Tuning a Corry CMIBF Cavity Filter

[JonMu]

Continuing the discussion from Do I Need A Filter?:

Concerning these filters:
High Power Low Loss UHF 900 MHz ISM BandPass Cavity Filter Repeater NEW

@mikkyo

You can get one of these cavity filters off eBay and tune it to 978Mhz (or pay to have it tuned when you order):
High Power Low Loss UHF 900 MHz ISM BandPass Cavity Filter Repeater NEW

https://www.ebay.com/itm/High-Power-Low-Loss-UHF-900-MHz-ISM-BandPass-Cavity-Filter-Repeater-NEW/362939232077?hash=item5480dd2f4d:g:iqgAAOSwKdZbZl3J
 
and concerning past experience!

@Jerryh47

I bought several of these filters from this vendor and tried retuning them. Two of 6 tuned to 978 MHz. Four wouldn’t tune that high until I took them apart and used a metal lathe to slightly shorten the resonator lengths.

 
Hi Jerry @Jerryh47 - I am looking for some help. I bought two of the Corry Micronics CMIBF Cavity Filters above and I want to try and tune one of them. And I am looking for some hints.

Are these the only screws (the ones around the edge) you removed to open up the filter?

It looks like there are seven (7) screws for tuning 7 rods.

About how much (a best guess is fine) did you end up grinding off?

[Jerryh47]

To remove the top all the small screws must come out.
I shortened each of the cylinders by 0.02” using a lathe. Sandpaper and patient effort would have been about as fast.

Then reassembled and tuned with a network analyzer. A noise source and SDR will probably work to align the filter, too.

The four tuning screws opposite the four resonators have stronger influence on the tuning than the ones between sections.

[JonMu]

@Jerryh47

To remove the top all the small screws must come out.

So even the divider small screws? Will do!

@Jerryh47

The four tuning screws opposite the four resonators have stronger influence on the tuning than the ones between sections.

That helps! So the four corner resonators adjust the band pass…

Maybe the divider screws tune band reject bandwidth?!?

Four cavities and four resonators made sense. Once I saw seven tuning screws it got confusing quick!

[JonMu]

Only 37 screws to remove to open the filter…

---

very nice!

[Dxista]

Indeed. Well worth the eBay price on the looks alone. Unfortunately they won’t ship to Canada, and it’s likely due to shipping costs.