I was in charge of installing a cellular antenna on the roof of the five-story building where I worked. The company designed cellular phones, and we had to see if they could transmit to a cellular tower then receive transmission from that tower. The modulation was TDMA (time division multiple access). I had to make sure that any antenna mounted on the lab roof did not interfere with or obstruct any other antenna on the roof. The new antenna also had to avoid impinging upon the cellular transmission to other cellular antenna towers.
When I went up on the roof, I noticed there was a cellular tower about five to seven miles away that was installed on a mountaintop. There was nothing obstructing the space between the planned roof antenna and the cellular tower system. It looked like we were set.
I ordered the new antenna and directed the workmen on how and where to install it. I also asked them to attach a coaxial cable from the antenna and bring the other end down to the lab where we could attach cellular phones through an adapter so we could use the external roof antenna as an extension of the cellular phones. That allowed us to test the transmit and receiving capabilities of newly-designed cellular phones.
Once the antenna was up, we started testing phones about three or four times a day. Right away, we noticed that the received signal had a low rate of AM modulation. Sometimes it lasted two minutes with a modulation rate of two to three seconds. Other times it lasted only 30 seconds with a modulation rate of 0.5 to one second. People on the receiving end of the cellphone transmission also experienced the same problem.
During the two weeks that this “ghost” modulation was occurring, I started taking notes of the time it was happening. It appeared the modulation happened at about 9:30 a.m. It again happened twice in the afternoon at about 2 p.m. and 4 p.m. One morning just prior to 9:30 a.m., I went up onto the roof to view the space between the antenna tower and our roof antenna. I saw a railroad track in the valley with a freight train running horizontally between the antenna sites.
I ran down to the lab and asked a technician about the cellular reception and he said that the AM modulation was occurring with a rate of two cycles per second. I deduced that the traveling railroad cars were causing a shudder effect because the tower transmitting wave not only has a direct path to our antenna, but the tower transmitting signal was also intercepted by the train’s movement. The train caused two signals to reach the roof antenna at a different time, causing a vector addition and subtracting, or, an AM modulation.
At 2 p.m., an AM modulation rate of one cycle per second lasting 30 seconds occurred when a passenger train passed. Since we could not change the railroad schedule, we simply stopped testing our cellular phones during the times the freight and passenger trains were passing.
This entry was submitted by William J. Garner and edited by Rob Spiegel.
William J. Garner is an RF microwave engineering consultant with 47 years design experience. He has published papers in trade journals and holds seven patents.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
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