These are the cases when it is necessary to quickly change the receiving and transmitting frequencies in a wide range (≥1MHz). Also, change the difference between the Rx/Tx frequencies within more than ±1.5 MHz from the nominal value. The given frequency values are indicative, determined by the possibility of setting mobile duplexers and depend on the range.
For high-quality operation of the repeater, it is necessary to ensure a sufficient level of decoupling between the receiving and transmitting paths.
We find this level in the graph below.
The horizontal scale shows the Tx-Rx difference in MHz, the vertical scale shows the required resolution level in dB. The table shows the correction for transmitter power Tx.
This level of resolution must be provided by the spatial separation of the receiving and transmitting antennas. The value of the resolution from the divergence is shown in the graphs below.
Vertical scale – resolution in dB. Horizontal – distance in feet and meters (in parentheses).
Dependence of isolation between antennas on their vertical separation
Dependence of isolation between antennas on their horizontal separation
Let’s consider the use of graphs on the example of building an AFU path in the VHF range with Rx/Tx spacing of 4.5mHz and Tx-25W power.
– On the first graph, we find the required level of resolution. It is approximately 50 dB (taking into account the power of the transmitter).
– Now let’s estimate the necessary spatial diversity of antennas. From the graph of horizontal diversity, we can see that in order to ensure resolution of 50dB, even without taking into account the gain of the antennas, the distance will be about 50m. With vertical distribution, this distance is 6 m. The optimal choice of antennas is a pin collinear receiving top of the mast and a dipole below it for transmission. If there is an opportunity to increase the spread of Rx/Tx frequencies, then, as an example, when the spread is increased to 10 MHz, the vertical spread distance is reduced to 3 meters.
It is often difficult to provide the necessary level of resolution only by the spatial spread of antennas, therefore it is recommended to install a bandpass filter (PF) in the pleasant path with a bandwidth that provides the entire band when reconfiguring the frequency of the receiving path. Additional decoupling is provided by connecting a band-pass filter (PRF), the rejection of which is adjusted to the frequency of the transmitter. The filter selection is determined by the Rx/Tx frequency difference and the required Tx suppression level.
Decreasing the power of the repeater in order to improve the reception path is not effective enough. So, for example, reducing the power by two times provides a signal attenuation of only 3dB. This, most likely, will be reflected in the quality of reception of subscriber radio stations.