Antenna Filter Project:
The PCB for the 900Mhz low pass antenna filter arrived. The bench tests show it works well. Insertion loss at 910 Mhz is about 0.4dB and it provides >-25dB of signal attenuation at the GPS's RF band. The second harmonic is >-55dB below the fundamental. The third harmonic has been suppressed and I can no longer see it on my SA. Some field testing will begin in a few days and hopefully the antenna filter helps those 900 MHz installations that suffer from persistent GPS problems.
Here is a photo of the assembled antenna filter:
Here it is mounted on the 900 MHz Tx:
Early this morning I found that even with the stock wire antenna (which I consider a worst case antenna), the GPS was decently immune to the Tx. I could get the Tx within five inches of the GPS module (I did not try any closer due to the fear of GPS damage) and could not see any decrease in HDOP or satellite count. With the filter removed, the GPS lost all satellites in seconds.
I repeated the test several hours later and the results were quite different. With the same setup, within three minutes I lost most of the satellites with the filter installed. I had to move ten inches away before it could maintain the full satellite fix. With the filter removed, I lost a couple satellites within a minute. I was surprised that the results were so different from this morning. It must be related to satellites that are in view at the time. Or evil spirits.Same subject, different required solution:
As I mentioned before, not all the interference is coupled through the air. In addition to the coupled noise, some installations seem to experience significant EMI/RFI noise that is conducted through the cables. So, the antenna filter may need some additional help to fully resolve hardship situations.
Toroids are helpful (every installation should include them!), but they are not always the complete fix. The GPS host cable filter that was created a few weeks ago was meant to cure the common mode noise. But, it seems to me that it might be more practical to filter the A/V cable on the 900 MHz Tx instead. So, as an experiment I created a companion Tx cable filter board that has LC filters for power, video, and audio. It is designed to reduce the common mode noise that travels out of the module via its wiring.
I haven't had time to test out the Tx cable filter, but here is a close up photo:
Here it is mounted on the 900MHz/500mW Tx module: