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Posted: Sat Dec 01, 2007 11:14 pm Post subject: Tomtom TMC antenna -- horizontal or vertical?
As far as I know, Classic FM broadcast in either slant or circular polarization. So why does the Tomtom RDS TMC manual say that the antenna must be mounted vertically?
On my car (BMW Z4 M Coupe) the windscreen is short and steeply raked, so if I ran the antenna horizontally across the bottom I'd have a much longer antenna (and, I presume, more 'tuned' to the wavelength of the transmission). TBH running it 'vertically' up the A-pillar results in it being inclined backwards by about 40 degrees anyway...
Joined: Feb 27, 2006 Posts: 14871 Location: Keynsham
Posted: Sun Dec 02, 2007 1:06 am Post subject:
I've never had TMC so I can't speak from experience. But I have read a lot of the posts, just in case it ever gets good enough performance to tempt me. And my recollection is that the advice given by experienced people on this forum was to have the aerial as straight as possible horizontally across the windscreen.
Whilst you wait for one of the experts to respond, why not give it a try? _________________ Dennis
Joined: Jan 14, 2005 Posts: 19638 Location: Blackpool , Lancs
Posted: Sun Dec 02, 2007 10:25 am Post subject:
I have mine deployed across the bottom of the screen before it goes up the screen to the top of the glass (or I sometimes tie it to the rear view mirror, depending upon the car) - I have tried many different locations for the aerial cable over the last 18 months or so, with most of the cable aranged vertically is what I have found to be best - Mike
Joined: Feb 10, 2005 Posts: 1951 Location: Mostly somewhere in Essex
Posted: Sun Dec 02, 2007 11:32 am Post subject:
I suppose the answer is 'do what works' I have a Navman T1 and it's aerial is about 7 to 8 inches long. In my motorhome it is vertical up the side of the windscreen and the cable is laid along the dash to the middle where it connects with the T1. In my car it is horizontal along the top of the screen with the cable running down the side, then across to the centre connection.
Both work fine! ( With the exception of the transmitter's dead spots around the country )
So, my contribution is, try the fuctionally most acceptable for your car and only change it you don't get a good signal. _________________ Gee-Pee
Lifetime member PGPSW - time rapidly decreasing
Update: coverage seemed to be patchy with the cable running up the passenger-side A-pillar. So I've relocated the oval blob to the base of the driver's side A-pillar, and run the cable across the bottom of the windscreen. I was encountering problems with the printed dots on the edges of the screen breaking the seal, so i just tucked it into the gap between the windscreen and dashboard.
Result: TMC now locks on instantly, and I can locate the Tomtom in my preferred position of the base of the driver's side A-pillar, rather than in the middle of the dash. Furthermore, I don't have any nasty cables in my line of sight, and the only part visible externally is the oval blob suckered to the windscreen.
Joined: Jan 14, 2005 Posts: 19638 Location: Blackpool , Lancs
Posted: Tue Dec 04, 2007 12:49 am Post subject:
As Gee-Pee said when you find a solution that works for your vehicle use it, each car is different so a little experimentation is required at times, nice to hear you got it working though, it is a usable system compared to the earlier versions - Mike
Posted: Tue Dec 11, 2007 10:44 am Post subject: Tmc Aerial
I struggled to get a decent signal either horizontal or vertical (i'm sure the A-Pillar stops the signal anyway), so i took the wire and ran it around the door and up the outside of the windscreen and discreetly taped it with black tape to the window rubber vertically.
After about 10-15 mins it gets a good signal, and the circle stays green; the problem is that i need to know where the delays are before that time, as by then i'm already on the motorway.
So in review its better outside than inside (less interferance) yet its still not as good as GPRS with monthly subscription.
Joined: Jul 07, 2006 Posts: 1649 Location: Newcastle, England, UK
Posted: Tue Dec 11, 2007 9:28 pm Post subject:
When im out driving, i have my tt in the middle of the windscreen, not on the right hand side. i then run the tmc cable to the left along to the corner where the tax disc is, then keeping about 2inches between the cable and pillar where the glass meets the chassis, i run the remainder of the cable straight up to the roof from the tax disc. Get perfect results everytime now, whether its supposed to be horizontal or vertical _________________ Tomtom GO 1005 LIVE
Hmm, more experimentation required. Today I drove my wife's car from Bishop's Stortford to Luton Airport, with the cable running along the bottom of the windscreen.
Result was that 10 minutes after I left home, the TMC locked on for about 1 minute. Then for the remainder of the journey (another hour...) it was searching. I didn't get any traffic updates during that minute either
Joined: Jul 07, 2006 Posts: 1649 Location: Newcastle, England, UK
Posted: Tue Dec 11, 2007 9:33 pm Post subject:
carl_w wrote:
Hmm, more experimentation required. Today I drove my wife's car from Bishop's Stortford to Luton Airport, with the cable running along the bottom of the windscreen.
Result was that 10 minutes after I left home, the TMC locked on for about 1 minute. Then for the remainder of the journey (another hour...) it was searching. I didn't get any traffic updates during that minute either
This may be a silly question, but you do have the latest software on your satnav and the latest TMC receiver? _________________ Tomtom GO 1005 LIVE
Posted: Tue Dec 11, 2007 11:03 pm Post subject: Re: Tomtom TMC antenna -- horizontal or vertical?
carl_w wrote:
As far as I know, Classic FM broadcast in either slant or circular polarization. So why does the Tomtom RDS TMC manual say that the antenna must be mounted vertically?
Normal FM transmissions are Horizontally polarised, but Classic FM transmissions are mixed polarisation (horizontal and vertical). Possibly the following will explain:
For long distance work, the polarisation is linear, horisontal. One reason is that it is easier to mount a single yagi horisontal than vertical, and of course that circular polarisation is more difficult to produce than linear. Another reason is that signals with vertical polarisation is strongly attenuated by passing some hundred metres of tree tops while horisontally polarised signals are much less affected.
A circularly polarised wave becomes more or less horisontal after having passed through the tree tops on its way into the troposphere because the vertical component is attenuated.
For local communication, vertical polarisation is the standard (FM). Of course the reason is that vertical antennas fit nicely on cars and hand held transceivers. Having opposite polarisation between local and DX traffic makes it easier to avoid interference. The hundreds of kilowatt ERP powers from DX stations is less likely to overload a vertically polarised local station (with poor dynamic range). The high levels of noise sidebands from local FM stations is less likely to cause interference to the DX-er.
A conclusion could be that a horizontally polarised aerial is preferable, as it is more likely to receive distant transmissions, a problem that seems to plague TMC receivers.
On many cars with roof mounted aerials, the polarisation is almost horizontal across the roof. _________________ Regards
Today's update: again no luck with the antenna running up a couple of inches from the A-pillar. So I took the vertical bit (about 6" worth) and ran it across the passenger side window. Result was coverage about 50% of the time. So I'm assuming I have a heat-reflective windscreen (note for anyone else speccing a BMW: it's "sun protection glass")
I now suspect the ideal solution for reception is the Tomtom in the driver's side A-pillar with the antenna running across the driver's side window. But that means the window won't go up and down and I have to allow a load of slack so the door can open. Roof panel is metal so I can't run it up there (would have been great if it was plastic).
An aerial socket would have solved the problem. Or keeping my last car (plastic-bodied TVR).
Does anyone know what the connector is? Then I could run an extension lead along the door sills and site the whole lot in the rear window.
Don't laugh at a possible solution that I may have found. I have a new viamichelin and never got a decent tmc signal except in France. Always the optimist, I slapped it on the screen this morning. After about 15 mins got a signal. Eh? Then realised that one of the two suckers had unsuckered and the unit was at 45 degrees. After that I had no probs (driving through Surrey and SW London). Hope this may help.
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I have a Navman T1 and it's aerial is about 7 to 8 inches long. In my motorhome it is vertical up the side of the windscreen and the cable is laid along the dash to the middle where it connects with the T1. In my car it is horizontal along the top of the screen with the cable running down the side, then across to the centre connection.
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