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Saturday, June 24, 2017

The reason for RF emission from new DVD/GPS Car unit - Active GPS antenna

Few weeks back I got a new Car DVD/GPS unit for my small Fiat Panda.
I am an EHS person and the car was already tested and found to be low EMF car.
In addition I have installed  RF protection film over the windows to block the RF coming from cellphone towers along the side of the road.
2 weeks ago I installed the new system in my car and was amazed that the system emitted RF radiation even after the WIFI was turned OFF and the Bluetooth was not operating.
I reported that on my latest post: http://norad4u.blogspot.com/2017/06/how-can-people-stand-car-multimedia-gps.html
It took me few days of trial and error  + testing the unit with an RF meter to find out where the radiation is coming from, it was the GPS antenna.

This a video I did after finding out that the GPS antenna was emitting the radiation
https://youtu.be/85yf96bamX4



I first thought that it must be a mistake, or a bug since I thought there should not be any RF emission from a GPS Antenna, since GPS units do not emit any RF, right? wrong!
Today I found-out that the GPS antenna main pin has 3.3V on it, I was begging to think why.
Later on tonight I came to the conclusion that the antenna must be an ACTIVE ANTENNA.
Did some research and it turns out that there are Active antennas for GPS out there and that some GPS units need them, and some come-with them.

So this in an other thing to test when you buy a car or a DVD/GPS unit for the car, does the GPS unit is Active or Passive.

I am still trying to work this out , maybe it is possible to just connect a passive antenna and all will be OK, maybe I will need to find a switch in the software or GUI. I hope I will manage to do it and to make this unit a NO RF EMISSION unit so I can use it in my car.


In the picture - the DVD/GPS Car unit in question, on my lab desk


The Solution:
I got and installed a passive GPS antenna.
Then I tested the RF radiation emitted from it, there was no radiation measured.
The GPS receiver gets all the data and gets the signal from the satellites.
It seems that the RF emitting Active Antenna has no added value.
This is perfect example of RF Exposure without any real need.

In the picture - Active (right) Vs Passive (left) GPS antenna


In the picture - no RF emission from the Passive antenna

Why is it radiating?
The unit I have has WIFI (which I turned off) and 3G data if you use dongles for communication (for getting updates of traffic) so I don't think there is a real function to the RF emission. I think it is just a RF signal that should not be there.
I don't think it was part of a tracking system as well (which is usually also relaying on UMTS or GPRS modems).

Active GPS external antenna gets 3.3V on the coax cable and use it to run a LNA (Low Noise Amp), basically a filter+amplifier.
The unit identify it is active by the load, and push the 3.3V over the coax.
If there is no load (in case of passive antenna) the unit does not find load, and does not push the 3.3V over the coax.
Please see - http://www.trakgps.com/en/index.php/information/gps-articles-information/67-gps-antenna-factsheet

I am guessing the Active GPS antenna that I got with my DVD/GPS car unit was one of the following:
1. Badly designed not to include an RF Faraday cage over the LNA amplifier in side it .
2. Badly designed to have the 3.3V reach the antenna it self and not just the LNA.
3. A malfunction antenna.
4. Maybe all active antenna are working this way - this is way I urge you all to use your RF meter to test as your GPS units with external antennas.

In this case, I used the RF meter right a way to make sure I turned the WIFI and Bluetooth correctly.
Then I saw the RF from the active antenna. 
It took me a while to understand what and where from, but if I did not use a RF meter it would have take me a lot more time and cost me many more headaches. 

So, the Bottom line is:
1. Use your RF meter every time you need to buy, install, operate a new device, you can never know.
2. If you have a GPS unit with external antenna make sure it is not active. If it is active measure it with the RF meter and let me know what you saw.
3. If you are find the external antenna to radiate, and you prefer it not to, try to change it with a passive antenna.

2 comments:

  1. It tracks you. The company can then sell data to third party or perhaps it is so the government can track you.

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    Replies
    1. Hello and thanks for your comment.
      In the case of a smart phone you are right.
      But in this case, the GPS unit wireless capabilities were turned off so the unit could not transmit the data outside the unit.
      I think this was just a bug in the design of the antenna.
      All the best
      amirb

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