Cross Band Interference with Flex 6600M and Elecraft KPA500

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  • Updated 2 months ago
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I have a strange thing happening in my SO2R setup with my 6600M and Elecraft KPA 500 / KAT500 setup.  When running High Power in a contest (i.e. 500 watts), I normally have the KPA500 on Slice A (Antenna 1) while still using Slice B (Antenna 2) at 100 watts.  In this configuration, I can transmit on Slice A at 500 W and have very little interference on Slice B which is great.  However, surprisingly, when I transmit on Slice B at 100 Watts, it almost totally wipes out Slice A.  In this example, I have Slice A on 20 CW and Slice B on 15 CW.


The strange thing is that this only happens if I have the KPA500 in "Operate" mode.  If I have the KPA500 in "Standby" or "Off", I don't have the problem.  In other words, having the KPA 500 in "Operate" on Antenna 1 (even though I am not transmitting) causes RX on Slice A it to be wiped out when transmitting on Slice B (Antenna 2).  

I have tried different band combinations (e.g. 40 and 20, etc.) with the same results.  Also, I do have external Bandpass Filters on both slices (Dunestar).  The Flex 6600M, KPA500, and KAT 500 all have separate ground wires running to my common station ground point.


Has anyone else experienced this or have any suggestion what I might try?  Like I said, it is not a problem when the amp in in standby or off but it is surprising to me that having the amp in operate would affect RX on that slice.


73,

AL, K0AD
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Al Dewey

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Posted 3 months ago

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K3SF

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hi Al

Would be good to have a better description or wiring diagram of the setup. That includes antennas, antenna switches and how things are run to and from flex to the amp and then the antennas.  

I am not able to visualize your setup based on the problem description.

i would look to see if some configuration to determine if the cause maybe RF going somewhere you think it is not.

also transmit and receive antenna configuration on the flex


Paul K3SF

For example here is my RF distribution setup


(Edited)
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Al Dewey

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I'll try to put something together.  Probably a good exercise anyway.  May take a few days.

73,

AL, K0AD
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Kari Gustafsson

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Hi, I had a similar experince with 2 x OM2500A in SO2R. When one in Operate mode I got rf feedback to other slice and also detected rf on power meter of the amp. De-taching PTT send cable to the amp avoided the phenomena. But no good solution in a real live contest .Unfortunately I forgot how I solved the problem.
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Al Dewey

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Kari - I had some time today so looked at this a little more given your hint.  Your experience is exactly what I am seeing.  I have the Elecraft KPA500 amp hooked to Slice A / Antenna 1 and no amp hooked to Slice B / Antenna 2.  When I transmit on Slice B (15 meters) with only 1 watt, it totally wipes out Slice A (20 meters).  This only happens when the Elecraft amp is in Operate.  There is no interference at all when the amp is on Standby.  I also noticed that, if I disconnect the PTT Keying Cable on the Elecraft amp (still on Slice A / Antenna 1), I get no interference at all when transmitting on Slice B / Antenna 2.

I thought I might be able to solve the problem by putting an isolation relay in the PTT Keying Cable but that did not work even if I used a battery to power the relay!

I am really stumped on this one.  It is hard to believe it is RF as the problem happened even if I have the power tuned all the way down to zero on Slice B.  Another idea I had was to add a relay that would put the amp in Standby except when I was transmitting on Slice A.  But there does not seem to be a way to externally switch from Standby to Transmit on the Elecraft amp.

If you happen to remember how you solved your problem with the OM2500As , let me know.  I am really stumped on this one.


73,

Al, K0AD
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Ken - NM9P, Elmer

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How are you getting PTT sent to the amp on ANT A? Is it accidentally keying the amp when transmittingnon ant B?
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Al Dewey

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Ken - Great tip!  Yes, I just checked and TX1 (which is connected to the amp on Antenna 1)  is indeed being keyed when I transmit on Slice B / Antenna 2.

Is there a way to configure things so that TX1, TX2, and TX3 can be assigned to a specific Antenna or Slice?  Seems like it would be something on the Transmit Tab but I can't find it.

Let me know.

73,

Al, K0AD
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bobby

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Unfortunately if you look at the profiles the slice, rcv, xmt antenna are in the global profile and the tx1,tx2, and tx3 are in the transmit profile. I see no way you can get the rig to change transmit profile based on which antenna you have or slice selected without changing global profiles everytime you want to use the other antenna and that would be a total pita for an operator.


The option I see would be a manual switch which would disconnect the rca plug from either the flex or the kpa when you wanted to use the other slice, once again a pita and not operator friendly.


The real solution is to have flex change the profile operation and allow an antenna to be assigned to each tx input independently, but you know what the chances of that happening would be, zilch.


Maybe you could use an mfj1708 to detect the xmt on ant b to close a ckt to cause a relay to open disconnecting the rca cable from the kpa as soon as you transmit on b.
(Edited)
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Al Dewey

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Bobby;

That's what I was afraid of.  I have studied the Flex 6600M documentation hoping I would find an output that tells what Slice or Antenna is current transmitting.  I would use such an output to drive a relay to disconnect the RCA PTT cable as you suggest.  So I'll have to find some other way around this.  Having a manual switch would not be practical during a fast paced contest when SO2R is being used.  Also, I could just hit the Operate / Standby switch on the amp instead of some other switch.


I feel this is "hole" in the design.  It is difficult to position the 6600M has an SO2R radio if it cannot drive two separate amps on each antenna / slice.  

73,

Al, K0AD
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bobby

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Al I found this under transmit profiles look at the bottom paragraph highlighted in yellow which indicates you should be able to set the transmit profile to make it work for you, see photo.
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bobby

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Transmit profiles are linked to the tx antenna setting in the transmit slice, that should do it.
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Rich - N5ZC

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Al,

I have to transmit profiles setup.  I call them SO2R TX1 and SO2R TX2.

SO2R TX1 enables RCA TX1 and disables all the other PTT outputs
SO2R TX2 enables RCA TX2 and disables all the other PTT outputs including TX1

SO2R TX1 is activated when I'm in slice A and its the active TX slice
SO2R TX2 is activated when I'm in slice B and its the active TX slice

It's been a while since I set it up. But I would setup something similar to SO2R TX1 while slice A is active, save the profile then switch to Slice B and make it the active transmit slice.  Set it up and save the profile.

After you get those two basic profiles setup, go to each band while in Slice A with SO2R TX1 selected and adjust your power output to drive your amplifer, save the profile.  Then go to the next band, adjust the power, save the profile, etc etc. 

Do the above for Slice B SO2R TX2 and adjust your power level, save the profile and move to the next band and repeat.

Don't forget to be in FDX (full duplex mode).

Hopefully I understood your original question and this helps.

Rich - N5ZC
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Al Dewey

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Rich;

Thanks for the great instructions.  I followed them last night and it solved my problem !  I was getting all ready to hook up a complex relay system that would disconnect the Slice A / Antenna 1 PTT relay when transmitting on Slice B / Antenna 2.  That is no longer necessary.  

Again thanks for the great tip.  Now I am all ready for SO2R in the  ARRL RTTY Roundup this weekend.


73,

Al, K0AD
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Rich - N5ZC

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Glad I was able to help. 

See you in the Roundup this weekend. 

I'm not very good at SO2R, but RTTY is easier to manage since the computer is doing most of the work. 

73,

Rich - N5ZC
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Rich - N5ZC

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Al,

** Correct **
I just read the manual you posted.  That last paragraph with the bullets is how I setup my profiles. My setup is probably driven by the Antenna selection not by the Slice.

I have a 4O3A antenna genius 8x2 switch. Ant A is connected to one side of the switch and Ant B is connected to the other side.

Rich - N5ZC
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Kari Gustafsson

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Hi, yes the way you and Rich discuss is the way I solved it. Good luck. Doing SO2R is a blast ....