I found three of these Celwave 8-cavity filters which are currently tuned to about 850 MHz. And if you think these can be re-tuned to 1090 MHz then please consider the following offer.
To anyone willing to take on the re-tuning task here’s my offer. I’ll ship you three filters, you re-tune one to 1090 MHz and ship it back to me. You keep the other two for yourself.
Thanks for your consideration, Tom.
Dimensions: 227mm long by 102mm wide by 32mm thick.
Input & Output connectors, N-type.
RF Sample connector, BNC.
You’re looking for someone with a network analyzer that covers that frequency range.
I have a scalar network analyzer that can do the job – but…
Usually cavity filters are designed for a particular band, or portion of a band, and with a high Q which implies a limited tuning range, like 5% or under. You’re talking about re-tuning an 8 section cavity filter off-spec by about 25%, which is freaking enormous (technical term).
The good news is you’re tuning to a higher frequency, so it might be doable, but that might (almost certainly will) require doing violence (sorry, making adjustments) to the cavities and particularly the coupling loops. I’d also worry that the insertion loss is going to go up with the shift in frequency.
If you can’t find anyone closer, I’m willing to take a shot at one, but I’d say just ship out one (unless you have a spouse who has become highly allergic to the things and wants all of them gone yesterday; this happens), and I’ll see if it’s doable, but I’m not really optimistic. The coupling loops are critical, and will need to be reworked, a lot of cut-and-try as these things are notoriously difficult to model.
Thank Bob,
I didn’t know if they were usable or not. And it sounds like too much of a shot in the dark to be worth trying, IMHO. No harm, no foul, they did not cost me anything they were in some old bays of analogue cellular equipment bound for the recycler. On the same shelf as these filter were two 24 VDC adjustable gain RF amps, which might be useful and these I can try out myself.
Next time I’m in the US I’ll drop one of the filters in the mail for you.
Cheers, Tom. http://i.ebayimg.com/00/s/NjgzWDEwMjQ=/$(KGrHqV,!p0E9d7lsntVBPiz0eJSpw~~60_35.JPG
The RF amps look like a winner – you can hook one up and do a frequency scan to see how it behaves.
I think it’s worth taking a whack at the cavity filters, but as I said, not overly optimistic. Another gotcha in retuning is the coupling between stages/cavities… The next time you wander south of the border, put one in the post.
Bob,
I have not taken one apart. It looks like the filter was built within a section of rectangular copper wave-guide with the end pieces inserted in either end and hard riveted in place. Do you think it is worth taking one apart? If so, any advice on what to look for or pay attention to?
Here’s a sweep of the Celwave cavity filter:
The first peak is at 841MHz, with a very respectable 2.5dB insertion loss. Bandwidth is around 50 MHz (that’s the -3dB points). The second peak is wider, and is at 2182MHz. The little blip to the right of that one is at 2414MHz, and is down 31dB.
Physically, the sucker is riveted together. I’ll take a whack at retuning over the weekend, or next week after I get a few other things off my plate.
I’m not too optimistic about being able to raise the operating frequency by 250MHz… A few percent is okay, but…
