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BPF Filter question #bpf

Jack Brabham - KZ5A
 

I am considering building a 10 band BPF filter set to function as front end filtering for an Afedri SDR. The filters all look good for this purpose except the 80M filter which appears to run out of bandwidth before getting to 3885.   80M digital is a priority as well as is the AM window around 3885.   So... I'm wondering if  someone has worked out a mod for a 75M filter?   I would just drop the 60M filter and have seperate filters for 75M and 80M in the 10 band set.

73 Jack KZ5A

Ronald Taylor
 

Hi Jack. According to the specifications on this filter, it has a 3 dB bandwidth of 465 kHz on 80 meters. You should be able to see from the digital segment to the AM segment with no more than 3 dB additional loss from the center. You should be able to peak the filter somewhere between those two band segments and come pretty close to those numbers. Of course, if you want the filter to have minimum loss at both band segments, you will need to use two separate filters as you mentioned.

Good luck … Ron

On Fri, Nov 15, 2019 at 7:18 AM Jack Brabham - KZ5A <kz5a@...> wrote:
I am considering building a 10 band BPF filter set to function as front end filtering for an Afedri SDR. The filters all look good for this purpose except the 80M filter which appears to run out of bandwidth before getting to 3885.   80M digital is a priority as well as is the AM window around 3885.   So... I'm wondering if  someone has worked out a mod for a 75M filter?   I would just drop the 60M filter and have seperate filters for 75M and 80M in the 10 band set.

73 Jack KZ5A

Hans Summers
 

Hi Jack

The 80m filter has plenty of bandwidth to cover the whole band.

If you do happen to want it wider this can be achieved by using a slightly larger coupling capacitor between the two tuned circuits, this is covered in the manual. 

73 Hans G0UPL 

On Fri, Nov 15, 2019, 17:18 Jack Brabham - KZ5A <kz5a@...> wrote:
I am considering building a 10 band BPF filter set to function as front end filtering for an Afedri SDR. The filters all look good for this purpose except the 80M filter which appears to run out of bandwidth before getting to 3885.   80M digital is a priority as well as is the AM window around 3885.   So... I'm wondering if  someone has worked out a mod for a 75M filter?   I would just drop the 60M filter and have seperate filters for 75M and 80M in the 10 band set.

73 Jack KZ5A

 

For info, atmospheric noise in a 2.5Khz window on 80m is about -100dbm or -111dbm in a CW window.
At 28Mhz that becomes -120dbm and -130dbm.

I think most rx's have more than enough gain to compensate for a few db lost in a filter especially at LF.

Spreadsheet for this stuff, and more is here www.qsl.net/g0ftd/other/misc/


73 de Andy