Page 24 - RTH3BA Preview

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In Figure 32-15, our transmitter is connected to port A. A 50Ω load is connected to
port C. Our antenna is connected to port B. Our transmitted signal enters port A and
exits port B to the antenna.

Any signals coming down our antenna (from other transmitters) will enter port B and
exit port C and be dissipated in the 50Ω load. What we have achieved is isolation
between our transmitter and all other transmitters on the same or nearby radio site.

Figure 32-15. Creating Isolation between transmitters
A single isolator reduces the levels of other nearby transmitters in our transmitter's
power amplifier by about 20dB. If we did not do this, external signals could enter our
transmitter's power amplifier and mix with our transmit signal to produce new
frequencies that are reradiated causing interference to other receivers. This is called
transmitter intermodulation.

If more isolation is needed, then two single isolators can be placed in series or a dual
isolator, which is two single isolators in the one package, can be used. Figure 32-16
is a picture of a dual isolator. With such an isolator you can expect at least 40dB of
isolation and up to 50dB. With the additional use of cavity filters and careful antenna
placement, an isolation of more than 100dB can be achieved.

Figure 32-16 Dual ferrite isolator
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