Catalog Filters

I was working on a project for the radio club and needed a couple of filters. One for 571 Mhz and the other for 735 Mhz. Previously I made a stripline filter and I have done copper pipe filters. These are rather large and I wanted something quick. Looking in the Digikey catalog I found filters by Toko. If you are buying a million of these filters I am sure Toko would make them exactly to your spec. For ham use in single quantity we have to take what is stocked in the catalog. Of course the catalog selection was made without consulting us hams to what we might need. The chance of finding something in stock value that is useful approches Epsilon. Much to my surprise I found a helical filter for 734 MHz with a 10 MHz bandwidth. Ok, lucked out on that one. For 571 the closest I could get was 550 MHz. Could I tune it up to 571? This little two section filter has tuning screws at the top of the cans. The screws were near the top so it looked like I was already at the top of the frequncy limit. As it turned out I was able to tune up to 571 MHz. For a test I ran the screws down to see how low I could get and still have the same overall filter response. The stock specification was Fo = 550 MHz, 10 MHz bandwidth. I don't recall the published insertion loss but I was looking at about 5DB. That was measured at the dip of the ripple which was approximatly 2 DB. My test jig was not ideal for grounding the cans, terminating the filter, and making connection to the RG188 teflon cable so I am sure I introduced some error in the response. The results:

Minimum Fo = 494.1 MHz
Maximum Fo = 586.0 Mhz

Other specs remained about the same but the insertion loss and bandwidth was increasing as I approched the upper limit.

For this filter I can pull about 8% from center but the catalog frequency was already near the top of the range. This would be unknown to the purchaser until after they examined the product.

If you need a filter check the catalog offerings from Digikey and Mouser. You may luck out and find something that might tune to the desired frequency but I would not try to go beyond 8 to 10 percent.

For reference this part was Toko type 7HW, Part number # 252HXPK-2736F (Digikey #TK3307-ND)