Motorola MICOR UHF BPF and Circulator tuning help

Motorola® MICOR UHF Band Pass Filter and Circulator Tuning

By Kevin K. Custer W3KKC & John Everson AB6LI

Concept:
Many times when trying to down band the MICOR UHF Station there are issues with getting enough power from the transmitter. This article deals with tuning the bandpass filters and circulator in the "Antenna Network" to work better in the UHF ham band.

Description:
Since there isn't a genuine Motorola tuning guide for the BPFs and circulator in a MICOR Unified Chassis UHF Station or Repeater, this information is presented to try and cover it.

Benefits:
Allow your MICOR UHF transmitter to have better efficiency and power output in the ham band. Spurious signals may also be eliminated.
The exciter and receiver usually tune okay, but other parts don't, read on.

Modification:
The information in the article referenced below can be used to tune the first BPF. The information is pertinent to the UHF MICOR Station since it actually uses a high-band exciter:
MICOR Bandpass Filter Tuning article.

Next, take the second bandpass filter (the one in the tripler section) and remove the 'soldered on' cover that hides the tuning screws. Tune the screws for maximum power from the tripler. Keep tuning and retuning till no further improvement. Throw the cover away if you wish or solder it back on.

The last thing is the MICOR UHF Band Pass Filter and Circulator Tuning:
Remove the antenna network from the station and the cover from the circulator unit. Inside you will see a circulator (magnetic discs under a 3 legged metal cover), the output filter (the sealed rectangular box with coax feeding it) and the circulator reject load (the small rectangular wafer mounted on the heat sink). There are six trimmer caps that you will need to adjust. Do not attempt to adjust them with out the lid on. The lid greatly affects the tuning and must be in place during the tuning process. You will need to drill access holes. Measure and drill the top cover as needed for access to the six trimmers. As an alternative to measuring I have used a piece of regular paper 2 3/4" square, laid it on the top of the circulator, fitting the paper into the corner of the enclosure, and simply rubbing a pencil over the trimmer caps to make a simple pattern. Then take the pattern, punch the holes in the paper, lay it into the cover (be sure to properly orient the pattern) and then mark and drill. I use a 3/16" drill bit which gives plenty of access. Be sure to clean the cover and remove any burrs or left over shavings as they will stick to the circulator and/or possibly short out some of the other circuitry. Re-install the cover and all four screws.

The tuning procedure varies depending on the equipment available. A service monitor and a return loss bridge are the best method but are most likely out of reach to most hams. A signal source, a good reliable wattmeter, and a good 50 Ohm load make the job quick and easy on the bench. Or, the unit may be tuned right in place in the station unit.

The antenna network also doubles as a directional coupler to provide forward and reflected power feedback to the power control board for power output control. These adjustments will provide proper throughput for RF in both directions.

Hook your signal source to the input connector, connect the wattmeter to the output cable, then terminate the wattmeter with the dummy load. Inject an on-frequency signal into the network and tune all six trimmers for the maximum output. Do this twice. Be sure to use a non-metallic tuning tool. Then, note the RF level. Next, reverse the input and output cables. Connect the signal source to the output and the wattmeter with dummy load to the input connector. Inject a signal into the output conector and adjust the two trimmer caps next to the circulator load on the heatsink for MINIMUM RF on the meter. This assures that any reflected power is going to the circulator load and not your PA! You may then reverse the connections back to the original configuration and re-check RF throughput. Your results should be very near your earlier readings. If not, you may touch up the other four trimmers but this isn't normally necessary. Re-install the network and verify proper output from the PA.

The network can also be tuned when installed in the station. Turn the power set control all the way up. Key the transmitter, then tune the trimmers for maximum power output into a load or resonant antenna. Another alternative (if you have a way to monitor PA current draw) is to adjust the trimmers for maximum power out with minimum current draw (maximum efficiency).

After tuning, replace the network and install all of the covers and enjoy. The Antenna Network is lossy even when tuned correctly, just do the best you can.