception all the way past J6. Also when parking in the car park in Oxford if I face south the reception will be over 40, but facing the car north drops the reception below 30.

I'm guessing that there could be environmental factors that may explain these discrepancies the couple of times I tested it, but on the face of it I'd guess that the Windscreen is impacting reception, so an external antenna seems to be the only option.

I've removed the plastic Hub at the end of the string and tried attaching to a longer wire, and a better antenna, but as the signal is not amplified it makes little to no difference in reception. I've also tried attaching ferrites to the GTM unit to reduce interference, but this actually reduced performance.

Also in London, I get no messages north of Camden, which is rubbish as I very rarely drive into central London. Around Crouch End, Highgate, Finsbury Park and Weeod green which is where I start most Journeys from I get nothing.
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Joined: Oct 09, 2007
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PostPosted: Wed Oct 10, 2007 11:54 am    Post subject: Reply with quote

Maybe as you come north with the GTM searching, it takes the time you travel from J6 to J8 for the GTM to hone in on 151 (FoxFM). Whereas going the other way, as it's already tuned and not searching you retain the updates to J6 and beyond.

FWIW, by testing it at home on AC power, the GTM12 seems to need a level (signal strength) of 30+ to reliably decode the info, below that it can lock on to a station (Mode 5 and N-Sync 0), but there are too many errors in the messages and the message takeup rate is low; much below level 27 and the unit will start scanning again.

Thanks for the info on ferrites - I was thinking of trying that but won't bother now. Might try to split the car aerial feed with an amplifier, but need to find out more about my cars aerial first.

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PostPosted: Tue Oct 16, 2007 6:49 am    Post subject: Reply with quote

Just a bit of an update. Went up to Oxford again, and had the GTM status page on the whole way. Straight after the Hills while I was still descending Channel 151 came up straight away (Signal over 40), but by the time I got to Junction 6 It had dropped to 27 or so and started scanning new channels. I guess the other day when I tried this it did not lock on whilst I was higher up.

Had a quick go at getting at the Radio Aerial. Anyone else tried that with a recent model Toyota Corolla?
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PostPosted: Thu Dec 20, 2007 10:38 pm    Post subject: Reply with quote

After much time spent on the internet (a lot on this site - it's a good resource) I ended up removing the existing shark-fin aerial from the roof (it was fitted with a 15mm square hole) and replacing it with a Hirshmann AUTA 16V flex available from CPC.. It's an amplified VHF FM aerial and I ran the coax/12V cables to the front of the car. The 12V supply and coax screen was connected to the +ve and -ve of the 12V cigarette lighter power supply at the front. The centre of the coax was connected to the cut-down shoe-string aerial of the GTM12. The GTM12 USB lead was extended so it would reach the NUVI on the dash.

Mixed results. The signal strength is greater and it will now lock onto stations in areas that were dead spots with the shoe-string. However there are still signal drop-outs. I expected the unit to hold onto a signal (Mode 5) until leaving the range of the transmitter and then start searching for another station. Unfortunately, even when the signal strength seems fairly high it will lose its lock in various areas and start searching again. After a few minutes it will lock onto the previous station again with mode 5 (but sometimes will scan past that station while searching even though it's signal strength is high!).

It seems overly sensitive to whatever is causing this drop-out - surely waiting 30 secs to a minute to see if the signal returns before starting a search is preferable to wasting several minutes searching in vain only to return to the same station. As to what is causing the drop-outs it's a mystery - it's not like the dead zones are in obviously bad reception areas.

All in all I think all the time spent was probably worth it, but getting the aerial installed and waterproof was a real PITA, it took me several attempts & lots of silicone, removing silicone etc. The people at Hirschmann were extremely helpful though, and even sent me some alternative seals.

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Joined: Aug 03, 2006
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PostPosted: Sun Feb 10, 2008 9:47 am    Post subject: Reply with quote

Just thought I would add my experience's with the GTM 12 so far.

I have only had the unit a short time now but never had any problems getting traffic data. The longest period was when I drove to the shops a mile away, it wasn't till I arrived back home that the icon became active.

With regard to quality of traffic information I am not in a position to comment. I will be up and down a usually congested stretch of road tomorrow (as already mentioned by a post before, the Team Valley) so will be able to gauge the reliability of messages.

But from what I gather from previous posters, the non-reception has been one of the major gripes. I also have a reasonably new Nuvi so they may have fixed the noisy power supply issues.
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