Welcome to 2018
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Recycling Circulators to Other Frequencies
- By Steve Fogerty -
Circulators are extensively used on LMR sites. The purpose is to reduce mixing (inter-modulation) products created in the transmitter’s final transistor. Without a circulator, the transmission frequency can mix with signals from other sources received by the transmitter antenna & travel down the antenna cable. They then mix & are re-radiated as sum & difference frequencies. It can cause interference to close by receivers on site. Circulators only allow signals to travel in one direction, in normal use from the transmitter to the antenna. The other important function is to protect the transmitter from damage due to high reflected power with an antenna-related problem. Any reflected power is dumped into the circulators dummy load and the transmitter still sees a perfect 50 Ohm load. Circulators have an external dummy load & Isolators have an internal dummy load, otherwise, they are the same.
A colleague who hasa LMR business mentioned he had a few old circulators on A & B band. He then asked if whether they could be modified to work on EN or even EE band. I said that is quite a stretch in frequency but I’d see what could be done. Some circulator modules are frequency specific & become lossy outside of the specified range. TDK Hexalators are an example & others too difficult to access mechanically. The first circulator model for modification was a Deltec IS85-3AN-20 (81-88 MHz- A Band). Similar Deltec models are CC85-3AN-2 also on 81-88 MHz & CC105-3AN-2 on 104.5 MHz (B Band). TDK223A on 84.5 or 104.5 MHz have identical circulator modules to the Deltec modules mentioned.
The first is always the most difficult & no exception here. After many hours of experimentation, I had gutted the internals, made new tuned circuits & matching components for 153 MHz on the E band. It also needed an external magnet to work correctly.
The results: The network Analyzer showed it works just as good as in its former life on 84 MHz. Insertion loss: less than 1 dB & around 50 dB reverse isolation. Later I reworked a version to EE band with the same results.
Worthwhile? Yes, for about 1/3rd the cost of a new circulator, an old never to be used again unit can be modified with the same performance as a new one.