Low power transmitters have problems similar to those encountered by big-time transmitters, but the fixes are not always the same.
Take AM radio for example. Using a 'combiner' it is possible to put two or more transmitters on a single antenna without the transmitters interfering with each other. But the way part 15 low power AM transmitters are designed according to FCC rules, there's no way to design a combiner that will preserve enough signal strength for combined transmitters to provide enough output to be effective. That means that when several AM transmitters are put in operation they must have separate antennas and be located far enough from each other so as not to interfere. Things get even more complicated when other types of transmitters are clustered near each other.
This is a complicated subject and it may be hard to follow along but it is also difficult for me to find the best way to talk about it without being incoherent.
The other day in a thread not too far away I mentioned that my C.Crane FM1 Transmitter had developed a whistle. Today that whistle was so piercing it drove me mad and I was about to remove the FM1 from service when I discovered that the whistle got either stronger or weaker when I moved the TP-Wireless TP-WT02 2.4 GHz Transmitter either closer or farther away from the FM1. So turning off the TP-Wireless solved the FM1 problem, but it also took KDX-AM off the air since the TP-WT02 provided the audio link out to the Procaster AM Transmitter, but the point is made moot by the fact that yesterday I closed KDX-AM because everytime I walked through a doorway it was losing audio as my body blocked the 2.4 GHz signal. The 2.4 GHz signal path needs to be raised above human head level to avoid being blocked by bodies walking around.
To compound all this I plan to add more AM transmitters as I've discussed on other nearby threads, but they can't be too close to the other transmitters, which brings us full circle back to what I started out talking about.
It also works the other way as well.
When I ran AM on a Hamilton Rangemaster, the antenna was located close to my amateur radio antenna. I could hear the broadcast in the background noise on the 40 meter (7 Mhz) band - no problems on the other amateur bands.
I've never had issues using FM, even when the Decade transmitter was located right beside my wireless router. Or on the 2 meter amateur band (144 Mhz), which is right beside the FM broadcast band.
AM seems particularly prone to both generating and receiving interference.
Same here. When I was using the procaster my home phone was near the antenna and the AM broadcast was in the phone when I picked it up. If someone left a message on the answering machine they heard my station. But with FM this doesn't happen. Seems with amplitude modulation other electronics can reproduce it.
To summarize what we are talking about... part 15 operators using multiple types of transmitters have a challenge of seeking physical places to locate the equipment that avoids interference between devices. IT'S FUN!
Today I will be trying to safely relocate the 2.4 GHz Transmitter so that audio can be sent to the Procaster AM Transmitter without causing a whistle on KDX-FM.
A straight wire connection would be a permanent solution, except the distance and number of doorways between the sending and receiving point will require a balanced line which will mean drilling into the baseboard, sending the wire through the basement and back up, and a small unbalanced-to-balanced amplifier for which I have the parts but it's not yet built.
Previously I tried to use KDX-FM to get signal to the Procaster, but chronic multi-pathing ended that effort.
What Got Done Today.
Some thinking and planning. I'm fairly sure that the whistle I was getting from interaction between the 2.4 GHz Transmitter and the FM1 Transmitter was caused by the 2.4 GHz antenna being up too close to an audio cable feeding both transmitters. Tomorrow at noon the radio station will be shut down while I re-install things keeping audio & power cables as far from the transmitters as possible. As always the result will get published here so we might all learn from the experience.
Result Report!
3:30 in the morning proved to be a good time to re-locate a few things to solve the bugs we've been talking about. The TP-Wireless 2.4 GHz Transmitter is now about 8-feet away from the FM1 Transmitter, and is raised three feet higher than before. Also the microwave antenna has a better shot through the doorway and is almost in line-of sight with its receiver at the Procaster AM Transmitter. The 2.4 GHz Transmitter is now in the same cluster as the Wi-Fi Router and Cordless Phone, all Ghz radiation happening in the same physical array, and luckily not appearing to collide with each other. The unwanted whistle is gone from the FM signal. The microwave signal is no longer prone to interruption by walking around and blocking its signal path with bodies.
Further improvement is possible by raising the TP-Wireless higher, which will require adding an extension to its DC power cable, we are now at the limit of its 5-foot length, and also a secure mast (mini-tower) will be necessary, always a daunting project when trying to fit with surrounding furniture.