Conversion to 1296 MHz for the DF1SR circulator

In need for a number of circulator / Isolators for my new 23 cm solid state PA, I got some circulators from Georg DF1SR. These were originally build for 1450 MHz and these did not have a too good specification on 1296 MHz. Typical I did see a isolation of 13-15dB and a insertion loss of 0,7dB.

With some good advice of Wolf-Henning DF9IC I tried to convert the first circulator.

I removed the top plate and inserted several plates of all different thickness. An aluminium plate of 0,8mm proved to be the best, resulting in a loss of 0.3dB and an isolation of 23dB  or so. Not too great but better than nothing! Following the advice of Wolf Henning I tried to tune by adding some copper foil on the PCB near the connector connection of the circulator. A common procedure on the uWave bands. It was a very tedious job, because one had to open the circulator, add the tuning vanes and close everything again. In the end after 2 evenings of playing I decided it was not for me. No real improvement was archived. It remained about the same value as with only the 0,8mm alu plate inserted. I must admit I was a bit disappointed as I had hoped for more. The reports from DF9IC were more hopeful. Measurements were done with my spectrum analyser and the tracking generator.


Measured isolation of the first circulator




Measured transmission of the first circulator


After that I decided to give up on the conversion, I decided to change the second circulator. To my surprise the second one proved to be done in 5 minutes! An isolation was archived of 30 dB. This shows that this magnetic isolator / circulator conversion is not an exact science. One would need to change more pieces to give a more accurate result on this. The most important is to test the isolation and see if you can get some improvement.

My 1296 amp consisted of 2 200W amps. I wanted to add a circulator to the output of each amplifier. BUT the phase of the 2 circulators was not anywhere near each other. Seeking advice from some professionals this is ONLY the case when the circulators were produced in same batch. Not to speak of home converted circulators.