K3/100 RF Feedback/Spurious oscillations ?
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K3/100 RF Feedback/Spurious oscillations ?
 
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Very strange effect noticed today when tuning up on 20m.
At power levels above 20 w on 14.175 my external Palstar SWR meter would suddenly leap from 1:1 to above 5:1. Also the K3 SWR display showed similar. High current was also reported. No problems with either a TS850S or K2, both OK up to 100W. Switching the RX Ant switch to the main antenna or removing the RX Antenna BNC lead cured the problem. I use the RX antenna input for 2 x Pennant loops for the LF bands, but find that they sometimes give a better SNR than the main antenna on higher bands. The main antenna system is a doublet fed with open wire feeders into a fully balanced (no toroid on output) ATU. My initial thoughts is that there might be some spurious output generated by an obscure feedback path from the RX antenna input. If this output were on a frequency well separated from the desired then it could show up as as high SWR. What think Ye ? 73 Stewart G3RXQ _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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> Switching the RX Ant switch to the main antenna or removing the RX
> Antenna BNC lead cured the problem. You may be receiving to much power on your RX, which could cause unwanted oscillations somewhere in the transmitter. It is always a good idea to automatically disconnect the RX antenna, e.g. with a small relay, when transmitting. A friend of mine damaged his RX front end several times before he figured out that he was getting to much power back into his rig via his beverages on 160m. vy 73 de toby _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
Re: K3/100 RF Feedback/Spurious oscillations ?
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In reply to this post by Stewart Baker
Stewart,
I reported something similar after CQWW CW, and there was some discussion about isolation between the RX antenna input and the TX stages of the radio. I'm not sure if there is a plan to do anything about it or not, but would be interested in solving this problem, as it impacted my ability to use our beverages on RX. 73, andy, ae6y, p49y ----- Original Message ----- From: "Stewart Baker" <[hidden email]> To: "Elecraft Reflector" <[hidden email]> Sent: Thursday, January 31, 2008 8:14 AM Subject: [Elecraft] K3/100 RF Feedback/Spurious oscillations ? Very strange effect noticed today when tuning up on 20m. At power levels above 20 w on 14.175 my external Palstar SWR meter would suddenly leap from 1:1 to above 5:1. Also the K3 SWR display showed similar. High current was also reported. No problems with either a TS850S or K2, both OK up to 100W. Switching the RX Ant switch to the main antenna or removing the RX Antenna BNC lead cured the problem. I use the RX antenna input for 2 x Pennant loops for the LF bands, but find that they sometimes give a better SNR than the main antenna on higher bands. The main antenna system is a doublet fed with open wire feeders into a fully balanced (no toroid on output) ATU. My initial thoughts is that there might be some spurious output generated by an obscure feedback path from the RX antenna input. If this output were on a frequency well separated from the desired then it could show up as as high SWR. What think Ye ? 73 Stewart G3RXQ _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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> about it or not, but would be interested in solving this problem, as it
> impacted my ability to use our beverages on RX. At the Bavarian Contest Club's contest QTH we use a simple relay to disconnect the rigs reception antennas whenever the rig goes to a TX state. A simple relay, a few simple parts and the PTT/KEY signal do the trick. Many rigs do not switch the RX antenna input when transmitting, so this always requires a bit of care - especially at high power levels. vy 73 de toby _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
Re: K3/100 RF Feedback/Spurious oscillations ?
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Toby,
Yes, that is a sensible way to do it. Does anyone have a simple circuit for this in an SO2R setup, where you would need to ground the RX input to the transmitting radio without grounding the receiving antenna that might also be connected to the second radio? 73, andy, ae6y ----- Original Message ----- From: <[hidden email]> To: <[hidden email]> Sent: Thursday, January 31, 2008 8:39 AM Subject: Re: [Elecraft] K3/100 RF Feedback/Spurious oscillations ? >> about it or not, but would be interested in solving this problem, as it >> impacted my ability to use our beverages on RX. > > At the Bavarian Contest Club's contest QTH we use a simple relay to > disconnect the rigs reception antennas whenever the rig goes to a TX > state. A simple relay, a few simple parts and the PTT/KEY signal do the > trick. > > Many rigs do not switch the RX antenna input when transmitting, so this > always requires a bit of care - especially at high power levels. > > vy 73 de toby > > _______________________________________________ > Elecraft mailing list > Post to: [hidden email] > You must be a subscriber to post to the list. > Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/subscribers.htm > Elecraft web page: http://www.elecraft.com > _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
Re: K3/100 RF Feedback/Spurious oscillations ?
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Any chance that part of the RF getting in through the RX antenna connection is
common-mode? If so, a simple ferrite toroid choke might solve the problem. Probably not, but I'd try that first, just because it's such a simple fix. Bill W5WVO Andrew Faber wrote: > Toby, > Yes, that is a sensible way to do it. Does anyone have a simple > circuit for this in an SO2R setup, where you would need to ground the > RX input to the transmitting radio without grounding the receiving > antenna that might also be connected to the second radio? > 73, andy, ae6y > ----- Original Message ----- > From: <[hidden email]> > To: <[hidden email]> > Sent: Thursday, January 31, 2008 8:39 AM > Subject: Re: [Elecraft] K3/100 RF Feedback/Spurious oscillations ? > > >>> about it or not, but would be interested in solving this problem, >>> as it impacted my ability to use our beverages on RX. >> >> At the Bavarian Contest Club's contest QTH we use a simple relay to >> disconnect the rigs reception antennas whenever the rig goes to a TX >> state. A simple relay, a few simple parts and the PTT/KEY signal do the >> trick. >> >> Many rigs do not switch the RX antenna input when transmitting, so this >> always requires a bit of care - especially at high power levels. >> >> vy 73 de toby >> >> _______________________________________________ >> Elecraft mailing list >> Post to: [hidden email] >> You must be a subscriber to post to the list. >> Subscriber Info (Addr. Change, sub, unsub etc.): >> http://mailman.qth.net/mailman/listinfo/elecraft >> Help: http://mailman.qth.net/subscribers.htm >> Elecraft web page: http://www.elecraft.com >> > > > _______________________________________________ > Elecraft mailing list > Post to: [hidden email] > You must be a subscriber to post to the list. > Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/subscribers.htm > Elecraft web page: http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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