satalite radio question

[dacflyer]

i have a sirius radio. i use it from vehicle to vehicle, so i typically just lay the antenna in the dash. it does not pick up so well as it would outside the car. is there something in the glass thats acting like a shield that is attenuating the signal ?
and is there available some sort of a signal booster or amp that i could get to compensate?

in one of the menus it shows satalite signal strength, and there is a terestial signal strength.
but the terestial signal never has any signal on it.. does anyone know what this is used for ?

i know the antenna is supposed to be mounted out of the vehicle, but whats the diff of having it inthe dash, it still should see the signal thru the glass. its got pleanty of sky view.

[Dean Huster]

The AM broadcast band DXer and SWLer in me just cringes at the oximoron "satellite radio".

[dyarker]

Window tint is a super thin layer of metal on a plastic film and can attenuate the signal. More likely, the roof of the car is blocking direct path to satellite. With antenna on dash, signal strength is stronger with car facing South than when facing North. Best signal would be when car faces exact direction to satellite.

Terrestrial refers to regular AM and/or FM stations on the suface of the Earth.

On the roof the antenna "sees" the whole sky regardless of which way the car is facing.

Cheers,

[jollyrgr]

The terrestrial signal strength refers to the satellite repeaters found in big cities or "Urban Canyons". You can find maps of Sirius repeaters on websites. I found this one:

http://www.dogstarradio.com/sirius_map.php

According to the above site the following cities have such repeaters:

Los Angeles
New York
Washington DC
San Francisco
Atlanta
Miami
Chicago
Detroit
Philadelphia

Unless you are in one of the above cities I doubt you'd ever see a signal strength for the terrestrial. This is to overcome the "shadows" caused by skyscrapers to the satellite signal.

[dyarker]

I which to amend as corrected by jollyrgr ... ground based yes, normal radio stations no

[dacflyer]

jollyrgr >> i was wondering why , when in atlanta, i never lost signal going under huge bridges etc..


the windows in my car are not tinted. and the window area is very large that very little if anything is blocked.. but i do suppose there is something inthe glass thats shielding it in some way,, but even driving down the road, trees will constantly knock me out..

still i am curious to find a signal amp / booster , i have had no luck to find one...

[jwax]

Are you paying for that service? Seems Sirius should provide ample signal for the $.

[dacflyer]

is there a free service ? NOT

[haklesup]

Assuming the window glass is 100% transparent to the sattelite signal, you will still be shielding much of the signal from above and behind because of the roof.

In general, if your eyes can't look in a particular direction, your antenna can't either. So unless you drive a convertable (even with the rag top up) or an AMC pacer (he he) then you're blocking much of the sky from view.

I didn't check any maps of where the sattelites are parked but I can imagine situations where you might get better reception driving one way as opposed to another (most of the time it will not be straight overhead but at some angle to the ground). It may be that for certain directions, the antenna might work better in the back window.

In my car, the combined GPS/Sirius antenna is clipped to the back of the rear view mirror. I let my subscription expire but I do recall occasional signal dropouts, particularly under heavy tree cover (not cool for eating lunch in the car under a shady tree)

boosters will generally not help much with blocked signals. It's not that they loose sufficient strength, but they are completely blocked by anything metal or having a high water content like leaves. A reciever with a data buffer will help but only if you keep moving. I don't know which models (if any) feature a buffer.

I don't see any reason you could not get a more sensitive antenna, a perminant mount or maybe even an array but that kind of defeats your intention to just move it from car to car.

Finding out where the sattelites are WRT your position is a good place to start in understanding what you can and cannot do.

[Dean Huster]

If your Magellan/Garmin/Tom-Tom/whatever works fine in the same position, you may be barking up the wrong tree.

[haklesup]

If your Magellan/Garmin/Tom-Tom/whatever works fine in the same position, you may be barking up the wrong tree.

Those GPS recievers generally have a sattelite view page. On it you can see which sattelites it is recieving data from and what the signal strength is. Since GPS really only needs to see 3 sats (usually finds 4-6) it will appear to work more often than the radio which relies on only one, loss of resolution is not always noticable unless you are watching the DOP parameter. Anyway that GPS reciever display should show which sat's are hard to see when its on the dash. Its not unusual to see lock on sats only on part of the horizon and overhead.

Furhtermore, GPS always buffers data and you need to loose contact for nearly a minute to loose a lock with the modern chipsets. GPS is just more tolerant of line of sight reception because it uses data buffering and have 12 channel recievers by and large.

Now go back to an old Garmin GPS II vintage reciever and you may get similar performance to the radio. That chipset was slow and had a limited buffer and only a 6 channel reciever if I recall. it lost lock easily and fast

[jollyrgr]

Hacklesup brings up an important point about satellite location. GPS satellites are in orbital planes with a orbital period of about 12 hours. There is a constellation of 24 to 32 GPS satellites at any one time. (I believe there are currently 31 actively broadcasting GPS satellites with two retired birds as in orbit spares.) GPS frequenices are at 1.57542 GHz (L1 signal) and 1.2276 GHz (L2 signal).

The XM and Sirius systems use "geosynchronous" orbits but in a slightly different manner from each other. The XM satellites are "parked" over the equator and remain more or less in the same place "apparent" to ground based observers. XM uses two satellites at a time. Sirius satellites use an elliptical orbit that cause the satellites to "appear" to travel north and south of the US. At least one of the three satellites is always overhead of the US.

I'm not sure about the glass in your car but modern energy saving automotive glass has a thin metallic coating (may not even apear as tint) to reflect infrared radiation (heat). This coating will reflect RF from satellite and cell systems.

[dacflyer]

well i guess the material is shielding the signal to a certain point.
and as of now i cannot find a signal booster of any kind that i can use to cmpensate for the windshield.
but its strange that the gps's work ok behind the glass..and my antenna is in the same area.
most antennas on gps's are concealed too..

[haklesup]

but its strange that the gps's work ok behind the glass..and my antenna is in the same area

We have pointed out several important differences between SatRadio and GPS. To compare the two is like comparing FM and AM only they are more different.

Have you looked at the signal strength display inside and outside the car. The outside reading isn't really fair unless you are comparing it in a car with and without a windshield because outside you would have better visibility of the sky even without factoring in a windshield coating that may attenuate the signal.

Surely you have justified the cost of an external antenna by now

[dacflyer]

ya, i have justified an external antenna, but i know it will wear out transfering it from vehicle to vehicle..
oh well..