Garmin GTM21 Traffic - signal problems

[Katabatics]

I have been using my first Garmin Streetpilot 550 for six months, great unit but very poor traffic reception in the UK. Traffic Master were very helpful in explaining how their service is output and with the comments in this forum about limits on transmitter output signal strength in the UK, it explained why I only receive a signal (green led on the unit) close to a radio transmitter, usually in or near a town.

Following the advice in this forum, I pulled the black plug of the end of the GTM21 ariel wire and jury rigged it into my car ariel plug at the radio. I have a Volvo V70. The signal was about 50% better, but it still only worked in the area of a transmitting station, but I picked up a few more stations. I went to my local Maplins to get something more permanent, and found their technical advice to be extremely helpful.

The permanent solution is to use screened cabling for the GTM21 airel (similar to car ariel cable but a thinner version). I have spliced this into the input run of the car ariel cable, close to the back of the radio, but I think any convenient point will do. This needs a good Stanley knife to separate the car's screening without breaking it so you can get at the core. I wrapped the inner cores together and then bound the inner splice with electrical tape, then with both screens are in contact taped up the whole splice.

The important part is to run the new screened cable from the splice all the way up to where the Garmin thin wire enters the black plastic block close to the satnav unit plug. This provides screening of the traffic signal as far as possible. Maplin suggested cutting the Garmin black wire close to the block, but I have not done this as it makes the GTM21 much less portable. I soldered the end of the inner core of the screened cable to the exposed end of the Garmin wire and then to closely coiled up the Garmin wire and placed the coil and the joined end of the screened cable close to the block, so as to provide screening almost to the back of the unit, and taped it together at the block. I also taped the two GTM21 cables together just to tidy up. I have inserted a co-ax m/f plug so I can separate the GTM21 from the car. The Volvo has a neat gap around the underside of the steering console that allows a feed back inside to the dash panels back to the ariel, so no need to drill any holes. The results have transformed reception (permanent green) to about 90% of the time. I suspect this is as good as it gets.

[inspiredron]

Would be interested to know what your signal strength is just sitting in the car a) with the new setup
b) with the normal spread out GTM21 aerial wire.

This all adds to the theory that keeping the signal clean is very important.

Several folk have pointed to suitable aerial splitters for those not inclined to take a Stanley knife to their car aerial cable!

Where are you based? You also may be interested in this post:
www.pocketgpsworld.com/modules.php?name=Forums&file=viewtopic&t=57828

[PaulJSmith]

Hi,

I have a C-class merc 2001 and tried plugging it into the aerial of the car by wedging it at the radio-to-aerial socket. It did improve the reception of the Garmin unit but also made the radio reception much worse for the car. Fringe stations that were clear before were really bad. Tried several times to adjust the positioning of the join to make sure it was not shorting with the aerial's earth ring but no improvements were notes.

I gave up in the end as the radio reception was more important to me. I ended up running a wire all the way around the windscreen, tucking it inside the A pillars and inside the headliner in the roof.

It is still crap!

I tried a powered booster on the garmin aerial (use the term "aerial" very loosely) and this also made no difference. I finally tried the ferrite rings trick and this made a small difference (positively) but not much really. I think garmin should provide a better aerial in the first place!!

I don't really want to risk cutting/splicing my car aerial in a more permenant way in case I knacker the car radio reception permenatly.
Cheers,
Paul

[philpugh]

[Katabatics]

Hi

I can understand the reluctance to cut into the ariel cable. There are some products avaialble that will split the ariel cable. I found that these were designed to fit a different radio into the car, but it does give you a connection to attach the garmin string to without interferring with the main feed. I think the most important issue is signal strength and the coaxial cable does seem to protect this. You could wire the end of the Garmin string up to the core of some coax and run that all round the windscreen and see if you get better reception, but coil the garmin string closely at the plug end.


GTM

[Katabatics]

Here are the results of GTM test done with the Garmin String (GS) and the Coax (CX), the latter connected up to the car aerial.

yellow light on in drive
GS Level 0-10
CX level 10-39

Drove to Chichester where signal connects - green light, but only with CX
CX Level 10-48 (message count incrementing at levels 21 -30
GS gave levels up to 25 but yellow light

M27 around Portsmouth was the only place where I ever got a green with GS so
GS Level 30-48
CX Level 50-65

Parked in office at Eastleigh CX always gives a signal. Turned on satnav and monitored GTM screen. Green light came on after about 1 minute at level 25 with message count. On journey home message count continues to increment at signal levels 10-30 range tuned to channel 157

Held signal back to my home east of Chichester with message count incrementing with levels in the teens. Below 10 green is maintained but message count is static.

It seems that signal level is not the only factor as CX gives results at the levels the GS achieves but GS does not lock on to green

Anyone have some ideas?

Katabatics

[portman]

You'll probably find that if it had a good signal there is a small delay before searching for a better signal, to allows for signal fluctuations. As you say, connecting the coax at the receiver will make a significant difference to the signal, i guess the best bet is to solder the coax straight into the receiver.
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