Hi,
Our local community FM radio station have a rebroadcast site about 40 miles away, receiving 101.1 Mhz and re-broadcasting at 105.1Mhz. They think they have a problem with the receive antenna there that receives the main station signal because the audio sometimes cuts out for fractions of a second or simply sound distorted. The receive signal is likely demodulated and then re-transmitted. But it also could be done without demodulating if the re-transmitter is designed for this purpose; I'm thinking the latter. They haven't had a technician in the last ten years that can answer questions about the installation and I haven't been there yet, but they have asked me to look at it. It would be nice to first test it from the ground. If we can be sure the problem is either the antenna or the coax in the tower, then we can call an expert climber to make the fix.
So what is the best way to test a receive antenna already in service in a tower?
I have a TinySA spectrum analyzer, so will be able to see the rf level out of the coax in the re-transmitter building. There likely is a steep notch filter for the re-transmitted frequency for that system to operate properly and not overpower the receiver front end. Would that normally be at the antenna or in the building?
If the signal if too low would it be OK to use a TinyVNA to measure return loss with it?
I am not sure how to go about this.
1) first the re-transmitter would either have to be turned off or at least the notch filter kept in the receive antenna circuit before going to the VNA.
2) But then, the signal received would be full of signals from all other broadcasters in the area making the VNA almost unusable to measure return loss.
Am I correct in these assumptions?
What about amplifying the signal with a small broadband amplifier and putting a directional coupler after this amp, and then use the spectrum analyzer to measure return loss port vs forward port and simply use the VNA as an RF generator. Does this make any sense?
If not, how should I go about this?
BTW, I used to do this for a living, but retired 15 years ago with no contact with this type of work since.
Your insight will be appreciated,
Daniel
Our local community FM radio station have a rebroadcast site about 40 miles away, receiving 101.1 Mhz and re-broadcasting at 105.1Mhz. They think they have a problem with the receive antenna there that receives the main station signal because the audio sometimes cuts out for fractions of a second or simply sound distorted. The receive signal is likely demodulated and then re-transmitted. But it also could be done without demodulating if the re-transmitter is designed for this purpose; I'm thinking the latter. They haven't had a technician in the last ten years that can answer questions about the installation and I haven't been there yet, but they have asked me to look at it. It would be nice to first test it from the ground. If we can be sure the problem is either the antenna or the coax in the tower, then we can call an expert climber to make the fix.
So what is the best way to test a receive antenna already in service in a tower?
I have a TinySA spectrum analyzer, so will be able to see the rf level out of the coax in the re-transmitter building. There likely is a steep notch filter for the re-transmitted frequency for that system to operate properly and not overpower the receiver front end. Would that normally be at the antenna or in the building?
If the signal if too low would it be OK to use a TinyVNA to measure return loss with it?
I am not sure how to go about this.
1) first the re-transmitter would either have to be turned off or at least the notch filter kept in the receive antenna circuit before going to the VNA.
2) But then, the signal received would be full of signals from all other broadcasters in the area making the VNA almost unusable to measure return loss.
Am I correct in these assumptions?
What about amplifying the signal with a small broadband amplifier and putting a directional coupler after this amp, and then use the spectrum analyzer to measure return loss port vs forward port and simply use the VNA as an RF generator. Does this make any sense?
If not, how should I go about this?
BTW, I used to do this for a living, but retired 15 years ago with no contact with this type of work since.
Your insight will be appreciated,
Daniel