[K2] KPA100 - Antenna Help
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Hi All,
A bit of a strange one here.. Short story, I have just built and installed a KPA100 internal amp for my K2. All is going well with the set-up and I have been using fine with a multiband vertical I have at home. Problem is I put up an end-fed 1/2 wave vertical to use in a local contest this weekend and I am seeing a strange phenomenon!! Below is a series of observed events. 1. Erected antenna, checked SWR with analyzer (SWR 1.2:1) 2. Connected to K2/100 via about 10m of RG-213 and checked SWR using 10w = SWR still 1.2:1, so far so good. 3. Turned up power (50w), checked SWR, getting Hi Refl reading on K2, and external meter, blown fuse in K2/100 lead. 4. Replaced fuse, check again at 10w OK, turn power up to 11w and engage KPA100, get Hi Refl.. 5. Check K2/100 using dummy load and my other multiband vertical, all OK at 10w and 100w. 6. Use K2 at 10w to drive my external amp, to 400w to check the 1/2wave antenna and all OK, SWR 1.2:1 7. With the KPA100 engaged the power seems to way overshoot on the K2, hence the blown fuse. However, this doesn't happen into a dummy load or other antenna. Looks like my K2 with KPA100 doesn't like the 1/2 wave end-fed antenna at all, but for the life of me not sure why. I first though I must have burnt out the 1/2wave matching circuit somehow on the antenna, but obviously this is OK as it will handle 400w. The end fed 40m halfwave is home-made, and is basically a airwound coil with a couple of doorknob capacitors in series to match the high impedance at the end of the 1/2w wire to 50 ohms. In short it is a beefed up Par-End Fedz antenna. Speaking of which, I have a couple Par-End Fedz antennas, which I plan to check to see if I get the same problems. Any suggestions would be appreciated. David, VK2NU ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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OK, to add to my last,
I've just tried with a different antenna on 40m. Again, once I engage the KPA100, I get high SWR, power overshoot and a blown fuse. So it doesn't seem to be antenna related but seems to be band related. 30m-10m is Ok. I don't have an antenna for 80/160 so can't check. All bands are fine with a dummy load... David ----- Original Message ----- From: David Douglass To: [hidden email] Sent: Saturday, June 12, 2010 12:04 PM Subject: [K2] KPA100 - Antenna Help Hi All, A bit of a strange one here.. ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by David VK2NU
David,
Is this a new KPA100 from Elecraft, or did you purchase the kit from a previous owner. What I am getting at is -- do you have the latest T/R switch components installed in this KPA100? Blue toroid cores at RFC1 and L16. If the toroid core at L16 is red, you have the older T/R switch design, and it is susceptible to a parasitic oscillation above the 40 meter band. The KPA100UPKIT is the cure should you have the lower level KPA100. 73, Don W3FPR David Douglass wrote: > Hi All, > > A bit of a strange one here.. > > Short story, I have just built and installed a KPA100 internal amp for my K2. All is going well with the set-up and I have been using fine with a multiband vertical I have at home. > > Problem is I put up an end-fed 1/2 wave vertical to use in a local contest this weekend and I am seeing a strange phenomenon!! Below is a series of observed events. > > 1. Erected antenna, checked SWR with analyzer (SWR 1.2:1) > 2. Connected to K2/100 via about 10m of RG-213 and checked SWR using 10w = SWR still 1.2:1, so far so good. > 3. Turned up power (50w), checked SWR, getting Hi Refl reading on K2, and external meter, blown fuse in K2/100 lead. > 4. Replaced fuse, check again at 10w OK, turn power up to 11w and engage KPA100, get Hi Refl.. > 5. Check K2/100 using dummy load and my other multiband vertical, all OK at 10w and 100w. > 6. Use K2 at 10w to drive my external amp, to 400w to check the 1/2wave antenna and all OK, SWR 1.2:1 > 7. With the KPA100 engaged the power seems to way overshoot on the K2, hence the blown fuse. However, this doesn't happen into a dummy load or other antenna. > > Looks like my K2 with KPA100 doesn't like the 1/2 wave end-fed antenna at all, but for the life of me not sure why. > > I first though I must have burnt out the 1/2wave matching circuit somehow on the antenna, but obviously this is OK as it will handle 400w. The end fed 40m halfwave is home-made, and is basically a airwound coil with a couple of doorknob capacitors in series to match the high impedance at the end of the 1/2w wire to 50 ohms. In short it is a beefed up Par-End Fedz antenna. Speaking of which, I have a couple Par-End Fedz antennas, which I plan to check to see if I get the same problems. > > Any suggestions would be appreciated. > > David, VK2NU > > Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by David VK2NU
Dave, I was typing a reply when Don's message appeared, which is likely the correct information. I can only add that the parasitic oscillation is probably out of the "range" of your antenna as indicated by the high SWR. The dummy load doesn't care / know if the RF from the PA is on a "correct" frequency / band. (:-) 73! Ken Kopp - K0PP ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by David VK2NU
David,
I just thought of something else - the KPA100 may be oscillating on you. That fact will not show up on a dummy load, but because an antenna is frequency selective, it will indicate Hi-refl. The KPA100 *should* detect that condition and reduce the power, but for some reason yours is not. and I strongly suspect the T/R switch area. Check the T/R switch components carefully - particularly the orientation of Q6 and Q7 and the diodes. Also check the soldering for C31. Make certain the value of C31 is .22 uF. Since you likely do not want to test on-the-air with that condition, if you have a tuner (preferrably a link coupled tuner), connect the output to a dummy load and then adjust the tuner for a 1:1 SWR at the frequency you will be testing. If the KPA100 is oscillating at some parasitic frequency, Hi-Refl will be indicated because of the tuned circuit between the KPA100 and the dummy load. 73, Don W3FPR David Douglass wrote: > OK, to add to my last, > > I've just tried with a different antenna on 40m. Again, once I engage the KPA100, I get high SWR, power overshoot and a blown fuse. So it doesn't seem to be antenna related but seems to be band related. 30m-10m is Ok. I don't have an antenna for 80/160 so can't check. > > All bands are fine with a dummy load... > > David > ----- Original Message ----- > From: David Douglass > To: [hidden email] > Sent: Saturday, June 12, 2010 12:04 PM > Subject: [K2] KPA100 - Antenna Help > > > Hi All, > > A bit of a strange one here.. > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > > Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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