Good investigation Scott, ties in with my findings. Protem I have installed a relay to short the loop pre-amp output on TX.
73 Stewart G3RXQ On Sat, 2 Feb 2008 18:22:57 -0800, Scott Prather wrote: > > When I read about this report from Stewart last night (as well as the > responses indicating that other had experienced a similar effect) I decided > to set up the bench test to see if I could emulate the problem. I've since > discovered that these reports are valid, the K3 PA will generate a severe > spectral "comb" if there's insufficient isolation between the RX and TX > antennas when the KXV3 RX input is used. > > To execute this test, I ran the K3 into a dummy load through an RF coupler > that displays 26 dB of isolation. The output of this coupler was split (3 dB > splitting loss) between a Tektronix 496 spectrum analyzer and a Tektronix > 2701 50-ohm step attenuator. The output of the 2701 was fed into the K3's RX > input. Initially, the attenuator was set to 0 dB. > > The K3 was keyed at full output (100W or +50 dBm), which results in a +21 > dBm signal back into the K3's RX input. Under this condition, if the RX port > is enabled, the K3 will generate a spectral comb with the most severe spurs > only 10 to 20 dB down from the carrier. Turning the RX input off clears the > problem. I was able to create this comb spectrum on 160, 80, and 40 meters, > with 40 displaying the highest-power spurs. > > Lowering the input power to the K3 via the 2701 attenuator indicates that > the "sweet spot" for generating this spurious oscillation occurs when > there's an input signal of about +20 dB. This would correspond to a minimum > of 30 dB of TX antenna to RX antenna isolation when operating at 100W output > power or 40 dB of isolation at 1 kW. I have spectrum analyzer photos of the > spurious signals generated by the K3 under these conditions if anyone is > interested. > > I ran the same test under the same conditions with my K2/100, and this > problem does not occur. > > The bottom line here is that a +20 dBm signal (100 mw) will create severe > spurs on the K3 but not the K2 when operating in the 40M band and below. > Above 40M I was unable to duplicate the problem, perhaps a higher power to > the RX input is required. > > It's important to note that anyone who has +20 dBm or more coming back on > the receive antenna is asking for trouble in the first place, and additional > isolation and/or a shorting relay during TX operation is probably a good > idea. > >> From the reports, it sounds like there have been a few folks who had rather > high power coming back in through the RX input for a while and didn't notice > it until this secondary symptom was noted in the K3. I haven't looked at the > RX input circuit in the K2 to see how it differs from the K3 so that I can > find the root-cause of this problem. > > Scott > N7NB > > K2 #1285 > K3 #219 > > > ------------------------------ > > Message: 34 > Date: Thu, 31 Jan 2008 16:14:13 GMT > From: Stewart Baker <[EMAIL PROTECTED]> > Subject: [Elecraft] K3/100 RF Feedback/Spurious oscillations ? > To: Elecraft Reflector <[email protected]> > Message-ID: <[EMAIL PROTECTED]> > Content-Type: text/plain; charset="iso-8859-1" > > 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: [email protected] > 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: [email protected] 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
