Isolation in Passive Repeater needed or not ?

[enrico]

Does anyone knows why isolation is needed when using one set of antennas (high gain donor and omni indoor), each other connected by a coax cable acting as passive repeater ?


Thanks

 

[jiripolivka]

Does anyone knows why isolation is needed when using one set of antennas (high gain donor and omni indoor), each other connected by a coax cable acting as passive repeater ?


Thanks

In really PASSIVE repeater, isolation does not matter much as there is no gain in the system. The system only picks up a good signal by one antenna in a location where it is good, and by a cable and another antenna the signal is retransmitted in another location where the first signal is poor.

If one inserts a suitable amplifier in the above cable, then a caution is advised. If amplifier gain only compensates for a cable loss, the system works as needed. If amplifier gain exceeds cable loss, then the system may start oscillating due to returning amplified signal from the "output" antenna back to the first "input"antenna, due to a possibly poor isolation between the two.


To check if the isolation is adequate, use an amplifier with a variable attenuator cascaded, and adjust the attenuator so that the system just stops oscillating. The problem can be if persons, trucks or other objects may move around system antennas, so they could reflect signals and affect isolation between the "output" and "input" antenna.

 

[biff44]

Even with a passive receiver, there might be multipath induced. I would say that the isolation would want to be at least 25 dB (an maybe more if a very complex modulation) to keep it from causing intersymbol interference

 

[enrico]

Hi Biff,

Thanks for all.
I guess your second post is more in-line to my thoughts.
I have done a calculation with a cable having similar characteristics to the RG-213 at frequency of interest and found out that a shift of 180 degrees is introduced by the dielectric cte (2.2) if the cable length is 4.88 long, for example. There might be other situations with cable length according to its multiple factor introducing same phase shift.

Thus, a destructive interference retracts this scenario, recalling the fact it happens not only with signals of same frequency but having also different amplitudes, so the resulting signal amplitude will be proportional to it. It can be the case the feedback signal from the indoor to donor antenna have a very weak amplitude not resulting in any impact. Isolation in this case play an important role when the resulting signal plus antenna gain cannot offer the indoor coverage.

I understand there is no oscillation involved in the exercise, since a passive circuit does not introduce gain greater than one and no sustained amplitude.

E.

 

[FvM]

Even with a passive receiver, there might be multipath induced.

Might be. Unfortunately nothing is said about the application (involved frequency band, modulation scheme etc), so everything is just a guess.