New Possible Modifications for CS-series (or KN-Q7A) with Sandwich


I came up with this idea while thinking about the options with the sandwich VFO.  Why not program the jumper closed for one band and jumper open for another band.  Then modify the two band-pass filter chains to work with both frequencies.

First: I modified a sandwich so that when the jumper (switch) is shorted, I get 40 Meters.  When the jumper (switch) is open, I get 80 Meters.  The sandwich does have to be powered off and restarted to make the switch.

Here is the code difference:

if (digitalRead(JP))  //JP10: Open - 80 Meter, Short - 40 Meter


    op_freq = 3800000;  //set op_freq to the mid of the band

    band_high_limit = 4000000;  //set band high limit to 4.00 MHz for 80 Meter Band

    band_high_limit = 3500000;  //set band low limit 3.50 MHz

    bfo_freq = 8190100; //IF freq = 8190.1 kHz, BFO freq is around IF freq depending on LSB or USB, default to LSB

    bfo_low_limit = 8189500;

    bfo_high_limit = 8193500;




    op_freq = 7200000;  //set op_freq to higher portion of the band

    band_high_limit = 7300000;  //set band high limit to 7.30 MHz

    band_high_limit = 7000000;  //set band low limit 7.00 MHz

    bfo_freq = 8190100;     //historical reason: for KN-Q7A kits IF freq = 8192 kHz

    bfo_low_limit = 8189500;

    bfo_high_limit = 8193500;


This was for my older KN-Q7A with the 8.192 MHz IF.

The sandwich works great!

Next experiment was to look at putting two Band-pass filters in parallel (for a total of 4 in radio).  One filter was calibrated for 40 Meters and the other 80 Meters.  NOTE: This experiment has not been placed in the radio itself, yet.  Only 40 Meter or 80 Meter signals passed through as I had anticipated.  This would mean that I don't have to add a relay to switch the filters in and out.  I would just put them in parallel.  I am not certain as to the overall affect on the signal path such as impedance, etc. but it is an interesting step.

The frequency of the VFO (sandwich) should be able to select the appropriate signal coming through.  I will see what happens.


Great work!  I'd love to do this for a 20/40 split and shrink it down to something that would fit inside the case.



The Sandwich modification would work for any band you desire.  The band-pass filter addition will easily fit in the case.  It appears that the receive side filter would have to have a switch or relay to control the filter.  I am not sure, yet, that you need to switch out the transmit side filter.
I will be experimenting in the next few weeks.  I have attached my first pass of the filters, uninstalled in the radio.


I would think that you'd need a switch of some sort on the band pass filters, there are some decent bilateral analog switch ICs available (like this one ) if you want to avoid using relays or an external switch.  Where did you source the filter cans?  I'd be interested in experimenting with this a bit myself.



Adam Rong ( sells the DIY7-xx coils on special order.  They are designed for the CS-Series and KN-Q7A kits special.
The setup seems to be working with my modified KN-Q7A kit using a modified sandwich code.  I can not find any issue yet, but further testing needs to be run with better instrumentation than I have at home.
I used a relay on the receive side filter only.  The transmit filters are in parallel.  Again, all seems to work.
I have attached some photos of the transmit filters.  This board basically replaces the receive filter chain in the radio and is mounted to the inside of the lid.  The transmit filter is attached to the existing coil cans with double sided sticky tape.  So far, I see no cross talk issue.
I am working on documentation.  Adam may design a PCB and sell this as an upgrade kit if this works well.  I have sent him my documentation for review.


Thanks for that, I may be adding this mod sooner than I expected.  What did you wind up doing about the final LPF filter?


The final low-pass filter has sufficient bandwidth that it is not an issue for 40 and 80 Meters.  It would probably work for 20 and 40 Meters.
I do not have "HighTech" instruments, so I can't tell if there are any major issues.  I can check power and look at the waveform on an oscilloscope.
Shortly, I will begin working on a PCB for the solution.
I have mailed one of my customers the parts to try it out on his radio.
I would suspect that if all goes well, Adam will offer it as a kit as well.
-Larry N7RGW


I am putting together a kit upgrade that can be added to a KN-Q7A with Sandwich or the new CS-Series radio.  The kit will include two PC boards along with all components to make the modification to adapt the radio to operate in dual band mode.  
My first experiments have been with 40 Meters and 80 Meters.  It appears to work very well.  The kit will be available for any two frequencies of operation, 40/80 or 40/20, etc.
I am estimating that the initial cost will be $35 per kit plus shipping.  It will be posted on my website at  You can reach me at n7rgw@... as well.  It probably will not be ready for shipping for a couple of weeks while I am waiting for the PC boards to arrive and be checked out.
The first kits will be limited in quantity to 9, assuming the PCB works properly.  I will post parts lists and instructions within the next week.
You can build it from scratch if you wish.  I will try to provide any help you might need.
Larry - N7RGW


This will be a very worthwhile upgrade to the KN-Q7A and CS radios.

Thank you for making it available Larry.

I'm anxiously waiting to order one.


---In CHINA_QRP@..., <larry.lovell@...> wrote :

I am putting together a kit upgrade that can be added to a KN-Q7A with Sandwich or the new CS-Series radio. 


OK, I am finally ready to announce the Dual Band modification to the KN-Q7A or CS-series radio using the Sandwich VFO.  The modification is easy.
I can provide the Sandwich code modification based on three things:
    1)  Base frequency of original radio (basically the crystal filter chain frequency)
    2)  Base Band operation for example a CS-40 or 40 Meter band operation.
    3)  Additional band you wish to add such as 80 Meters to the 40 Meter base radio.

The cost of the kit includes a PCB for both the Receive and Transmit Frequencies, all parts including the appropriate DIY7 coils.  The introductory price will be $35 USD plus shipping. 

Software will be downloadable from my website once I complete the various permutations of the code.  The software and schematic will be available whether you purchase my kit or not.  However, I support what I sell.

I made a slight error to the first PCB, so you will need to follow the instructions regarding the coils.  I will fix the problem on the next revision of the PCB.  The error does not affect the signal or the filter.

The first run will have limited quantities available.  I have for example 4 kits ready for shipment 80/40 for Base 40 Meter.  I will kit others as the orders come in.

I am finishing the touches of the manual which should be available by the end of the day.