If separate antennas are used for receive and transmit, they must be placed exatly above each other, to achieve maximum decoupling. The mast will influence the radiation pattern of the lower antenna, cousing a minimum in the direction of the mast, especially when the distance between the mast and the antenna is 1/4 lambda.
In those situations, where little room exists on the mast, long coaxcables have to be used, or two repeaters on different frequency bands must work on the same location, it is often more economical to use one antenna and one cable. Combined operation for a 2mtrs and a 70cms repeater on one antenna is simple, and reduces costs and installationproblems. PI2RTD and PI3RTD work that way.
With one antenna and cable:
|1.||The radiationpattern (coverage) is the same for transmit and receive.|
|2.||Simpler and cheaper cabling and installation.|
|3.||Greater duplexfilter isolation is needed (up to 40dB). The filter will be costlier and will have more insertionloss.|
|4.||Less wideband isolation between transmitter and receiver. Extra bandpass cavities can be needed to suppress transmittersideband noise.|
|5.||For optimum results sometimes a circulator must be added (depending on the quality of the duplexfilter).|
Suitable antennas for one-antenne-operation.
If one antenna is used, all components of the whole antenna installation (including mast and guywires) must be of top quality.
Bad contacts are desatrous (cracking and intermodulation). Remember: The transmitter signalstrength is 160dB or so above the sensitivity level of the receiver. A small non-linear resistance (oxidation) couses intermod, cracking near the repeater antenna induces loud widebandnoises on the receiver input frequency.
Becouse of the more complicated duplexfilter and therefor higher insertionloss, an antenna with higher gain is needed for the same coverage.
Especially when a circulator is used, flawless match between antenna, cable and circulator under all weather circumstances is needed.
pa0nhc: If the antenna radiates exactly towards the horizon (CHECK this!), is a high-gain antenna allways better, as the vertical radiation angle is smaller. Then noises from below the antenna are better suppressed, enhancing signal/noise ratio, especially in a busy city-environment. Reflections to the roof are less. This can significantly improve the repeater-range, as it results in a lower radiation angle towards the horizon ("Fresnell zone"). For this reason too, allways put the antenna as high as possible above the roof.
pa0nhc: If, due to mismatch, the antenna returns transmitter energy to the repeater, a circulator directs this energy to the
receiver input. Possible result: blocking and/or intermodulation.