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hegedusa
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Joined: Mar 27, 2004
Posts: 55
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 Posted: Wed May 25, 2005 10:53 pm Post subject: Signal strength on status screen decreases when on car mount |
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My GO 300's signal strength is low, and sometimes it just won't get a signal - well, it won't lock on. The strengths of the bars are 20-30. I found that if I place the tomtom near the side window the strength goes up (to 30-45), and I put this down to having some sort of reflective windscreen.
However, my old tomtom bluetooth GPS had perfect reception in the car...
I then noticed that the signal strength drops as soon as I latch the GO onto the car mount. Even if the power isn't connected.
What's going on here? Has anyone else noticed this?
Anthony |
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Oldboy
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Joined: Dec 08, 2004
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Location: Suffolk, UK
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| Posted: Thu May 26, 2005 6:16 am Post subject: |
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With the standard car mount there is nothing on it to change the signal strength. It might be that the mount just puts it at the wrong angle (place) to receive a good signal. It sounds like a good case of needing an Ext Aerial.
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Richard
TT 910 V7.903: Europe Map v1045
TT Via 135 App 12.075: Europe Map v1145 |
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hegedusa
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Joined: Mar 27, 2004
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| Posted: Thu May 26, 2005 11:11 am Post subject: |
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| Oldboy wrote: | | With the standard car mount there is nothing on it to change the signal strength. It might be that the mount just puts it at the wrong angle (place) to receive a good signal. It sounds like a good case of needing an Ext Aerial. |
No I don't think that's the case. Even if I unclip the mount, and leave the tomtom in place, the signal goes up. I tried even removing the mount from the suction cup section, and I can replicate the problem by holding it in my hand. I "fixed" the problem by inserting a small piece of paper in the mount so that the contacts don't make contact. The difference is consistent. Of course now I have to power the tomtom from the rear power connector rather than through the mount.
What I don't know is whether it's the tomtom that's faulty or some sort of short in the mount.
The problem happens even if the power cable is not connected to the mount.
Odd. |
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Oldboy
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Location: Suffolk, UK
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| Posted: Thu May 26, 2005 6:20 pm Post subject: |
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I can't think of a solution. What happened when the car lead was connected? Did this reduction the signal levels?
It might be worthwhile asking TT for a possible answer. You never know they might have an answer.
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Richard
TT 910 V7.903: Europe Map v1045
TT Via 135 App 12.075: Europe Map v1145 |
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hegedusa
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Joined: Mar 27, 2004
Posts: 55
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| Posted: Thu May 26, 2005 6:34 pm Post subject: |
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| Oldboy wrote: | I can't think of a solution. What happened when the car lead was connected? Did this reduction the signal levels?
It might be worthwhile asking TT for a possible answer. You never know they might have an answer. |
It makes no difference if the 12V lead is connected - I'll email tomtom and see what they say |
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Meester-Martinho
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Joined: May 26, 2005
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Location: GPS Hell
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| Posted: Tue May 31, 2005 12:34 pm Post subject: Re: Signal strength on status screen decreases when on car m |
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From what i can remember the tomtom go gps receiver is in the bottom of the unit i have heard of people mounting it upside down and flipping the screen.
the reason your blutooth rcvr worked was because you could push that right up against the windscreen whereas your go unit is closer to you if you have a reflective windscreen the go unit will not be seeing enough of the sky whereas reflective screens dont go all the way to the bottom of the screen which yours old rcvr saw more of the sky hence better reception
| hegedusa wrote: | My GO 300's signal strength is low, and sometimes it just won't get a signal - well, it won't lock on. The strengths of the bars are 20-30. I found that if I place the tomtom near the side window the strength goes up (to 30-45), and I put this down to having some sort of reflective windscreen.
However, my old tomtom bluetooth GPS had perfect reception in the car...
I then noticed that the signal strength drops as soon as I latch the GO onto the car mount. Even if the power isn't connected.
What's going on here? Has anyone else noticed this?
Anthony |
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Eldar
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Location: London
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| Posted: Tue May 31, 2005 1:52 pm Post subject: Re: Signal strength on status screen decreases when on car m |
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| Meester-Martinho wrote: | From what i can remember the tomtom go gps receiver is in the bottom of the unit i have heard of people mounting it upside down and flipping the screen.
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TTG GPS receiver is at the top of the unit. As for hegedusa's problem - report it to TT, there might be a problem there. |
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JimmyTheHand
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Joined: Apr 16, 2005
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| Posted: Tue May 31, 2005 10:05 pm Post subject: Re: Signal strength on status screen decreases when on car m |
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| hegedusa wrote: | I then noticed that the signal strength drops as soon as I latch the GO onto the car mount. Even if the power isn't connected.
What's going on here? Has anyone else noticed this?
Anthony |
I haven't noticed - but I'll try it tomorrow and see if I see similar (I have to use Unit off Dash at moment until I get external Aerial)
Looking at the sticky about creating an external aerial using mount that came with the Unit I wonder if the unterminated aerial circuitry is interfering with the internal aerial.
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Eldar
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| Posted: Tue May 31, 2005 11:03 pm Post subject: Re: Signal strength on status screen decreases when on car m |
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| JimmyTheHand wrote: | | Looking at the sticky about creating an external aerial using mount that came with the Unit I wonder if the unterminated aerial circuitry is interfering with the internal aerial. |
It does not. The external antenna input is disconnected when the antenna is not present and the isolation of the antenna switch is fairly high. |
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JimmyTheHand
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Joined: Apr 16, 2005
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| Posted: Wed Jun 01, 2005 10:42 pm Post subject: Re: Signal strength on status screen decreases when on car m |
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| hegedusa wrote: | I then noticed that the signal strength drops as soon as I latch the GO onto the car mount. Even if the power isn't connected.
What's going on here? Has anyone else noticed this? |
I had a play in the car today - on the dash board not connected to mount I had reasonable signals most of the way to work and quickly recovers when it looses - try the same thing in the mount and cuts out most of the journey. So I think I am seeing similar.
| Eldar wrote: | | JimmyTheHand wrote: | | Looking at the sticky about creating an external aerial using mount that came with the Unit I wonder if the unterminated aerial circuitry is interfering with the internal aerial. |
It does not. The external antenna input is disconnected when the antenna is not present and the isolation of the antenna switch is fairly high. |
I have to ask what you mean by disconnected? IIRC the GPS is in the GHz range, that gives wave lengths around 20cm, at that sort of frequency short wires (such as those from base of TomTom to aerial switch/connection) aren't simple conductors - I am sure you don't really want lessons on transmission line theory (and I am pretty rusty anyway).
However it could just be a simple case of the base causing something to switch in the TomTom that shouldn't.
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hegedusa
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Joined: Mar 27, 2004
Posts: 55
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| Posted: Wed Jun 01, 2005 11:08 pm Post subject: Re: Signal strength on status screen decreases when on car m |
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[quote="JimmyTheHand"] | hegedusa wrote: | I then noticed that the signal strength drops as soon as I latch the GO onto the car mount. Even if the power isn't connected.
What's going on here? Has anyone else noticed this? |
I had a play in the car today - on the dash board not connected to mount I had reasonable signals most of the way to work and quickly recovers when it looses - try the same thing in the mount and cuts out most of the journey. So I think I am seeing similar.
I did contact tomtom, and they failed to even understand what I'd written - they replied "if the problem happens when out of the car it must be the tomtom unit itself and not the mount that is faulty". What I actually said was that it was not related to the windscreen as it seemed to go wrong even if I placed the tomtom on the mount outside of the car.
Anyway on Saturday I'll have the chance to test this using my brother-in-law's tomtom 300 (and mount). |
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Eldar
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Location: London
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| Posted: Thu Jun 02, 2005 8:00 am Post subject: Re: Signal strength on status screen decreases when on car m |
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| JimmyTheHand wrote: |
I have to ask what you mean by disconnected? IIRC the GPS is in the GHz range, that gives wave lengths around 20cm, at that sort of frequency short wires (such as those from base of TomTom to aerial switch/connection) aren't simple conductors - I am sure you don't really want lessons on transmission line theory (and I am pretty rusty anyway).
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By disconnected I mean that there is a switch (an IC) in the GPS receiver which disconnects the path to the external antenna including the co-ax cable leading from the receiver to the RF connector at the bottom of the TTG. It's current sensing arrangement, and the switch itself should have about 15 dB of isolation. When the external antenna is connected it starts drawing current and the switch disconnects the internal antenna and connects the external one to the input of the SIRF RF IC.
You are quite right - I really don't want lessons on transmission lines theory, and BTW the wavelength is not around 20cm in the cable 8) |
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Frazzle
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Joined: Nov 05, 2004
Posts: 66
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| Posted: Thu Jun 02, 2005 12:40 pm Post subject: |
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A thought....
I may be wrong here but I seem to remember seeing a large spring "washer" incorporated into the base swivel plate on my XRay images. Someone took a base apart recently so there should be pictures in the Forums somewhere.
I'm going out on a limb here but I wonder if the washer (which is a full type and not split) is having a detrimental effect on the GPS patch antenna (aerial). It could possibly act as a "shorted turn" at GPS RF frequencies in the near field of the patch to the detriment of the polar pattern.
This is pure speculation and I'm not even confident that this is possible so don't bite my head off if I'm talking out of my rear orofice  |
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JimmyTheHand
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Joined: Apr 16, 2005
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| Posted: Thu Jun 02, 2005 7:55 pm Post subject: Re: Signal strength on status screen decreases when on car m |
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| Eldar wrote: | | By disconnected I mean that there is a switch (an IC) in the GPS receiver which disconnects the path to the external antenna including the co-ax cable leading from the receiver to the RF connector at the bottom of the TTG. It's current sensing arrangement, and the switch itself should have about 15 dB of isolation. When the external antenna is connected it starts drawing current and the switch disconnects the internal antenna and connects the external one to the input of the SIRF RF IC. |
That seems a strange way to do it. We are talking very low currents, therefore very low voltages to detect and therefore prone to error. I would have expected a physical link type connection to switch, much more reliable! - however based on the problem it is highly likely TomTom have done it the "difficult" way!
15 db (voltage ratio 1:5.6) seems very low for isolation, especially when the signal to noise ratio needs to be better!
| Quote: | | You are quite right - I really don't want lessons on transmission lines theory, and BTW the wavelength is not around 20cm in the cable |
It is only an approximation – and the wavelength is shorter in wire so making the effects more likely
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J. |
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Eldar
Pocket GPS Moderator
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Joined: Sep 24, 2004
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Location: London
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| Posted: Fri Jun 03, 2005 12:18 am Post subject: Re: Signal strength on status screen decreases when on car m |
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| JimmyTheHand wrote: |
That seems a strange way to do it. We are talking very low currents, therefore very low voltages to detect and therefore prone to error. I would have expected a physical link type connection to switch, much more reliable! - however based on the problem it is highly likely TomTom have done it the "difficult" way!
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That is a very common way to switch an active external antennain and out. We are talking 10-20 mA of current. Say 10mA across 1kR, how much voltage that might be?
| JimmyTheHand wrote: |
It is only an approximation – and the wavelength is shorter in wire so making the effects more likely |
Sure. I take it you are referring to 1/4 wavelength effects? If so you are quite right, that might happen. |
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