Antenna Spacing

[tush]

Ok, I have tried to search this and either Im blind, or I am searching the wrong thing. If I place 2 antennas on a tower (GMRS Band) one for TX, mounted at the top of my mast, and a RX mounted below. What is the spacing to avoid over powering the receiver when transmitted? I have seen everything for 1/4 wave to 100 Foot. So the information is a little foggy. Yes a filter would be best, but I am finding it hard to find one for these frequencies. Any Ideas?

 

[tweiss3]

The minimum performance for a UHF duplexer is 70db isolation. To meet that isolation with vertical separation (one antenna directly over the other one), the minimum space from top of the bottom antenna to the bottom of the top antenna is 30', and that increases if they are not perfectly one over the other.

 

[Ubbe]

It depends of the antennas radiation pattern, how much signal it actually will spread between antennas. It also depends of the receiver. With a good receiver I can input 1W and receiver sensitivity or intermod handling doesn't flinch. That would be impossible with a scanner type of receiver or most portable and many mobile 2-way radios. Even base station receivers can sometimes not be good enough to handle strong signals.

/Ubbe

 

[tush]

So as I only have 35' to play with, Running a local repeater in simplex would be the simpler option? At least until I can set aside the finances to get a duplexer.

 

[prcguy]

Its common on commercial towers to have about 20ft minimum spacing where the top section of the tower is reserved for receive only antennas then starting about 20ft below the transmit and full duplex antennas are installed. However you need much more than just antenna spacing and at the very least a 1/4 wave band pass cavity on both the transmit and receive path, not to mention an isolator on the transmitter.

A cheaper 4" band pass cavity at UHF will be down about 15dB at 5MHz spacing so one on the transmit side and another on the receive side will get you about 30dB isolation. My goal would be a good 85dB isolation for a 50w repeater so that would be looking at antenna spacing to give you the remaining 55dB which is unlikely. Instead I would look for 8" cavities which will be down about 27dB at 5MHz with a pair giving about 54dB of isolation. That will leave about 31dB isolation from vertical antenna spacing to achieve 85dB which is more realistic.