Shielded ?

[TechTwo]

If I use a scanner inside a vehicle with the antenna on the scanner, using vehicle power, won't this shield the antenna as the metal body/chassis of the vehicle is now tied to the scanner ground? This would attenuate the signal depending to where the scanner is located and the amount of windows, so in a closed van the scanner would almost be deaf.

Is this correct?

I don't have a vehicle power adapter to try it for myself, but I believe this to be the case.

Any ideas?

Thanks

Michael

 

[MacombMonitor]

No, that is not correct. While it is true that the negative side of the 12VDC power will be grounded to the metal body of the car, it is totally isolated from the antenna itself. The only portion of the antenna that is grounded is the outer metal of the BNC connector, or what ever type of connector you're using. The actual antenna element is the center pin/wire of the antenna assembly.

 

[Al42]

All of which has nothing to do with whether having the antenna inside the car will shield the antenna from signals coming from outside the car.

The radio doesn't have to be grounded for a Faraday Cage (which is what the car is, even if it's a very poor one) to work. Signals with wavelengths much larger than 4 times the largest opening in the cage just won't penetrate it very well. Of course, for scanning this means that there's some slight attennuation for low band, and not much above that.

Of course you will notice a difference between putting the scanner on the floor of the car and holding it next to a window - but it will receive strong signals - even in the trunk.

 

[MacombMonitor]

Wouldn't the car's metal body actually need to be grounded, to ground, to even begin to become a Faraday Cage? Considering the car is sitting high on four insulators, aka; rubber tires. As I recall, Grove Enterprises used to sell a short piece of coax cable with a BNC on one end for your scanner, and nothing buy a ring terminal on the center conductor of the coax, to be attached anywhere, to the metal body of the car. Therefore using the car's metal body as the antenna. It worked...maybe not well, but it worked. I think in this case the scanner itself could not be grounded to the cars metal. I think this was intended to be used with handhelds.

 

[Al42]

Wouldn't the car's metal body actually need to be grounded, to ground, to even begin to become a Faraday Cage?

No. It doesn't work by grounding the signal, so ground has no effect. A plane at 30,000 feet is a pretty good Faraday Cage.

As I recall, Grove Enterprises used to sell a short piece of coax cable with a BNC on one end for your scanner, and nothing buy a ring terminal on the center conductor of the coax, to be attached anywhere, to the metal body of the car. Therefore using the car's metal body as the antenna. It worked...maybe not well, but it worked. I think in this case the scanner itself could not be grounded to the cars metal. I think this was intended to be used with handhelds.

If you connect a receiver in a Faraday Cage to the cage, using the cage as an antenna, it'll work as well as any random-length wire antenna.