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JimmyTheHand
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Joined: Apr 16, 2005
Posts: 386
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 Posted: Sat Jun 04, 2005 10:50 am Post subject: Re: Signal strength on status screen decreases when on car m |
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| Eldar wrote: | | 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? |
about 1-2V out of 5V available (doesn't seem efficient for a battery device), it is more likely you would aim for a 0.5/0.25v - you would then aim to switch over at something like half of this to ensure that both it has fully switched and potential variations in components and voltage levels do not effect the switch over. But the more interesting thing is what happens when you disconnect the current drawing device, but leave a wire - then basically you have an aerial attached to the input of the switch, so you consider low input impedances for the switch to reduce the pickup levels, but this will effect the "off" current flow though your current detector, further reducing the voltage differential.
Detecting attached devices by current flow is something that sounds easy but is much hard in practise - however I have no doubt that there are ICs that can do this by throwing a few dozen transistors and diodes at it - but it would be much simpler to have a link to switch circuit on or off.
| Eldar wrote: | | Sure. I take it you are referring to 1/4 wavelength effects? If so you are quite right, that might happen. |
I am actually talking of the impedance variation on transmission lines due to impedance mismatch - it is what causes standing waves (hence increases noise levels) and the quarter wavelength effect is merely the most extreme case (i.e. where open circuit appears as short circuit)
I don't think there is any point pursing this line of discussion further, because I don't know the circuitry of the TomTom and its mount and haven't done any electronic design for over a decade so to go any deeper will require me to do more work than I really care to do.
Dispite everything it seems that TomTom have a potential problem with the 300 and its mount (another user seems to have same problem) - I don't anticipate it effecting me much longer since I have an external aerial on order - the signal is still not strong enough in the car without mount to be 100% OK in all situation, so as long as the new mount and aerial work OK I'll be happy.
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hegedusa
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Joined: Mar 27, 2004
Posts: 55
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| Posted: Sun Jun 05, 2005 8:48 am Post subject: Same problem with another tomtom and some odd effects |
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I've just confirmed that my brother-in-law's tomtom 300 is behaving in exactly the same way - singal drops significantly when on the mount. We tried it with my mount and his, and with both tomtoms - same effect.
Now here's the strange part. If I leave one tomtom just sitting right next to the other, with one sitting on the mount but not connected, the signal strengths are high (38-41) on both tomtoms. As soon as I push the tomtom down onto the mount, the signal strengths drop as seems to have been the problem so far. But here's the odd thing - the other tomtom sitting next to it suffers the same effect - the signal strengths drops too!
How's this happening? Not having any knowledge of radio theory, the best I can come up with is could the tomtom that's on the mount be "retransmitting" the signal into the mount and this is causing noise and interference so much so that it affects another tomtom sitting right next to it?
I'm not going to go to the effort of sending my tomtom back - it seems quite obvious to me that there's a design flaw in there somewhere. THe thing is, Tomtom will probably never admit to it. |
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JimmyTheHand
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Joined: Apr 16, 2005
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| Posted: Sun Jun 05, 2005 9:57 am Post subject: Re: Same problem with another tomtom and some odd effects |
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| hegedusa wrote: | Now here's the strange part. If I leave one tomtom just sitting right next to the other, with one sitting on the mount but not connected, the signal strengths are high (38-41) on both tomtoms. As soon as I push the tomtom down onto the mount, the signal strengths drop as seems to have been the problem so far. But here's the odd thing - the other tomtom sitting next to it suffers the same effect - the signal strengths drops too!
How's this happening? Not having any knowledge of radio theory, the best I can come up with is could the tomtom that's on the mount be "retransmitting" the signal into the mount and this is causing noise and interference so much so that it affects another tomtom sitting right next to it? |
If, as I believe, the TomTom is switching/partially switching to the missing external Aerial it is possible that some noise is being transmitted out of the Aerial socket - which though is very low power, the adjacent TomTom is close enough to pickup.
I have heard that some GPS units will interfere with others when close by, this could be either re-radiation of intermediate frequencies used internally or issues with the Aerial of one blocking signals to another.
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hegedusa
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Joined: Mar 27, 2004
Posts: 55
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| Posted: Sun Jun 05, 2005 10:06 am Post subject: Re: Same problem with another tomtom and some odd effects |
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| JimmyTheHand wrote: |
If, as I believe, the TomTom is switching/partially switching to the missing external Aerial it is possible that some noise is being transmitted out of the Aerial socket - which though is very low power, the adjacent TomTom is close enough to pickup.
I have heard that some GPS units will interfere with others when close by, this could be either re-radiation of intermediate frequencies used internally or issues with the Aerial of one blocking signals to another. |
The tomtoms don't interefere with each other until one of them is actually connected to the mount. Whatever effect the mount has on the unit that's plugged into it, it's having the same effect on a nearby tomtom. |
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hegedusa
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Joined: Mar 27, 2004
Posts: 55
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| Posted: Sun Jun 05, 2005 10:09 am Post subject: Where to get antenna? |
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Where does one purchase an external antennna anyway? All the posts I've seen involve making one somehow.
Or perhaps someone makes a different mount that doesn't have this problem. |
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Eldar
Pocket GPS Moderator
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Joined: Sep 24, 2004
Posts: 1294
Location: London
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| Posted: Sun Jun 05, 2005 12:21 pm Post subject: Re: Same problem with another tomtom and some odd effects |
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| hegedusa wrote: |
How's this happening? Not having any knowledge of radio theory, the best I can come up with is could the tomtom that's on the mount be "retransmitting" the signal into the mount and this is causing noise and interference so much so that it affects another tomtom sitting right next to it? |
I'm well impressed. That is the most likely explanation for this problem. I think that the signals from the processor/memory are getting onto the DC supply wires in the mount and interfere with GPS reception. |
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JimmyTheHand
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Joined: Apr 16, 2005
Posts: 386
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| Posted: Sun Jun 05, 2005 1:37 pm Post subject: Re: Where to get antenna? |
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| hegedusa wrote: | Where does one purchase an external antennna anyway? All the posts I've seen involve making one somehow.
Or perhaps someone makes a different mount that doesn't have this problem. |
A couple of places
GPS Warehouse
or Total GPS (note: I am note happy, having ordered this last week it seems that they messed up the delivery address - sending it to the home address and not where I requested, and I have yet to see any signs  - going to have to phone them again)!
Amazon also have them, for £10 more.
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rayflan
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Joined: Apr 06, 2005
Posts: 277
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| Posted: Sun Jun 05, 2005 4:59 pm Post subject: Re: Where to get antenna? |
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| JimmyTheHand wrote: | | hegedusa wrote: | Where does one purchase an external antennna anyway? All the posts I've seen involve making one somehow.
Or perhaps someone makes a different mount that doesn't have this problem. |
A couple of places
GPS Warehouse
or Total GPS (note: I am note happy, having ordered this last week it seems that they messed up the delivery address - sending it to the home address and not where I requested, and I have yet to see any signs - going to have to phone them again)!
Amazon also have them, for £10 more. |
Quote from totalgps.co.uk
"This will only work with the Tomtom classic it will NOT work with the Classic Model"..............????????????????????
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JimmyTheHand
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Joined: Apr 16, 2005
Posts: 386
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| Posted: Sun Jun 05, 2005 5:27 pm Post subject: Re: Where to get antenna? |
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| rayflan wrote: | Quote from totalgps.co.uk
"This will only work with the Tomtom classic it will NOT work with the Classic Model"..............???????????????????? |
I think they mean it is for the Go range excluding the original model (title says for 300) - they have another similar item that they say doesn't work on the 300/500/700. Someone else has brought one from them and it works on their 300 - I let you know if it turns up!
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EQtomtom300
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Joined: May 20, 2005
Posts: 30
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| Posted: Sun Jun 05, 2005 9:50 pm Post subject: |
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Just to throw a spanner into the topic.
I seem to get better results when my TT300 is on the mount in the car. I'm not using the TT mount though as i have recently bought a Brodit mount and proclip.
Strange world...........
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oldbwl
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Joined: Oct 24, 2003
Posts: 12
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| Posted: Sun Jun 26, 2005 11:25 am Post subject: |
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Try moving the power supply to the back of the TTG and not on the mount. I tested the signal strength as per the above post and can also see a drop as soon as the TTG is located on the mount.
This only happens though only if the power is coming via the mount.
If the power lead goes into the TTG directly or I leave it on batteries totally, I can see no difference in my tests today. |
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Andy_P
Pocket GPS Moderator
Joined: Jun 04, 2005
Posts: 19991
Location: West and Southwest London
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| Posted: Wed Jul 06, 2005 12:47 am Post subject: |
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I've been talking about this on the MYTomTomGo forum.
When I first heard about this problem I had a quick look and it didn't seem to happen with mine. Then a couple of days ago I experienced TOTAL signal loss when my Go300 was clicked on the mount, but fine as soon as it was off.
Ever since then, I seem to have the 'about 30% reduction in signal problem' that others have seen.
It definitely seems to be something to do with the multipin connector, I see the change of level straight away when I just rock the unit a couple of mm so it just mates and unmates with the connector.
I've emailed TomTom, lets see what they say.... |
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Andy_P
Pocket GPS Moderator
Joined: Jun 04, 2005
Posts: 19991
Location: West and Southwest London
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| Posted: Wed Jul 06, 2005 12:51 am Post subject: |
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Sorry admin, pls delete duplicate post. I only hit 'submit' once, I promise!
Admin Edit: have deleted duplicate. |
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