Re: Integration of External Band Pass Filters

Drew Vonada-Smith K3PA

Hi Bud,

I use switched low power BPFs at the input of each transceiver (K3 and FT5k), so I have not tried your RX loop method.  But I took a look at the block diagram and those two inputs are not electrically equivalent, so a few things occurred to me:

1) If there is no filter at the main ANT1 (when its in the RX loop), is it possible that the ingress is so high as to fire the GDT on that input?   I can't find the value of GDT they used.  If that happens all bets are off, of course.  But it seems unlikely as I believe you have high power filters in the system also.  

2) The same thought with regards to the linearity of the ANT1/2 T/R switch.  It is not in the RX antenna loop.

3) The RX loop is on the other side of the TX low pass filter array from ANT1.  So, the out of band impedances presented to the filter by RX in/out may be a lot different than they are at ANT1.  Those high order filters might be pretty sensitive to that.  Perhaps that plays havoc with rejection?  

4) There's quite a bit of circuitry in the transverter/RX ant board that I have not had time to go through in detail.  But if the RF amplitudes were above a diode would have to take a peek.

I guess you would need a spec an or VNA to investigate some of that unless someone has already done the exercise.  Do you know the amplitude of the 20M energy in the 10M radio front end when there is no filter?  That would narrow the likely scenarios.   

Drew K3PA

---In Elecraft_K3@..., <aa3b.bud@...> wrote :

Clarification to my last message:

"I transmitted a 500W CW signal on 20M and the K3 was receiving on 10M."

---In Elecraft_K3@..., <aa3b.bud@...> wrote :

I am experiencing an issue with band pass filters (BPF) here that I don’t understand and I am hoping that someone can help me work through it.


I have been experimenting with BPFs to eliminate interference while doing single operator two radio contesting (SO2R).


My K3 includes the KRX3 sub receiver, KAT3 ATU and KXV3 option.  The BPFs are 100W units manufactured by LPS.


My experiment tested two configurations as follows:


1.  BPFs connected in series with the coax going to the rear panel Antenna 1 connector.

2.  BPFs connected between the rear panel connectors RX ANT IN and RX ANT OUT.  The RX antenna was activated from the front panel.


I transmitted a 500 CW signal on 10M and the K3 was receiving on 10M.


Configuration 1 worked fine with no interference heard on 10M.   Configuration 2 did not work – strong interference was present and I got a “High Signal” warning.  I would have expected similar performance in both configurations. 


I really would like to use Configuration 2 as a permanent solution but it appears not to work.  Any advice?




Bud AA3B


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