Let's Build Something Construction of the 40M Band Pass Filter 

Thanks for watching the video. Charlie Morris is a friend and undoubtedly a superb engineer. He has built some really great rigs. 73's Pete N6QW

Hi Peter, The 42IF123 IF Cans but disappeared from the world and so now I am forced to use discrete coils and caps. LT Spice the simulation program (free) from Linear Technology sure makes that task easier. I have one design developed using that program where with the addition or removal of two 150 PF caps the network can be tuned to either 40 or 20 Meters. See my searchable blog n6qw.blogspot.com and you can see the component values. also my website www.n6qw.com and look at the Sudden Transceiver project for the values. Thanks for your posting. 73's Pete N6QW

I am not sure what you are asking. Wire up the circuit as shown, and you will have proper phasing given how the IF cans are constructed.

@@N6QW Actually my question is about parts 42IF123 if not being available , and we get a green or blue or pink (IF 10.7 ) out of a used radio.

@@fariborzsadeghivafa4473 Long answer. I believe the other colors are not 10.7 MHz and so you will need the inductance of the inductor for that core and then you can just use the right size of capacitor to resonate with the that value for the band pass you are using. OR you could visit n6qw.com and tour through the projects as there many band pass filters with powdered iron cores and caps that will work in your project, Check the PSSST and the Sudden Transceiver projects Another desirable featire of the non-IF can is tha they can be scaled using an LT Spice simulation and so you can put the band pass on any range you want..

Hi,Not an easy answer that can be done without some significant calculation and modeling. There are several methods to do this (and I am not going to do it for you). The video involved the use of packaged transformers which are no longer available. Thus the only option is to build the band pass network from scratch. You have a requirement to have at least 600 kHz of pass frequencies with a base frequency of 8.8 MHz.One way is to hand calculate the component values using methodology shown in a publication entitled Solid State Design for the Radio Amateur by Wes Hayward. That calculations are not that difficult but may require you to make a couple of runs at it. A second method is to use free software programs that let you adjust values so that you can get a centered filter with a specific band pass. Two such programs are LT Spice (free) from Linear Technology. and the second is Elsie ( free from Tonne Software). Both programs let you simulate the filter and you can visually see the band pass. By making several runs you can finally arrive at the desired filter. Keep in mind these are simulations and that what you build in practice may have some effects from physical layout and stray capacitance.What was shown in the video had a smaller band width like 300 KHz and was a bit of a stretch for the packaged units. For a larger bandwidth (like 600 kHz) you will probably need to do the calculation. Another publication called EMRFD (Experimental Methods In RF Design) has programs to design BPF's --this is not a favorite publication of mine. My 1st choice would actually be a combination (and how I do it) of first hand calculating the component values and then use LT Spice to validate the design. The video was associated with a published project and was intended to provide a design that used standard components (now not available) so that the builder could just slap something together without having to do some serious engineering. Now you will have to engineer the answer. If you can actually find the specified transformers you can trial and error the values. The coupling caps most likely would be in the 1 to 2 PF range and since the transformer original design was 10.7 MHz and was moved down to 7.2 MHz with 68 PF --something in the 22 PF to 39 PF range should move it down to the range you want but making it do 600 KHz is very likely not a reality -- you will get several hundred kHz of pass but most likely not 600 kHz. To meet your specifications will require the rigorous calculation as described in the publication cited.Good luck with your project.73'sPete N6QW

Check the webpage for this project here www.jessystems.com/LBS_Detail.html There are two options we have listed: an alternate source for the transformers "Kit and Parts" or you can roll your own by building your own band pass filter. Kits and parts is also a source for the toroids used in the homebrew filter. Thank you for your interest in this project. Pete N6QW

kiriakos k Hi -- Thanks for your email. If you can buy the 42IF123 transformers then it would take about 15 minutes to build as it only requires three capacitors and the two IF cans. The cost could be around $10 USD depending if you have to buy everything. The version with discrete components would cost only slightly more and perhaps 1/2 to 1 hour to build depending on your skill level. Either is fairly simple to build. 73's Pete N6QW

Pete Juliano thank you a lot