Duplexer Question

[PDMR25]

Scenario….Two rooftop antennas mounted approximately15 feet apart. One is a UHF repeater and the other is a UHF mobile being used as a base station. The UHF repeater is transmitting at 40 W and the other antenna is transmitting at 80 W. Will the 80 W transmission damage or detune the duplexer in the repeater because the antennas are so close to each other? The 80 W base station will be transmitting about one megahertz higher than what the repeater duplexer is tuned to. Also, the antennas are 15 feet apart, but also the repeater antenna is about 10 feet higher than the other antenna.

Thank you.

 

[jeepsandradios]

It really depends. Without proper test equipment it would be hard to say what effect it would have. With that said I have one SAR site with 2 UHF repeaters. Both have quality duplexers and both are on the same 40' tower with one antenna on top of tower and one 10' below the other. I do see an increased noise floor but the filtering in the duplexers and multicoupler clean up most of it before hitting repeaters. But I dont run a cheap mobile duplexers. They are designed BP/BR duplexers for an RF site.

 

[RCBi-Dave]

Hello and welcome PDMR25

Scenario….Two rooftop antennas mounted approximately15 feet apart. Also, the antennas are 15 feet apart, but also the repeater antenna is about 10 feet higher than the other antenna.

The mounting of the 2 UHF antennas is good and is better than most installations that are trying to avoid interfering with each other. The 10 foot vertical spacing is the most helpful.

Will the 80 W transmission damage or detune the duplexer in the repeater because the antennas are so close to each other? The 80 W base station will be transmitting about one megahertz higher than what the repeater duplexer is tuned to.

The 80w transmit signal will usually not damage, detune, or harm a good quality repeater duplexer.

I would be concerned with the potential for RF inter-modulation (IMD) interference that will occur if/when both transmitters transmit at the same time. The transmit signals could mix together inside each other’s transmitter amplifier output circuits and create additional unwanted RF signals that will pass back out to their antennas.

A circulator or isolator device with harmonic filtering would be need to be added to the output of each transmitter to prevent the unwanted IMD problems. The devices would stop all unwanted RF signals from entering the outputs of the transmitter amplifiers. Each circulator or isolator is tuned to pass only the frequency of the transmitter output that it is connected to. It is best to be a good RF neighbor and keep a "clean house" especially in urban areas.

If the UHF repeater duplexer is a simple “reject only” type of filtering without any band pass filtering, the repeater receiver will usually be desensitized due to RF overloading from the 80w transmitter. Adding a RF notch filter tuned to the 80w transmitter frequency and connecting it inline with the repeater receiver antenna circuit will help reduce/remove the specific 80w transmit frequency from reaching the repeater receiver front end.

Best Regards

 

[PDMR25]

Hello and welcome PDMR25

The mounting of the 2 UHF antennas is good and is better than most installations that are trying to avoid interfering with each other. The 10 foot vertical spacing is the most helpful.


The 80w transmit signal will usually not damage, detune, or harm a good quality repeater duplexer.

I would be concerned with the potential for RF inter-modulation (IMD) interference that will occur if/when both transmitters transmit at the same time. The transmit signals could mix together inside each other’s transmitter amplifier output circuits and create additional unwanted RF signals that will pass back out to their antennas.

A circulator or isolator device with harmonic filtering would be need to be added to the output of each transmitter to prevent the unwanted IMD problems. The devices would stop all unwanted RF signals from entering the outputs of the transmitter amplifiers. Each circulator or isolator is tuned to pass only the frequency of the transmitter output that it is connected to. It is best to be a good RF neighbor and keep a "clean house" especially in urban areas.

If the UHF repeater duplexer is a simple “reject only” type of filtering without any band pass filtering, the repeater receiver will usually be desensitized due to RF overloading from the 80w transmitter. Adding a RF notch filter tuned to the 80w transmitter frequency and connecting it inline with the repeater receiver antenna circuit will help reduce/remove the specific 80w transmit frequency from reaching the repeater receiver front end.

Best Regards

Awesome! Thank you!

 

[speedway_navigator]

Vertical separation increases the isolation between antennas better than horizontal.
If the 2 antennas are on the same horizontal plane you have about 39 dB attenuation at 460 MHz from the 80W antenna if it does not have gain (and if my math is right.)

In theory the mobile antenna will see about 10 dBm signal strength, then you lose about a decibel or so for coax and connector loss depending on coax type and length.